Banner for this page

Here you can find recent articles from Nature, Acta Materialia, Scripta Materialia, Computational Materials Science, Physical Review Letter, Physical Review B, Journal of Physics: Condensed Matter and Science and Engineering A. If you have more suggestions, feel free to write me!

Sun Feb 25 2018

Study of pattern selection in 3D phase-field simulations during the directional solidification of ternary eutectic Al-Ag-Cu

Author(s): Philipp Steinmetz, Johannes Hötzer, Michael Kellner, Amber Genau, Britta Nestler

During the directional solidification of ternary eutectic alloys, different arrangements of the three solid phases evolve, leading to multiple microstructure patterns. These patterns influence the macroscopic properties of the alloy. Different arrangements are even found in single micrographs for the same imposed process conditions. Gaining a better understanding of the complex mechanisms leading to these different patterns is crucial to obtain tailored microstructures with specific properties. To exploit the different patterns which appear in ternary eutectic Al - Ag - Cu , a coupled approach is undertaken using a large-scale simulation and extensive parameter studies on smaller 3D domains. Three patterns found in both large-scale simulations as well as experimental micrographs are investigated with smaller domain size simulations by systematically varying the lamellar spacings. The resulting undercooling-spacing correlations of all investigated phase arrangements follow a Jackson-Hunt-type shape. Different stability ranges of the initially set microstructure are found, and depending on the lamellar spacings, the pattern with the lowest undercooling changes. The combination of both outcomes gives an explanation for the presence of different patterns within single micrographs of directionally solidified ternary eutectic alloys.

Computational Materials Science

Advanced atomistic models for radiation damage in Fe-based alloys: Contributions and future perspectives from artificial neural networks

Author(s): N. Castin, M.I. Pascuet, L. Messina, C. Domain, P. Olsson, R.C. Pasianot, L. Malerba

Machine learning, and more specifically artificial neural networks (ANN), are powerful and flexible numerical tools that can lead to significant improvements in many materials modelling techniques. This paper provides a review of the efforts made so far to describe the effects of irradiation in Fe-based and W-based alloys, in a multiscale modelling framework. ANN were successfully used as innovative parametrization tools in these models, thereby greatly enhancing their physical accuracy and capability to accomplish increasingly challenging goals. In the provided examples, the main goal of ANN is to predict how the chemical complexity of local atomic configurations, and/or specific strain fields, influence the activation energy of selected thermally-activated events. This is most often a more efficient approach with respect to previous computationally heavy methods. In a future perspective, similar schemes can be potentially used to calculate other quantities than activation energies. They can thus transfer atomic-scale properties to higher-scale simulations, providing a proper bridging across scales, and hence contributing to the achievement of accurate and reliable multiscale models.

Computational Materials Science

Atomistic simulation of shear-coupled motion of [1 1 0] symmetric tilt grain boundary in α-iron

Author(s): Jian Yin, Yi Wang, Xiaohan Yan, Huaiyu Hou, Jing Tao Wang

Shear-coupled grain boundary (GB) motion (SCGBM) is an important and efficacious plasticity mechanism in the deformation of metals, especially nanocrystalline metals. In this work, molecular dynamic (MD) simulation has been performed to investigate the SCGBM of two [1 1 0] symmetric tilt GBs, Σ9[1 1 0](2 2 1) and Σ17[1 1 0](2 2 3), in α-iron, and the effects of temperature and strain rate on SCGBM have been studied. The coupling factor β which is defined as the ratio of the velocities of GB lateral translation and migration was calculated, and a geometric model of β depending on the misorientation angle was constructed in [1 1 0] symmetric tilt GBs of BCC metals. The model was branched into two modes (〈1 0 0〉 and 〈1 1 1〉) corresponding to the perfect dislocation Burgers vectors in BCC metals. The β values calculated in the 〈1 1 1〉 mode were in good agreement with the MD simulation results for both the GBs. Further, the atomistic mechanisms of the SCGBM processes were also investigated. A same structural unit transformation was observed for the two GBs, which confirmed that both Σ9[1 1 0](2 2 1) and Σ17[1 1 0](2 2 3) GBs moved in the 〈1 1 1〉 mode during the SCGBM process.

Computational Materials Science

Density functional theory study on the stability, electronic structure and absorption spectrum of small size g-C3N4 quantum dots

Author(s): Shuncheng Zhai, Ping Guo, Jiming Zheng, Puju Zhao, Bingbing Suo, Yun Wan

The geometric stabilities, electronic properties as well as ultraviolet–visible (UV–Vis) absorption spectra of the g-C3N4 quantum dots were systematically investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. Taking the tri-s-triazine as a basic building unit, three types of planar structures of the g-C3N4 quantum dots were designed. The chain-type g-C3N4 quantum dots are the lowest in energy. The growth pattern of the g-C3N4 quantum dots had been speculated. The absorption spectra of the triangular sheet structure are in good agreement with the experimental results, and the absorption spectra of the (g-C3N4)15 quantum dots with a 3.41 nm lateral size has covered most of the visible light area. These small size g-C3N4 quantum dots are promising to be used as graphitic carbon nitride-based composite materials for energy conversion.

Computational Materials Science

First principles studies of superhard BC6N phases with unexpected 1D metallicity

Author(s): Yufei Gao, Yingju Wu, Quan Huang, Mengdong Ma, Yilong Pan, Mei Xiong, Zihe Li, Zhisheng Zhao, Julong He, Dongli Yu

Three novel sp 2-sp 3 hybridized BC6N phases with a sandwich structure, including a type of orthorhombic BC6N (o-BC6N) and two types of tetragonal BC6N (t-BC6N-1 and t-BC6N-2), are investigated through first principles calculations. The structural stabilities are confirmed by the calculated elastic constants and phonon dispersions. Calculated electronic band structures, density of states (DOS), and partial DOS show that the o-BC6N, t-BC6N-1, and t-BC6N-2 crystals may possess the metallicity with the conducting electrons from the p orbits of sp 2-hybridized C atoms. Calculations of electron orbits indicate that the electrons in o-BC6N and t-BC6N-1 structures can conduct through the π bonds along the orientation parallel to the [1 0 0] and [0 1 0] directions in different layers. Moreover, the electrons in t-BC6N-2 structure can conduct along the orientation parallel to the [1 1 0] and [ 1 ¯  1 0] directions in different layers. The behavior of the linear electron conductivity in the layer and vertical direction of conduction between the adjacent layers imply that the three kinds of crystals have potential applications in the field-effect devices. Calculation results using the semi-empirical microscopic model show that o-BC6N, t-BC6N-1, and t-BC6N-2 are potential superhard materials with Vickers hardness of 52.4, 45.3, and 40.1 GPa, respectively.

Computational Materials Science

Modeling amorphous silicon nitride: A comparative study of empirical potentials

Author(s): Atreyi Dasmahapatra, Peter Kroll

We perform a comparative study of empirical potentials for atomistic simulations of amorphous silicon nitride (a-Si3N4). We choose 5 different parameterizations of the Tersoff potential, the Marian-Gastreich two-body (MG2) and three-body (MG3) potential, the Vashishta (V) potential, and the Garofalini (SG) potential. Amorphous models of Si3N4, comprising of 448 atoms, are generated by each empirical potential using a melt-and-quench procedure. Subsequently, models are optimized using Density Functional Theory calculations, and structures resulting from these DFT optimizations are compared. We emphasize local coordination of atoms and the enthalpies of formation (ΔHf) relative to crystalline β-Si3N4. The SG and MG2 potentials prove to be best options for modeling of a-Si3N4. Models generated with these potentials are close to their DFT local minimum, exhibit the smallest number of defects, and have realistic enthalpies of formation.

Computational Materials Science

The influence of chemical heterogeneities on the local mechanical behavior of a high-entropy alloy: a micropillar compression study

Author(s): Anita Heczel, Megumi Kawasaki, Dávid Ugi, Jae-il Jang, Terence G. Langdon, Jenő Gubicza

The effect of the chemical inhomogeneities on the local mechanical behavior was studied in a CoCrFeMnNi high-entropy alloy. Micropillar compression revealed that, despite the difference in the chemical composition, the stress-strain behaviors in the two regions were almost identical. The size effect was negligible in the micropillar compression experiments.

Science and Engineering A

Mechanical Twinning in Ni-based Single Crystal Superalloys during Multiaxial Creep at 1050°C

Author(s): Jean-Briac le Graverend, Florence Pettinari-Sturmel, Jonathan Cormier, Muriel Hantcherli, Patrick Villechaise, Joël Douin

Multiaxial high-temperature creep tests have been performed at 1050°C on a first-generation Ni-based single-crystal superalloy through the use of asymmetric notched specimens. Mechanical twins have been observed by EBSD and TEM analyses. A finite element simulation in a crystal plasticity framework has also been carried out to better understand the presence of mechanical twins at stress concentrators depending on stress triaxiality, plastic strain rate, and damage level.

Science and Engineering A

An experiment-based model of combined hardening and non-hardening embrittlement in an interstitial free steel

Author(s): Yu Zhao, Shenhua Song

The fracture appearance transition temperatures (FATTs) are evaluated for the samples of a Nb-stabilized and P-strengthened interstitial free (IF) steel. Based on the measurements, the combined effect of hardening and phosphorus grain boundary segregation on the embrittlement of the steel is investigated. Both hardening and P boundary segregation can raise the FATT of the steel, causing hardening embrittlement and non-hardening embrittlement, respectively. Meanwhile, the grain size influences both kinds of embrittlement, i.e., there is a synergistic effect between grain size and hardening or P boundary segregation on the FATT of the steel. With the aid of the Taylor expansion along with the experimental data, a combined hardening and non-hardening embrittlement equation is established, being expressed as FATT = 2.1C p + 3.48σ s − 22.36d −1⁄2 + 0.64(C p − 14)(d −1⁄2 − 3.06) + 0.896(σ s − 14)(d −1⁄2 − 3.06) − 13.7, where FATT is the fracture appearance transition temperature in °C, C p is the phosphorus boundary concentration in at.%, σ s is the yield strength in 10MPa, and d is the grain size in mm. A comparison of the calculated and measured FATTs is made, demonstrating that the calculated FATTs are well consistent with the measured ones.

Science and Engineering A

Multi-phase intermetallic mixture structure effect on the ductility of Al3Ti alloy

Author(s): Zichuan Lu, Fengchun Jiang, Yunpeng Chang, Zhongyi Niu, Zhenqiang Wang, Chunhuan Guo

In this work, to improve the ductility of Al3Ti alloy, a multi-phase intermetallic mixture structure was achieved by the post annealing treatments (AT) on the as-fabricated continuous shape memory alloy NiTi fiber reinforced Al3Ti composite (CSMAR-Al3Ti). Experimental results revealed that the multi-phase intermetallic mixture structure displays a multi-layer characteristic, including the newly formed intermetallic layer and the eutectic area. Microstructure characterization and formation mechanism confirmed that the newly formed intermetallic layer consists of NiAl, Al3Ni2 and Al3Ti phases, and the eutectic area still consists of Al3Ni and Al3Ti phases. The results of compression and tensile tests indicated that the multi-phase intermetallic mixture structure can simultaneously improve the maximum strength and the failure strain of the monolith Al3Ti alloy. Based on the systematic investigations, it is found that the toughening mechanism is related to the multi-layer characteristic of the multi-phase intermetallic mixture structure, which is beneficial to the crack blunting, crack deflecting and load transformation during the deformation process. Furthermore, the multi-phase intermetallic mixture structure achieved by the 48h post annealing treatment is the most effective method to improve the ductility of Al3Ti alloy.

Science and Engineering A

Realizing the Haldane Phase with Bosons in Optical Lattices

Author(s): Junjun Xu, Qiang Gu, and Erich J. Mueller

We analyze an experimentally realizable model of bosons in a zigzag optical lattice, showing that, by rapidly modulating the magnetic field, one can tune interaction parameters and realize an analog of the Haldane phase. We explain how quantum gas microscopy can be used to detect this phase’s nonloc...

Physical Review Letters

Overcoming the Time Limitation in Molecular Dynamics Simulation of Crystal Nucleation: A Persistent-Embryo Approach

Author(s): Yang Sun, Huajing Song, Feng Zhang, Lin Yang, Zhuo Ye, Mikhail I. Mendelev, Cai-Zhuang Wang, and Kai-Ming Ho

The crystal nucleation from liquid in most cases is too rare to be accessed within the limited time scales of the conventional molecular dynamics (MD) simulation. Here, we developed a “persistent embryo” method to facilitate crystal nucleation in MD simulations by preventing small crystal embryos fr...

Physical Review Letters

Damage in a Thin Metal Film by High-Power Terahertz Radiation

Author(s): M. B. Agranat, O. V. Chefonov, A. V. Ovchinnikov, S. I. Ashitkov, V. E. Fortov, and P. S. Kondratenko

We report on the experimental observation of high-power terahertz-radiation-induced damage in a thin aluminum film with a thickness less than a terahertz skin depth. Damage in a thin metal film produced by a single terahertz pulse is observed for the first time. The damage mechanism induced by a sin...

Physical Review Letters

Johari-Goldstein Relaxation Far Below ${T}_{g}$: Experimental Evidence for the Gardner Transition in Structural Glasses?

Author(s): K. Geirhos, P. Lunkenheimer, and A. Loidl

Experimental evidence for the Gardner transition, theoretically predicted to arise deep in the glassy state of matter, is scarce. At this transition, the energy landscape sensed by the particles forming the glass is expected to become more complex. In the present Letter, we report the dielectric res...

Physical Review Letters

Traditional Semiconductors in the Two-Dimensional Limit

Author(s): Michael C. Lucking, Weiyu Xie, Duk-Hyun Choe, Damien West, Toh-Ming Lu, and S. B. Zhang

Interest in two-dimensional materials has exploded in recent years. Not only are they studied due to their novel electronic properties, such as the emergent Dirac fermion in graphene, but also as a new paradigm in which stacking layers of distinct two-dimensional materials may enable different funct...

Physical Review Letters

Band Jahn-Teller structural phase transition in ${\mathrm{Y}}_{2}\mathrm{In}$

Author(s): E. Svanidze, C. Georgen, A. M. Hallas, Q. Huang, J. M. Santiago, J. W. Lynn, and E. Morosan

The number of paramagnetic materials that undergo a structural phase transition is rather small, which can perhaps explain the limited understanding of the band Jahn-Teller mechanism responsible for this effect. Here we present a structural phase transition observed in paramagnetic ${\mathrm{Y}}_{2}...

Physical Review B

Relaxation of strongly coupled electron and phonon fields after photoemission and high-energy part

A E Myasnikova, E A Zhileeva and D V Moseykin

An approach to considering systems with a high concentration of correlated carriers and strong long-range electron–phonon interaction and to calculating the high-energy part of the angle-resolved photoemission spectroscopy (ARPES) spectra of such systems is suggested. Joint relaxation of strongly coupled fields—a field of correlated electrons and phonon field—after photoemission is studied to clarify the nature of characteristic features observed in the high-energy part of the ARPES spectra of cuprate superconductors. Such relaxation occurs in systems with strong predominantly long-range electron–phonon interaction at sufficiently high carrier concentration due to the coexistence of autolocalized and delocalized carriers. A simple method to calculate analytically a high-energy part of the ARPES spectrum arising is proposed. It takes advantage of using the coherent states basis for the phonon field in the polaron and bipolaron states. The approach suggested yields all the high-en...

Journal of Physics Condensed Matter

Anomalous DC Hall response in noncentrosymmetric tilted Weyl semimetals

S P Mukherjee and J P Carbotte

Weyl nodes come in pairs of opposite chirality. For broken time reversal symmetry (TR) they are displaced in momentum space by {${\bf Q}$} and the anomalous DC Hall conductivity {$\sigma_{xy}$} is proportional to {${\bf Q}$} at charge neutrality. For finite doping there are additive corrections to {$\sigma_{xy}$} which depend on the chemical potential as well as on the tilt ( {$C$} ) of the Dirac cones and on their relative orientation. If inversion symmetry (I) is also broken the Weyl...

Journal of Physics Condensed Matter

Role of four-fermion interaction and impurity in the states of two-dimensional semi-Dirac materials

Jing Wang

We study the effects of four-fermion interaction and impurity on the low-energy states of 2D semi-Dirac materials by virtue of the unbiased renormalization group approach. The coupled flow equations that govern the energy-dependent evolutions of all correlated interaction parameters are derived after taking into account one-loop corrections from the interplay between four-fermion interaction and impurity. Whether and how four-fermion interaction and impurity influence the low-energy properties of 2D semi-Dirac materials are discreetly explored and addressed attentively. After carrying out the standard renormalization group analysis, we find that both trivial insulating and nontrivial semimetal states are qualitatively stable against all four kinds of four-fermion interactions. However, while switching on both four-fermion interaction and impurity, certain insulator–semimetal phase transitions and the distance of Dirac nodal points can be respectively induced and modified due to ...

Journal of Physics Condensed Matter

Study of irradiation effect of Xe +22 and Kr +14 ions on structural properties of Zn nanotubes

M V Zdorovets, D B Kadyrzhanov, I E Kenzhina, I A Ivanov and A L Kozlovskiy

The paper presents the results of synthesis and directed modification of structural properties of Zn nanotubes by irradiating with heavy ions. The nanotubes were obtained by electrochemical deposition in pores of template polymer matrices. It was established using SEM, XRD and EDS methods where irradiation with Xe +22 and Kr +14 ions makes it possible to modify the crystal structure of nanotubes. As a result of irradiation with Xe +22 ions, partial destruction of nanotubes is observed, which indicates an increase in the number of defects in the structure and a decrease in strength properties. Change in the crystal structure parameters is observed when irradiation with Kr +14 ions with fluence below 5  ×  10 11 ion cm −2 . That indicates the possibility of using Kr +14 ions for directional modification of nanostructures, while irradiation with Xe +22 ions leads to amorphization and destruction of nanot...

Journal of Physics Condensed Matter

The elusive role of Nb Li bound polaron energy in hopping charge transport in Fe: LiNbO 3

Laurent Guilbert, Laura Vittadello, Marco Bazzan, Imed Mhaouech, Simon Messerschmidt and Mirco Imlau

Charge transport due to small polarons hopping among defective (bound polarons) and regular (free polarons) sites is shown to depend in a non-trivial way on the value of the stabilization energy provided by the lattice distortion surrounding the charge carriers. This energy, normally not directly accessible for bound polarons using spectroscopic techniques, is determined here by a combination of experimental and numerical methods for the important case of small electron polarons bound to {${\rm Nb}_{{\rm Li}}$} defects in the prototype ferroelectric oxide lithium niobate. Our findings provide an estimation of the {${\rm Nb}_{{\rm Li}}$} polaron stabilization energy {$E_{\rm GP}=(0.75\pm0.05)~{\rm eV}$}

Journal of Physics Condensed Matter

Local Bi–O bonds correlated with infrared emission properties in triply doped Gd 2.95 Yb 0.02 Bi

Liping Tong, Katsuhiko Saito, Qixin Guo, Han Zhou, Xingmei Guo, Tongxiang Fan and Di Zhang

A correlation function between the Raman intensities and the nearest-neighbor mean-square relative displacement (MSRD) {$\sigma^2$} of local Bi–O bonds is successfully established based on x-ray absorption fine structure (XAFS) and temperature-dependent Raman spectra in the temperature range 77–300 K in amorphous and crystalline Gd 2.95 Yb 0.02 Bi 0.02 Er 0.01 Ga 5 O 12 . The structural symmetries of Gd 2.95 Yb 0.02 Bi 0.02 Er 0.01 Ga 5 O 12 are described by using {$\sigma^2$} of local Bi–O bonds. More importantly, Gd 2.95 Yb 0.02 Bi 0.02 Er 0.01 Ga 5 O 12 is found to show excellent infrared (IR) emission properties due...

Journal of Physics Condensed Matter

Spin-flop and magnetodielectric reversal in Yb substituted GdMnO 3

A Pal, W Prellier and P Murugavel

The evolution of various spin structures in Yb doped GdMnO 3 distorted orthorhombic perovskite system was investigated from their magnetic, dielectric and magnetodielectric characteristics. The Gd 1− x Yb x MnO 3 (0  ⩽   x   ⩽  0.15) revealed an enhanced magnetodielectric coupling when their magnetic structure is guided from ab to the bc -cycloidal spin structure upon Yb doping. The compounds exhibit magnetic field and temperature controlled spin-flop from c to a -axis. Additionally, magnetodielectric reversal is observed for the x   =  0.1 sample which depends on both magnetic field and temperature. The resultant correlation between magnetic and electric orderings is discussed in the frame of symmetric and antisymmetric exchange interaction models. These findings provide further insight in understanding the magnetoelectric materials and importantly show a way to tune the magnetic and magn...

Journal of Physics Condensed Matter

Tunable dynamic moduli of magnetic elastomers: from characterization by x-ray micro-computed

Giorgio Pessot, Malte Schümann, Thomas Gundermann, Stefan Odenbach, Hartmut Löwen and Andreas M Menzel

Ferrogels and magnetorheological elastomers are composite materials obtained by embedding magnetic particles of mesoscopic size in a crosslinked polymeric matrix. They combine the reversible elastic deformability of polymeric materials with the high responsivity of ferrofluids to external magnetic fields. These materials stand out, for example, for significant magnetostriction as well as a pronounced increase of the elastic moduli in the presence of external magnetic fields. By means of x-ray micro-computed tomography, the position and size of each magnetic particle can be measured with a high degree of accuracy. We here use data extracted from real magnetoelastic samples as input for coarse-grained dipole-spring modeling and calculations to investigate internal restructuring, stiffening, and changes in the normal modes spectrum. More precisely, we assign to each particle a dipole moment proportional to its volume and set a randomized network of springs between them that mimics ...

Journal of Physics Condensed Matter

Fri Feb 23 2018

Atomic self-diffusion in TiNi

Author(s): A.V. Bakulin, T.I. Spiridonova, S.E. Kulkova

The projector augmented wave method was applied to investigate the energetics of point defect formation at finite temperatures and the Ni-vacancy jumps in the intermetallic B2-TiNi alloy. It was shown that the effective formation energy of the Ni-vacancy (1.14 eV) is significantly lower than that of Ti-vacancy (1.74 eV); however, the antistructural atoms are dominant defects irrespective of alloy composition. The obtained results reveal that the migration barrier of the Ni-vacancy to the nearest-neighbor site is less (0.19 eV) than that for the Ni-vacancy jump to the next-nearest-neighbor site (1.64 eV). The Ni-vacancy implements two sequential nearest-neighbor jumps with short-lived intermediate configuration and some of such jumps initiate six-jump cycles of the Ni-vacancy along [0 0 1] and [1 1 0] directions with [0 0 1] bent mechanism as preferential one. In the latter case the calculated migration barrier of 0.82 eV is found to be in good agreement with experiment. The energy barrier for four-jump cycle flat mechanism is calculated by 0.27 eV higher. It is shown that the Ni-vacancy diffusion in B2-TiNi is strongly dominated by both six-jump cycle [0 0 1] bent and flat mechanisms at low temperatures but the contribution of the next-nearest-neighbor jumps become important with temperature increase.

Computational Materials Science

Identifying interatomic potentials for the accurate modeling of interfacial segregation and structural transitions

Author(s): Yang Hu, Jennifer D. Schuler, Timothy J. Rupert

Chemical segregation and structural transitions at interfaces are important nanoscale phenomena, making them natural targets for atomistic modeling, yet interatomic potentials must be fit to secondary physical properties. To isolate the important factors that interatomic potentials must capture in order to accurately model such behavior, the performance of four interatomic potentials was evaluated for the Cu-Zr system, with experimental observations used to provide validation. While experimental results show strong Zr segregation to grain boundary regions and the formation of nanoscale amorphous complexions at high temperatures and/or dopant compositions, a variety of disparate behaviors can be observed in hybrid Monte Carlo/molecular dynamics simulations of doping, depending on the chosen potential. The potentials that are able to recreate the correct behavior accurately reproduce the enthalpy of mixing as well as the bond energies, providing a roadmap for the exploration of interfacial phenomena with atomistic modeling. Finally, we use our observations to find a reliable potential for the Ni-Zr system and show that this alloy should also be able to sustain amorphous complexions.

Computational Materials Science

Structural stability and mechanical properties of Co3(Al, M) (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W) compounds

Author(s): Min Jin, Naihua Miao, Wenyue Zhao, Jian Zhou, Qiang Du, Zhimei Sun

The structural stability and mechanical properties of Co3(Al, M) (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W) compounds with cubic L12-type and hexagonal D019-type structures have been investigated by first-principles calculations. Calculated temperature-dependent formation energies indicate that all the L12-type Co3(Al, M) can be generated at high temperature and show better stability than their D019-type according to the quasi-harmonic Debye model. Furthermore, we reveal that the element Al plays a major role in promoting L12 structure more stable than D019 structure for the Co3(Al, W), and the element W reduces metastability as well as improves the strength of L12. We also find that most of the L12-Co3(Al, M) compounds possess good mechanical stability and ductility, which are verified by the elastic constants and Poisson’s ratio. More importantly, the element Cr can be used to replace the W of L12-Co3(Al, W) to increase the strength to weight ratio as the L12-Co3(Al, Cr) possesses comparable elastic properties to the L12-Co3(Al, W), including the Young’s and shear moduli. It is also observed that all the L12-Co3(Al, M) compounds show a high degree of elastic anisotropy. The electron localized function and suggests that the rise of the Young’s moduli in Co3(Al, M), with the alloying element M changing from group IVB to VIB, is mainly attributed by the increasing bonding strength of the nearby transition-metal atoms. Our results will be useful for the study of thermodynamic and mechanical properties as well as the design of Co-based high-temperature alloys.

Computational Materials Science

Effects of grain boundary configuration and characteristics on the demagnetization process and coercivity of anisotropic NdFeB magnets

Author(s): W. Li, L.Z. Zhao, Q. Zhou, X.C. Zhong, Z.W. Liu

A systematic investigation on the effects of grain boundary configuration and characteristics on the demagnetization process and magnetic properties of NdFeB magnets is carried out by micromagnetic simulation based on a periodic anisotropic model. The results indicate that, when the grain boundary phase is perpendicular to the easy axis, the demagnetization field is along the positive Z axis and the magnetic moments rotate difficultly due to the dipolar coupling. When the grain boundary phase is parallel to the easy axis, the demagnetization field is along the negative Z axis and the magnetic moments rotate easily. The best hard magnetic properties can be obtained by reducing the thickness and saturation magnetization of grain boundary phase distributed parallel to the easy axis and reducing the exchange stiffness of grain boundary phase perpendicular to the easy axis.

