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!

Thu Dec 14 2017

First-principles modeling of superlattice intrinsic stacking fault energies in Ni3Al based alloys

Author(s): A. Breidi, J. Allen, A. Mottura

High-throughput quantum mechanics based simulations have been carried out to establish the change in lattice parameter and superlattice intrinsic stacking fault (SISF) formation energies in Ni3Al-based alloys using the axial Ising model. We had direct access to the variation in SISF energies due to finite compositional change of the added ternary transition metal (TM) element through constructing large supercells, which was equally necessary to account for chemical disorder. We find that most added TM ternaries induce an important quasi-linear increase in the SISF energy as a function of alloying composition x. The most pronounced increase corresponds to Fe addition, while Co addition decreases the SISF energy monotonically. Our results shed light on the role played by TM elements on strengthening L12 Ni3Al precipitates against stacking fault shear. The data are of high importance for designing new Ni-based superalloys based on computational approaches.

Acta Materialia

Strain-induced indium clustering in non-polar a-plane InGaN quantum wells

Author(s): Ja Kyung Lee, Bumsu Park, Kyung Song, Woo Young Jung, Dmitry Tyutyunnikov, Tiannan Yang, Christoph T. Koch, Chan Gyung Park, Peter A. van Aken, Young-Min Kim, Jong Kyu Kim, Junhyeok Bang, Long-Qing Chen, Sang Ho Oh

In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar a-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar a-plane QW structure with 10 nm-thick In0.1Ga0.9N double QWs was investigated and compared with the polar c-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar a-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.

Acta Materialia

Nano-phase separation sintering in nanostructure-stable vs. bulk-stable alloys

Author(s): Mansoo Park, Tongjai Chookajorn, Christopher A. Schuh

Accelerated sintering through nanoscale-phase separation is explored in the Cr-Ni binary system. When the processing requirements of both supersaturation and nanocrystallinity are met in the initial powders, Cr-Ni alloys show an onset of sintering at a low temperature and a rapid rate of densification. Independent characterization techniques, namely x-ray diffraction, thermomechanical analysis, and electron microscopy, confirm that Ni-rich phases develop upon sintering in a way that enhances consolidation. However, this system is shown to be unique in that the Ni addition facilitates rapid nano-phase separation sintering but does not promote stability of a nanoscale grain structure; the nanostructure is a transient feature of the system and the fully consolidated material is microcrystalline with a phase-separated structure. A thermodynamic stability analysis explains this structural evolution, and clarifies the roles of alloying elements that stabilize nanostructure and those that accelerate sintering in nanostructured systems. This work may broaden the applicability of nano-phase separation sintering and inform alloy design based on sinterability.

Acta Materialia

Low-temperature structure and the ferroelectric phase transitions in the $\mathrm{CdTi}{\mathrm{O}}_{3}$ perovskite

Author(s): Brendan J. Kennedy, Qingdi Zhou, Shipeng Zhao, Fanhao Jia, Wei Ren, and Kevin S. Knight

The paraelectric-ferroelectric transition in $\mathrm{CdTi}{\mathrm{O}}_{3}$ has been monitored using high-resolution neutron diffraction data. This necessitated preparing a sample enriched in $^{114}\mathrm{Cd}$. A subtle, but significant, anisotropy in the thermal expansion of the lattice paramete...

Physical Review B

Hydrodynamic instabilities in miscible fluids

Domenico Truzzolillo and Luca Cipelletti

Hydrodynamic instabilities in miscible fluids are ubiquitous, from natural phenomena up to geological scales, to industrial and technological applications, where they represent the only way to control and promote mixing at low Reynolds numbers, well below the transition from laminar to turbulent flow. As for immiscible fluids, the onset of hydrodynamic instabilities in miscible fluids is directly related to the physics of their interfaces. The focus of this review is therefore on the general mechanisms driving the growth of disturbances at the boundary between miscible fluids, under a variety of forcing conditions. In the absence of a regularizing mechanism, these disturbances would grow indefinitely. For immiscible fluids, interfacial tension provides such a regularizing mechanism, because of the energy cost associated to the creation of new interface by a growing disturbance. For miscible fluids, however, the very existence of interfacial stresses that mimic an effective surfa...

Journal of Physics Condensed Matter

Pressure dependence of Ce valence in CeRhIn 5

Z E Brubaker, R L Stillwell, P Chow, Y Xiao, C Kenney-Benson, R Ferry, Z Jenei, R J Zieve and J R Jeffries

We have studied the Ce valence as a function of pressure in CeRhIn 5 at 300 K and at 22 K using x-ray absorption spectroscopy in partial fluorescent yield mode. At room temperature, we found no detectable change in Ce valence greater than 0.01 up to a pressure of 5.5 GPa. At 22 K, the valence remains robust against pressure below 6 GPa, in contrast to the predicted valence crossover at P   =  2.35 GPa. This work yields an upper limit for the change in Ce-valence and suggests that the critical valence fluctuation scenario, in its current form, is unlikely.