Computational Materials Science

Predictive modeling of dynamic fracture growth in brittle materials with machine learning

Author(s): Bryan A. Moore, Esteban Rougier, Daniel O’Malley, Gowri Srinivasan, Abigail Hunter, Hari Viswanathan

We use simulation data from a high fidelity Finite-Discrete Element Model to build an efficient Machine Learning (ML) approach to predict fracture growth and coalescence. Our goal is for the ML approach to be used as an emulator in place of the computationally intensive high fidelity models in an uncertainty quantification framework where thousands of forward runs are required. The failure of materials with various fracture configurations (size, orientation and the number of initial cracks) are explored and used as data to train our ML model. This novel approach has shown promise in predicting spatial (path to failure) and temporal (time to failure) aspects of brittle material failure. Predictions of where dominant fracture paths formed within a material were ∼85% accurate and the time of material failure deviated from the actual failure time by an average of ∼16%. Additionally, the ML model achieves a reduction in computational cost by multiple orders of magnitude.

Computational Materials Science

The intrinsic low lattice thermal conductivity in the rock salt SnSe

Author(s): Yun Xie, Yang Zhou, Xin-Gao Gong

The rock-salt IV-VI compounds usually have low thermal conductivity, while the orthorhombic Tin Selenide (SnSe) was reported to show extremely low lattice thermal conductivity ( κ L ) and thus achieves a record high zT value. To understand why SnSe has low thermal conductivity and how important role played by the orthorhombic structure, we studied κ L for both forms of SnSe, the rock salt and the orthorhombic, based on first-principles calculations and the Boltzmann transport equation. We found an even smaller value of κL for the rock salt structure, which establishes a possibility for even larger zT materials. Detailed analyses were performed on thermal properties for both structures. We attributed the low κL of the rock salt SnSe to its large Grüneisen parameters, which stem from the strongly anharmonic phonons. Our findings indicate the indecisive role that the orthorhombic structure is playing in achieving low κL for SnSe and provide fresh insight in understanding the thermal properties of SnSe.

Computational Materials Science

Phase filed simulation of dendritic growth of copper films irradiated by ultrashort laser pulses

Author(s): Ning Xue, Yunpeng Ren, Xudong Ren, Naifei Ren, Qing Lin, Qiqi Wang, Kai Qin

A phase field model (PFM) is combined with a two-dimensional two-temperature model (TTM) to simulate the evolution of dendritic growth during re-solidification of ultrafast laser-material interaction. The dynamic solidification conditions at different locations of the melting pool obtained from TTM are fed into the quantitative PFM based on the macro-micro coupled method. A series of simulations are executed to investigate the influence of laser parameters, such as laser influence and pulse duration, on melting pool characteristics and local dendrite morphology. Besides, dendrite structures calculated at different areas of the melting pool were discussed based on local solidification conditions, and the simulated dendrite arm spacing (DAS) for various cooling rates was made comparison with previously published experimental data. The simulated results reveal that the maximum temperature gradient has significant influence on the local dendritic competitive growth, while the laser parameters effect the local microstructure distinctly due to the changes of solidification conditions. This work demonstrates the potential application of PFM to predict the microstructure morphology presented in ultrashort laser-material interaction and other industrially relevant conditions with complex solidification conditions.

Computational Materials Science

Melting temperature of CoCrFeNiMn high-entropy alloys

Author(s): M.A. Gutierrez, G.D. Rodriguez, G. Bozzolo, H.O. Mosca

Atomistic modeling of CoCrFeNiMn (0 < xMn < 25 at.%) high entropy alloys shows that the melting temperature as a function of Mn concentration does not follow the behavior consistent with a homogeneous solid solution, exhibiting a maximum value for 8.7 at.% Mn. Using the concepts of the BFS method for alloys, a description of the phenomenon is provided, showing that sluggish diffusion generates changes in the atomic distribution that lead to this anomalous behavior. This theoretical analysis is meant to provide a template for studying complex compositions and fine effects in high entropy alloys, which would be hard to detect experimentally but that could have an impact on potential applications of these complex materials.

Computational Materials Science

A molecular dynamics investigation into nanoscale scratching mechanism of polycrystalline silicon carbide

Author(s): Yao Liu, Beizhi Li, Lingfei Kong

In this study, 3D molecular dynamics simulations were conducted to investigate the nanoscale scratching mechanism of polycrystalline SiC constructed by Voronoi site-rotation and cut method. The scratching process, SiC crystal structure evaluation, scratching force, stress, and temperature, surface morphology, and subsurface damage (SSD) were discussed after simulation. The results indicate that the ductile scratching process of polycrystalline SiC could be achieved in the nanoscale depth of cut through the amorphous crystal structure phase transition, which is the primary material removal mechanism in nanoscale SiC scratching. The silicon atom can penetrate into the diamond grit which may cause the wear of diamond tool. The disorder grain boundary (GB) atoms can transit to hexagonal diamond structure and generate dislocation during the scratching. Furthermore, the higher scratching speed results in smaller scratching force, smaller normal stress, and higher temperature due to the larger impaction breaking more SiC bonds, which makes the SiC material more ductile and easier to remove. The tangential stress shows great dependence on the grain and GB geometry and position due to the stress concentration in the GB. It is also found that the higher scratching speed encourages the pileup atoms in the front of grit to flowing to the grit side to form the groove protrusion, which can get a shallower SSD thickness and a wider SSD layer. SSD results also indicate that at least three types of the material removal mechanism – amorphous transition, intergranular fracture, and transgranular fracture – exists in the polycrystalline SiC scratching process.

Computational Materials Science

Half metallic ferromagnetism in Ni based half Heusler alloys

Author(s): A. Amudhavalli, R. Rajeswarapalanichamy, K. Iyakutti

The search for emerging materials with ferromagnetic and spin flip properties has attracted widespread interest in material science. Ni based half Heusler alloys have been one of the benchmark half metallic ferromagnetic material owing to their magnetic interaction and promising figure of merit in magnetic memory element. A systematic pathway to design novel Heusler materials is by analyzing structural phase stability, electronic structure, mechanical and magnetic properties. Specifically, we carried out first principles calculations to identify the ferromagnetic and half-metallic properties of Ni based half Heusler alloys XYZ (X = Ni; Y = Cr; Z = Si, Ge, Ga, Al, In, As). The predicted phase stability shows that α-phase is found to be the lowest energy phase compared with β and γ phases. A pressure-induced structural phase transitions from α-phase to γ-phase in NiCrSi, NiCrGe, NiCrGa, NiCrAl, NiCrIn and α-phase to β-phase in NiCrAs are found. Due to the presence of d8 and d5 electronic configuration within Ni2+ and Cr2+ ions respectively, all the Heusler materials show half metallic behavior. Furthermore, the magnetic moments for these half Heusler alloys in all the three different phases (α, β and γ) have been reported. Our work paves the way for designing novel Heusler materials at normal and high pressure.

Computational Materials Science

Calibration of nonlocal strain gradient shell model for vibration analysis of a CNT conveying viscous fluid using molecular dynamics simulation

Author(s): Kianoosh Mohammadi, Ali Rajabpour, Majid Ghadiri

In this article, a nonlocal strain gradient cylindrical shell model is developed to study vibration analysis and instability of a single-walled carbon nanotube conveying viscous fluid. The fluid flow is modeled by modified Navier-Stokes equation considering slip boundary condition and Knudsen number. The obtained governing equations of motion and corresponding boundary conditions are discretized using generalized differential quadrature method. Simplifying the nonlocal strain gradient theory, the results of this theory are compared to those of classical, strain gradient, and nonlocal theories. The effects of material length scale and nonlocal parameters on natural frequency and critical flow velocity are further investigated. Also, for the first time, the effect of fluid flow on vibration behavior of the carbon nanotube is studied by molecular dynamics simulation. In the simulations, TIP4P/2005 water model is used, which accurately considers thermo-physical properties of water such as viscosity. Moreover, in order to apply carbon–carbon interaction, modified Tersoff potential is used, with Lennard-Jones potential being employed for other interactions. In this study, size-dependent parameters of nonlocal strain gradient theory are calibrated and variation of calibrated values under the effect of flow velocity is explored.

Computational Materials Science

Enhancing oxygen iron conductivity of 8YSZ electrolytes in SOFC by doping with Fe2O3

Author(s): Yen-Hsin Chan, Hsin-Yi Lai, Cha'o-Kuang Chen

The present work investigates Fe2O3 doping as a method of improving the oxygen ion conductivity (OIC) of electrolytes in solid oxide fuel cells (SOFCs) working in a temperature range between 773 K and 973 K. Yttria-stabilised zirconia (YSZ), which alone does not have an OIC sufficiently high for working in the specified temperature range, is used as the electrolyte in the present work. A molecular dynamics (MD) modelling approach is employed to simulate the electrolyte, and the oxygen ion conductivity is characterised and examined for a range of doping proportions and working temperatures. Modelling results indicate that there exists an upper limit of oxygen vacancy concentration for doped 8YSZ electrolytes, and that the optimum OIC is achieved by an 8YSZ electrolyte doped with approximately 4% Fe2O3. Also reported are several other findings that might be of interest to future studies on the performance of electrolytes.

Computational Materials Science

An interatomic potential for simulation of defects and phase change of zirconium

Author(s): Yifang Ouyang, Jizheng Wu, Minghui Zheng, Hongmei Chen, Xiaoma Tao, Yong Du, Qing Peng

We introduce a long-range interaction analytical embedded atom method (namely la-EAM) interatomic potential, which has been developed by fitting the lattice constants, cohesive energy, mono-vacancy formation energy and elastic constants of α-Zirconium. We validate this la-EAM potential by extensive investigation of the bulk, surface, and defect properties of Zirconium using molecular dynamics simulations compared with available experiments and theoretical results. We examine the lattice constants, cohesive energy, elastic constants, phonon dispersion curves of α-, β-, and ω-Zirconium and find a good agreement with available experiments. We have studied the 0D (zero-dimension) defects including vacancies and self-interstitial atoms, 1D defects (dislocations), 2D defects including surface and stacking fault, and 3D bulk properties. Furthermore, our phase transformation energy barrier of α → ω agrees with the experimental observation. The success of our potential could attribute to the correctly accounting for the long-range interactions of the Zr atoms. Our results suggest that the developed la-EAM potential of Zr is useful in molecular dynamics simulations of bulk, surface and defect properties and phase transitions of Zirconium at various temperatures and pressures.

Computational Materials Science

Ordering effects in 2D hexagonal systems of binary and ternary C-B-N alloys

Author(s): Agnieszka Jamróz, Jacek Adam Majewski

We present theoretical study of ordering phenomena in binary C 1 - x B x , C 1 - x N x , and ternary C 1 - 2 x B x N x alloys forming two-dimensional, graphene-like systems. In order to find equilibrium distribution of different species corresponding to minima of the energy, we use Monte Carlo approach in Metropolis regime. We employ empirical Tersoff potential to account for the interactions between atoms and to calculate the total energy. We model alloys considering large systems containing up to 20 000 atoms with various N and B concentrations. For quantitative description of ordering phenomena in the studied alloys, we determine Warren-Cowley Short Range Order (SRO) parameters for the first coordination shell. Our studies clearly demonstrate the presence of the SRO in the studied alloys. Binary alloys are neither random nor the clustering of N (or B) dopants takes place. In the ternary alloys, the h-BN domains are formed within the carbon areas.

Computational Materials Science

Phase-field lattice Boltzmann simulations of multiple dendrite growth with motion, collision, and coalescence and subsequent grain growth

Author(s): Tomohiro Takaki, Ryotaro Sato, Roberto Rojas, Munekazu Ohno, Yasushi Shibuta

In the formation of a typical equiaxed structure during the solidification of metals and alloys, multiple equiaxed dendrites typically grow with motion, collision, and coalescence and subsequently grain growth occurs after the formation of grain boundaries. In this study, we develop a phase-field lattice Boltzmann model that can simulate these complex formation processes involving equiaxed structures. In this model, multiple dendrites are represented by employing multiple phase-field variables, and the formation of grain boundaries is modeled by simply introducing an interaction term between the phase-field variables. Liquid flow is computed using the lattice Boltzmann method, and the motion of a solid is described by solving the equations of motion. Collision-coalescence representation in the present model was validated by performing simulations of collisions between two circular objects. Furthermore, grain growth was validated through static and dynamic conditions in a simple three-grain system. Good agreements with theoretical solutions were obtained for both cases. Finally, using the developed model, a series of formation processes of multiple-dendrite growth with motion, collision, and coalescence and the subsequent grain growth are successfully performed for the first time.

Computational Materials Science

Effect of atomic orderdisorder on vacancy clustering in concentrated NiFe alloys

Author(s): D. Chakraborty, A. Harms, Mohammad W. Ullah, W.J. Weber, D.S. Aidhy

Using molecular dynamics simulations, we elucidate the effect of atomic structure on vacancy clustering in ordered (L10) and random NiFe. Based on our simulations, we predict the vacancy evolution to be in complete contrast between the two systems. While large vacancy clusters, i.e., stacking fault tetrahedra (SFT) are formed in the random structure, no clustering is observed in the ordered-L10 structure. Similar simulations are performed on L10-CuAu and L10-TiAl to understand whether SFT formation is generic in L10 structures, or is specific to NiFe. Both materials show SFT formation, thereby highlighting specific defect energetics in L10 NiFe that lead to the lack of vacancy clustering. We elucidate that L10-NiFe has unique thermodynamic and kinetic defect energetics, i.e., antisite energies, vacancy sublattice preference, and directional migration energy barriers that collectively lead to the lack of vacancy clustering. Understanding such defect energetics could open avenues to prevent defect clustering in the vision towards development of radiation-tolerant concentrated alloys for nuclear reactor applications.

Computational Materials Science

Phase-field study of the transient phenomena induced by ‘abnormally’ large grains during 2-dimensional isotropic grain growth

Author(s): Ramanathan Perumal, P.G. Kubendran Amos, Michael Selzer, Britta Nestler

‘Abnormally’ large grains, whose sizes are greater than twice the critical radius ( 2 R c ), are known to alter the isotropic grain growth phenomena. In the present work, phase-field simulations of 2-dimensional microstructures are extensively analysed to elucidate the deviations from the normal grain growth introduced by the presence of abnormal grains. Polycrystalline microstructures that are ’artificially’ made to resemble physical structures, by governing the distribution and the sizes of the abnormal grains, is employed to analyse the grain growth in the presence of large grains. This study unravels that the abnormal grains induce a period of transition during which its grain size distribution is shifted and confined within 2 R c , indicating a complete disappearance of the abnormality in the microstructure. Furthermore, it is identified that this transition period establishes a bimodal distribution, which subsequently evolves into a unimodal time-invariant distribution. This behaviour noticeably reveals the misconception that the disappearance of the abnormality signifies the onset of normal grain growth. Moreover, despite the apparent disappearance of the abnormal grains, a continued increase in the volume-fraction of these ‘pre-existing’ abnormal grains is recognized, and in the steady-state condition, it is observed that the microstructure predominantly consists of these pre-existing abnormal grains. Influence of the factors like initial volume-fraction of the abnormal grains F o and degree of abnormality U max on the duration of the transient period is quantified by investigating close to hundred microstructures with unique F o and U max .

Computational Materials Science

Cubic diamondlike BC7 predicted from first principles as a superhard material

Author(s): Dan Zhou, Jiashi Zhao, Bingjun Shen, Ying Xu, Yonggang Zou, Jian Tian

A cubic diamondlike BC7 structure with I-43m space group and 64 atoms per cell is theoretically designed from first-principles calculations. Our calculated phonon spectra and elastic constants confirmed that BC7 is both dynamically and mechanically stable. Based on the calculated electronic band structure and density of states, a hole-conducting behavior is predicted in BC7. The simulated high elastic constants, modulus, hardness and ideal strength reveal that BC7 exhibits excellent mechanical characters and belongs to a superhard material with prospects for potential multi-functional material in electronic and mechanical application.

Computational Materials Science

Analytic model of the γ-surface deviation and influence on the stacking fault width between partial dislocations

Author(s): B.A. Szajewski, A. Hunter, D.J. Luscher

The stacking fault width ( R eq ) between partial dislocations within an FCC crystalline lattice characterizes the onset of numerous plastic flow mechanisms, as well as the relationship between material strength and grain size. Continuum models traditionally consider a complete unit of dislocation slip ( b ) along the 110 direction distributed between two discrete partial dislocations, each with a fixed partial Burgers vector ( b p ), which bound a stacking fault. Across the stacking fault, the vectorial slip is assumed to be constant, yielding a constant intrinsic stacking fault energy density, γ isf . Here, we demonstrate that the vectorial displacement path taken in accomplishing a complete unit of slip ( b ) deviates from the expected displacement path containing the local minima, γ isf , leading to a correction in the nominally derived stacking fault width. The magnitude of the correction depends on both the net orientation of the dislocation within the lattice, and also the degree of relaxation of each partial Burgers vector along the 1 ¯ 1 2 direction. We derive a simple analytic model for the corrected stacking fault width, explicitly accounting for the deviation, by introducing a variable ξ (0  < ξ <  1), which governs the magnitude of each partial dislocation component along the 1 ¯ 1 2 direction. Significantly, our model predicts a correction to R eq of O ( | b | ) for the nominally screw dislocation, and has no noticeable influence on nominally edge dislocations. We apply our model towards computing the stacking fault width of several FCC metals, and demonstrate excellent agreement both with our own numerical data as well as that obtained from ab initio and Molecular Statics (MS) methods within the literature. The results from this study demonstrate that, upon judicious application, discrete linear elastic models are successful in reproducing elastic interactions as computed from higher fidelity models on the spatial scale of metallic dislocation cores.

Computational Materials Science

Investigation of cobalt and silicon co-doping in quaternary Heusler alloy NiFeMnSn

Author(s): Yu Feng, Xiaohui Xu, Wenxi Cao, Ting Zhou

Recent study on nearly half-metallic Heusler compound NiFeMnSn pointed out that its high spin polarization was destroyed by the existence of site disorder, hindering its further applications in spintronics devices [Mukadam M D, et al., 2016 Phys. Rev. B 94 214423]. In the present work, by employing the first-principles calculations, we study the influence of Co + Si co-doping on the magnetic properties and electronic structures of NiFeMnSn by substituting Ni with Co as well as Sn with Si. The calculated relative formation energy indicates that the co-doped compound could be easily obtained. The total magnetic moments at various co-doping concentrations are consistent with the Slater-Pauling rule. Analyses on electronic structures reveal that four co-doped compounds, i.e., Ni0.5Co0.5FeMnSn0.25Si0.75, Ni0.5Co0.5FeMnSi, Ni0.25Co0.75FeMnSn0.25Si0.75 and Ni0.25Co0.75FeMnSi possess perfect 100% spin polarization. Furthermore, owing to the fact that the Fermi levels of Ni0.5Co0.5FeMnSn0.25Si0.75, Ni0.5Co0.5FeMnSi and Ni0.25Co0.75FeMnSi are adjusted to the middle of the spin down energy gap, their half metallicity is enhanced.

Computational Materials Science

The shock response of crystalline Ni with H-free and H-segregated 〈1 1 0〉 symmetric tilt GBs

Author(s): Yaxin Zhu, Zhenhuan Li, Minsheng Huang, Qilin Xiong

The shock responses of crystalline Ni with two types of symmetric tilt grain boundaries (STGBs) are studied by large-scale molecular dynamics simulations, with special concerns on the effect of hydrogen segregated STGBs on them. The crystalline Ni with different kinds of H-free STGBs have significantly different plastic responses during the shock processes due to different mechanisms of dislocation emission from the STGBs. The segregation of hydrogen atoms at the STGBs significantly affects the dislocation emission from the H-segregated STGBs, and then heavily affects the shock plastic responses of crystalline Ni with H-segregated STGBs. On the one hand, dislocation emission from the H-segregated STGBs is delayed compared with that from the H-free ones at the low shock velocity, showing the so-called lagging effect. On the other hand, multi-mode dislocation emissions (or dislocation emission on the multi slip systems) are frequently activated from the H-segregated STGBs at the high shock velocity. In addition, the spallation behavior of the crystalline Ni under shock loading is also significantly affected by hydrogen segregation at the STGBs. In the targets with the H-free STGBs, spallation originates mainly from the interior of the grains due to strong interaction and reaction between dislocations there, however, it originates definitely from the H-segregated STGBs due to the reduced GB cohesive strength by hydrogen segregation.

Computational Materials Science

Atomistic simulation of tension-compression asymmetry and its mechanism in titanium single-crystal nanopillars oriented along the [1 1 2¯ 0] direction

Author(s): Junqiang Ren, Qiaoyan Sun, Lin Xiao, Jun Sun

Molecular dynamics simulations were performed with a Finnis-Sinclair many-body potential to investigate the mechanical properties and deformation mechanisms under applied uniaxial tensile and compressive loads in α-titanium (Ti) single-crystal nanopillars oriented along the 〈 1 1  2 ¯  0 〉 direction. The results indicate that the mechanical properties and plastic deformation mechanism display tension–compression asymmetry. The non-linear elastic behavior is attributed to the difference in friction between the neighboring atomic planes at the elastic deformation stage under push and pull loading conditions. Increasing the friction leads to hardening with compression. Decreasing the friction leads to softening with tension. Increasing the friction may also lead to higher yield stress with compression compared with tension. Perfect nanopillars are yielded via the nucleation and propagation of { 1 ¯  0 1 0} 〈1  2 ¯  1 0 〉 dislocations on the surface and corners of the nanopillars. Prismatic slip is the dominant mode of plastic deformation in the Ti nanopillars under compressive loading. However, {1 0  1 ¯  1} 〈1 0  1 ¯ 2 ¯ 〉 twinning is the dominant plastic deformation mechanism together with prismatic slip under tensile loading. Two typical intrinsic stacking faults (SFs) with different propensities exist in the nanopillars. The microstructural evolution of the SFs was also simulated.

Computational Materials Science

Configurational thermodynamics of C in body-centered cubictetragonal Fe: A combined computational study

Author(s): Jia-Yi Yan, A.V. Ruban

Configurational thermodynamics of C in body-centered cubic (bcc) or tetragonal (bct) Fe is investigated combining several computational techniques. Pairwise CC interaction energies in bcc Fe are calculated by density functional theory (DFT) and embedded atom method (EAM) potential respectively. The interaction between C atom and homogeneous strain is calculated assuming C acts as force dipoles in linear elastic medium of Fe. The CC and C–strain interactions are input into Monte Carlo (MC) simulations to find equilibrium C configuration on octahedral interstitial sublattices (OISs) in bcc/bct Fe and corresponding thermodynamic properties. In bcc Fe, DFT-MC and EAM-MC both give a single-phase region with C distributed with disorder on all the three OISs (α) at high temperature, and a two-phase region with ferrite and an ordered compound ( α ) at low temperature. The compound is Fe16C1 according to DFT or Fe16C2 according to EAM inputs, both having two OISs occupied. When a homogeneous tetragonal lattice strain is applied, the disordered phase exhibits a preferential sublattice occupation (Zener ordered α′), which is primarily caused by C–strain interaction and is mitigated by CC interactions. The ordered compound may also have two ( α ) or one (α″) OIS occupied. C clustering in bcc/bct Fe follows a conditional spinodal mechanism, namely long-range order being a prerequisite of spinodal decomposition. This is verified by the difference in solution thermodynamics of α/α′ (ordering-type) and α /α″ (clustering-type), as well as kinetic Ising model simulations which reveal a temporal sequence of short-range ordering, long-range ordering, and eventually C clustering.

Computational Materials Science

Accelerating band gap prediction for solar materials using feature selection and regression techniques

Author(s): Fadoua Khmaissia, Hichem Frigui, Mahendra Sunkara, Jacek Jasinski, Alejandro Martinez Garcia, Tom Pace, Madhu Menon

We present a novel approach to apply machine learning techniques to build a more robust prediction model for band-gap energies (BG-E) of chalcopyrites, a class of materials for energy applications in the fields of solar energy, photocatalysis, and thermoelectrics. Guided by knowledge from domain experts and by previous works on the field, we aim to accelerate the discovery of new solar materials. Our objectives are two folds: (i) Identify the optimal set of features that best describes a given predicted variable. (ii) Boost prediction accuracy via applying various regression algorithms. Ordinary Least Square, Partial Least Square and Lasso regressions, combined with well adjusted feature selection techniques are applied and tested to predict the band gap energy of chalcopyrites materials. Compared to the results reported in Zeng et al. (2002), Suh et al. (1999, 2004), and Dey et al. (2014), our approach shows that learning and using only a subset of relevant features can improve the prediction accuracy by about 40 % .

Computational Materials Science

Mechanical responses of pristine and defective C3N nanosheets studied by molecular dynamics simulations

Author(s): A.H.N. Shirazi, R. Abadi, M. Izadifar, N. Alajlan, T. Rabczuk

The purpose of this study is to investigate the mechanical properties of a new two-dimensional graphene like material, crystalline carbon nitride with the stoichiometry of C3N. The extraordinary properties of C3N make it an outstanding candidate for a wide variety of applications. In this work, the mechanical properties of C3N nanosheets have been studied not only in the defect-free form, but also with critical defects such as line cracks and notches using molecular dynamics simulations. Different crack lengths and notch diameters were considered to predict the mechanical response at different temperatures under the uniaxial tensile loading. Our simulation results show that larger cracks and notches reduce the strength of the nanosheets. Moreover, it was shown the temperature rise has a weakening effect on the tensile strength of C3N. Our study can provide useful information with respect to the thermo-mechanical properties of pristine and defective graphene like C3N 2D material.

Computational Materials Science

Effect of alloying elements on mechanical, electronic and magnetic properties of Fe2B by first-principles investigations

Author(s): Xiang Wei, Zhiguo Chen, Jue Zhong, Li Wang, Wei Yang, Yipeng Wang

First-principles calculations using density functional theory (DFT) were performed to explore the effect of common 3d elements M (M = Ti, V, Cr, Mn, Co, Ni and Cu) on the mechanical, electronic and magnetic properties of Fe2B. A formula of (Fe0.875,M0.125)2B was used. Firstly, the negative cohesive energy and formation enthalpy suggest that all the compounds are thermodynamically stable. With the knowledge of calculated elastic constants, the moduli, the Pugh’s modulus ratio G/B, the Poisson’s ratio v and the hardness of Fe2B and (Fe0.875,M0.125)2B were further predicted. Overall, with increasing atomic number, the moduli and hardness of the borides initially increase and then decrease. (Fe0.875,Cr0.125)2B possesses the largest bulk, shear and Young’s modulus simultaneously, while (Fe0.875,Mn0.125)2B has the largest hardness. The G/B and v values indicate that all the alloying elements are able to enhance the ductility of the Fe2B except Mn, but they do not change the nature of intrinsic brittleness of the Fe2B. Combined with the electronic structures, we revealed that the mechanical properties of the borides are mostly determined by FeB and MB bonds. FeFe bonds in 〈2 2 0〉 and 〈1 1 3〉 orientations are both covalent bonding. It can also be predicted that all the alloying elements reduce the magnetic moments (Ms) of the Fe2B mainly because the Ms of the substituted M atom is smaller than that of Fe atom.

Computational Materials Science

First-principles investigation on the chemical bonding, elastic properties and ideal strengths of MoAlB and WAlB nanolaminated MAB phases

Author(s): Fu-Zhi Dai, Zhihai Feng, Yanchun Zhou

The present work deals with first-principles calculations on chemical bonding characteristics, elastic properties, and ideal strengths under both shear and tensile deformations of MoAlB and WAlB nanolaminated MAB phases. Diverse chemical bonds were found in MoAlB and WAlB, including strong BB covalent bonds, AlB covalent-ionic bonds, MoB bonds and metallic MoAl and AlAl bonds. Elastic properties of MoAlB and WAlB are not highly anisotropic with shear modulus of (0 1 0)[1 0 0], (0 1 0)[0 0 1], (0 0 1)[1 0 0] similar to each other. Nevertheless, ideal strengths display significant anisotropy. The results show that ideal shear strength τ m of basal plane shears are lower than those of non-basal plane shears, while ideal tensile strength σ m of the basal plane is lower than those of non-basal planes. In addition, ideal tensile strength of the basal plane is also much lower than ideal shear strengths of basal plane shears. The basal plane cleavage and shear failure both take place along the weakly bonded AlAl layers. Moreover, the basal plane shear failure is found accompanied by remarkable expansion of the AlAl layers, which reduces the resistance against basal plane shears. It suggests that MoAlB and WAlB tend to cleavage along the AlAl layer or dislocations tend to multiply and slip in the AlAl layer, which is similar to MAX phases.

Computational Materials Science

Fast simulations of a large number of crystals growth in centimeter-scale during alloy solidification via nonlinearly preconditioned quantitative phase-field formula

Author(s): Tong Zhao Gong, Yun Chen, Yan Fei Cao, Xiu Hong Kang, Dian Zhong Li

Ameliorating the computing efficiency is always of importance in phase-field simulations of material microstructure formation and evolution. Borrowing from the nonlinear preconditioning treatment of diffuse interface models, the usual quantitative phase-field model for a binary alloy has been transformed to make it easier to compute accurately. The transformation yields a new variable whose value changes linearly across the interface. The dependences of simulated results of the nonlinearly preconditioned phase-field formula on the interface grid size and the discretization time step have been examined in detail through numerical experiments, including the growth velocity, the radius and the solute concentration of a steady tip. The results show that the new evolution equations are able to be solved on a computational mesh with interface grids 2–4 times coarser than those used in the conventional method. In combination with the front-tracking method to capture the crystallographic orientation of each crystal, the orientation gradient energy is incorporated into the nonlinearly preconditioned phase-field model, which enables simulations of grain boundary behaviors. The algorithm of the distributed parallel finite element method on an adaptive mesh is applied to further raise the computing efficiency. Simulations of multi-dendrites growth of Al-4 wt.%Cu alloy in undercooled melt cooling down continuously are performed. The results demonstrate that the proposed fast simulation approaches allow quantitative simulations of a large number of dendrites growth on the scale of centimeters or millimeters, respectively in two or three dimensions, just using an ordinary workstation instead of clusters or supercomputers.

Computational Materials Science

Discovering mechanisms relevant for radiation damage evolution

Author(s): Blas Pedro Uberuaga, Enrique Martínez, Danny Perez, Arthur F. Voter

The response of a material to irradiation is a consequence of the kinetic evolution of defects produced during energetic damage events. Thus, accurate predictions of radiation damage evolution require knowing the atomic scale mechanisms associated with those defects. Atomistic simulations are a key tool in providing insight into the types of mechanisms possible. Further, by extending the time scale beyond what is achievable with conventional molecular dynamics, even greater insight can be obtained. Here, we provide examples in which such simulations have revealed new kinetic mechanisms that were not obvious before performing the simulations. We also demonstrate, through the coupling with higher level models, how those mechanisms impact experimental observables in irradiated materials. Finally, we discuss the importance of these types of simulations in the context of predicting material behavior.

Computational Materials Science

Role of pre-width reduction in deformation behavior of AZ31B alloy during break-down rolling and finish rolling

Author(s): Weitao Jia, Fangkun Ning, Yunpeng Ding, Qichi Le, Yan Tang, Jianzhong Cui

Present study aimed at investigating the effect of introducing pre-width reduction (Pre-WR) on microstructure evolution and rolling formability variation during both the break-down rolling and finish rolling of AZ31B alloy. Different rolling routes relevant to the above two rolling processes were performed and compared. Results show that pre-induced twins by pre-rolling (8% thickness reduction) can significantly reduce the edge-cracking depth level during subsequent super-high reduction rolling (SHRR) (80% thickness reduction) through replacing the formation of shear bands by the promotion of the twinning-induced recrystallization to coordinate the severe deformation. However, the dominant behavior of twinning and twinning-induced DRX within the edge region induced a unique partial recrystallization with coarse grains and further a poor rolling formability in SHRR. Introducing more {10 1 ¯ 2} tensile twins at the edge by Inter-WR (a Pre-WR type with 10% width reduction) before the SHRR can significantly refine the grains and obtain almost a recrystallized homogeneous microstructure. It can also weaken the basal texture of the edge material and further decrease the number/depth level of edge crack by enhancing the activity of both non-basal slips and dominant continuous dynamic recrystallization (CDRX). During finish rolling, heavy twins and shear bands resulted in the larger edge crack level. The improvement of Pre-WR on rolling formability can be inherited to the strip finishing rolling process by evidently reducing the twins and shear bands.

Science and Engineering A

Microstructure, texture and mechanical properties evolution of extruded fine-grained Mg-Y sheets during annealing

Author(s): G.H. Huang, D.D. Yin, J.W. Lu, H. Zhou, Y. Zeng, G.F. Quan, Q.D. Wang

The microstructure, texture and mechanical properties evolution of the extruded fine-grained Mg-(1, 5) Y (wt%) alloy sheets were investigated during annealing. The as-extruded sheets exhibited a fully dynamic recrystallized (DRXed) microstructure consisting of uniform and fine equiaxed grains (5.7–8.7 µm) and a small amount of YH2 second phase particles. Both sheets exhibited significant thermal stability at 300 °C annealing and remarkable grain growth at temperatures higher than 400 °C. The measured grain growth activation energy (Q) of Mg-1Y at all temperatures tested was 91 ± 3 kJ/mol, suggesting that the growth was controlled by grain boundary diffusion. Meanwhile, for Mg-5Y alloy at lower temperatures (300–400 °C), the Q value (59 kJ/mol) indicated that the grain growth was controlled by grain boundary diffusion while the Q value (175 kJ/mol) at higher temperatures (400–450 °C) implied that lattice self-diffusion controlled the process. A representative rare-earth texture was present for both sheets in the as-extruded condition, and remarkable basal texture weakening was observed with the grain growth during each isothermal annealing process. The texture weakening can be ascribed to the preferential growth of non-basal oriented grains based on the EBSD analysis, which was likely related to the segregation of Y atoms at grain boundaries. The microhardness and grain size relationship can be described well by the Hall-Petch relation for all the annealed sheets, while the yield stress was not the case indicating that the macroscopic strength was more sensitive to the texture than the localized microhardness.

Science and Engineering A

The effect of annealing on the microstructural evolution and mechanical properties in phase reversed 316LN austenitic stainless steel

Author(s): D.M. Xu, G.Q. Li, X.L. Wan, R.D.K. Misra, X.G. Zhang, G. Xu, K.M. Wu

The present study aims to investigate the effect of annealing on microstructural evolution and mechanical properties in phase reversion-induced ultrafine/fine-grained 316LN austenitic stainless steel. The commercial 316LN austenitic stainless steel was cold rolled at room temperature to 90% thickness reduction and subsequently annealed in the temperature range of 600–1000 °C for 1–100 min. Evolution of phases in selected samples was identified and quantified by X-ray diffraction together with the corresponding microstructural characterization through optical, scanning and transmission electron microscopy, and electron backscattered diffraction. Mechanical properties of selected samples were determined by the tensile test. The results indicated that 46% α′-martensite and 54% deformed untransformed austenite were obtained in 316LN austenitic stainless steel after 90% cold reduction. Ultrafine/fine austenite grains nucleated at α′-martensite and deformed untransformed austenite via nucleation and growth process on annealing. The average grain size increased gradually with increased annealing temperature and time, with consequent decrease in yield strength and increased elongation.

Science and Engineering A

Deformation mechanisms and texture evolution of in-situ magnesium matrix composites containing polymer derived SiCNO dispersoids during hot compression

Author(s): Nagaraj M. Chelliah, Pambannan Padaikathan, M.K. Surappa

In-situ magnesium matrix composite was fabricated by injecting a liquid polymer directly into, and having it converted into 2.5 vol% SiCNO ceramic dispersoids, within molten Mg using a stir-casting method. The deformation mechanisms and texture evolution for these as-cast composites were investigated in a strain rate range of 10−3− 1 s−1 within a temperature range of 150–350 °C under uniaxial compression. It was observed that the deformed composites follow a power-law creep having a stress exponent, n = 8 and activation energy, Q = 149 kJ mol−1 which suggest that deformation mechanism is controlled by lattice self-diffusion for constant structure creep. It was found that in the range of 150–250 °C, with a ratio of rate of work-softening to rate of work-hardening of about 0.80, twinning induced shear bands nucleate and propagate along the direction of maximum shear stress. When the temperature approaches 350 °C, the plastic flow is dominated by dislocation assisted slip. Analysis of Zener-Hollomon parameter (Z) revealed that the transition from twinning into dislocation slip dominated deformation progresses at 1013 s−1 < Z < 1015 s−1. Macro-textural studies confirm that while basal plane assists deformation by twinning mechanism, the non-basal prismatic planes favor significant plastic deformation by dislocation assisted slip for the in-situ composites.

Science and Engineering A

Microstructure and mechanical properties of an in-situ TiB2Al-Zn-Mg-Cu-Zr composite fabricated by Melt-SHS process

Author(s): He Li, Xiaoming Wang, Lihua Chai, Haijing Wang, Ziyong Chen, Zhilei Xiang, Tounan Jin

A 10 vol% TiB2/Al-Zn-Mg-Cu-Zr composite was prepared by Melt-SHS techniques. The microstructures and mechanical properties of the composite were analyzed in this work. The results show that TiB2 particles with an average particle size of 0.67 µm are dispersed in the composite uniformly well combined with the matrix alloy after hot extrusion and T6 treatment. The addition of TiB2 particles resulted in a grain size reduction by 64.4%. The hardness, UTS, TYS and elastic modulus of the composite are increased by 9%, 20.2%, 22.3% and 24.6%, respectively. However, the elongation is reduced with the addition of TiB2 particles. The improvement of yield strength is attributed to the uniform dispersion of the fine TiB2 particles in the alloy matrix and the major contribution is from the dislocation strengthening mechanism, which has an additional contribution of 13.8%. In addition, the Orowan strengthening mechanism, grain refinement and load bearing strengthening mechanism have also contributed.

Science and Engineering A

An investigation on microstructure, texture and formability of AZ31 sheet processed by asymmetric porthole die extrusion

Author(s): Qinghang Wang, Jiangfeng Song, Bin Jiang, Aitao Tang, Yanfu Chai, Tianhao Yang, Guangsheng Huang, Fusheng Pan

This paper provided an effective plastic deformation technique, asymmetric porthole die extrusion, for fabricating AZ31 magnesium alloy sheets. Three kinds of asymmetric porthole extrusion dies were designed and entitled as APE-45, APE-60 and APE-90 die in terms of asymmetric porthole die angle, respectively. The effect of different APE processes on the microstructures, texture evolutions and mechanical properties of AZ31 sheets was investigated at room temperature. For comparison, conventional extrusion (CE) and symmetric porthole die extrusion (PE) were also conducted on processing AZ31 sheets. Shear deformation induced by APE declined the grain size and promoted a broad angular distribution of basal planes in the APE sheets compared with the CE and PE sheets. Especially, the APE-90 sheet obtained finest grain size of 5.2 µm and made basal planes tilted towards the extrusion direction by ~ 21° rotation in the sheet plane. With increasing asymmetric porthole die angle, the volume fraction of recrystallized grains gradually increased, resulting in the decrease of basal pole intensity. Due to the increased activity of basal <a> slip, APE sheets exhibited the decrease in yield strength and r-value and increase in elongation to failure, especially for the APE-90 sheet. The improved formability of the APE sheets was attributed mainly to texture weakening. The APE-90 sheet exhibited the highest index Erichsen value and improved by ~ 74% and ~ 94% compared to the CE and PE sheets, respectively. Consequently, microstructure-texture control induced by APE could enhance the room-temperature stretch formability of AZ31 sheets.

Science and Engineering A

The effect of precipitates on voiding, twinning, and fracture behaviors in Mg alloys

Author(s): Wei Fu, Ruihong Wang, Jinyu Zhang, Kai Wu, Gang Liu, Jun Sun

The effect of precipitates on fracture behaviors was comparatively investigated among three kinds of Mg alloys with different precipitates, i.e., Mg-Gd alloy with prismatic plate-shaped precipitates, Mg-Zn alloy with [ 0001 ] α rod-shaped precipitates, and Mg-Gd-Zn-Zr alloy with basal plate-shaped precipitates. By comparing the fracture behaviors of the alloys before and after aging treatment, it was evident that the presence of precipitates greatly promoted the formation of microvoids that were initiated at primary intermetallic particles. Voiding was the most accelerated in the Mg-Gd alloy, where the precipitate-dislocation interactions are the strongest. While in the Mg-Gd-Zn-Zr alloy with the weakest precipitate hardening, the precipitate-facilitated voiding was the least significant. The precipitate-dependent voiding suppressed the twinning behaviors, causing the volume fraction of deformation twins decreased in turn from Mg-Gd-Zn-Zr to Mg-Zn, and finally to Mg-Gd alloys. The tensile ductility of present Mg alloys approximately scaled with the volume fraction of deformation twins, and was highly sensitive to the precipitates. The fracture scenario of present Mg alloys was proposed that the voiding suppressed twinning and the competition between voiding and twinning can be mediated by precipitates. In the Mg-Gd-Zn-Zr alloy with limited voiding, the twinning dominated the deformation process and concomitantly resulted in a great ductility. The formation of microvoids or the fracture of primary intermetallic particles was quantitatively analyzed by applying a Weibull model, where both the alloy strength and the volume fraction of primary intermetallic particles were considered to rationalize the remarkable difference in fracture behaviors among the present three Mg alloys. Furthermore, the coupling contribution of precipitates and twins to the work hardening was modelled in the aged Mg-Gd-Zn-Zr alloy that displayed a large room temperature tensile ductility of ~ 13.5%.

Science and Engineering A

High-temperature strengthening mechanisms of Laves and B2 precipitates in a novel ferritic alloy

Author(s): Tianyi Chen, Chad M. Parish, Ying Yang, Lizhen Tan

Precipitates of the Laves and B2 phases were engineered in a newly-designed advanced ferritic alloy. Under creep test at 650 °C with 120 MPa, the material showed a steady-state minimum creep rate of 1 × 10−4 h−1, about one order of magnitude lower than T91. Microstructural characterization of the ferritic alloy revealed primarily ductile and partially brittle fractures after the creep test. Coarse Laves phase (~ 1 µm) was observed associating with the brittle fracture, resulting in reduced creep ductility. However, fine Laves phase precipitates (~ 100 nm) helped the dimple-ductile fracture and strengthened the material through impeding the motion of dislocations and boundaries. Unlike the B2 precipitates remained coherent exerting the classic Orowan bypassing mechanism at the brittle location, some of the B2 precipitates at the ductile location became incoherent and can develop an attractive interaction with dislocations. This coherency change of B2 precipitates, together with the nucleation of ultrafine (~ 40 nm) Laves phase precipitates during the creep test, would compensate for the coarsening-induced loss of Orowan strengthening of coherent B2 precipitates.

Science and Engineering A

Dynamic compressive response of a dendrite-reinforced Ti-based bulk metallic glass composite

Author(s): Jitang Fan, Fufa Wu, Dongfeng Li

Dynamic compressive response of a dendrite-reinforced Ti-based bulk metallic glass composite is investigated. Strain rate dependencies of the max strength and strain energy density are illustrated. The inherent mechanisms are revealed by studying the dynamic deformation behaviors at lower and higher strain rates. Under high-rate loading, the deformation and fracture of the soft crystalline dendrites and the hard amorphous matrix are characterized, respectively. Then, their contributions to the failure of the composite material are discussed. Finally, the dynamic damage efficiency of the composite material is addressed.

Science and Engineering A

Strengthening of AA5052 aluminum alloy by equal channel angular pressing followed by softening at room temperature

Author(s): M. Howeyze, H. Arabi, A.R. Eivani, H.R. Jafarian

Effects of equal channel angular pressing (ECAP) on the microstructure and tensile properties of AA5052 aluminum alloy was investigated. XRD analysis showed that the maximum dislocation density was achieved after 4 passes ECAP and reduced with further deformation which was in line with formation of ultrafine grain structure with a high fraction of high angle grain boundaries (HAGBs). Increasing dislocation density in 4 passes deformed specimen resulted in increasing hardness, yield strength (YS) and ultimate tensile strength (UTS). With 6 passes ECAP, the hardness and YS were not changed while the UTS was further increased. Unchanged values of hardness and YS were attributed to the reduction of dislocation density and simultaneous formation of ultrafine grain structure. Room temperature softening was observed in the severely deformed samples and consequently, YS reduces, UTS remains unchanged and ductility and toughness improve significantly.

Science and Engineering A

Microstructural Characterization of Deformation-Induced Martensite in an Ultrafine-Grained Medium Mn Advanced High Strength Steel

Author(s): C.L. Wu, C.P. Chang, D. Chen, J.F. Tu, C.Y. Huang

Microstructural characterization of deformation-induced martensite in its early stage of formation has been performed in an ultrafine-grained (UFG) medium Mn advanced high strength steel (AHSS). By using field emission scanning electron microscope (FESEM) and electron backscattered diffraction (EBSD), the nucleation site of deformation-induced martensite has been studied. Grain sizes, Schmid factors and aspect ratios of the parent austenite grains of deformation-induced martensite were measured in order to know their influence on the formation of deformation-induced martensite. Orientation relationship between deformation-induced martensite and its parent austenite has also been analyzed. Discussion on the effect of microstructural parameters on the formation of deformation-induced martensite in AHSS is given.

Science and Engineering A

Mechanical Properties and Uniformity of Fe-MgB2 Wires upon Various Wire Drawing Steps

Author(s): Fırat Karaboğa, Asaf Tolga Ulgen, Hakan Yetiş, Mustafa Akdoğan, Murat Pakdil, Ibrahim Belenli

Effect of drawing process on the mechanical properties and cross-sectional uniformity of Fe/MgB2 mono-filamentary wires were investigated in this study. Successive cold drawing steps were applied with and without intermediate strain relief annealing steps. Micro-hardness measurements of the drawn wires were taken from the polished cross-sections. Tensile tests were performed on the 120 mm long wires samples. Microstructural properties of the sheathing material and that of the superconducting core were studied using SEM. Uniformity of the cross-sectional area in final mono filamentary wires was studied on SEM images of the polished cross-sections. Excessive mechanical deformation of the soft iron sheathing metal via drawing was assessed and discussed.

Science and Engineering A

Through thickness variations of deformation texture in round profile extrusions of 6063-type aluminium alloy: experiments, FEM and crystal plasticity modelling

Author(s): Kai Zhang, Knut Marthinsen, Bjørn Holmedal, Trond Aukrust, Antonio Segatori

The deformation texture and its through-thickness heterogeneity for an extruded round profile are experimentally measured by the electron back-scatter diffraction technique (EBSD), and numerically modelled by coupling FEM flow simulation and crystal plasticity simulations. Deformation histories are extracted from the FEM flow simulation. The billet material was a type of 6063 aluminium alloy. Small-scale round profiles were extruded at 300°C in a lab extrusion setup featuring immediate water quenching at the end of deformation, keeping the profile essentially in the deformed state. The deformation texture is a strong <111> and weak <100> duplex fibre texture in the centre region, whereas the texture is weaker and rotated when approaching the surface. The full-constraint Taylor and the Advanced-lamel model (Alamel) were employed and evaluated for texture predictions. Both models give similar predictions as the experiments. The Alamel model performs better in terms of both global and through-thickness texture predictions. Analysing the deformation history from the FEM simulations reveals that the deformation condition is close to ideal uniaxial tensile deformation in the centre, whereas it is an approximate plane-strain deformation with superimposed simple shear deformation near the surface region. It is concluded that the texture through-thickness gradient can be successfully predicted by coupling FEM and Alamel model for the studied profile. The texture heterogeneity is mainly attributed to the deformation heterogeneity across the thickness.

Science and Engineering A

First-Order Interfacial Transformations with a Critical Point: Breaking the Symmetry at a Symmetric Tilt Grain Boundary

Author(s): Shengfeng Yang, Naixie Zhou, Hui Zheng, Shyue Ping Ong, and Jian Luo

First-order interfacial phaselike transformations that break the mirror symmetry of the symmetric $∑5\text{ }(210)$ tilt grain boundary (GB) are discovered by combining a modified genetic algorithm with hybrid Monte Carlo and molecular dynamics simulations. Density functional theory calculations con...

Physical Review Letters

Quasitopological rotational waves in mechanical granular graphene

Author(s): Li-Yang Zheng, Georgios Theocharis, Vincent Tournat, and Vitalyi Gusev

Granular crystals are periodic structures of elastic beads arranged in crystal lattices. One important feature of granular crystals is that the interactions between beads can take place via noncentral contact forces, leading to the propagation of rotational and coupled rotational-translational waves...

Physical Review B

Transmission/reflection behaviors of surface plasmons at an interface between two plasmonic systems

Fuxin Guan, Shulin Sun, Shaojie Ma, Zhening Fang, Baocheng Zhu, Xin Li, Qiong He, Shiyi Xiao and Lei Zhou

Although surface plasmon polaritons (SPPs) have been intensively studied in past years, the transmission/reflection properties of SPPs at an interface between two plasmonic media are still not fully understood. In this article, we employ a mode expansion method (MEM) to systematically study such a problem based on a model system jointing two superlattices, each consisting of a periodic stacking of dielectric and plasmonic slabs with different material properties. Such a generic model can represent two widely used plasmonic structures (i.e. interfaces between two single dielectric/metal systems or between two metal–insulator–metal waveguides) under certain conditions. Our MEM calculations, in excellent agreement with full-wave simulations, uncover the rich physics behind the SPP reflections at generic plasmonic interfaces. In particular, we successfully derive from the MEM several analytical formulas that can quantitatively describe the SPP reflections at different plasmonic inte...

Journal of Physics Condensed Matter

Straightforward measurement of anisotropic thermal properties of a Bi 2 Se 3 single crystal

D Fournier, M Marangolo, M Eddrief, N N Kolesnikov and C Fretigny

We demonstrate here a simple measurement protocol which allows the thermal properties of anisotropic crystalline materials to be determined. This protocol is validated by the measurement of Bi 2 Se 3 , a layered material consisting of covalently bonded sheets with weak van der Waals bonds between each layer, which has highly anisotropic thermal properties. Thermoreflectance microscopy measurements were carried out on a single-crystal Bi 2 Se 3 sample, firstly on the bare sample and then after capping with a 100 nm thick gold layer. Whereas on the bare sample lateral heat diffusion is dominated by the in-plane thermal diffusivity, on the metal-capped substrate heat diffusion perpendicular to the sample surface dominates. Using a simple theoretical model, we show how this double measurement protocol allows the anisotropic thermal conductivity coefficients of bulk Bi 2 Se 3 to be evaluated.

Journal of Physics Condensed Matter

Transport theory for femtosecond laser-induced spin-transfer torques

Pavel Baláž, Martin Žonda, Karel Carva, Pablo Maldonado and Peter M Oppeneer

Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser pulse is accompanied by a nonthermal spin-polarized current of hot electrons. These spin currents are studied here theoretically in a spin valve with noncollinear magnetizations. To this end, we introduce an extended model of superdiffusive spin transport that enables the treatment of noncollinear magnetic configurations, and apply it to the perpendicular spin valve geometry. We show how spin-transfer torques arise due to this mechanism and calculate their action on the magnetization present, as well as how the latter depends on the thicknesses of the layers and other transport parameters. We demonstrate that there exists a certain optimum thickness of the out-of-plane magnetized spin-current polarizer such that the torque acting on the second magnetic layer is maximal. Moreover, we study the magnetization dynamics excited by the superdiffusive spin-transfer torque due to the flow of hot electrons employi...

Journal of Physics Condensed Matter

The melting points of MgO up to 4 TPa predicted based on ab initio thermodynamic integration

Takashi Taniuchi and Taku Tsuchiya

The melting curve of MgO is extended up to 4 TPa, corresponding to the Jovian core pressure, based on the one-step thermodynamic integration method implemented on ab initio molecular dynamics. The calculated melting temperatures are 3100 and 16 000 K at 0 and 500 GPa, respectively, which are consistent with previous experimental results, and 20 600 K at 3900 GPa, which is inconsistent with a recent experimental extrapolation, which implies the molten Jovian core. A quite small Clapeyron slope ( {${\rm d}T/{\rm d}P$} ) of {$0.0\pm 0.5$} is found at 3900 GPa due to comparable densities of the liquid and B2 phases under extreme compression. The Mg–O coordination number in the liquid phase is saturated at around 7.5 above 1 TPa and remains smaller than that in the B2 phase (8) ev...

Journal of Physics Condensed Matter

Thu Feb 22 2018

Resolving the controversy

Jannik Meyer

The structure of the platelet defect in diamond has been determined by transmission electron microscopy, distinguishing the best-matched atomic model that settles a long-standing debate.

Nature

A thirst for advancement

Benjamin S. Hsiao, Samuel Chigome & Nelson Torto

The resource-rich continent of Africa is showing signs of significant progress in materials science research and is harnessing a plethora of human and material resources to tackle a wide range of challenges.

Nature

Intragranular void formation in shock-spalled tantalum: Mechanisms and governing factors

Author(s): M. Cheng, C. Li, M.X. Tang, L. Lu, Z. Li, S.N. Luo

Intragranular void formation in polycrystalline tantalum is investigated with plate impact experiments and molecular dynamics simulations, as regards its mechanisms and governing factors: grain boundary (GB) misorientation, grain orientation, grain size and shock pressure. Free-surface velocity history measurements are performed to obtain spall strength. Electron backscatter diffraction characterizations are used to obtain spatial distribution of voids, GB misorientation, and grain orientation associated with twins and intragranular voids. Intragranular voids are smaller in size but larger in number than intergranular voids, and nucleate in the GB vicinities. Smaller grains are favorable for the formation of intergranular and intragranular voids. At higher shock pressure, intragranular voids increase in fraction and tend to distribute at grain centers. Deformation twinning depends on grain orientation and prefers to form at high-angle GBs. However, intragranular voids have negligible dependence on grain orientation and favor medium-angle GBs, and the preference is enhanced slightly with increasing impact velocity and decreasing grain size. The simulations show GB-induced multiple-slip, slip-slip intersections and strain localizations act as the prerequisites to intragranular void formation.