Journal of Physics Condensed Matter

Perfect transmission of 3D massive Kane fermions in HgCdTe Veselago lenses

Y Betancur-Ocampo and V Gupta

The transmission properties of three-dimensional (3D) massive Kane fermions in HgCdTe (MCT) heterojunctions have been studied using the simplified Kane–Melé model. Based on our theoretical calculations, we propose the design of an electronic device, called a mass inverter, which consists of the junction of a narrow-gap semiconductor and semimetal. Such a device can be used in electron optics applications, since it operates as a Veselago lens and presents Klein tunneling (KT) of 3D massive Kane fermions under normal incidence. We found that KT and Veselago lensing can also be observed for general MCT heterojunctions with a specific value of doping level. We show that non-resonant perfect transmission of massive Kane fermions persists in a potential barrier for heterojunctions formed by a semimetal between two standard semiconductors. This effect is quite robust when the ideal conditions of a possible experimental test are deviated. Our findings may have important implications in ...

Journal of Physics Condensed Matter

Spin Hall magnetoresistance in the non-collinear ferrimagnet GdIG close to the compensation

Bo-Wen Dong, Joel Cramer, Kathrin Ganzhorn, H Y Yuan, Er-Jia Guo, Sebastian T B Goennenwein and Mathias Kläui

We investigate the spin Hall magnetoresistance (SMR) in a gadolinium iron garnet (GdIG)/platinum (Pt) heterostructure by angular dependent magnetoresistance measurements. The magnetic structure of the ferromagnetic insulator GdIG is non-collinear near the compensation temperature, while it is collinear far from the compensation temperature. In the collinear regime, the SMR signal in GdIG is consistent with the usual {${\rm si}{{{\rm n}}^{2}}\theta $} relation well established in the collinear magnet yttrium iron garnet, with {$\theta $} the angle between magnetization and spin Hall spin polarization direction. In the non-collinear regime, both an SMR signal with inverted sign and a more complex angular dependence with four maxima are observed within one sweep cycle. The number of ma...

Journal of Physics Condensed Matter

Nonlinear modeling of crystal system transition of black phosphorus using continuum-DFT model

A R Setoodeh and H Farahmand

In this paper, the nonlinear behavior of black phosphorus crystals is investigated in tandem with dispersion-corrected density functional theory (DFT-D) analysis under uniaxial loadings. From the identified anisotropic behavior of black phosphorus due to its morphological anisotropy, a hyperelastic anisotropic (HA) model named continuum-DFT is established to predict the nonlinear behavior of the material. In this respect, uniaxial Cauchy stresses are employed on both the DFT-D and HA models along the zig-zag and armchair directions. Simultaneously, the transition of the crystal system is recognized at about 4.5 GPa of the applied uniaxial tensile stress along the zig-zag direction on the DFT-D simulation in the nonlinear region. In order to develop the nonlinear continuum model, unknown constants are surveyed with the optimized least square technique. In this regard, the continuum model is obtained to reproduce the Cauchy stress–stretch and density of strain–stretch results of t...

Journal of Physics Condensed Matter

Demagnetizing fields in all-optical switching

F Hoveyda, E Hohenstein, R Judge and S Smadici

A model of demagnetizing fields and micromagnetic simulations are applied to examine the evolution of a demagnetized cylinder. In addition to three expected final magnetic structures, a fourth switched state is obtained over a range of magnetic energy densities. The switched state is absent when demagnetizing fields are neglected. The connection to all-optical switching of materials with perpendicular magnetic anisotropy is discussed.

Journal of Physics Condensed Matter

Planar screening by charge polydisperse counterions

M Trulsson, E Trizac and L Šamaj

We study how a neutralising cloud of counterions screens the electric field of a uniformly charged planar membrane (plate), when the counterions are characterised by a distribution of charges (or valence), {$n(q)$} . We work out analytically the one-plate and two-plate cases, at the level of non-linear Poisson–Boltzmann theory. The (essentially asymptotic) predictions are successfully compared to numerical solutions of the full Poisson–Boltzmann theory, but also to Monte Carlo simulations. The counterions with smallest valence control the long-distance features of interactions, and may qualitatively change the results pertaining to the classic monodisperse case where all counterions have the same charge. Emphasis is put on continuous distributions {$n(q)$} , for which new power-laws c...