Acta Materialia

An analysis of the influence of grain size on the strength of FCC polycrystals by means of computational homogenization

Author(s): Sarra Haouala, Javier Segurado, Javier LLorca

The effect of grain size on the flow stress of FCC polycrystals is analyzed by means of a multiscale strategy based on computational homogenization of the polycrystal aggregate. The mechanical behavior of each crystal is given by a dislocation-based crystal plasticity model in which the critical resolved shear stress follows the Taylor model. The generation and annihilation of dislocations in each slip system during deformation is given by the Kocks-Mecking model, which was modified to account for the dislocation storage at the grain boundaries. Polycrystalline Cu is selected to validate the simulation strategy and all the model parameters are obtained from dislocation dynamics simulations or experiments at lower length scales and the simulation results were in good agreement with experimental data in the literature. The model is applied to explore the influence of different microstructural factors (initial dislocation density, width of the grain size distribution, texture) on the grain size effect. It is found that the initial dislocation density, ρ i , plays a dominant role in the magnitude of the grain size effect and that dependence of flow stress with an inverse power of grain size ( σ y σ d g x ) breaks down for large initial dislocation densities ( > 10 14 m−2) and grain sizes d g > 40 μm in FCC metals. However, it was found that the grain size contribution to the strength followed a power-law function of the dimensionless parameter d g ρ i for small values of the applied strain ( < 2%), in agreement with previous theoretical considerations for size effects in plasticity.

Acta Materialia

Rapid solidification of non-stoichiometric intermetallic compounds: Modeling and experimental verification

Author(s): Jianbao Zhang, Haifeng Wang, Wangwang Kuang, Yachan Zhang, Shu Li, Yuhong Zhao, D.M. Herlach

The thermodynamic extremal principle was applied to model of rapid solidification of non-stoichiometric intermetallic compounds and the Co-xat.%Si alloys (x = 50, 53, 55) were undercooled to test the model. It is the model with but not the model without solute drag that can be derived self-consistently in thermodynamics. Unique dual sluggish and abrupt growth stages were found in undercooled Co-53 at.%Si and Co-55 at.%Si alloys. The first (second) sluggish stage is solute-controlled (thermal-controlled). The first (second) abrupt growth stage is ascribed to the sharp occurrence of solute trapping (inverted partitioning) and disorder trapping that initiates the transition from solute-controlled (thermal-controlled) to thermal-controlled (kinetic-controlled) growth. Since the predictions by the current (previous) model with (without) solute drag predicted well (deviate drastically from) the experimental results, solute drag was suggested be significant upon rapid solidification. The current work solved such an open problem, i.e. solute drag in solidification, and is helpful for not only understanding the non-equilibrium phenomena that is of theoretical importance but also controlling the non-equilibrium microstructures that is of technological importance.

Acta Materialia

Interfacial thermal and electrical transport properties of pristine and nanometer-scale ZnS modified grain boundary in ZnO polycrystals

Author(s): Xin Liang, Lei Shen

Grain boundary plays an important role in energy carrier transport. By choosing ZnO as a model system of technological importance, and by measuring the thermal and electrical transport properties of ZnO polycrystals with a wide range of grain boundary spacing, we determine the interfacial thermal (Kapitza) resistance of the ZnO grain boundary R k = 4.0 ± 0.7 × 10 9 m2 K W−1, which is relatively independent of grain size. The effective electron potential barrier height and the depletion width of the grain boundary generally increase with spacing, but they collapse below ∼100 nm and become almost invariant above ∼1 μm. When the grain boundary is locally modified by nanometer-thick ZnS thin film, the Kapitza resistance increases by more than three times, up to about 12.9 × 10 9 m2 K W−1, and the depletion region expands more than twice. The charge carrier concentration is influenced by the effective potential barrier height due to the grain boundary energy filtering effect whereas the electron mobility is related to the depletion width. Our investigations demonstrate the significance of grain boundary characteristics for interfacial and effective bulk transport properties. The findings and the approach are broadly important for polycrystalline materials of which the functional performance can be adjusted via grain boundary engineering.

Acta Materialia

Uncovering the influence of common nonmetallic impurities on the stability and strength of a Σ5 (310) grain boundary in Cu

Author(s): Zhifeng Huang, Fei Chen, Qiang Shen, Lianmeng Zhang, Timothy J. Rupert

Impurities are often driven to segregate to grain boundaries, which can significantly alter a material's thermal stability and mechanical behavior. To provide a comprehensive picture of this issue, the influence of a wide variety of common nonmetallic impurities (H, B, C, N, O, Si, P and S) incorporated during service or materials processing are studied using first-principles simulations, with a focus on identifying changes to the energetics and mechanical strength of a Cu Σ5 (310) grain boundary. Changes to the grain boundary energy are found to be closely correlated with the covalent radii of the impurities and the volumetric deformations of polyhedra at the interface. The strengthening energies of each impurity are evaluated as a function of covalent radius and electronegativity, followed by first-principles-based tensile tests on selected impurities. The strengthening of a B-doped grain boundary comes from an enhancement of the charge density among the adjacent Cu atoms, which improves the connection between the two grains. Alternatively, the detrimental effect of O results from the reduction of interactions between the Cu atoms. This work deepens the understanding of the possible beneficial and harmful effects of impurities on grain boundaries, providing a guide for materials processing studies.

Acta Materialia

Role of microstructure on twin nucleation and growth in HCP titanium: A statistical study

Author(s): M. Arul Kumar, M. Wroński, R.J. McCabe, L. Capolungo, K. Wierzbanowski, C.N. Tomé

A detailed statistical analysis is performed using Electron Back Scatter Diffraction (EBSD) to establish the effect of microstructure on twin nucleation and growth in deformed commercial purity hexagonal close packed (HCP) titanium. Rolled titanium samples are compressed along rolling, transverse and normal directions to establish statistical correlations for {10–12}, {11–21}, and {11–22} twins. A recently developed automated EBSD-twinning analysis software is employed for the statistical analysis. The analysis provides the following key findings: (I) grain size and strain dependence is different for twin nucleation and growth; (II) twinning statistics can be generalized for the HCP metals magnesium, zirconium and titanium; and (III) complex microstructure, where grain shape and size distribution is heterogeneous, requires multi-point statistical correlations.

Acta Materialia

Epitaxial growth of the intermetallic compound NiAl on low-index Ni surfaces in NiAl reactive multilayer nanofoils

Author(s): F. Baras, O. Politano

Crystal growth in the case of self-propagating reactions in Ni-Al nanofoils was investigated by means of molecular dynamics simulations. We studied the heteroepitaxial growth of NiAl on Ni during mixing and alloying at interfaces. Three low-index Ni surfaces were considered. In the case of the (001) and (111) orientations of Ni, a layer-by-layer formation occurred. Four orientations of NiAl grains were observed in (001) and six in (101). The orientation relationships of NiAl(101) with Ni(001) and with Ni(111) were derived. For the (101) orientation of Ni, massive crystallization was observed in the form of grains tilted in relation to the interface. The microstructure evolution was tracked along with grain orientation dynamics. In all cases, a simple geometric construction based on the relationship between unit cells of Ni and NiAl explains crystal growth specificities. The nucleation process and growth kinetics were also investigated. The present study proves that crystal growth varies considerably, in relation to Ni orientation.

Acta Materialia

A study of size effects in bioinspired, “nacre-like”, metal-compliant-phase (nickel-alumina) coextruded ceramics

Author(s): Ryan P. Wilkerson, Bernd Gludovatz, Jeremy Watts, Antoni P. Tomsia, Gregory E. Hilmas, Robert O. Ritchie

Coextrusion has been shown to be a viable processing route for the fabrication of bioinspired ceramic materials that exhibit improved damage tolerance. This provides one of the few examples of the synthesis of “nacre-like” ceramic hybrid structures with “brick-and-mortar” architectures using a method that includes the “mortar” (i.e., the metallic or polymeric compliant phase) during the entire fabrication process, instead of infiltrating it into a pre-fabricated ceramic scaffold. In this study, we examine how manipulation of filament size can lead to improved mechanical performance in the resulting biomimetic ceramics by reduction of the brick size and mortar thickness while still maintaining a model high volume fraction (∼90 vol.%) ceramic containing a metallic compliant phase. Specifically, we synthesized model brick-and-mortar alumina hybrid structures (Al2O3/Ni) containing small volume fractions (<10%) of nickel which we made by the coextrusion of alumina and nickel oxide in a thermoplastic (polyethylene-ethyl acrylate) suspension. Flexural strength and crack-initiation fracture toughness values were used to compare the performance of various ceramic brick sizes, with full crack-growth resistance-curves (R-curves) measured and compared to similar bioinspired ceramics to ascertain how brick size and mortar thickness affect the final mechanical performance. It was found that even though these structures are significantly coarser than those made with other processing methods, they still exhibit comparable crack-growth resistance, despite their lower strength, as well as improving R-curve behavior that tracks closely with brick size. Indeed, these structures display some of the highest fracture toughness values of any high-volume fraction alumina with a metallic compliant phase to date. Toughening was found to be induced by marked crack deflection as the crack path followed the metallic “mortar” phase, coupled with significant crack bridging and “brick” pull-out in the image of the toughening mechanisms seen in nacre.

Acta Materialia

Transient porous nickel interlayers for improved silver-based Solid Oxide Fuel Cell brazes

Author(s): Quan Zhou, Thomas R. Bieler, Jason D. Nicholas

Silver-copper oxide reactive air brazes are the most widely used Solid Oxide Fuel Cell (SOFC) brazes. However, the conventional Ag-4 wt.% Cu composition has a high wetting angle of ∼40° in air on yttria-stabilized-zirconia (YSZ) that leads to manufacturing defects (denoted as Type I pores). Also, many elements that oxidize during brazing to promote braze wetting (such as Cu) are easily reduced by SOFC fuels. This results in Type II pores that decrease the braze interfacial strength and provide a quick path for hydrogen permeation into the braze (where the hydrogen reacts with diffused oxygen to form gaseous water pockets, denoted as Type III pores). The present work demonstrates that transient porous nickel interlayers, instead of reactive element additions, can be used to promote Ag wetting on YSZ and produce high-quality YSZ-stainless steel braze joints. Mechanical tests on these reactive-element-free, silver-based SOFC braze joints, both before and after 500 h of 750 °C oxidation in air, show that the braze and braze interface strengths are higher than the underlying YSZ|NiO-YSZ substrate. The elimination of Type I and Type II porosity enabled by this new technique should result in improved braze lifetimes for SOFC and other ceramic-to-metal sealing applications.

Acta Materialia

Understanding precipitate evolution during friction stir welding of Al-Zn-Mg-Cu alloy through in-situ measurement coupled with simulation

Author(s): J.F. dos Santos, P. Staron, T. Fischer, J.D. Robson, A. Kostka, P. Colegrove, H. Wang, J. Hilgert, L. Bergmann, L.L. Hütsch, N. Huber, A. Schreyer

Friction Stir Welding (FSW) imparts both heat and deformation to the metal being joined, producing profound microstructural changes that determine the weld properties. In the case of welding of aerospace aluminium alloys, the most important change is the modification of the size, nature, and fraction of strengthening precipitates. To understand these changes requires the ability to measure the microstructural evolution during the welding process. This paper describes a new tool, the FlexiStir system, a portable friction stir unit designed for use in a high-energy synchrotron beamline that enables in-situ studies of microstructural evolution during FSW. FlexiStir has been used to measure precipitate evolution during FSW of aluminium alloy 7449-TAF and provide time-resolved measurement of precipitate size and volume fraction via small angle X-ray scattering (SAXS). These measurements have been interpreted with the aid of a previously developed microstructural model. The model predictions and SAXS measurements are in good qualitative agreement and demonstrate the complex precipitate transformation, dissolution, and reprecipitation events that occur during welding.

Acta Materialia

Segregation of alloying elements to planar faults in γ'-Ni3Al

Author(s): Y. Rao, T.M. Smith, M.J. Mills, M. Ghazisaeidi

Systematic first principles calculations are performed to determine the interaction energies between solutes (Co, Cr, Nb, Ta) and stable stacking faults in γ ' -Ni3Al. Our results show that there is no obvious driving force for segregation to already formed SISF, SESF, or twin boundaries, which is in contrast to direct experimental evidence for segregation as provided by high spatial resolution structure and chemical analysis using scanning transmission electron microscopy techniques. This apparent discrepancy provides evidence for possible formation mechanisms of these planar faults via metastable precursor structures that are found to be attractive for solutes. In addition, we study the underlying reasons for the segregation in terms of size and chemical mismatch and find that a favorable change in bonding environment, rather than the solute misfit volume, dominates the solute/fault interaction energy.

Acta Materialia

Dependence of hydrogen-absorption and -desorption characteristics on density of lithium–zirconium oxides exposed in air at room temperature

Author(s): B. Tsuchiya, S. Nagata, Y. Mizoguchi, Y. Takagi, M. Ito, Y. Oya, K. Okuno, K. Morita

The dependence of the hydrogen (H)-absorption and -desorption characteristics of air-exposed Li2ZrO3 samples at room temperature on Li2ZrO3 densities of 2.53–3.78 g/cm3, related to the surface area, and the most stable trapping sites of H were investigated using weight gain measurement (WGM), elastic recoil detection (ERD), Fourier-transform infrared (FT-IR) spectroscopy, thermal desorption spectroscopy (TDS), gas chromatography (GC), and first-principles calculations. The WGM, ERD, and FT-IR spectra revealed that the H concentration in Li2ZrO3 increased with increasing air exposure time and relative humidity and decreasing density by splitting water (H2O). The TDS and GC analyses revealed that hydrogen molecules (H2), as well as H2O, were released from the air-exposed Li2ZrO3 at annealing temperatures less than 323 K. In addition, it was found by using a density functional theory (DFT) code that H atoms dissociated from H2O preferred to occupy oxygen (O) substitutional sites in Li2ZrO3.

Acta Materialia

Design of strain tolerant porous microstructures – A case for controlled imperfection

Author(s): David Jauffrès, Christophe L. Martin, Rajendra K. Bordia

Porous materials, especially ceramics, are used in an ever-expanding range of functional applications. In most cases there are minimum mechanical requirements which limit the porosity level and thus the functional performance provided by the pore surface or volume. In order to design porous materials with the best compromise between functional and mechanical performance, a sound understanding of microstructure-mechanical properties relationships is required. In the current work, discrete simulations are used to assessed the Young's modulus and fracture toughness of various realistic porous microstructures obtained via partial sintering of powders. Scaling laws relating these quantities to microstructural parameters are derived and it is demonstrated that the proportionality between Young's modulus and fracture toughness, often claimed for partially sintered materials, is actually an approximation of a more general relationship. The proposed scaling laws suggest new strategies to build microstructurally tougher and strain tolerant porous materials. It is shown that strain tolerant microstructures can be designed by introducing controlled heterogeneity and hierarchy. Finally, the proposed scaling relationship between Young's modulus and fracture toughness is simplified to give it a practical use and verified for a wide range of porous microstructures, including hierarchical ones.

Acta Materialia

Longitudinal twinning in a TiAl alloy at high temperature by in situ microcompression

Author(s): Thomas Edward James Edwards, Fabio Di Gioacchino, Gaurav Mohanty, Juri Wehrs, Johann Michler, William John Clegg

The stress required to activate twinning of the longitudinal < 11 2 ¯ ] { 111 } system in the lamellar γ-TiAl phase of the alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol.% TiB2 was measured at several temperatures up to 700 °C by in situ micropillar compression of soft mode oriented γ-TiAl/α2-Ti3Al lamellar stacks. The lamellae undergoing deformation twinning were identified by electron backscatter diffraction orientation mapping. In some cases, such lamellae were not constrained by domain or colony boundaries and longitudinal twinning was the only deformation mechanism observed based on digital image correlation strain maps. The resolved shear stress for such unconstrained twinning was found to increase monotonically with temperature from 25 °C to 700 °C. This is consistent with the stacking fault energy increasing with temperature as found in many metallic alloys, suggesting that the increased ease of deformation twinning at high temperature in bulk TiAl alloys is due to the increased ease with which the twinning shear can be accommodated by the neighbouring domains and lamellae with increasing temperature, rather than a thermal softening of the intrinsic twinning mechanism.

Acta Materialia

Half-Heusler phase formation and Ni atom distribution in M-Ni-Sn (M = Hf, Ti, Zr) systems

Author(s): Matylda N. Guzik, Cristina Echevarria-Bonet, Marit D. Riktor, Patricia A. Carvalho, Anette E. Gunnæs, Magnus H. Sørby, Bjørn C. Hauback

High resolution synchrotron radiation powder X-ray diffraction (SR-PXD) and scanning transmission electron microscopy (STEM) have been employed for structural characterization of MNiSn, M0.5M′0.5NiSn and M0.5M′0.25M″0.25NiSn (M, M′, M″ = Hf, Ti, Zr) half-Heusler compounds, synthesized by arc melting and thermal annealing. Rietveld refinement results demonstrate that ternary Ti- and Hf-based compositions crystallize with only one half-Heulser phase, while two cubic phases are found in samples with nominal composition ZrNiSn. The performed analysis does not suggest obvious presence of excess Ni in any of the ternary compounds. Instead, it shows disordered distribution of the stoichiometric Ni atoms over 4c and the nominally vacant 4d sites in ZrNiSn as well as formation of Ni vacancies at the 4c site in ZrNi0.98Sn.

Acta Materialia

Integrated imaging in three dimensions: Providing a new lens on grain boundaries, particles, and their correlations in polycrystalline silicon

Author(s): R. Keinan, H. Bale, N. Gueninchault, E.M. Lauridsen, A.J. Shahani

Most technologically-relevant materials exhibit a microstructural heterogeneity over multiple length scales, and it is this heterogeneity that ultimately determines their performance. For example, the efficiency of polycrystalline silicon (poly-Si) photovoltaic cells is critically dependent on the nature of the grain boundaries and foreign metal impurities in the bulk. Here, we probe the characteristics and distributions of these defects in three dimensions by using a novel, integrated, and non-destructive imaging platform. In particular, recent advances in laboratory-based diffraction contrast tomography (LabDCT) enable us to measure grain centroid, volume, orientation, and shape. From this crystallographic information, we extract the five-parameter grain boundary distributions in poly-Si. By using a combination of LabDCT, attenuation-based tomography, and electron microscopy, we determine that the location of the impurity particles is non-random in the bulk and strongly dependent on grain boundary character. The correlative analysis not only demonstrates the degree of interaction between foreign metal impurities and structural defects in poly-Si, but also highlights the viability of burgeoning tomographic methods such as LabDCT. It is anticipated that our integrated approach can be extended to other complex microstructures with minimal sample-specific tuning.

Acta Materialia

Stacking-fault energy, mechanical twinning and strain hardening of Fe-18Mn-0.6C-(0, 1.5)Al twinning-induced plasticity steels during friction stir welding

Author(s): Seung-Joon Lee, Yufeng Sun, Hidetoshi Fujii

The effect of friction stir welding (FSW) on the microstructure, stacking-fault energy (SFE) and strain hardening rate (SHR) of Fe-18Mn-0.6C-(0 and 1.5)Al (wt.%) twinning-induced plasticity steels using three welding speeds (50, 100 and 200 mm min−1) was investigated. The yield strength of the FSWed 0Al and 1.5Al steels improved due to both grain boundary strengthening by grain refinement and dislocation hardening by the introduction of dislocations with an increase in the welding speed. Their SHR with three stages and without the yield drop increased due to the active mechanical twinning and the introduction of dislocations during the FSW when the welding speed was increased. Among the 0Al steels, 0Al-200 steel with a fine grain exhibited more active twinning than the coarse-grained specimen (0Al-50), which is contrast to the 1.5Al steel. Regardless of the specimens, the slight increase in the SFE, which was attributed to both the shear strain energy caused by the introduction of dislocations and the excess free energy by the grain refinement during the FSW, leads to an increase in the critical twinning stress (σ tw ). Despite the fine grain of the 0Al steel, the origin of its active twinning was the highly increased yield strength relative to σ tw , and the promoted dislocation interactions, giving rise to an increase in the number of sites at which twin nucleation occurred.

Acta Materialia

The influence of silicon additions on the deformation behavior of austenite-ferrite duplex medium manganese steels

Author(s): Binhan Sun, Fateh Fazeli, Colin Scott, Nicolas Brodusch, Raynald Gauvin, Stephen Yue

The present study systematically investigated the effect of Si additions from 0 to 3 wt.% on the deformation mechanisms of a 0.2C-10Mn-3Al medium Mn steel. Two austenite-ferrite duplex microstructures, characterized by the different austenite characteristics (i.e. fraction and stability), were produced for different Si-alloyed steels by intercritical annealing and subsequently subjected to tensile testing. The influence of Si content on the activation and kinetics of transformation-induced plasticity (TRIP) effect, the formation of deformation twins, as well as the strain partitioning between phase constituents during deformation was studied in detail. For the low austenite fraction (∼30%) duplex structure with a high austenite stability (low Ms temperature), the tensile strength was only slightly changed with Si levels, whereas the uniform elongation was significantly influenced, with first an increase followed by a decreasing trend with higher Si contents. This was attributed to changes in strain partitioning between austenite and ferrite and to different extent of deformation twinning in austenite, which were highly dependent on the solute Si contents and the related microstructural changes. It was found that strain partitioning in this type of microstructure can result in a substantially high strain hardening rate and high uniform elongation. Conversely, for the samples with a high austenite fraction (∼45%) and low stability, the tensile properties were insensitive to the Si content, being mainly controlled by the TRIP effect. In this structure, a higher fraction of martensite was formed at the beginning of the plastic deformation, which decreased the strain partitioning between austenite and ferrite. The TRIP effect after the Lüders strain is believed to be the main factor influencing the strain hardening rate of this type of structure.

Acta Materialia

Probing the entropy hypothesis in highly concentrated alloys

Author(s): Cláudio Geraldo Schön, Thien Duong, Yuhao Wang, Raymundo Arróyave

High Entropy Alloys (HEAs) designate a class of multicomponent metallic alloys in nearly equiatomic compositions. According to the constituting postulate, a larger number of constituents in a solid solution increases its configurational entropy, which has a maximal value when constituents exist in equiatomic concentrations and this is sufficient to overcome enthalpic contributions between the alloy components which would otherwise favor compound formation or phase separation. This entropy effect would, thus, stabilize disordered, crystallographically simple, solid solutions. Since then, numerous HEA candidate systems have been experimentally studied. It is unclear, however, if, and to which extent, the configurational entropy can be accessed by the system in the way it is suggested. The present work deals with this question using theoretical/computational methods. First, a series of model Body Centered Cubic (BCC) systems involving strong symmetric interactions between unlike atom pairs (referred to as “equinteracting” systems), containing up to five components, is investigated using the Cluster Variation Method in the irregular tetrahedron approximation. The symmetry of interactions, though artificial, allows for the straightforward presentation of phase diagram sections even for quaternary and quinary systems. Next, a “real” HEA candidate system, VNbTaMoW, which presents the BCC structure, is investigated by ab initio calculations, allowing to extend the conclusions to a realistic case with asymmetric interactions. The results show that configurational entropy has a small, even marginal, effect on phase transitions and the competition between conflicting interactions in the solid solution (i.e. frustration) seems to be the relevant factor behind the observed stabilization in the disordered states in HEA systems.

Acta Materialia

Influence of hydrogen on the elastic properties of nickel single crystal: A numerical and experimental investigation

Author(s): G. Hachet, A. Metsue, A. Oudriss, X. Feaugas

A theoretical formalism based on density functional theory was conducted to examine the influence of hydrogen on the elastic constants of nickel at finite temperature. In this investigation, we also performed uniaxial tensile tests on < 001 > oriented nickel single crystals with different hydrogen concentrations. We highlighted large discrepancies between the elastic properties determined with these two approaches. Thus, further investigations were carried out on the influence of defects induced by the incorporation of hydrogen. Differential scanning calorimetry on samples charged with hydrogen showed that the vacancy concentration was in the same range as the hydrogen concentration. Moreover, voids around 2 nm in diameter were visible with transmission electron microscopy. Additional calculations were performed on nickel and nickel-hydrogen systems with vacancy, which confirmed that defects had a higher impact on the elastic constants of nickel than the solute. Finally, we used an analytical model derived from linear elastic theory to quantify the impact of vacancy clusters on the elastic constants. We reproduced the experimental degradation of < 001 > Young's modulus with increasing hydrogen concentration for spherical isotropic clusters with a diameter 10 times smaller than the voids observed with transmission electron microscopy.

Acta Materialia

A Nucleation Progenitor Function approach to polycrystalline equiaxed solidification modelling with application to a microgravity transparent alloy experiment observed in-situ

Author(s): Shaun McFadden, Robin P. Mooney, Laszlo Sturz, Gerhard Zimmermann

A Nucleation Progenitor Function (NPF) approach that accounts for the interdependence between nucleation and growth during equiaxed solidification is proposed. An athermal nucleation density distribution, based on undercooling, is identified as a progenitor function. A Kolmogorov statistical approach is applied assuming continuous nucleation and growth conditions. The derived progeny functions describe the (supressed) distribution of actual nucleation events. The approach offers the significant advantage of generating progeny functions for volumetric (3D) data and projected image (2D) data. The main difference between 3D and 2D data in transparent alloy experiments is due to a stereological correction for over-projection. Progeny functions can be analysed to obtain statistical output information, e.g., nucleation counts, average nucleation undercooling and standard deviation. The statistical output data may be calculated in a formative (running) or a summative (final) mode. The NPF kinetics have been incorporated into a transient thermal model of equiaxed solidification. The model has been applied to characterise a microgravity solidification experiment with the transparent alloy system Neopentylgycol-30 wt%(d)Camphor. The model predicted thermal and observed nucleation and growth data with a good level of agreement.

Acta Materialia

Microstructural evolution and high-temperature oxidation mechanisms of a titanium aluminide based alloy

Author(s): S.J. Qu, S.Q. Tang, A.H. Feng, C. Feng, J. Shen, D.L. Chen

Oxidation resistance of titanium aluminide (TiAl) based alloys is a fundamental aspect for the high-temperature structural applications such as in the advanced hypersonic aircraft engines and gas turbines. The aim of this study was to identify oxidation kinetics and mechanisms through detailed microstructural characterization of a newly-developed Ti-44Al-4Nb-1.5Cr-0.5Mo-0.1B-0.1Y alloy via focused ion beam (FIB), transmission electron microscopy (TEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), scanning transmission electron microscopy (STEM), along with density functional theory (DFT) calculations. The alloy consisting mainly of γ-TiAl/α2-Ti3Al lamellar structure exhibited a superior oxidation resistance at 700 °C, and followed parabolic oxidation kinetics at 800 °C and 900 °C. The observed multi-layered scale structure consisted of TiO2, Al2O3-rich, Al2O3+TiO2, H-Ti2AlN+Al2O32-Ti3Al, Z-Ti5Al3O2+AlNb2+Laves-(Ti,Nb)Cr2, and H-Ti2AlN/α2-Ti3Al lamellae from the outside to inside after high-temperature oxidation. The γ-TiAl/α2-Ti3Al lamellae near the scale/substrate interface were first transformed into H-Ti2AlN/α2-Ti3Al lamellae, with orientation relationships identified as ( 0001 ) α 2 / / ( 0001 ) T i 2 A l N , ( 10 1 ¯ 0 ) α 2 / / ( 10 1 ¯ 0 ) T i 2 A l N and [ 1 2 ¯ 10 ] α 2 / / [ 1 2 ¯ 10 ] T i 2 A l N . The H-Ti2AlN/α2-Ti3Al lamellae were then transformed into a metastable Z-Ti5Al3O2 phase at the scale/substrate interface. The Z-phase was decomposed to Ti3Al and Al2O3 as the scale/substrate interface moved inwardly. Ti3Al reacted further with oxygen and nitrogen to form Ti2AlN, which was finally oxidized to form TiO2 and α-Al2O3. A Nb-rich layer was present beneath the scale along with the formation of AlNb2 and Laves phase, and the doping effect of Nb to suppress the diffusion of oxygen occurred mainly in the TiO2+Al2O3 compound layer. The results obtained in this study would pave the way for the development of advanced oxidation-resistant TiAl-based materials for high-temperature applications.

Acta Materialia

Atomistic modeling of capillary-driven grain boundary motion in Cu-Ta alloys

Author(s): R.K. Koju, K.A. Darling, K.N. Solanki, Y. Mishin

Nanocrystalline Cu-Ta alloys are emerging as a new class of structural materials preserving the nano-scale grain size up to the melting point of Cu. This extraordinary structural stability is caused by the strong pinning of grain boundaries (GBs) by Ta nano-clusters precipitating from the unstable solid solution after mechanical alloying. Many aspects of the Ta stabilization effect remain elusive and call for further experimental and simulation work. In previous atomistic computer simulations of stress-driven GB migration [JOM 68, 1596 (2016)], the GB–cluster interactions in Cu-Ta alloys have been studied for several different compositions and GB velocities. The results have pointed to the Zener pinning as the main mechanism responsible for the grain stabilization. This paper extends the previous work to the motion of individual GBs driven by capillary forces whose magnitude is similar to that in real nanocrystalline materials. Both the impingement of a moving GB on a set of Ta clusters and the GB unpinning from the clusters are studied as a function of temperature and alloy composition. The results demonstrate a quantitative agreement with the Zener pinning model and confirm the “unzip” mechanism of unpinning found in the previous work. In the random Cu-Ta solid solution, short-circuit Ta diffusion along stationary and moving GBs leads to the nucleation and growth of new GB clusters, which eventually stop the GB motion.

Acta Materialia

Surface energies, segregation, and fracture behavior of magnesium aluminate spinel low-index grain boundary planes

Author(s): Fiona Yuwei Cui, Animesh Kundu, Amanda Krause, Martin P. Harmer, Richard P. Vinci

Fracture toughness of single crystal and bicrystal boundaries in magnesium aluminate spinel (MgAl2O4) specimens was measured using micro scale fracture tests as a means of assessing relative surface energies of the low-index planes and the effect that Eu doping has on grain boundary energy. Single crystal specimens with {111}, {110}, and {100} surfaces were bonded together using hot pressing to create bicrystal interfaces. The bicrystals were cut into smaller pieces and half were doped with Eu. Micro cantilever deflection tests were employed to measure fracture toughness within each single crystal and at the bicrystal boundaries. Of the single crystals, the {111} plane was found to have the highest surface energy and the {100} plane the lowest. The structure and chemistry of the grain boundaries was characterized with atomic resolution scanning transmission electron microscopy (STEM). The clean {111}/{100} interface had a lower toughness and therefore a higher boundary energy than the clean {100}/{110} interface. Eu segregated to the {111}/{100} interface of the doped bicrystal boundary where it bonded strongly to the {111} plane but not to the {100} plane. As a result, the boundary strength was unchanged despite the presumed decrease in the energy of the Eu-bonded {111} surface because the bonding to the other side of the boundary was not improved. The Eu-doped {100}/{110} boundary has non-uniform segregation, and the same fracture toughness as its non-doped counterpart. Therefore, Eu was not found to significantly lower the energy of the {100} and {110} surface or to improve bonding across the interface. This study demonstrates how micro-cantilever fracture toughness measurements on single crystals and interfaces can indicate surface and interface energy trends that aid in interpreting grain growth and delineating extrinsic and intrinsic toughening mechanisms.

Acta Materialia

Defect-mediated vortex multiplication and annihilation in ferroelectrics and the feasibility of vortex switching by stress

Author(s): Shuai Yuan, W.J. Chen, L.L. Ma, Ye Ji, W.M. Xiong, J.Y. Liu, Y.L. Liu, Biao Wang, Yue Zheng

The possibility of switching the direction of the dipole toroidal moment in ferroelectrics provides exciting opportunities for development of novel nanoscale memory and logic devices. However, a practical control of vortex chirality is rather challenging at present stage, not to mention via mechanical methods. In this paper, we performed the phase-field simulations to show that mechanical switching of vortex chirality can be achieved in ferroelectric nanoplatelet via defect engineering. After introducing a void defect in the nanoplatelet, relative stability of single-vortex state and multi-vortices state is found to be altered. Importantly, during stress-induced vortex multiplication process, the void is a favored nucleation core of new vortex; meanwhile, vortices tend to annihilate away from the void during a vortex annihilation process. As the favored regions of vortex nucleation and annihilation are not the same, a deterministic mechanical switching of vortex chirality can be achieved. The effects of temperature, shape of the nanoplatelet, void size, as well as void position, on the defect-mediated vortex switching behaviors are systematically revealed. Our study demonstrates the feasibility of vortex switching by mechanical loads and provides a route to control and develop electromechanical devices based on ferroic vortices.

Acta Materialia

Warm ductility enhanced by austenite reversion in ultrafine-grained duplex steel

Author(s): Guan-Ju Cheng, Baptiste Gault, Cheng-Yao Huang, Ching-Yuan Huang, Hung-Wei Yen

The current work investigated the relationship between microstructure and warm deformation properties in a strong but ductile Mn-rich steel. A cold-rolled Fe-11.3Mn-0.068C-0.3Si-1.1Al-0.25Mo-0.01P-0.01S-0.0003N (in wt. %) steel was deformed isothermally after inter-critical annealing at temperatures from 550 °C to 720 °C. Deformation at 650 °C led to exceptional ductility, corresponding to total elongation of over 100%. The microstructure was characterized by electron backscattered diffraction, transmission Kikuchi diffraction, and transmission electron microscopy. It was found that the rate of austenite reversion can be accelerated by deformation, and that the transformation makes strained austenite into equiaxed grains. Exceptional ductility can be achieved when warm deformation is accompanied by austenite reversion. This research will provide metallurgical principles for warm deformation of steel under reversed transformation.

Acta Materialia

Peierls stresses estimated via the Peierls-Nabarro model using ab-initio γ-surface and their comparison with experiments

Author(s): Y. Kamimura, K. Edagawa, A.M. Iskandarov, M. Osawa, Y. Umeno, S. Takeuchi

In order to rationalize the homologous nature of the Peierls stress reported in a previous paper (Y. Kamimura et al., Acta Mater. 61 (2013) 294–309), we have calculated numerically the Peierls stresses via the Peierls-Nabarro model (P-N model) of the planar core dislocation, using calculated ab-initio γ-surface for a variety of crystals: fcc metals, bcc metals, NaCl type crystals, CsCl type ionic crystals and various tetrahedrally coordinated crystals. The obtained γ-surfaces for the same group of crystals are found to have similar shape and height when normalized by materials parameters, indicating homologous nature. Except for some very soft crystals ( τ P exp / G ≪ 1 × 10−4), the calculated Peierls stresses for majority of crystals ( τ P exp / G > 1 × 10−4) are shown, for the first time, to have a correlation with experimental ones reflecting the homologous nature of the γ-surface, although the former are generally larger than the latter by half an order on the average, with an exception of bcc metal in which the strain field of the controlling screw dislocation is known to be non-planar. For very soft crystals, the calculated Peierls stresses are orders of magnitude larger than experimental ones; possible origins of this large discrepancy are discussed.

Acta Materialia

Isothermal decomposition of carbon and nitrogen-enriched austenite in 23MnCrMo5 low-alloy steel

Author(s): H.P. Van Landeghem, S.D. Catteau, J. Teixeira, J. Dulcy, M. Dehmas, M. Courteaux, A. Redjaïmia, S. Denis

The industrial importance of carbonitriding is owed to the exceptional wear and fatigue resistance it imparts to treated steel parts. This resistance is related to microstructural changes occurring during the enrichment treatment and upon cooling. Here, the effects of interstitial contents, in particular nitrogen, and transformation temperature were investigated in 23MnCrMo5 steel. Samples were homogeneously enriched in the austenitic phase and the isothermal transformation of the enriched austenite between 750 °C and 600 °C was studied. CrN was found to precipitate during the enrichment treatment. During subsequent isothermal holding, CrN precipitate as fine platelets in nitrogen containing samples. At equal carbon content, ferrite formed faster and in finer grains in presence of nitrogen. Preexisting CrN facilitate ferrite nucleation resulting in more numerous ferrite grains. The intense nitride precipitation is the main origin for enhanced hardness in nitrogen-enriched alloys. The exact mechanism leading to the observed microstructures could not be determined and remains under investigation. In particular, the high nitrogen supersaturation of ferrite required to produce the observed fraction of CrN has to be explained.

Acta Materialia

The microstructural origin of work hardening stages

Author(s): D.A. Hughes, N. Hansen

The strain evolution of the flow stress and work hardening rate in stages III and IV is explored by utilizing a fully described deformation microstructure. Extensive measurements by transmission electron microscopy reveal a hierarchical subdivision of grains by low angle incidental dislocation boundaries (IDBs) and medium to high angle geometrically necessary boundaries (GNBs). This universal evolution is demonstrated for nickel, copper, and aluminum deformed by cold rolling from strains of 0.05–5.5. Microstructural morphology evolves with increasing strain through a transition resulting in a lamellar cell-block structure aligned with the deformation. This transition is caused by the emergence of new slip systems and a stable texture. Four parameters describe the microstructure, the misorientation angle across each boundary type and their respective spacing. Universal scaling characterizes the normalized distributions of three separate parameters. A new scaling law connects the strain evolution of two strength parameters: the dislocation density of IDBs and the spacing between GNBs. Strengthening mechanisms and strength contributions for those two parameters are expressed respectively as a linear addition of the classical Taylor and Hall-Petch formulations. Model predictions agree closely with experimental values of flow stress and work hardening rate in stages III and IV. Strong connections between the evolutionary stages of the deformation microstructure and work hardening rates create a new (modern) basis for the classic problem of work hardening in metals and alloys. These connections lead the way for the future development of ultra high strength ductile metals produced via plastic deformation.

Acta Materialia

Mechanical rejuvenation in bulk metallic glass induced by thermo-mechanical creep

Author(s): Y. Tong, W. Dmowski, H. Bei, Y. Yokoyama, T. Egami

Using high energy X-ray diffraction we studied the temperature, stress, and time effect on structural changes in a Zr-based bulk metallic glass induced by thermo-mechanical creep. Pair distribution functions obtained from two-dimensional diffraction patterns show that thermo-mechanical creep induces structural disordering, but only when the stress beyond a threshold is applied. A similar threshold behavior was observed for anelastic strain. We conclude that anelastic creep strain induces rejuvenation, whereas plastic strain does not.

Acta Materialia

The effects of microstructure and microtexture generated during solidification on deformation micromechanism in IN713C nickel-based superalloy

Author(s): G. Liu, J. Salvat Cantó, S. Winwood, K. Rhodes, S. Birosca

Nickel-based superalloy IN713C produced through investment casting route is widely used for turbocharger turbine wheels in the automotive industry. The produced microstructure and microtexture are not homogeneous across the turbine component due to geometrical factors and localised cooling rate during the casting process, which give rise to inhomogeneous deformation during service. In the present paper, two kinds of in-house fatigue tests, Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF), were conducted at 600 °C in attempt to simulate the actual fatigue conditions experienced by turbine wheels in turbocharger. From Geometrically Necessary Dislocation (GND) distributions and strain analyses, it is concluded that microstructure heterogeneity such as carbide precipitates distribution within dendritic structure network determine the failure micromechanics during LCF tests. In the early stage of LCF loading, crack principally initiated within near surface carbides that have been oxidised during high temperature exposure. The higher GND density at the tip of carbide precipitates due to oxidation volume expansion are found to facilitate easy cracks initiation and propagation. Moreover, the cluster-like carbides network and its distribution can accelerate oxidation process and crack growth effectively. Furthermore, in the later stage of crack propagation during LCF, the weak interdendrite areas rotate to accommodate increased strain leading to faster cracks propagation and hence final catastrophic failure. Serial section technique for 3-D visualisation was employed to investigate the crystallographic grain orientation correlation with fracture mechanics during HCF loading. It appears that the microtexure and grain orientations are more critical than the alloy microstructure in an area with a relatively uniform carbides distribution and weak dendrite structure where HCF failure occurred. Based on the slip trace analysis, it was found that most faceting occurred in Goss grains (<110>//LD) and on slip system with the highest Schmid factor. It is concluded that cracks were initiated on planes with high Schmid factors and assisted by the presence of porosity.

Acta Materialia

Dynamic shear deformation of a CrCoNi medium-entropy alloy with heterogeneous grain structures

Author(s): Yan Ma, Fuping Yuan, Muxin Yang, Ping Jiang, Evan Ma, Xiaolei Wu

Single-phase CrCoNi medium-entropy alloys (MEA) are emerging recently as an interesting class of metallic materials, but the dynamic response of this MEA at high strain rates remains unknown. Here we have produced this MEA with various heterogeneous microstructures, using cold rolling followed by annealing at various temperatures. The high-strain-rate response of the MEAs was characterized using hat-shaped specimens in Hopkinson-bar experiments. A combination of high dynamic shear yield strength and large uniform dynamic shear strain was observed, exceeding all other metals and alloys reported so far. Even better dynamic shear properties was revealed when the experiments were conducted at cryogenic temperature. The strong strain hardening under dynamic shear loading can be attributed to the dynamic grain refinement and deformation twinning that accompany the homogeneous shear deformation. When compared to room temperature, the efficiency of grain refinement was found to be enhanced at cryogenic temperature, with a higher density of multiple twins, stacking faults, Lomer-Cottrell locks, and hcp phase via phase transformation inside the grains, which could be responsible for the better dynamic shear properties under cryogenic environment.

Acta Materialia

Thermodynamic dislocation theory of adiabatic shear banding in steel

Author(s): K.C. Le, T.M. Tran, J.S. Langer

The statistical-thermodynamic dislocation theory developed in our earlier studies is used here in an analysis of the experimental observations of adiabatic shear banding in steel by Marchand and Duffy (1988). Employing only a small set of physics-based parameters, we are able to explain experimental stress-strain curves, including yielding transitions and strain hardening, over wide ranges of temperatures and strain rates. We make a simple model of weak notch-like perturbations that, when driven hard enough, trigger shear banding instabilities that are quantitatively in agreement with those seen in the experiments.

Scripta Materialia

Effect of the amplitude of the training stress on the fatigue lifetime of NiTi shape memory alloys

Author(s): Yahui Zhang, Ziad Moumni, Jihong Zhu, Weihong Zhang

This paper presents a mechanical training process that allows enhancing resistance to low cycle fatigue of shape memory alloys. To this end, three training stresses were tested (0–509.6 MPa, 0–637.0 MPa, 0–764.3 MPa); for each case, NiTi wires were first subjected to the corresponding load during first 20 cycles, and then tested to failure under strain-controlled fatigue loading. Results show that fatigue lifetime is training-dependent in the sense that specimens with higher training stresses present a better fatigue lifetime. Indeed, for sufficiently high training stress, fatigue lifetime can be 10 times extended.

Scripta Materialia

Effect of trace lanthanum hexaboride and boron additions on microstructure, tensile properties and anisotropy of Ti-6Al-4V produced by additive manufacturing

Author(s): M.J. Bermingham, S.D. McDonald, M.S. Dargusch

In this work trace lanthanum hexaboride (LaB6) and elemental boron are alloyed with Ti-6Al-4V and their effects on the microstructure, tensile properties (including anisotropy) and melt pool shape during Additive Manufacturing (AM) are investigated. During the melting process, the LaB6 scavenges oxygen and decomposes into La2O3 and TiB. The presence of the rare earth element drastically changes the apparent surface tension and shape of the deposited layers. This is attributed to the Heiple-Roper effect and could have benefits during AM in producing components with unsupported overhangs. The formation of eutectic TiB during the final stages of solidification results in highly directional TiB needles in between columnar grains that are aligned with the build direction. The slow cooling rate during deposition of approximately 90–100 °C s−1 produces very large TiB particles which can exceed 50 µm in length. Although improving strength by up to 10%, under tensile stress the high aspect ratio TiB particles are sites for crack opening which leads to a decline in ductility in the longitudinal test direction and a corresponding increase in anisotropy over unmodified Ti-6Al-4V.

Science and Engineering A

Influence of the kissing bond on the mechanical properties and fracture behaviour of AA5083-H112 friction stir welds

Author(s): Nan Zhou, Dongfu Song, Wenjun Qi, Xiaohui Li, Ji Zou, Moataz M. Attallah

The kissing bond phenomenon in AA5083-H112 friction stir butt welds was investigated in joints welded using a matrix of welding parameters, with tool rotation speeds of 800, 1000, and 1200 rpm and feed speeds 100, 200, and 300 mm/min. The length of the kissing bond along the cross-section normal to the welding direction was measured to quantify its influence on the mechanical properties. A combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile and fatigue testing were used to elucidate the impact of the kissing bond on the microstructural and mechanical properties development. The fracture type, location, and morphology were studied for the various conditions. The results showed that the welding parameters had a substantial effect on the length of the kissing bond, which was found to decrease with the increase in the welding heat input, as estimated based on the rotation and feed speeds. Moreover, the length and morphology of the kissing bond had a significant influence on the tensile and fatigue fracture type. A shear fracture type was characteristic for welds showing high tensile properties and long fatigue life, whereas fracture along the kissing bond was characteristic for poor tensile properties and short fatigue life.

Science and Engineering A

Microstructural evolution and its influence on the mechanical properties of a thermomechanically processed β Ti–32Zr–30Nb alloy

Author(s): Sertan Ozan, Yuncang Li, Jixing Lin, Yaowu Zhang, Hongwei Jiang, Cuie Wen

In this study, microstructural evolution and its influence on the mechanical properties of a newly developed titanium–zirconium–niobium (TZN) alloy after cold rolling and recrystallization annealing have been investigated. With an increase in the cold rolling reduction rate (CRRR) of the Ti–32Zr–30Nb TZN alloy, a change in the plastic deformation mechanisms has been observed. Deformation-induced α” and kink bands were observed in the TZN alloy after cold rolling at 20% and 56% CRRR; however, with a further increase in CRRR, while the deformation mechanisms including kink bands, {332} <113> β mechanical twinning and shear bands were increased, the formation of deformation-induced α” was suppressed. The Young's modulus of the TZN alloy specimen after cold rolling at 86% CRRR was found to be higher than those of the specimens after cold rolling at 20%, 56% and 76% CRRR, due to the reverse transformation of the deformation-induced martensite α” into β. The TZN alloy is evaluated as a promising candidate material for orthopaedic implant applications by virtue of its unique combination of values for Young's modulus, tensile strength, elongation at rupture and elastic admissible strain, which were measured in the ranges of 57–69 GPa, 692–961 MPa, 4–10% and 1.11–1.31%, respectively, after various thermomechanical treatments.

Science and Engineering A

Creep deformation of Co-Re-Ta-C alloys with varying C content–investigated in-situ by simultaneous synchrotron radiation diffraction

Author(s): Lukas Karge, Ralph Gilles, Debashis Mukherji, Andreas Stark, Premek Beran, Norbert Schell, Michael Hofmann, Pavel Strunz, Johannes Häusler, Joachim Rösler

The creep deformation of precipitation hardened Co-Re-Ta-C alloys is investigated during in-situ synchrotron diffraction experiment at 1373 K. At room temperature, the alloys have a structure consisting of ϵ -Co (hcp) and metastably retained γ -Co (fcc) and are strengthened by precipitates of the mono-carbide of Ta, which are finely dispersed in the alloy matrix. The alloy exhibits an allotropic ϵ γ -Co phase transformation when heating to > 1173 K . A lower C content in the alloy generally promotes this transformation. It is shown that this transformation is strongly influenced by application of compressive load. The transformation ϵ γ -Co at high temperature under load leads to microstructure refinement and subsequently to dissolution of hardening precipitates. This results in a considerable acceleration of the creep rate. Further, the equilibrium ratio of γ / ϵ -Co phase is significantly altered under compressive load. This behavior is attributed to a volume relaxation as the ϵ - and γ -Co phase have different unit cell volumes.

Science and Engineering A

In-situ TiB2-NiAl composites synthesized by arc melting: Chemical reaction, microstructure and mechanical strength

Author(s): Heng Zhang, Heguo Zhu, Jiewen Huang, Jianliang Li, Zonghan Xie

TiB2 particles (5 vol% and 10 vol%) reinforced NiAl matrix composites were fabricated in-situ from Ti-B-Ni-Al system by arc melting. The reaction mechanism and mechanical properties of the composites were studied. When the reaction system was heated to 917 K, Ni and Al reacted with Ti to form the transient phase AlNi2Ti, which continued to react with B to yield ultrafine TiB2 particles acting as reinforcement agent. The apparent activation energy for these two reactions were calculated and found to be 497.99 kJ/mol and 2354.78 kJ/mol, respectively. The reinforcement agent exerted dispersion strengthening effect on the matrix. The room temperature compressive strength of the composites reinforced by 10 vol% of TiB2 particles was determined to be 538.3 MPa, representing a 35.2% increase over NiAl alloy. To further refine the grain size, the rare earth element cerium (2 wt% and 4 wt%) was introduced to the composites. The compressive strength of the composites containing 4 wt% Ce was determined to be 571.1 MPa, representing a 43.5% increase over NiAl alloy.

Science and Engineering A

Hydrogenation effect on microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys

Author(s): Rimma Lapovok, Emil Zolotoyabko, Alex Berner, Vladimir Skripnyuk, Eugene Lakin, Natalya Larianovsky, Chunjie Xu, Eugen Rabkin

This work explores the ways of manipulating the microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys of various compositions using hydrogen treatment. Changes to phase composition, microstructure, and mechanical properties of the alloys upon hydrogenation were studied. Prior to hydrogenation, the alloys were extruded at different temperatures with or without subsequent aging. Hydrogen treatment was performed on bulk rods after thermo-mechanical processing. X-ray diffraction and scanning electron microscopy studies showed that a single rare-earth (RE) hydride phase, Gd0.5Y0.5H2, was formed in all samples. As a result, the 14H long period stacking ordered (LPSO) structure, detected before hydrogenation, is completely destroyed due to the clustering of RE atoms into large hydride crystals and annihilation of the specific spatial order within the superlattice. Prolonged hydrogen treatment at high temperature (≈ 0.74·Tm) causes recrystallization and grain growth in the magnesium matrix, defect annealing and reduction of preferred orientation, which together with the complete destruction of the LPSO phase lead to the substantial decrease of alloys’ strength and concomitant increase of their ductility to the record-high value of above 20%.

Science and Engineering A

Dynamic mechanical behaviors and failure thresholds of ultra-high strength low-alloy steel under strain rate 0.001s to 106s

Author(s): Jie Ren, Yuxin Xu, Xiaoxu Zhao, Pengduo Zhao

In this study, the dynamic mechanical behaviors of the typical ultra-high strength low-alloy martensite steel 35CrMnSiA under strain rate 0.001/s~106/s were studied through quasi-static compressive tests (0.001/s), SHPB tests (2000/s~5000/s) and planar plate impact tests together with DISAR tests (105/s~106/s). The XRD analysis and metallographic observation were conducted to investigate the microstructure evolutions and failure mechanism of 35CrMnSiA under different stress states. Under uniaxial stress state, the adiabatic shear failure occurs as long as the rise rate of plastic strain energy density reaches 10.58 × 106 J·m−3 µ s−1 and the plastic strain energy density is above 4.51 × 108 J·m−3. For 35CrMnSiA samples under uniaxial strain state, the critical pressure for reversible phase transformation ( α ε , BCC HCP) falls in the range of 17.57–19.19 GPa. Characterized by prominent temperature rise and volume shrinkage, the α ε phase transformation induced by continuous dynamic recrystallization contributed to increasing the strength but weakening the ductility of 35CrMnSiA. In addition, the Hugoniot coefficients for 35CrMnSiA under a wide range of pressures have been determined. Moreover, the failure thresholds for 35CrMnSiA were obtained by dynamic fracture experiments and high velocity impact experiments: 35CrMnSiA projectiles fractured over impact pressure of 2.59 GPa, and when the impact pressure exceeded 21.25 GPa above which 35CrMnSiA suffered phase transformation, the projectiles had severe mass abrasion.

Science and Engineering A

Effect of Ti and C additions on the microstructure and mechanical properties of the FeCoCrNiMn high-entropy alloy

Author(s): Hu Cheng, Wei Chen, Xiaoqiang Liu, Qunhua Tang, Yanchong Xie, Pinqiang Dai

To improve the yield strength of an FeCoCrNiMn high-entropy alloy (HEA), elemental Ti and C were doped into the alloy. Subsequently, an in situ synthesized carbides particle-strengthened HEA matrix composite was prepared by mechanical alloying (MA), followed by a vacuum hot-pressing sintering (VHPS) method. The TiC nanoparticles were distributed along the grain boundaries. The microstructure of the alloy contained a face-centered cubic (FCC) solid solution as the matrix phase and small amounts of TiC, M23C6 and M7C3 (where M = Cr, Mn, Fe) carbides. The addition of elemental Ti and C significantly improved the room-temperature compressive yield strength of the FeCoCrNiMn HEA from 774 MPa to 1445 MPa (an 86.7% increase), accompanied by a decrease in the compressive strength and plasticity. Grain boundary strengthening and precipitation strengthening are the main strengthening mechanisms of the alloy doping with elemental Ti and C.

Science and Engineering A

Dynamic co-precipitation of α-Mn Mg12Ce in creep resistant Mg-Sr-Mn-Ce alloys

Author(s): M. Celikin, R. Gauvin, M. Pekguleryuz

Quaternary Mg-Sr-Mn-Ce alloys with trace Ce additions exhibit four times lower creep strain than the ternary Mg-Sr-Mn alloys at 200 °C and 50 MPa. Upon thermal exposure, the main phase transformation is the precipitation of both α-Mn and Mg12Ce / α-Mn in intradendritic regions from supersaturated Mg matrix. It was seen that 2D plates of Mg12Ce phases formed with their habit plane perpendicular to g = 2 1 ¯ 1 ¯ 0 and affected the size, morphology and orientation of the α-Mn precipitates. 2D plates form obstacles to <a> dislocations impeding basal and prismatic slip. Dynamic co-precipitates of Mg12Ce / α-Mn act a key role in creep strengthening via dislocation pinning.