Journal of Physics Condensed Matter

Role of interbranch pumping on the quantum-statistical behavior of multi-mode magnons in

Zahra Haghshenasfard and M G Cottam

Theoretical studies are reported for the quantum-statistical properties of microwave-driven multi-mode magnon systems as represented by ferromagnetic nanowires with a stripe geometry. Effects of both the exchange and the dipole–dipole interactions, as well as a Zeeman term for an external applied field, are included in the magnetic Hamiltonian. The model also contains the time-dependent nonlinear effects due to parallel pumping with an electromagnetic field. Using a coherent magnon state representation in terms of creation and annihilation operators, we investigate the effects of parallel pumping on the temporal evolution of various nonclassical properties of the system. A focus is on the interbranch mixing produced by the pumping field when there are three or more modes. In particular, the occupation magnon number and the multi-mode cross correlations between magnon modes are studied. Manipulation of the collapse and revival phenomena of the average magnon occupation number and...

Journal of Physics Condensed Matter

Electrical resistivity and wave character of free electrons in amorphous and nanoglass Sc 75 Fe 25

M Ghafari, W D Hutchison, S J Campbell, H Gleiter, H Hahn and T Feng

The residual electrical resistivity of metallic amorphous alloys, ρ 0 , is typically in the range 50 µ Ω cm  <   ρ 0   <  310 µ Ω cm corresponding to a mean free path of conduction electrons of order a few interatomic distances. In crystalline metals with low defect levels such as Cu however, the residual electrical resistivity is about ρ 0   ≈  1.54  ×  10 −2 µ Ω cm, leading to extensive progression of free electrons through the crystalline material, of typically up to 4  ×  10 6 nm. The relatively ‘high’ values for the electrical resistivity of distorted Sc 75 Fe 25 alloys are discussed here within the framework of the wave character of electrons. The present investigation of amorphous and nanoglass Sc 75 Fe 25 over the temperature range 1.9–320 K, focuses on clarification of the temperature dependence of the resistivity, ρ ( T ). These...

Journal of Physics Condensed Matter

Interfacial rheology of model particles at liquid interfaces and its relation to (bicontinuous)

J H J Thijssen and J Vermant

Interface-dominated materials are commonly encountered in both science and technology, and typical examples include foams and emulsions. Conventionally stabilised by surfactants, emulsions can also be stabilised by micron-sized particles. These so-called Pickering–Ramsden (PR) emulsions have received substantial interest, as they are model arrested systems, rather ubiquitous in industry and promising templates for advanced materials. The mechanical properties of the particle-laden liquid–liquid interface, probed via interfacial rheology, have been shown to play an important role in the formation and stability of PR emulsions. However, the morphological processes which control the formation of emulsions and foams in mixing devices, such as deformation, break-up, and coalescence, are complex and diverse, making it difficult to identify the precise role of the interfacial rheological properties. Interestingly, the role of interfacial rheology in the stability of bicontinuous ...

Journal of Physics Condensed Matter

Electrical conductivity of In 2 O 3 and Ga 2 O 3 after low temperature ion irradiation; implications

L Vines, C Bhoodoo, H von Wenckstern and M Grundmann

The evolution of sheet resistance of n-type In 2 O 3 and Ga 2 O 3 exposed to bombardment with MeV 12 C and 28 Si ions at 35 K is studied in situ . While the sheet resistance of Ga 2 O 3 increased by more than eight orders of magnitude as a result of ion irradiation, In 2 O 3 showed a more complex defect evolution and became more conductive when irradiated at the highest doses. Heating up to room temperature reduced the sheet resistivity somewhat, but Ga 2 O 3 remained highly resistive, while In 2 O 3 showed a lower resistance than as deposited samples. Thermal admittance spectroscopy and deep level transient spectroscopy did not reveal new defect levels for irradiation up to {$2 \times 10^{12}$} cm −2 . A model where larger defect com...

Journal of Physics Condensed Matter

(Tl, Au)/Si(1 1 1) ##IMG## [http://ej.iop.org/images/0953-8984/30/2/025002/toc_cmaa9e2dieqn001.gif]

A N Mihalyuk, C R Hsing, C M Wei, S V Eremeev, L V Bondarenko, A Y Tupchaya, D V Gruznev, A V Zotov and A A Saranin

Formation of the highly-ordered {$\sqrt7 \times \sqrt7$} -periodicity 2D compound has been detected in the (Tl, Au)/Si(1 1 1) system as a result of Au deposition onto the Tl/Si(1 1 1) surface, its composition, structure and electronic properties have been characterized using scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density-functional-theory calculations. On the basis of these data, the structural model of the Tl–Au compound has been proposed, which adopts 12 Tl atoms and 10 Au atoms (in total, 22 atoms) per {$\sqrt7 \times \sqrt7$} unit cell, i.e.  ∼1.71 ML of Tl and  ∼1.43 ML of Au (in total, ∼3.14 ML). Qualitatively, the model can be visualized as consisting of truncated-pyramid-like Au clusters with a Tl atom on top, while the other Tl atoms for...