Science and Engineering A

Microstructure, mechanical properties and oxidation behavior of short carbon fiber reinforced ZrB2-20voSiC-2voB4C composite

Author(s): Jiten Das, B.Chenna Kesava, J. Janardhana Reddy, V. Srinivas, Sweety Kumari, VV Bhanu Prasad

Present study aims at observing the effect of short carbon fiber addition on the microstructure, mechanical properties and oxidation behavior of the ZrB2-20v/oSiC-2v/oB4C composite. Microstructure of the composite shows uniformly distributed SiC particles along with the occasional presence of short carbon fiber and B4C particles in the ZrB2 matrix. Carbon fiber addition leads to refinement of both ZrB2 and SiC grains. Both the flexural strength and maximum strain value of carbon fiber reinforced ZrB2-20v/oSiC-2v/oB4C composite is observed to be higher than those of the base i.e. ZrB2-20v/oSiC-2v/oB4C composite. Although the fracture toughness of the composite improves, hardness of the composite does not change significantly due to addition of carbon fiber. After the oxidation treatment in air furnace at 1600 °C for 2 h, the carbon fiber reinforced composite shows formation of SiO2 rich, continuous and protective top layer of about 30 µm, ZrO2+SiO2-middle layer of about 80–90 µm and a relatively thin SiC depleted layer (~100 µm). On the other hand, the base composite shows the formation of a discontinuous SiO2-rich top layer of about 20–30 µm, ZrO2+SiO2-middle layer of about 60–70 µm, and a thick (~200 µm) and cracked SiC depleted layer after the same oxidation treatment. Hardness deterioration is not observed beneath the oxide layer including SiC depleted region in case of carbon fiber reinforced composite, while a slight deterioration is observed in the base composite. Thermal diffusivity of the carbon fiber reinforced ZrB2-20v/oSiC-2v/oB4C composite is observed to be slightly lower than that of the base ZrB2-20v/oSiC-2v/oB4C composite.

Science and Engineering A

A comprehensive analysis of extrusion behavior, microstructural evolution, and mechanical properties of 6063 Al–B4C composites produced by semisolid stir casting

Author(s): Amir Pakdel, Agnieszka Witecka, Gaulthier Rydzek, Dayangku Noorfazidah Awang Shri, Valeria Nicolosi

In this study, composites of aluminum alloy 6063 reinforced with 10wt.% boron carbide microparticles were successfully fabricated by a combination of spark plasma sintering and stir casting methods, followed by hot extrusion. A systematic study on the relationship between extrusion process variables (i.e. extrusion ratio, temperature, and punch speed) and porosity, particle refinement, particle distribution and consequently tensile properties and fracture behavior of the composites was performed. Extensive electron microscopy analysis and tensile testing of the composites revealed a multifactoral interdependency of microstructural evolution and mechanical properties on the extrusion process variables. For example, while increasing the extrusion ratio at higher temperatures led to moderate particle refinement, better densification of the composites, and improvement in mechanical properties, concurrent particle fragmentation and microvoid formation around the particles at lower temperatures had opposing effects on the mechanical behavior. We show that the dependency of mechanical properties on all such microstructural factors makes it difficult to predict optimum extrusion conditions in aluminum matrix composites. That is, unlike the common approach, extruding the composites at higher temperatures and achieving more reduction in area may not necessarily lead to the most favorable mechanical properties.

Science and Engineering A

Formability and Fracture Behaviour of Cryorolled Al-3 Mg-0.25 Sc Alloy

Author(s): S. Vigneshwaran, K. Sivaprasad, R. Narayanasamy, K. Venkateswarlu

The rolling of an Al-3Mg-0.25Sc alloy at room and cryogenic temperature to 50 percent and 75 percent reduction in thickness resulted in the formation of a bimodal microstructure. The grain size distribution based on electron backscattered diffraction (EBSD) analyses showed an enhanced fraction of ultra-fine grains with nearly 100nm in cryorolled samples whereas; room temperature rolled samples exhibited sub-micron grains with 300nm. The transmission electron microscopy (TEM) studies revealed the dense dislocation cell structures for cryorolled samples due to the restriction of dynamic recovery. The better forming and fracture limit strains were noted for cryorolled samples compared to room temperature rolled ones in the forming and fracture limit diagrams. A good correlation of formability with void coalescence parameters based on larger void size, lower ligament thickness and lower d-factor were obtained. This indicated better accumulation of plastic deformation and improved formability of cryorolled samples. Further, the reduced aspect ratio (L/W) of void signified delayed fracture behaviour of cryorolled samples compared to room temperature rolled samples and showed better fracture resistance. The improved fracture limit strain in combined forming and fracture limit diagram exhibited by cryorolled samples was consistent with the void coalescence parameters.

Science and Engineering A

New Structured Laves Phase in the Mg-In-Ca System with Nontranslational Symmetry and Two Unit Cells

Author(s): Hongbo Xie, Hucheng Pan, Yuping Ren, Liqing Wang, Yufeng He, Xixi Qi, and Gaowu Qin

All of the $A{B}_{2}$ Laves phases discovered so far satisfy the general crystalline structure characteristic of translational symmetry; however, we report here a new structured Laves phase directly precipitated in an aged Mg-In-Ca alloy by using aberration-corrected scanning transmission electron m...

Physical Review Letters

Electronic transport property in Weyl semimetal with local Weyl cone tilt

Liwei Jiang, Lanting Feng, Haibo Yao and Yisong Zheng

In realistic materials of Weyl semimetal (WSM), the Weyl cone tilt (WCT) is allowed due to the absence of Lorentz invariance in condensed matter physics. In this context, we theoretically study the electronic transport property in WSM with the local WCT as the scattering mechanism. In so doing, we establish an electronic transport structure of WSM with the WCT occurring only in the central region sandwiched between two pieces of semi-infinite WSM without the WCT. By means of two complementary theoretical approaches, i.e. the continuum-model method and the lattice-model method, the electronic transmission probability, the conductivity and the Fano factor as functions of the incident electron energy are calculated respectively. We find that the WCT can give rise to nontrivial intervalley scattering, as a result, the Klein tunneling is notably suppressed. More importantly, the minimal conductivity of a WSM shifts in energy from the Weyl nodal point. The Fano factor of the shot nois...

Journal of Physics Condensed Matter

Charge screening-controlled Verwey phase transition in Fe 3 O 4 /SrTiO 3 heterostructure

H Ji, Y G Wang and Y Li

Despite intensive investigations into the Verwey phase transition of Fe 3 O 4 over half a century, the mechanism of this phase transition remains controversial and needs further research. In this work, we build the Fe 3 O 4 /SrTiO 3 multiferroic heterostructure and investigate the temperature dependence of its saturation magnetization under various electric fields. It is found that the charge-screening effect not only influences the magnetization but also induces the temperature of the Verwey phase transition shifting ~13 K. It suggests that the Verwey phase transition has certain correlations with the electron distribution and the change of the number of minority spin electrons in the trimerons plays a dominant role in the temperature shift of the phase transition.

Journal of Physics Condensed Matter

Water at surfaces with tunable surface chemistries

Stephanie E Sanders, Heather Vanselous and Poul B Petersen

Aqueous interfaces are ubiquitous in natural environments, spanning atmospheric, geological, oceanographic, and biological systems, as well as in technical applications, such as fuel cells and membrane filtration. Where liquid water terminates at a surface, an interfacial region is formed, which exhibits distinct properties from the bulk aqueous phase. The unique properties of water are governed by the hydrogen-bonded network. The chemical and physical properties of the surface dictate the boundary conditions of the bulk hydrogen-bonded network and thus the interfacial properties of the water and any molecules in that region. Understanding the properties of interfacial water requires systematically characterizing the structure and dynamics of interfacial water as a function of the surface chemistry. In this review, we focus on the use of experimental surface-specific spectroscopic methods to understand the properties of interfacial water as a function of surface chemistry. Inves...

Journal of Physics Condensed Matter

First principles study of LiAlO 2 : new dense monoclinic phase under high pressure

Guangtao Liu and Hanyu Liu

In this work, we have systematically explored the crystal structures of LiAlO 2 at high pressures using crystal structure prediction method in combination with the density functional theory calculations. Besides the reported α , β , γ , δ and ε- phases, here we propose a new monoclinic ζ -LiAlO 2 ( C 2/ m ) structure, which becomes thermodynamically and dynamically stable above 27 GPa. It is found that the cation coordination number increases from 4 to 6 under compression. Consisting of the compact {LiO 6 } and {AlO 6 } octahedrons, the newly-discovered ζ -phase possesses a very high density. Further electronic calculations show that LiAlO 2 is still an insulator up to 60 GPa, and its bandgap increases upon compression. The present study advances our understanding on the crystal structures and high-pressure phase transitions of LiAlO 2 that may trigger applications in m...

Journal of Physics Condensed Matter

Asymmetric transmission of a planar metamaterial induced by symmetry breaking

Yu Bai, Yuyan Chen, Yongyuan Zhang, Yongkai Wang, Tudahong Aba, Hui Li, Li Wang and Zhongyue Zhang

Asymmetric transmission (AT) is widely used in polarization transformers and polarization-controlled devices. In this paper, a planar metamaterial nanostructure with connected gammadion-shaped nanostructure (CGN) is proposed to achieve AT effect for forward and backward propagations of circular polarized light. The CGN arrays can produce magnetic moment oscillation that is normal to the metamaterial plane, which is weakly coupled to free space and generates transmission valleys. The introduction of symmetry breaking exerts a strong influence on the AT effects, and these effects can be tuned by the structural parameters. Our planar metamaterials may have potential for application in the future design of polarization-controlling devices.

Journal of Physics Condensed Matter

Wed Feb 21 2018

Simple data and workflow management with the signac framework

Author(s): Carl S. Adorf, Paul M. Dodd, Vyas Ramasubramani, Sharon C. Glotzer

Researchers in the fields of materials science, chemistry, and computational physics are regularly posed with the challenge of managing large and heterogeneous data spaces. The amount of data increases in lockstep with computational efficiency multiplied by the amount of available computational resources, which shifts the bottleneck in the scientific process from data acquisition to data processing and analysis. We present a framework designed to aid in the integration of various specialized data formats, tools and workflows. The signac framework provides all basic components required to create a well-defined and thus collectively accessible and searchable data space, simplifying data access and modification through a homogeneous data interface that is largely agnostic to the data source, i.e., computation or experiment. The framework’s data model is designed to not require absolute commitment to the presented implementation, simplifying adaption into existing data sets and workflows. This approach not only increases the efficiency with which scientific results can be produced, but also significantly lowers barriers for collaborations requiring shared data access.

Computational Materials Science

A tight-binding molecular dynamics study of the noble metals Cu, Ag and Au

Author(s): S. Silayi, D.A. Papaconstantopoulos, M.J. Mehl

We have used the Naval Research Lab (NRL) tight-binding (TB) method to study the electronic and mechanical properties of the noble metals. In order to perform molecular dynamics simulations, we used new TB parameters that work well at smaller interatomic distances. The TB parameters were fitted to the fcc, bcc and sc periodic structures and were demonstrated to be transferable and robust for calculating additional dynamical properties which they had not been fitted to. We calculated the phonon frequencies and density of states at finite temperature and we also performed simulations to determine the temperature dependence of the coefficient of thermal expansion and the mean squared displacement. The energy for vacancy formation as well as energy for fcc-based, bcc-based clusters and icosahedral clusters of different sizes were also calculated. The results compared very well with experimental observations and independent (non-fitted) first-principles density functional calculations.

Computational Materials Science

The important role of oxygen defect for NO gas-sensing behavior of α-Fe2O3 (0 0 1) surface: Predicted by density functional theory

Author(s): Feifei Li, Changmin Shi, Xiaofeng Wang, Guangliang Cui, Dongchao Wang, Li Chen

Using density functional theory (DFT), we investigated and discussed the adsorption characteristics, gas-sensing response and gas-sensing mechanism of NO molecule on α-Fe2O3 (0 0 1) surface with and without oxygen defect (VO). The pure and oxygen-defective α-Fe2O3 (0 0 1) surface exhibited opposite electron transfer. The theoretical results proved that the NO molecule acted as a donor for pure α-Fe2O3 (0 0 1) surface. However, it failed to explain the increasing resistance of n-type metal oxide materials. For oxygen-defective α-Fe2O3 (0 0 1) surface, NO molecule acted as an acceptor. The results leaded to a decreasing electron-carrier concentration, and then resulted in an increasing resistance of oxygen-defective α-Fe2O3 (0 0 1) surface after NO molecule was introduced into. The direction of electron transfer was reversed by oxygen defect. In addition, the VO-NO adsorption configuration induced more stable adsorption structure and more significant electron transfer effects between NO molecule and oxygen-defective α-Fe2O3 (0 0 1) surface. The VO-NO adsorption configuration would have better gas-sensing performance for α-Fe2O3 (0 0 1) surface.

Computational Materials Science

Influence of SiC surface defects on materials removal in atmospheric pressure plasma polishing

Author(s): Guanglu Jia, Bing Li, Jufan Zhang

To study the influence of SiC surface defects on atmospheric pressure plasma polishing (APPP) process, quantum chemistry simulation and analysis is used to reveal the reaction features of typical defect topographies. Three groups of typical structures are modeled, including edge dislocation, screw dislocation and perfect crystal lattice. By using quantum chemistry calculation software, it is demonstrated that the existence of surface defects improves probability for chemical etching. The densities of states (DOS) and number of bonding electrons indicate that the defect structures have poor stability compared with perfect crystal lattice, which means defects are favorable for increasing the removal rate. The calculation results on activation energy also verify the conclusion further. Experimental machining and measurement have been performed to prove the theoretical analysis. Tests are made on selected single crystal SiC samples with different defect densities. Removal profiles measured by white light interferometer indicate that surface defects are helpful for raising the machining efficiency. But, measured surface topographies show that within certain range, surface defects deteriorate the surface roughness during the polishing process. Until most surface damage is removed, the surface roughness will be improved effectively which makes the interface smoother. Thus, the experimental investigation accords well with theoretical analysis.

Computational Materials Science

Electronic and magnetic properties of structural defects in pristine ZrSe2 monolayer

Author(s): Yonghui Gao, Xu Zhao, Haiyang Wang, Tianxing Wang, Shuyi Wei

In the present study, we research the electronic and magnetic properties of structural defects in pristine ZrSe2 by using the first-principles methods based on density functional theory. Twelve cases of vacancy defects are considered by removing Zr, Se or Zr+Se atoms in pristine monolayer ZrSe2. In selecting vacancy defects, we consider the order of ascending defect concentration of Zr and Se, and also calculate the different relative positions of Se atoms with the same defect concentration of Se, compare the electronic and magnetic properties with them. The results show that vacancy defects in pristine ZrSe2 monolayer, which all induce to the increase of the total magnetic moment except V1Se. In addition, we find the total magnetic moment increases as the number of defective atoms increases in case of Se atoms vacancy defects, and the largest total magnetic moment appears on the case of V6Se. Moreover, we also found that almost all the vacancy defects in monolayer ZrSe2 show metallic property and ferromagnetism and total energies can be increased with the increasing of the number of vacant atoms in ZrSe2 monolayer. In particular, when the relative position of the defective atom changes, the total magnetic moment changes dramatically. In three cases of Zr atoms vacancy defects the lowest total energy is V1Zr, the most stable case is V1Se in the case of twelve vacancy defects we considered. Its vacancy-defect formation energy is 2.287 eV. These results have a few guiding significance for relevant experiments based on ZrSe2.

Computational Materials Science

Computational study of the nanoscale mechanical properties of C-S-H composites under different temperatures

Author(s): X.F. Wang, T.R. Li, P. Wei, D.W. Li, N.X. Han, F. Xing, Y. Gan, Z. Chen

In this computational study, the nanoscale mechanical properties of C-S-H composite materials under different temperatures are evaluated with molecular dynamics (MD). The effects of different proportions of crystals, levels of porosity, and degrees of temperature on the nanoscale mechanical properties of the C-S-H composites are investigated. The initial computational models are built up based on tobermorite and jennite crystals, and the geometries of the models are then optimized before performing MD simulations at 25 °C, 80 °C, and 200–1500 °C with the interval of 100 °C, respectively. The resulting mechanical properties of the composites are evaluated under different conditions. The self-consistent (SC), Mori-Tanaka (MT), and Voigt methods are used to understand the effects of crystal proportions and porosities on the mechanical properties of the C-S-H composites at three representative temperatures. It is found that both the bulk and shear moduli of the C-S-H composites are decreased with the increasing temperature. Regarding the influence of porosity, a specific proportional relationship is obtained based on the results with the SC, MT, and Voigt methods. The reported findings provide a better insight into the thermomechanical response of the C-S-H composites.

Computational Materials Science

Overdriven dislocation-precipitate interactions at elevated temperatures

Author(s): Amirreza Keyhani

The two-dimensional dislocation dynamics approach has been recently used for analyzing plastic deformation in metals and alloys at elevated temperatures. The two-dimensional approach, however, only accounts for the dislocation climbing process, and it assumes that dislocation bypassing and shearing of precipitates are negligible. To examine the validity of this assumption, the present study quantifies dislocation bypassing and shearing of precipitates in terms of critical resolved shear stress, interaction time, and thermal activation energy for various precipitate strength levels, temperatures, and precipitate spacings. This study uses a modified dislocation dynamics approach that accounts for shearable and non-shearable precipitates. Simulations focus on the overdriven dislocation dynamics regime wherein the climbing process is limited by fast interactions between dislocations and precipitates. The results show that even though the resolved shear stress level required for a dislocation to overcome an array of precipitates decreases at higher temperatures, the interaction time between the dislocation and the precipitates increases. In addition, the maximum ratio of thermal activation energy to the precipitate energy barrier is only 0.15.

Computational Materials Science

Molecular dynamics simulation on the shape memory effect and superelasticity in NiTi shape memory alloy

Author(s): Xiang Chen, Teng Liu, Rui Li, Jiushan Liu, Yang Zhao

Molecular dynamics (MD) simulations Ni-Ti based on a second nearest neighbor modified embedded-atom method potential were performed to study atomic-scale mechanical behavior and microstructural evolution of Ni-Ti alloy at different temperatures. By using four calculated characteristic transformation temperatures, the optimum simulated temperature ranges were determined. At temperatures lower than Ms , a thermally induced self-accommodating martensitic phase consisting of three variants formed. Each of the two variants was twin structures. In the subsequent loading process, the most favorable variant grew with the movement of interfaces among the variants, which produced remnants of the residual strain after unloading. After heating up to over A f , the residual strain disappeared and the shape recovered to the original. The shape memory effect under austenite state was also performed, wherein the sample exhibited a typical stress–strain temperature curve and remained in a martensitic state after unloading. After heating at a high temperature, the martensite transformed back into its parent phase accompanied with the recovery of residual strain. The material exhibited superelastic behavior above A f , and the critical transformation stress increased with increased temperature. Some parts of B2 structures remained at the end of the plateau stage and continued transforming into martensites during the hardening stage for both T > Af and Ms < T < Af . The critical stresses for transformation satisfied the Clausius–Clapeyron relationship at T > Ms .

Computational Materials Science

Anharmonicity of vibrational modes in fullerenes

Author(s): Hengjia Wang, Murray S. Daw

We report a computational study of the anharmonicity of the vibrational modes of fullerenes using the “moments method” (Gao et al., 2015) with a Tersoff-style potential for carbon. We find that the frequencies of all vibrational modes drop systematically with temperature and that the size of the fullerene does not strongly determine the anharmonicity of its modes. Favorable comparison is made with experiments.

Computational Materials Science

Effect of different solute diffusivities on precipitate coarsening in ternary alloys

Author(s): M.S. Bhaskar, T.A. Abinandanan

According to Gibbs phase rule, ternary two phase alloys have a single degree of freedom at equilibrium at a given temperature. Thus, multiple precipitate-matrix equilibrium compositions are possible at the interface. From Philippe-Voorhees’ (PV) theory (Philippe and Voorhees, 2013), it is known that during coarsening in multicomponent alloys, precipitate-matrix compositions at the interface during coarsening are not just dependent on the Gibbs-Thomson effect but also on the relative mobilities of the solute elements. Our computer simulations, based on a phase field model, show that this effect of different solute diffusivities on size-dependent particle composition is more pronounced in alloys richer in the slower diffusing solute.

Computational Materials Science

Description of light-element magnetic systems via density functional theory plus U with an example system of fluorinated boron nitride: An efficient alternative to hybrid functional approach

Author(s): Wanxue Li, Xiaojun Xin, Hongyan Wang, Chunsheng Guo, Hong Jiang, Yong Zhao

It is well known that for light-element magnetic materials density functional theory (DFT) in the local density approximation or generalized gradient approximation (LDA or GGA) underestimates the electron localization effects and tends to give misleading results. Hybrid functionals such as Heyd-Scuseria-Ernzerhof (HSE) perform much better while being computationally expensive, especially for extended systems. In order to go beyond semi-local DFT without needing to calculate the expensive Fock exchange, here we explore the performance of the more efficient GGA plus the Hubbard U correction (GGA+U) approach to light-element magnetic materials by considering fluorinated boron nitride (F-BN) sheets and nanotubes as model systems. By applying the Hubbard U correction to the N-2p orbitals with the value of U determined by fitting the HSE results in a particular F-BN sheet, it is found that the GGA+U approach shows a great improvement to GGA in describing the magnetic properties of F-BN systems with an accuracy close to that of the HSE hybrid functional approach. It indicates the possibility of using the ad hoc correction approach as an efficient alternative to study light-element magnetic materials, especially for large systems where calculations based on hybrid functionals become cost-demanding.

Computational Materials Science

Monte Carlo simulations of bulk and nano amorphous silica (a-SiO2) melts

Author(s): Naveen Kumar Kaliannan, Karthik Krishnamurthy, SivaKartheeka Sreerama, Anto Michael Ronson Joseph Jesu Rathnam

Monte Carlo simulations were carried on bulk and nano amorphous silica (SiO2) melts under different thermodynamic conditions and nano-sizes. The potential developed by Ersan, Tahir, Norman and William (ETNW) was used to model the interatomic interactions of SiO2 melts in both cases (bulk and nano systems). The bulk system was developed using periodic boundary conditions. The nano SiO2 melts were prepared by cutting out a sphere from the bulk amorphous SiO2 melt structure to the required size, and then simulated under free boundary conditions. The high-pressure, high-temperature and nano-size effects on the structure and properties of the melts were investigated and studied via radial distribution functions, radial density profiles, potential energy, density, ring size analysis, coordination number distributions, bond distances and angles. In addition, to understand the difference in properties of bulk and nano silica melts, a comparative study was done at the same thermodynamic conditions. Details of the modelling, simulation results and the study are presented in this paper.

Computational Materials Science

Quantitative phase field modelling of precipitate coarsening in Ni-Al-Mo alloys

Author(s): M.S. Bhaskar

In this paper, we attempt a critical and quantitative comparison between our phase field model and experimental results on coarsening in ternary two phase alloys. For experimental results, we have chosen a specific Ni-Al-Mo alloy [Ni-7.7% Al-7.9% Mo] (at.%). Since the grand potential formulation (Choudhury and Nestler, 2012) has the advantage of being amenable to incorporation of thermodynamic data for all the relevant phases, it is our phase field model of choice for this study. Our attempt is aimed at identifying specific gaps between theory/computation and experiments that may be addressed through future research. A comparison of coarsening rate from our 3D simulations with the experimentally observed rate has led to an insight into possible inaccuracies in data available for this system. In particular, it has pointed to the possibility that the curvature of the free energy surface (for the α phase) being overestimated by ThermoCalc.

Computational Materials Science

Computational study of CNT based nanoscale reversible mass transport archival memory with Fe, Co and Ni nano-shuttles

Author(s): Bikash Sharma, Amretashis Sengupta, C.K. Sarkar

We report the atomistic study of a carbon nanotube (CNT) mass transport memory with Fe, Co and Ni nanoparticle shuttles encapsulated within it. For our calculation the extended-Hückel theory has been employed to study the various electronic, electrostatic and transport of such devices. The simulation results show that all the three sets of CNT devices with Fe, Co and Ni nano-shuttle are efficient in performance in terms of distinguishable electronic properties, with regard to nanoparticle position and type of nanoparticles. There is observable change in transmission w.r.t. change of positions and type of nanoparticles. Fe@CNT shows more metallic nature of transmission as compared to Co@CNT and Ni@CNT. All the devices show minimal loss of coherence in transmission in terms of conducting eigenstates and elastic backscattering. The Ni and Co nanoparticle captured more amount of charge as compared to Fe nanoparticle, and can offer superior performance in case of charge sensing detection of memory states.

Computational Materials Science

Polyimide electrode materials for Li-ion batteries via dispersion-corrected density functional theory

Author(s): Huili Lu, Shaorui Sun

Compared with organic electrode materials that are used for lithium ion batteries that are constructed using small organic molecules, polymer electrode materials have a better cycling stability, which may be due to their stable long chain structure, and currently, the mechanism of energy storage has not been thoroughly elucidated. In this study, dispersion-corrected density functional theory (DFT-D2) was used to explore the charge/discharge process of polyimide, which is an organic polymer electrode material that could be used in Li-ion batteries. The calculated potentials of PI-1 (polyimide-1) and PI-2 (polyimide-2) are 2.03 and 2.07 V, respectively, and they significantly agree with experimental values, which implies that DFT-D2 is a powerful method to investigate polymer electrodes for use in lithium ion batteries. The calculated potential of pyromellitimide (DPI) is 1.79 V, and DPI is a novel electrode material that has not been reported to date. For each of the three polyimides, lithium ions do not diffuse along the polymer chains but diffuse in the vertical direction, and the migration barriers of PI-1, PI-2, and DPI are 0.47, 0.84, and 0.088 eV, respectively; thus, they have good ionic conductivities (beyond PI-2). Although the calculated band gaps of the three polyimides are all approximately 1.0 eV, the effective electron (or hole) masses are too large, which may limit their electronic conductivities and rate performances. The calculated results show that polyimides are potential Li-ion electrode materials, and this theoretical method could be applied to design novel polymer electrode materials.

Computational Materials Science

Mechanism of NbC heterogeneous nucleation on TiN in microalloyed steel: A first-principles study

Author(s): Haihui Zhang, Huihui Xiong, Dezhi Wang, Wanlin Wang

In this work, the interfacial properties and bonding feature of NbC(1 0 0)/TiN(1 0 0), NbC(1 1 0)/TiN(1 1 0) and NbC(1 1 1)/TiN(1 1 1) interfaces were investigated by using the first-principles method. And the mechanism of NbC heterogeneous nucleation on TiN in microalloyed steel was revealed. The results showed that the (1 0 0) surface is more stable than (1 1 0) for both NbC and TiN. Besides, NbC(1 1 1)/TiN(1 1 1) interface-III (with C3Ti1 termination) possesses the largest work of adhesion, followed by the NbC(1 1 0)/TiN(1 1 0) interface and NbC(1 0 0)/TiN(1 0 0) interface. Additionally, NbC(1 1 1)/TiN(1 1 1) interfaces with CTi bonding are more stable than NbC(1 1 1)/TiN(1 1 1) interfaces with NbN, NbTi and CN bondings. The bonding of NbC(1 0 0)/TiN(1 0 0) interface-A (with NbN termination) and NbC(1 1 0)/TiN(1 1 0) interface-A (with CTi termination) are mainly consist of covalent bonds. While NbC (1 1 1)/TiN(1 1 1) interface-III has strong covalent bonding and weak metallic bonding. Furthermore, in the entire range of Ti and C chemical potentials, the interfacial energy of NbC(1 1 1)/TiN(1 1 1) interface-III is smaller than those of NbC(1 0 0)/TiN(1 0 0) interface-A and NbC(1 1 0)/TiN(1 1 0) interface-A. Thus, the priority about NbC/TiN heterogeneous nucleation interface is in the order of (1 1 1), (1 0 0), and (1 1 0).

Computational Materials Science

Electronic and magnetic properties of two dimensional cluster-assembled materials based on TM@Si12 (TM = 3d transition metal) clusters

Author(s): Zheng Nie, Ping Guo, Jiming Zheng, Puju Zhao, Yun Wan, Zhenyi Jiang

Electronic and magnetic properties of two dimensional (2D) cluster-assembled materials based on TM@Si12 (TM = 3d transition metal) clusters were systematically investigated by using the density functional method. Taking the hexagonal prism TM @Si12 as a building block, we constructed four different kinds of 2D crystal structures, each with a higher stability than the corresponding individual clusters. The hexagonal honeycomb and porous structures are proved to be thermodynamically stable at room temperature by first-principles molecular dynamics simulations, and the honeycomb structure is more favorable in energy than the porous structure. The magnetic coupling properties of the honeycomb and porous structures based on TM@Si12 were further studied in detail. The results show that almost all of the hexagonal TM@Si12 2D lattice exhibit a certain degree of magnetic ordering. These studies provide insights into the effective design of 2D spintronic materials.

Computational Materials Science

An open-source code to generate carbon nanotubegraphene junctions

Author(s): Hao Zhang, Zhencheng Ren, Chang Ye, Yalin Dong

Carbon nanotube (CNT)/graphene nanostructure has the potential to extend the superior mechanical, thermal, and electrical properties of graphene from two dimensions to three. While the theoretical investigation of CNT/graphene nanostructure based on atomistic modeling is garnering great attention, an open-source numerical tool to generate covalently bonded CNT/graphene junctions is still in lack for material scientists. In this work, a pathfinding algorithm is used to exhaust all possible configurations on graphene to seamlessly connect to a given CNT. The least squares approach method follows to sort out the configuration with minimum energy. The combined methods are able to generate CNT/graphene junction for any CNT type (m, n). Molecular dynamics simulation further reveals that the formed junctions are thermodynamically stable, and thus ready to serve as basic block for a CNT/graphene network. By providing an easy-to-use numerical tool in the form of MATLAB code, the intention is to free material scientists from the tedious preparation of atomic configuration.

Computational Materials Science

Electronic structure and optical properties of SrTiO3 codoped by WMo on different cationic sites with CN from hybrid functional calculations

Author(s): Yanyu Liu, Wei Zhou, Chao Wang, Lili Sun, Ping Wu

The effect of C, N, Mo and W doping on the electronic structures and optical absorption in SrTiO3 system was systematically explored in this work. The optical absorption edge is substantially redshifted to the visible region due to the induced impurity states within the band gap after doping. However, the monodoping generally comes with the compensating defects, which decreases the photo-catalytic efficiency. Fortunately, the charge compensated combinations, such as ((C@O/TM@Ti), (2N@2O/TM@Ti) and (2C@2O/TM@Sr), TM = Mo, W) could overcome the shortage of monodoping case. And the band-edge alignment confirms that the water redox potentials lie between the highest occupied and lowest unoccupied states for the C@O/TM@Ti and 2N@2O/TM@Ti models, which guarantees their visible-light photocatalytic activity. Although the highest occupied states of 2C@2O/TM@Sr cases are higher than the oxygen evolution potential, the conduction band minimum still keeps strong reduction capability. Furthermore, the formation energy of defect indicates that the codoping can increase the doping concentration of C for C@O/TM@Ti cases under O-rich condition and C@O/W@Ti case under O-poor condition.

Computational Materials Science

On the chemical effects in molten Ni1−xMx alloy

Author(s): Jianbo Ma, Yongbing Dai, Jiao Zhang, Shihao Chen, Jian Yang, Hui Xing, Qing Dong, Yanfeng Han, Baode Sun

A study of molten Ni1−xMx (M = Ta, W, Re, Os, Nb, Mo, Ru, Ti or Cr) at 1773 K was carried out by ab initio molecular dynamics simulation. Static structure factors, pair correlation functions and bond-angle distribution functions were calculated. Chemical short range order, the structure of coordination short range order (SRO), compactness of coordination shell and electronic density of states were studied. Hetero-coordination is preferred in molten Ni1−xMx (M = Ta, W, Re, Nb, Mo, Ti or Cr), while, self-coordination is preferred in molten Ni1−xOsx and Ni1−xRux. The affinity between unlike species in molten Ni1−xMx is decreasing with solute species shifting to the right in the periodic table. The structure difference between M- and Ni-centered coordination SROs is narrowed when solute species shifts to the right in the periodic table. With an increase of solute concentration, the affinity between bonding Ni-M and Ni-Ni atomic pairs both decrease, the solute and solvent centered coordination shells both become looser, in general. The structures of locally favored coordination SROs in molten Ni1−xMx are diversified and independent of the structure of crystal state. Few FCC- and HCP-type coordination SROs are found in these melts. No BCC type coordination SRO is detected. For these melts, the compactness difference of solute- and solvent-centered coordination shells is correlated with the equilibrium partition coefficients, k 0, of solute elements. Namely, when the solute centered coordination shell is averagely denser than the Ni centered one, k 0 < 1, vice versa. Our findings shed light on understanding the nature of segregation from the aspect of molten structure at atomic scale.

Computational Materials Science

Nanomechanics and modelling of hydrogen stored carbon nanotubes under compression for PEM fuel cell applications

Author(s): V. Vijayaraghavan, Jacob F.N. Dethan, A. Garg

Compressive strength of single walled carbon nanotubes (SWCNTs) filled with hydrogen molecules is analysed in this work utilising molecular dynamics simulation. Understanding mechanical characteristics of SWCNTs with hydrogen interactions are urgently important for providing solid groundwork to design robust and effective on-board hydrogen storage systems for proton exchange membrane fuel cell (PEMFC). This study analyses impact of SWNCTs’ geometry, weight percentage of hydrogen storage, temperature variation and vacancy defects. It is obtained that effect of hydrogen storage increases compressive resistance of SWCNTs. Furthermore, temperature increase and presence of defects negatively impacts compressive strength of SWCNTs. The compressive resistance of hydrogen stored SWCNTs were also found to depend on inter-molecular spacing of hydrogen molecules. It is expected that this work could provide important fundamentals in understanding mechanical behaviour of hydrogen stored SWNCTs subjected to compression that could assist in the development of effective on-board hydrogen storage systems for fuel cells.

Computational Materials Science

Predicting the volumes of crystals

Author(s): Iek-Heng Chu, Sayan Roychowdhury, Daehui Han, Anubhav Jain, Shyue Ping Ong

New crystal structures are frequently derived by performing ionic substitutions on known crystal structures. These derived structures are then used in further experimental analysis, or as the initial guess for structural optimization in electronic structure calculations, both of which usually require a reasonable guess of the lattice parameters. In this work, we propose two lattice prediction schemes to improve the initial guess of a candidate crystal structure. The first scheme relies on a one-to-one mapping of species in the candidate crystal structure to a known crystal structure, while the second scheme relies on data-mined minimum atom pair distances to predict the crystal volume of the candidate crystal structure and does not require a reference structure. We demonstrate that the two schemes can effectively predict the volumes within mean absolute errors (MAE) as low as 3.8% and 8.2%. We also discuss the various factors that may impact the performance of the schemes. Implementations for both schemes are available in the open-source pymatgen software.

Computational Materials Science

Nano-patterning of surfaces by ion sputtering: Numerical study of the anisotropic damped Kuramoto-Sivashinsky equation

Author(s): E. Vitral, D. Walgraef, J. Pontes, G.R. Anjos, N. Mangiavacchi

Nonlinear models for pattern evolution by ion beam sputtering on a material surface present an ongoing opportunity for new numerical simulations. A numerical analysis of the evolution of preexisting patterns is proposed to investigate surface dynamics, based on a 2D anisotropic damped Kuramoto-Sivashinsky equation, with periodic boundary conditions. A finite-difference semi-implicit time splitting scheme is employed on the discretization of the governing equation. Simulations were conducted with realistic coefficients related to physical parameters (anisotropies, beam orientation, diffusion). The stability of the numerical scheme is analyzed with time step and grid spacing tests for the pattern evolution, and the Method of Manufactured Solutions has been used to verify the proposed scheme. Ripples and hexagonal patterns were obtained from a monomodal initial condition for certain values of the damping coefficient, while spatiotemporal chaos appeared for lower values. The anisotropy effects on pattern formation were studied, varying the angle of incidence of the ion beam with respect to the irradiated surface. Analytical discussions are based on linear and weakly nonlinear analysis.

Computational Materials Science

Microstructural evolution during temperature gradient zone melting: Cellular automaton simulation and experiment

Author(s): Qingyu Zhang, Hua Xue, Qianyu Tang, Shiyan Pan, Markus Rettenmayr, Mingfang Zhu

The microstructural evolution in mushy zones of alloys due to temperature gradient zone melting (TGZM) is studied by simulations using a two-dimensional quantitative cellular automaton (CA) model and in situ observations of directional solidification with a transparent organic SCN-ACE alloy. The present model is an extension of a previous CA model by involving the mechanisms of both solidification and melting. The present CA model is adopted to simulate the temporal evolution of the position and velocity of a liquid pool migrating in the solid matrix of a SCN–0.3 wt% ACE alloy under conditions that the pulling velocity is either lower or higher than the critical pulling velocity. The CA simulated position and velocity curves agree well with analytical solutions. Simulations are also performed for the microstructural evolution of columnar dendrites in a SCN–2.0 wt% ACE alloy held in a stationary temperature gradient using the present CA model and a previous CA model that does not include the melting mechanism under otherwise identical conditions for comparison. The results show how melting is essential to dendrite arm migration in a temperature gradient. The time-averaged velocities of arm migration obtained from the present CA simulations increase with increasing temperature gradient and with decreasing the length between the initial arm position and the liquidus. This agrees reasonably well with experimental measurements and analytical predictions. The mechanisms of dendrite arm migration are investigated in detail by comparing the local equilibrium and actual liquid compositions at solid/liquid interfaces. The simulations render visualizing the complex interactions among local temperature, solute distribution/diffusion, and solidification/melting during the TGZM process.

Computational Materials Science

Tuning the magnetic properties of DyNiO3 by high-pressures

Author(s): Sihan Wang, Ying Xu, Haijun Li, Changbo Chen

At ambient pressure, the perovskite antiferromagnetic insulator DyNiO3 undergoes a metal to insulator transition at T MI = 460 K which is associated with the degree distortion of perovskites, leading to charge disproportionation. The change of the structural, magnetic and electronic structure properties of DyNiO3 with pressure (up to 10 GPa) have been investigated within generalized gradient approximation plus the Hubbard-U parameter (GGA + U) method by using density functional theory. We find that pressure reduces the unit cell volume and shrinks NiO bond lengths while straightening the bond angle. Since the pressure-induced the crystal structure transforms from P21 to P21/n space group occurs at about 3 GPa. We further show that with increasing pressure, the local magnetic moment of Ni1 remains unchanged (1.703μB ), but that of Ni2 decreases from 0.710μB to zero when the pressure exceeds 6 GPa. The change of the magnetic structure of DyNiO3 under pressures is attributed to a favorable change in Double-exchange (DE) interaction between Ni and O atoms and the transition of spin states in Ni2. Our results provide a coherent picture of pressure effects in rare-earth nickelates and have implications for the experimental and theoretical work on these compounds.

Computational Materials Science

Theoretical investigations on novel energetic salts composed of 4-nitro-7-(4-nitro-1,2,3-triazol-1-olate)-furazano[3,4-d]pyridazine-based anions and ammonium-based cations

Author(s): Ke Wang, Xiao-long Fu, Qiu-fan Tang, Huan Li, Yuan-jie Shu, Jun-qiang Li, Wei-piang Pang

Based on density functional theory and volume-based thermodynamics methods, the crystal densities (ρ), heats of formation (HOFs), detonation performance, specific impulse (I sp), impact sensitivities (H 50) and Gibbs free energies of formation of eight series novel energetic salts composed of 4-nitro-7-(4-nitro-1,2,3-triazol-1-olate)-furazano[3,4-d]pyridazine-based anions and ammonium-based cations were studied. Results show that all title salts possess high ρ and positive HOFs. Therein, ammonium and hydroxylammonium salts exhibit the highest ρ and detonation performance in each series and several even surpass those of HMX and RDX. For guanidinium-based salts in every series, when the number of NH2 group in cations increases, the HOFs, I sp and H 50 of corresponding salts improve, but their ρ values decrease. Consequently, detonation performance of guanidinium-based salts are close to each other (H series are similar to RDX). Otherwise, introducing N → O oxidation bond to anions is an effective method to improve ρ, detonation performance and I sp of the corresponding salts compared A with B-H series, but it decreases H 50. However, all guanidinium-based salts show lower impact sensitivities than RDX and HMX. Meanwhile, the position of N → O oxidation bond also has an effect on these properties.

Computational Materials Science

Effect of intercritical rolling temperature on microstructure-mechanical property relationship in a medium Mn-TRIP steel containing δ ferrite

Author(s): Z.P. Hu, Y.B. Xu, Y. Zou, R.D.K. Misra, D.T. Han, S.Q. Chen, D.Y. Hou

We elucidate the influence of intercritical rolling temperature on the microstructural evolution, mechanical properties and work-hardening behavior of a hot-rolled Fe-0.2C-6.5Mn-3Al-0.1V medium Mn transformation-induced-plasticity (TRIP) steel containing δ-ferrite. Tensile strength of 966 MPa, total elongation of 42.6% and yield strength of 705 MPa was obtained in the annealed steel subjected to a low intercritical rolling temperature. Rolling at a high intercritical rolling temperature promoted the partitioning of Mn from δ ferrite to prior austenite grains, and led to a martensitic matrix characterized by a fine lath structure. Subsequently, after intercritical annealing, the reversed austenite transformed from the martensitic matrix had high stability and small size. However, the reversed austenite with a high degree of Mn enrichment, fine lath structure and high stability provided a slow and less active TRIP effect. This was responsible for low work-hardening rate during deformation. In contrast, the high content of reversed austenite in the annealed steel subjected to low intercritical rolling temperature had relatively low stability and large lath width, exhibited serrated work-hardening behavior indicative of discontinuous TRIP effect. Additionally, the low intercritical rolling temperature led to a high density of dislocations in δ-ferrite, which effectively promoted VC precipitation after intercritical annealing, and enhanced yield strength. Furthermore, the formation of high angle boundary in δ-ferrite and the formation of pro-eutectoid ferrite at low intercritical rolling temperature also enhanced yield strength through grain boundary strengthening.

Science and Engineering A

Pressureless sintering of TiB2 with low concentration of Co binder to achieve enhanced mechanical properties

Author(s): Zhezhen Fu, Rasit Koc

This paper studies the pressureless sintering of TiB2 based materials with a low concentration of 3wt% Co binder to result in enhanced mechanical properties. Utilizing ultra-fine TiB2 powders obtained from a special carbon coated precursors method, TiB2-3wt% Co composite can be densified to a relative density of ~98.6% at a temperature of 1500°C without external pressure, which is over ~200°C lower than literature reported temperatures. The Co binder partially reacts with TiB2 and converts into Co2B and Ti-B-Co with good wettings with TiB2. Due to the low sintering temperature, the microstructure is fine with a grain size of ~1.75±0.16µm. The sample also combines superior mechanical properties including Vickers hardness of ~28.4±0.6 GPa, elastic modulus of ~519.6±17.2 GPa, indentation fracture toughness of ~7.0±0.4 MPa√m, and flexural strength of ~638.3±34.9 MPa. The variation of the Co content to 1, 10, or 20 wt% either leads to low relative density (~91.5% for 1 wt% Co even at a temperature of 1600°C) or deteriorated mechanical properties (the combination of hardness and fracture toughness, for samples containing 10 and 20 wt% Co) due to the formation of significant amount of brittle Co2B. Correlations between mechanical properties and microstructure are further discussed.

Science and Engineering A

Temperature dependent cyclic mechanical properties of a hot work steel after time and temperature dependent softening

Author(s): Andreas Jilg, Thomas Seifert

In this paper, the temperature dependent cyclic mechanical properties of the martensitic hot work tool steel 1.2367 after tempering are investigated. To this end, hardness measurements as well as monotonic and cyclic tests at temperatures in the range from room temperature to 650°C are performed on material tempered for different tempering times and temperatures. To describe the observed time and temperature dependent softening during tempering a kinetic model for the evolution of the mean size of secondary carbides based on Ostwald ripening is developed. Furthermore, mechanism-based as well as phenomenological relations for the cyclic mechanical properties of the Ramberg-Osgood model depending on carbide size and temperature are introduced. A good overall agreement of the measured and the calculated stress-strain hysteresis loops for different temperatures and heat treatments is obtained using the determined material properties of the kinetic and mechanical model.

Science and Engineering A

Elastic, mechanical, and thermodynamic properties of Bi-Sb binaries: Effect of spin-orbit coupling

Author(s): Sobhit Singh, Irais Valencia-Jaime, Olivia Pavlic, and Aldo H. Romero

Using first-principles calculations, we systematically study the elastic stiffness constants, mechanical properties, elastic wave velocities, Debye temperature, melting temperature, and specific heat of several thermodynamically stable crystal structures of ${\mathrm{Bi}}_{x}{\mathrm{Sb}}_{1−x}$ ($0...

Physical Review B

Room-temperature relaxor ferroelectricity and photovoltaic effects in tin titanate directly deposited on a silicon substrate

Author(s): Radhe Agarwal, Yogesh Sharma, Siliang Chang, Krishna C. Pitike, Changhee Sohn, Serge M. Nakhmanson, Christos G. Takoudis, Ho Nyung Lee, Rachel Tonelli, Jonathan Gardner, James F. Scott, Ram S. Katiyar, and Seungbum Hong

Tin titanate ($\mathrm{SnTi}{\mathrm{O}}_{3}$) has been notoriously impossible to prepare as a thin-film ferroelectric, probably because high-temperature annealing converts much of the $\mathrm{S}{\mathrm{n}}^{2+}$ to $\mathrm{S}{\mathrm{n}}^{4+}$. In the present paper, we show two things: first, pe...

Physical Review B

Tue Feb 20 2018

Modeling dynamic recrystallization during hot deformation of a cast-homogenized Mg-Zn-Zr alloy

Author(s): A. Hadadzadeh, F. Mokdad, M.A. Wells, D.L. Chen

Hot deformation of a cast-homogenized ZK60 magnesium alloy was studied in terms of dynamic recrystallization (DRX) kinetics. The alloy was isothermally compressed under uniaxial loading at 400°C and 450°C with varying strain rates of 0.001, 0.01 and 0.1s−1. The DRX fraction of each deformation condition was determined via electron backscatter diffraction (EBSD) analysis. DRX kinetics during hot deformation was investigated using an Avrami-DRX equation. For the sake of comparison, Avrami-SRX (static recrystallization) modeling was also employed. Both models were calibrated and validated for hot deformation of the alloy. It was observed that the Avrami-DRX model could be successfully used to predict the DRX kinetics except for the lowest DRX fraction (i.e., deformation at 400°C/0.1s−1). Moreover, the Avrami-SRX model was in good agreement with the Avrami-DRX model for deformation at the lowest strain rate (i.e., 0.001s−1). With increasing strain rate, Avrami-SRX model became increasingly less viable. This was mainly attributed to the smaller than 50% recrystallized fractions at 0.01–0.1s−1.

Science and Engineering A

On dynamical properties of electrons in Anderson–Mott insulators

M Pollak

Properties of electrons in non-crystalline (alias disordered) systems has been a very active research topic for over half a century, since Anderson’s ground breaking paper on localization. In strongly disordered systems electrons become Anderson and Mott localized. Interactions become important because screening by localized electrons is ineffective. Dynamical theories for such systems have long been controversial. Nevertheless one theory came to prominence in the literature and is often invoked and/or used. It is shown here that that theory is unsatisfactory in several aspects. It is based on the one-particle density of states, which turns out to be irrelevant to the problem it addresses. Another shortcoming is an implicit conjecture that interacting electrons move independently of each other. The theory is also in questionable agreement with experiment. It is shown that two other theories are free of those problems. They are useful for different types of studies, are compatibl...

Journal of Physics Condensed Matter

Origin of distorted 1 T -phase ReS 2 : first-principles study

Ji-Hae Choi and Seung-Hoon Jhi

Group-VIIB transition metal dichalcogenides (TMDCs) are known to be stabilized solely in a distorted 1 T phase termed as 1 T″ phase, which is compared to many stable or metastable phases in other TMDCs. Using first-principles calculations, we study the structural origin of 1 T″ phase group-VIIB TMDCs. We find that quasi 1D Peierls-like instability is responsible for the transition to the 1 T″ phase ReS 2 monolayer from the 1 T ′ phase, another distorted 1 T phase. Two half-filled bands in 1 T ′-ReS 2 make sharp peaks in the Lindhard function that prompt the charge density wave (CDW) phase with large band gap opening. Our calculations show that overlapping of the two bands in a broad energy range leads to robust CDW phase or stable 1 T″ phase in group-VIIB TMDCs against compositional variation, which is in stark contrast to typical Peierls instability driven by a single band. Calculated total energy curve near the ...

Journal of Physics Condensed Matter

Emergence of a new valence-ordered structure and collapse of the magnetic order under high pressure

Akihiro Mitsuda, Shigeki Manabe, Masafumi Umeda, Hirofumi Wada, Kazuyuki Matsubayashi, Yoshiya Uwatoko, Masaichiro Mizumaki, Naomi Kawamura, Kiyofumi Nitta, Naohisa Hirao, Yasuo Ohishi and Naoki Ishimatsu

The layered hexagonal EuPtP is a rare substance that exhibits two successive valence transitions occurring simultaneously with valence ordering transitions and an antiferromagnetic order. Anticipating that the application of pressure to this sample would induce a new valence-ordered structure and/or a new phenomenon associated with valence fluctuation, we examined the electrical resistivity ρ , the Eu L 3 -edge x-ray absorption spectroscopy, and the powder x-ray diffraction under high pressure. We found a new valence transition at around P   =  2.5 GPa. After the transition, a new valence-ordered structure is realized at the lowest temperature. The valence-ordered structure is inferred to be stacking of {$\cdots-(2+)-(3+)-(3+)-(2+)-(3+)-(3+)-\cdots$} (2+: Eu 2+ layer, 3+: Eu 3+ layer) along the c -axis. Upon further increases in pres...

Journal of Physics Condensed Matter

Surface charge method for molecular surfaces with curved areal elements I. Spherical triangles

Yi-Kuo Yu

Parametrizing a curved surface with flat triangles in electrostatics problems creates a diverging electric field. One way to avoid this is to have curved areal elements. However, charge density integration over curved patches appears difficult. This paper, dealing with spherical triangles, is the first in a series aiming to solve this problem. Here, we lay the ground work for employing curved patches for applying the surface charge method to electrostatics. We show analytically how one may control the accuracy by expanding in powers of the the arc length (multiplied by the curvature). To accommodate not extremely small curved areal elements, we have provided enough details to include higher order corrections that are needed for better accuracy when slightly larger surface elements are used.

Journal of Physics Condensed Matter

Topological insulation in a ladder model with particle-hole and reflection symmetries

B Hetényi and M Yahyavi

A two-legged ladder model, one dimensional, exhibiting the parity anomaly is constructed. The model belongs to the C and CI symmetry classes, depending on the parameters, but, due to reflection, it exhibits topological insulation. The model consists of two superimposed Creutz models with onsite potentials. The topological invariants of each Creutz model sum to give the mirror winding number, with winding numbers which are nonzero individually but equal and opposite in the topological phase, and both zero in the trivial phase. We demonstrate the presence of edge states and quantized Hall response in the topological region. Our model exhibits two distinct topological regions, distinguished by the different types of reflection symmetries.

Journal of Physics Condensed Matter

Sun Feb 18 2018

A promising material for thermal barrier coating: Pyrochlore-related compound Sm2FeTaO7

Author(s): Jun Yang, Yi Han, Muhammad Shahid, Wei Pan, Meng Zhao, Wangjie Wu, Chunlei Wan

Pyrochlore-related compound Sm2FeTaO7 was synthesized and investigated. It is shown that the thermal conductivity values are approximately half of the Yttria-Stabilized Zirconia (YSZ) over the temperature range of 25–1000°C. Especially, thermal conductivity at high temperature is close to that of minimum thermal conductivity, as predicted by theoretical investigation, signifying better heat insulation capabilities. Considering lower Young modulus, improved fracture toughness, and better high-temperature phase stability, Sm2FeTaO7 compound can be a promising material for thermal insulating applications, such as thermal barrier coatings.

Scripta Materialia

Pure climb of [001] dislocations in TiAl at 850°C

Author(s): Soumaya Naanani, Jean-Philippe Monchoux, Catherine Mabru, Alain Couret

This paper presents a study by Transmission Electron Miscroscopy of the deformation microstructure in the γ phase of a TiAl alloy crept at 850°C under 150MPa. A never observed population of dislocations is evidenced and characterized. It is shown that their Burgers vector is of [001] type and that they are moving by pure climb in the (001) planes. The reasons of the presence of these dislocations are discussed.

Scripta Materialia

Attractive interaction between interstitial solutes and screw dislocations in bcc iron from first principles

Author(s): B. Lüthi, L. Ventelon, D. Rodney, F. Willaime

Plasticity in steels depends largely on how dislocations interact with solute atoms. We consider here typical interstitial solutes (B, C, N, O) in body-centered cubic (bcc) Fe and show using ab initio calculations that, systematically, when a row of interstitial solutes is in the vicinity of a 1 / 2 1 1 1 screw dislocation, the dislocation adopts a hard core, forming regular prisms of Fe atoms centered on the solute atoms. This low-energy configuration, previously known only for C atoms, induces attractive dislocation-solute interaction energies ranging from −1.3 to −0.2 eV, depending on the nature of the solutes and their separation distance along the dislocation line. This attractive reconstruction is explained by the larger Voronoi volume of the prismatic sites in the dislocation core compared to bulk octahedral sites. Moreover, an analysis of the local density of states gives a first insight into the chemical contribution responsible for the solute dependence of the interaction energy.

Computational Materials Science

Nanoscale Zr-containing precipitates; a solution for significant improvement of high-temperature strength in Al-Si-Cu-Mg alloys

Author(s): Mehdi Rahimian, Sajjad Amirkhanlou, Paul Blake, Shouxun Ji

This work aims to reveal the valuable role of Zr in cast Al-Si-Cu-Mg alloys utilised at elevated temperatures. Furthermore, this work wants to improve high temperature tensile properties of the industrially popular Al7Si0.5Cu alloy by tuning alloying elements. The Al7Si2Cu0.2Zr alloy, subjected to well-tuned heat treatment process, was benchmarked against the conventional Al7Si0.5Cu alloy. Microstructural investigation showed that the main strengthening phases in the Al7Si2Cu0.2Zr alloy are θ´, Q´, Al-Si-Cu-Zr and Al-Si-Zr precipitates. Two Zr-containing precipitates (Al-Si-Cu-Zr and Al-Si-Zr) with the size of 80–200nm are formed during solutionising at530 °C, which can be considered as the first ageing step. Other two Cu-containing precipitates (θ´and Q´) at the size of 20nm are formed during ageing (170°C). Nano-sized Zr-containing precipitates are mostly exhibited elliptical morphology with coherent/semi-coherent interfaces with the α-Al matrix, making them more stable at elevated temperatures. As a result, the yield strength is improved at room temperature from 261 to 291MPa, and the ultimate tensile strength (UTS) is improved from 282 to 335MPa for the Al7Si2Cu0.2Zr alloy, compared with the Al7Si0.5Cu alloy. Moreover, the mechanical properties are significantly improved at elevated temperatures. The yield strength and UTS at 200°C are 177 and 186MPa, respectively, for the Al7Si0.5Cu alloy. But these are224 and 246MPa, respectively, for the Al7Si2Cu0.2Zr alloy. The improvement of mechanical properties at elevated temperatures is mainly attributed to the refined microstructure and the precipitation of strengthening phases containing slow-diffused Zr element to retard the precipitation coarsening. Furthermore, the addition of Cu changes the precipitates from θ´ and β´´ in the Al7Si0.5Cu alloy to θ´ and Q´ in the Al7Si2Cu0.2Zr alloy which, in turn, induce a complementary effect on the improvement of mechanical properties.

Science and Engineering A

Utilization of hot deformation to trigger Strain Induced Boundary Migration (SIBM) in Ni-base Superalloys

Author(s): Joshua McCarley, Sammy Tin

The effect of strain on the resultant microstructure of an experimental low stacking fault energy Nickel based superalloy containing 24wt. pct. Co was investigated. Billets subjected to a preliminary heat treatment at 1110°C were compressed to strain limits of 0.15 and 0.5 at strain rates ranging from 0.1/s to 0.01/s and temperatures at 1020°C and 1060°C. The as-deformed microstructures were assessed and characterized using electron backscatter diffraction, as were microstructures corresponding to a super-solvus anneal heat treatment at 1160°C for one hour. This study sought to identify a critical strain limit at which conditions indicative of Strain induced boundary migration (SIBM) could be effectively triggered for the experimental Ni-based superalloy over a set range of thermal-mechanical parameters. Microstructures corresponding to SIBM were then compared to more extensively deformed billets which contained notable fractions of dynamically recrystallized grains to quantify differences in the length fraction and density of ∑3 twin boundaries of the respective microstructures. Though billet samples deformed to both 0.15 and 0.5 contained notable magnitudes of stored strain energy, microstructures deformed to 0.15 were noted as having maintained larger length fractions of ∑3 twins due to a predominant absence of dynamic recrystallization. Annealed samples originally deformed to 0.15 yielded annealing twin length fractions as high as 59% when compared a sample deformed to the 0.5 strain limit under equivalent thermal-mechanical conditions that resulted in a twin length fraction of 50%. Although samples deformed to the lower strain limit exhibited higher length fractions of annealing twins, samples deformed to the higher strain limit of 0.5 were noted to yield ∑3 densities as high as 0.65 μ m 1 , whereas the annealed sample deformed under equivalent thermal-mechanical parameters to the 0.15 strain limit produced ∑3 densities as low as 0.32 μ m 1 .

Science and Engineering A

A rapid route for synthesizing Ti-(AlxTiyUFG Al) core-multishell structured particles reinforced Al matrix composite with promising mechanical properties

Author(s): Zheng-Yang Hu, Zhao-Hui Zhang, Hu Wang, Sheng-Lin Li, Shi-pan Yin, Qi Song, Xing-Wang Cheng

A new type of Al-matrix composites reinforced by in-situ Ti-(AlxTiy/Ultrafine-grained Al) core-multishell structured particles were successfully fabricated at relatively low temperature (450 to 580°C) by SPS technique within a very short time. The composite sintered at 580°C exhibits a yield strength improvement of 64.5% over the unreinforced matrix and a satisfactory elongation of 27%. The defects, UFG-Al grains and local-connection accelerated the Al-Ti inter-diffusion and consolidation process of composites. Although residual voids near AlxTiy/Al interface may induce the cracks, discontinuous tough intermetallic-phases inside AlxTiy layer and soft Ti-core can effectively decelerate the crack-propagation.

Science and Engineering A

Effect of rare earth addition on {101̅2} twinning induced hardening in magnesium

Author(s): Aidin Imandoust, Christopher D. Barret, Haitham El Kadiri

We investigated the effect of Gd and Ce additions in magnesium on sigmoidal hardening behavior associated with { 10 1 ̅ 2 } twinning after extrusion. Compression along extrusion direction revealed that rare earth additions enhance the flow stress, while c + a softening was expected. This phenomenon has been explained by the enhancement of c + a dislocations activity and the ensuing increase in forest hardening due to solute drag. It is expected that dislocation transmutation of basal to prismatic will be enhanced during { 10 1 ̅ 2 } twin growth in rare earth containing alloys, which would exacerbate c + a entanglement with the dislocation forests inside the twins. Forest hardening events overcompensated for the softening effects from lowering the critical resolved shear stresses of c + a dislocations, and resulted in higher flow stress for the binary rare earth containing alloys.

Science and Engineering A

Sat Feb 17 2018

Viscoelasticity of Cu- and La-based bulk metallic glasses: Interpretation based on the quasi-point defects theory

Author(s): J.C. Qiao, Y.X. Chen, J.M. Pelletier, H. Kato, D. Crespo, Y. Yao, V.A. Khonik

The dynamic mechanical relaxation of metallic glasses is closely associated with the physical and mechanical properties. In the current work, the dynamic mechanical relaxation behaviors of Cu46Zr45Al7Y2 and La65Al14(Cu5/6Ag1/6)11(Ni1/2Co1/2)10 bulk metallic glasses are investigated by mechanical spectroscopy. In general, metallic glasses display two relaxation modes: main ( α ) relaxation and the slow secondary ( β ) relaxation. The α relaxation is linked to the dynamic glass transition phenomenon and viscous flow while the slow β relaxation is associated with many fundamental issues, such as diffusion and glass transition phenomenon. The experimental study shows La65Al14(Cu5/6Ag1/6)11(Ni1/2Co1/2)10 bulk metallic glass displays a noticeable slow β relaxation. Contrarily, the Cu46Zr45Al7Y2 bulk metallic glass relaxation process takes the form of an “excess wing”. In the framework of quasi-point defects (QPD) theory, the dynamic mechanical response of the metallic glasses is discussed.

Science and Engineering A

Anisotropy of high temperature creep properties of a Co-base single crystal superalloy

Author(s): YuZhang Lu, Guang Xie, Dong Wang, ShaoHua Zhang, Wei Zheng, Jian Shen, LangHong Lou, Jian Zhang

The orientation dependence of creep properties of a Co-base single crystal superalloy at 982°C/248MPa was investigated. The orientations of specimens tested were near [001, 011] and [111]. The results showed the deformation of these specimens was all controlled by a/3<112> slip and stacking faults. The specimens with orientation near [001] exhibited the best creep resistance. There were a few stacking faults with two orientations in the specimens with [001] orientation. Specimens with orientations near [111] and [011] showed shorter creep lives, and stacking faults with single orientation were observed.

Science and Engineering A

Deposition characteristics of cold sprayed Inconel 718 particles on Inconel 718 substrates with different surface conditions

Author(s): Wen Sun, Adrian Wei Yee Tan, Ayan Bhowmik, Iulian Marinescu, Xu Song, Wei Zhai, Feng Li, Erjia Liu

This article examined the coating/substrate interface characteristics of IN718 coatings deposited on IN718 substrates using a high pressure cold spray technique. The study was aimed at a greater insight into the coating/substrate bonding mechanism under varying substrate conditions, namely surface roughness and surface preheating, from both experimental data and simulations. The coating/substrate interface microstructure and bonding strength were systematically investigated. The coating/substrate strengths were measured by using tensile and shear tests. A finite-element model on the single particle impact was used to explain and validate the experimental data and analyze the particle bonding behavior with the substrate at different preheating temperatures. The analysis of the coating/substrate interface revealed that a polished IN718 substrate surface could promote adiabatic shear instability as well as the formation of material jetting and thus resulting in a high interface bonding strength. In addition, the preheating of the substrates could increase the metallurgical bonding and promote the coating/substrate adhesion strength.

Science and Engineering A

Crystal plasticity modeling and characterization of the deformation twinning and strain hardening in Hadfield steels

Author(s): Matti Lindroos, Georges Cailletaud, Anssi Laukkanen, Veli-Tapani Kuokkala

The deformation and strain hardening due to dislocation slip and twinning in Hadfield steels is investigated with a crystal plasticity model. A phenomenological interaction and hardening formulation is incorporated to the numerical model based on the microscopic characterization and deformation behavior. Single and polycrystal simulations on a Hadfield steel are conducted to prove the soundness of the model in describing the deformation and hardening behavior of the steel. The competition between dislocation slip and twin dominated deformation plays an essential role in the asymmetry between tension-compression as well as in the strain hardening behavior of the steel. The simulations performed on another Hadfield alloy verifies the model's capability to represent the strain rate sensitivity of the material, when a positive strain rate dependence exists. The use of realistic 3D polycrystalline aggregates imitating the microstructure of the Hadfield steels provides new insight into the inter-grain and intra-grain behavior of austenitic microstructures exhibiting twinning, particularly revealing the nature of local stress and twin concentrations. The strain rate sensitivity of the alloyed Hadfield steel is observed also at the grain scale, intensifying twinning at higher strain rates depending on the loading direction. The local gradients were found to arise from the neighbouring grains and from the intra-grain reorientation, explaining the experimental observations of only partially twinned grains. The simulation results and the 3D aggregate approach used in this paper provide information for the validation of crystal plasticity model as well as about the local deformation behavior of Hadfield steels.

Science and Engineering A

Plastic Strain Partitioning in Dual phase Al13CoCrFeNi High Entropy Alloy

Author(s): Wael Abuzaid, Huseyin Sehitoglu

High entropy alloys present opportunities to develop new materials with unique mechanical properties. Through careful selection of constituent elements and thermal processing, different microstructures with varying properties can be achieved. This study is focused on an interesting class of high entropy alloys with dual phase microstructure, a soft FCC and a hard BCC phase. Specifically, the local material response of Al 13 CoCrFeNi (atomic %), at the microscale and in the vicinity of phase boundaries, is analyzed using high resolution strain and grain orientation measurements. Different heat treatments resulting is varying phase volume fractions and deformation temperatures were considered. The local response of this high entropy alloy displayed significant heterogeneity in plastic strain accumulation with preferential accumulation in the FCC grains and localizations at phase boundaries. The preferential accumulation of plastic strains in FCC grains (33 – 85% higher than BCC) was further enhanced with very high temperature heat treatments conducted at 1300 °C. These changes in plastic strain partitioning were associated with the increase in BCC phase volume fraction which was altered during heat treatment. At the macro-scale, the unloading response of Al 13 CoCrFeNi revealed a nonlinear unloading behavior with large magnitudes of recoverable strains (0.9 – 1.4%). Deformation at cryogenic temperatures revealed slip dominated plasticity and no changes in the underlying deformation mechanism due to temperate reduction. However, the plastic strain partitioning between the FCC and BCC phases is shown to be affected with larger magnitudes of plastic strains accumulating in the FCC phase, and less in the BCC phase, compared to the room temperature deformation response.

Science and Engineering A

Suppressing Ice Nucleation of Supercooled Condensate with Biphilic Topography

Author(s): Youmin Hou, Miao Yu, Yuhe Shang, Peng Zhou, Ruyuan Song, Xiaonan Xu, Xuemei Chen, Zuankai Wang, and Shuhuai Yao

Preventing or minimizing ice formation in supercooled water is of prominent importance in many infrastructures, transportation, and cooling systems. The overall phase change heat transfer on icephobic surfaces, in general, is intentionally sacrificed to suppress the nucleation of water and ice. Howe...

Physical Review Letters

Distinguishing quantum dot-like localized states from quantum well-like extended states across the

Sumi Bhuyan, Richarj Mondal, Bipul Pal and Bhavtosh Bansal

We have closely examined the emission spectrum at the heavy-hole exciton resonance in a high-quality GaAs multi-quantum well sample using picosecond excitation-correlation photoluminescence (ECPL) spectroscopy. Dynamics of the ECPL signal at low and high energy sides of the excitonic photoluminescence (PL) peak show complementary behavior. The ECPL signal is positive (negative) below (above) the PL peak and it changes sign within a narrow band of energy lying between the excitonic absorption and emission peaks. The energy at which this sign change takes place is interpreted as the excitonic mobility edge as it separates localized excitons in quantum dot-like states from mobile excitons in quantum well-like states.

Journal of Physics Condensed Matter

Spin filter effect of hBN/Co detector electrodes in a 3D topological insulator spin valve

Kristina Vaklinova, Katharina Polyudov, Marko Burghard and Klaus Kern

Topological insulators emerge as promising components of spintronic devices, in particular for applications where all-electrical spin control is essential. While the capability of these materials to generate spin-polarized currents is well established, only very little is known about the spin injection/extraction into/out of them. Here, we explore the switching behavior of lateral spin valves comprising the 3D topological insulator Bi 2 Te 2 Se as channel, which is separated from ferromagnetic Cobalt detector contacts by an ultrathin hexagonal boron nitride (hBN) tunnel barrier. The corresponding contact resistance displays a notable variation, which is correlated with a change of the switching characteristics of the spin valve. For contact resistances below ~5 kΩ, the hysteresis in the switching curve reverses upon reversing the applied current, as expected for spin-polarized currents carried by the helical surface states. By contrast, for higher contact resis...

Journal of Physics Condensed Matter

Suppression of material transfer at contacting surfaces: the effect of adsorbates on Al/TiN and

Gregor Feldbauer, Michael Wolloch, Pedro O Bedolla, Josef Redinger, András Vernes and Peter Mohn

The effect of monolayers of oxygen (O) and hydrogen (H) on the possibility of material transfer at aluminium/titanium nitride (Al/TiN) and copper/diamond (Cu/C dia ) interfaces, respectively, were investigated within the framework of density functional theory (DFT). To this end the approach, contact, and subsequent separation of two atomically flat surfaces consisting of the aforementioned pairs of materials were simulated. These calculations were performed for the clean as well as oxygenated and hydrogenated Al and C dia surfaces, respectively. Various contact configurations were considered by studying several lateral arrangements of the involved surfaces at the interface. Material transfer is typically possible at interfaces between the investigated clean surfaces; however, the addition of O to the Al and H to the C dia surfaces was found to hinder material transfer. This passivation occurs because of a significant reduction of the adhesion energy ...

Journal of Physics Condensed Matter

Determination of Debye temperatures and Lamb–Mössbauer factors for LnFeO 3 orthoferrite perovskites

A Scrimshire, A Lobera, A M T Bell, A H Jones, I Sterianou, S D Forder and P A Bingham

Lanthanide orthoferrites have wide-ranging industrial uses including solar, catalytic and electronic applications. Here a series of lanthanide orthoferrite perovskites, LnFeO 3 (Ln  =  La; Nd; Sm; Eu; Gd), prepared through a standard stoichiometric wet ball milling route using oxide precursors, has been studied. Characterisation through x-ray diffraction and x-ray fluorescence confirmed the synthesis of phase-pure or near-pure LnFeO 3 compounds. 57 Fe Mössbauer spectroscopy was performed over a temperature range of 10 K–293 K to observe hyperfine structure and to enable calculation of the recoil-free fraction and Debye temperature ( θ D ) of each orthoferrite. Debye temperatures (Ln  =  La 474 K; Nd 459 K; Sm 457 K; Eu 452 K; Gd 473 K) and recoil-free fractions (Ln  =  La 0.827; Nd 0.817; Sm 0.816; Eu 0.812; Gd 0.826) were approximated through minimising the difference in the temperature dependent experimental centre shift and theore...

Journal of Physics Condensed Matter

Exploring the validity and limitations of the Mott–Gurney law for charge-carrier mobility

Jason A Röhr, Davide Moia, Saif A Haque, Thomas Kirchartz and Jenny Nelson

Using drift-diffusion simulations, we investigate the voltage dependence of the dark current in single carrier devices typically used to determine charge-carrier mobilities. For both low and high voltages, the current increases linearly with the applied voltage. Whereas the linear current at low voltages is mainly due to space charge in the middle of the device, the linear current at high voltage is caused by charge-carrier saturation due to a high degree of injection. As a consequence, the current density at these voltages does not follow the classical square law derived by Mott and Gurney, and we show that for trap-free devices, only for intermediate voltages, a space-charge-limited drift current can be observed with a slope that approaches a value of two. We show that, depending on the thickness of the semiconductor layer and the size of the injection barriers, the two linear current–voltage regimes can dominate the whole voltage range, and the intermediate Mott–Gurney regime...

Journal of Physics Condensed Matter

Strain-induced bi-thermoelectricity in tapered carbon nanotubes

L A A Algharagholy, T Pope and C J Lambert

We show that carbon-based nanostructured materials are a novel testbed for controlling thermoelectricity and have the potential to underpin the development of new cost-effective environmentally-friendly thermoelectric materials. In single-molecule junctions, it is known that transport resonances associated with the discrete molecular levels play a key role in the thermoelectric performance, but such resonances have not been exploited in carbon nanotubes (CNTs). Here we study junctions formed from tapered CNTs and demonstrate that such structures possess transport resonances near the Fermi level, whose energetic location can be varied by applying strain, resulting in an ability to tune the sign of their Seebeck coefficient. These results reveal that tapered CNTs form a new class of bi-thermoelectric materials, exhibiting both positive and negative thermopower. This ability to change the sign of the Seebeck coefficient allows the thermovoltage in carbon-based thermoelectric device...

Journal of Physics Condensed Matter

Reaction paths of alane dissociation on the Si(0 0 1) surface

Richard Smith and David R Bowler

Building on our earlier study, we examine the kinetic barriers to decomposition of alane, AlH 3 , on the Si(0 0 1) surface, using the nudged elastic band approach within density functional theory. We find that the initial decomposition to AlH with two H atoms on the surface proceeds without a significant barrier. There are several pathways available to lose the final hydrogen, though these present barriers of up to 1 eV. Incorporation is more challenging, with the initial structures less stable in several cases than the starting structures, just as was found for phosphorus. We identify a stable route for Al incorporation following selective surface hydrogen desorption (e.g. by scanning tunneling microscope tip). The overall process parallels PH 3 , and indicates that atomically precise acceptor doping should be possible.

Journal of Physics Condensed Matter

Bismuth oxide film: a promising room-temperature quantum spin Hall insulator

Ya-Ping Wang, Sheng-Shi Li, Wei-Xiao Ji, Chang-Wen Zhang, Ping Li and Pei-Ji Wang

Two-dimensional (2D) bismuth films have attracted extensive attention due to their nontrivial band topology and tunable electronic properties for achieving dissipationless transport devices. The experimental observation of quantum transport properties, however, are rather challenging, limiting their potential application in nanodevices. Here, we predict, based on first-principles calculations, an alternative 2D bismuth oxide, BiO, as an excellent topological insulator (TI), whose intrinsic bulk gap reaches up to 0.28 eV. Its nontrivial topology is confirmed by topological invariant Z 2 and time-reversal symmetry protected helical edge states. The appearance of topological phase is robust against mechanical strain and different levels of oxygen coverage in BiO. Since the BiO is naturally stable against surface oxidization and degradation, these results enrich the topological materials and present an alternative way to design topotronics devices at room temperatu...

Journal of Physics Condensed Matter

High mobility In 0.75 Ga 0.25 As quantum wells in an InAs phonon lattice

C Chen, S N Holmes, I Farrer, H E Beere and D A Ritchie

InGaAs based devices are great complements to silicon for CMOS, as they provide an increased carrier saturation velocity, lower operating voltage and reduced power dissipation (International technology roadmap for semiconductors ( www.itrs2.net [http://www.itrs2.net] )). In this work we show that In 0.75 Ga 0.25 As quantum wells with a high mobility, 15 000 to 20 000 cm 2 V −1 s −1 at ambient temperature, show an InAs-like phonon with an energy of 28.8 meV, frequency of 232 cm −1 that dominates the polar-optical mode scattering from  ∼70 K to 300 K. The measured optical phonon frequency is insensitive to the carrier density modulated with a surface gate or LED illumination. We model the electron scattering mechanisms as a function of temperature and identify mechanisms that limit the electron mobility in In 0.75 Ga 0.25 As quantum wells. Background impurity scattering starts to dominate for tem...

Journal of Physics Condensed Matter

Fri Feb 16 2018

Texture and yielding anisotropy of Zircaloy-4 alloy cladding tube produced by cold Pilger rolling and annealing

Author(s): Chengze Liu, Geping Li, Linhua Chu, Hengfei Gu, Fusen Yuan, Fuzhou Han, Yingdong Zhang

Crystal orientation and mechanical properties of Zircaloy-4 (Zr-4) alloy cladding tube manufactured by cold Pilger rolling and annealing were systematically investigated. Electron Back-Scattered Diffraction (EBSD) analysis showed that grains, which were fully recrystallized in α phase, were strongly oriented with their c axis parallel to the tangential direction (TD) of the tube and, meanwhile, their normal direction of {11 2 ¯ 0 planes parallel to the axial direction (AD). Compression tests carried out along three loading directions (TD-0°, TD-45° and AD, respectively) at room temperature revealed that the Zr-4 alloy tube exhibit strong yielding anisotropy: TD-0°>TD-45°>AD. Meanwhile, the alloy exhibited strain rate hardening effect, since the yield strength increased with the growing strain rate. The relationship between texture and yielding anisotropy of the Zr-4 alloy tube was further discussed based on Schmid factor theory. The Schmid factor values for basal, prismatic and pyramidal <a> slip systems, which were three important yielding mechanisms in Zr-4 alloy during the compression, were calculated depending on the geometric relationship between loading directions and crystallography. Calculation results revealed that there was an inverse relationship between Schmid factor value for slip modes and the compression yield strength of the Zr-4 alloy.

Science and Engineering A

Strain rate effects of dynamic compressive deformation on mechanical properties and microstructure of CoCrFeMnNi high-entropy alloy

Author(s): Jeong Min Park, Jongun Moon, Jae Wung Bae, Min Ji Jang, Jaeyeong Park, Sunghak Lee, Hyoung Seop Kim

In this work, the effects of strain rate on mechanical deformation and microstructural evolution of CoCrFeMnNi high-entropy alloy (HEA) under quasi-static and dynamic compression were investigated. The HEA exhibited high strain-rate sensitivity values (m = 0.028) of yield strength under quasi-static conditions. In particular, due to the viscous drag effect, the variation of yield strength with strain rate under dynamic compression was much larger than that under quasi-static compression. Microstructural analysis using electron backscatter diffraction shows profuse twinning under both conditions. The dynamically deformed specimens exhibited strongly localized deformation regions (i.e., adiabatic shear bands). The process of dynamic compressive behavior in this HEA is competitive between hardening by dislocation and twinning, and thermal softening. To analyze numerically the flow behavior of the HEA under dynamic conditions, the modified Johnson-Cook model considering adiabatic temperature rise was employed. The modified Johnson-Cook model offered good agreement with experimental results regarding dynamic flow curves of this HEA.

Science and Engineering A

Microstructure and mechanical properties of laser-welded dissimilar DP780 and DP980 high-strength steel joints

Author(s): Shuling Gao, Yuntao Li, Lijun Yang, Wencong Qiu

The purpose of this paper is to research the effect of heat input on the microstructure and mechanical properties of DP780 and DP980 dissimilar steel welded joints by using laser welding and analyze the transformation mechanism from austenite to martensite in laser welding. The distribution of martensite volume fraction in each region was similar to the hardness value distribution. The hardness of the fusion zone was significantly improved by the laser welding, but the highest hardness value of the dissimilar steel welded joint appeared in the hardening zone on the DP980 side. At the same time, there was an obvious softening phenomenon at the side of DP980, but the minimal hardness value of DP780 and DP980 dissimilar steel welded joints appeared in the sub-critical HAZs on the side of DP780. This was why the DP780 and DP980 dissimilar welded joints were broken at the DP780 side of the sub-critical HAZ. Compared with the DP780 base metal and DP980 base metal, the plastic deformation and strength-ductility of DP780 and DP980 dissimilar welded joints were significantly reduced. The largest deformation zone of the dissimilar steel welded joints occurred in the heat-affected zone of the DP780 side. The elongation of the heat-affected zone of the DP780 side decreased with decreasing heat inputs. In addition, through the analysis of the transformation mechanism of austenite to martensite for DP780 and DP980 steels, when the peak temperature was lower than Ac1, the tempered martensite led to DP steels softening. When the peak temperature was higher than Ac1 but lower than Ac3, the decrease of martensite content of samples resulted in DP steels softening. When the peak temperature was higher than Ac3, the DP steels began to harden, and the microstructure was dominated by massive martensite and lath martensite.

Science and Engineering A

Effect of cooling temperature field on formation of shelf-like bainite in high carbon silicon steel

Author(s): Qingsuo Liu, Xu Zhao, Xin Zhang, Huibin Wang

The effect of temperature gradient field on microstructure of Fe-0.88C-1.36Si-1.01Cr-0.43Mn (wt.%) is investigated. The distribution of bainite is related to cooling temperature gradient field. The oriented bainitic ferrite with subunit structure is formed due to the high transformation driving force formed by large cooling temperature gradient, which displays large hardness.

Science and Engineering A

Sun Feb 25 2018

Computational Materials Science
Science and Engineering A
Physical Review Letters
Physical Review B
Journal of Physics Condensed Matter

Fri Feb 23 2018

Computational Materials Science
Science and Engineering A
Physical Review Letters
Physical Review B
Journal of Physics Condensed Matter

Thu Feb 22 2018

Nature
Acta Materialia
Scripta Materialia
Science and Engineering A
Physical Review Letters
Journal of Physics Condensed Matter

Wed Feb 21 2018

Computational Materials Science
Science and Engineering A
Physical Review B

Tue Feb 20 2018

Nature
Science and Engineering A
Journal of Physics Condensed Matter

Sun Feb 18 2018

Scripta Materialia
Computational Materials Science
Science and Engineering A

Sat Feb 17 2018

Science and Engineering A
Physical Review Letters
Journal of Physics Condensed Matter

Fri Feb 16 2018

Science and Engineering A