Journal of Physics Condensed Matter

Wed Dec 13 2017

Level-set simulation of anisotropic phase transformations via faceted growth

Author(s): M.M. Moghadam, P.W. Voorhees

Level-set method is used to simulate phase transformations with anisotropic kinetics where the transforming interface is faceted. The method overcomes previous limitation of this simulation methodology in tracking dynamic evolution of a large number of growing grains. The method is then used to simulate multi-grain phase transformations where the facets are three low-index ([1 0 0], [1 1 1], [1 1 0]) planes that yield morphologies including cube, octahedron and rhombic dodecahedron. The microstructure evolves under site-saturated nucleation and constant nucleation rate. The cube morphology undergoes fastest transformation followed by octahedron, rhombic dodecahedron and sphere. It is also shown that the Johnson-Mehl-Avrami-Kolmorgorov theory can be used to describe the kinetics of the faceted phase transformation. The resulting microstructure shows non-convex grain shapes with highly corrugated surfaces. The structures are also characterized using the average grain length along certain low index crystallographic directions, the coherent length. This measurement shows that for a cubic morphology, there is a significant difference in the coherent length for low index directions, while there is no meaningful difference in the coherent length for kinetic Wulff shapes of other anisotropies examined.

Computational Materials Science

PAOFLOW: A utility to construct and operate on ab initio Hamiltonians from the projections of electronic wavefunctions on atomic orbital bases, including characterization of topological materials

Author(s): Marco Buongiorno Nardelli, Frank T. Cerasoli, Marcio Costa, Stefano Curtarolo, Riccardo De Gennaro, Marco Fornari, Laalitha Liyanage, Andrew R. Supka, Haihang Wang

PAOFLOW is a utility for the analysis and characterization of materials properties from the output of electronic structure calculations. By exploiting an efficient procedure to project the full plane-wave solution on a reduced space of atomic orbitals, PAOFLOW facilitates the calculation of a plethora of quantities such as diffusive, anomalous and spin Hall conductivities, magnetic and spin circular dichroism, and Z2 topological invariants and more. The computational cost associated with post-processing first principles calculations is negligible. This code, written entirely in Python under GPL 3.0 or later, opens the way to the high-throughput computational characterization of materials at an unprecedented scale.

Computational Materials Science

Softening of nanocrystalline nanoporous platinum: A molecular dynamics simulation

Author(s): Yuehui Xian, Jiejie Li, Runni Wu, Re Xia

Spinodal decomposition and voronoi tessellation are combined for the generation of nanocrystalline nanoporous model with grain size corresponding to ligament diameter and randomly distributed crystal orientations. A series of large-scale molecular dynamics (MD) simulations were performed and utilized to research on the mechanical behaviors of nanocrystalline nanoporous platinum (nc-NPPt) undergoing uniaxial tension. The relation between relative density and mechanical properties, together with related atomistic deforming mechanisms of as-generated nc-NPPt were analyzed. It is found that the existence of nanocrystals and substantial surface may lead to decreased tensile strength and increased plasticity of nc-NPPt, and the plastic deformation scheme of nc-NPPt is dominated by grain boundary movements. The present results will provide atomistic insights for understanding deformation mechanisms of nanocrystalline nanoporous metals as well as future mechanical optimization of nanoporous metals.

Computational Materials Science

Thermal decomposition of energetic MOFs nickel hydrazine nitrate crystals from an ab initio molecular dynamics simulation

Author(s): Dong Xiang, Weihua Zhu

We report the initiation mechanisms and subsequent chemical decompositions of energetic metal-organic frameworks (MOFs) nickel hydrazine nitrate (NHN) crystal at high temperatures and high temperatures coupled with high pressures by ab initio molecular dynamics simulations. The initial decomposition step is the collapse of the MOF cage at 518 K and 518 K coupled with 1, 2, and 3 GPa. But their cleavage steps are different. It is found that the pressure decelerates the initial decomposition of the NHN crystal. The decomposition mechanisms of the NHN crystal at 518 K coupled with 20.2 GPa are much different from those at 518 K coupled with low pressures. The results indicate that the temperature is the foremost factor that affected the decomposition of the NHN crystal. At 4000 K and 4000 K coupled with 20.2 GPa, the initial decomposition mechanisms of NHN are both the cage collapse by several NiN bond breaking. The variation tends and the release times of H2O are similar at the two cases. In the early decomposition stage, NHN decomposed very fast and drastically. But the later decomposition was slow. During 1.0 ps, there are ten H2O released fast at the two extreme conditions. Then the population of H2O keeps nearly stable during the later decomposition stage. At 7.959 ps, last one H2O was formed. Our results may provide useful information for understanding the chemical decompositions of energetic metal-organic frameworks compounds.

Computational Materials Science

An effective method to calculate the composition-dependent interdiffusivity with one diffusion couple

Author(s): Kaiming Cheng, Jixue Zhou, Huixia Xu, Shouqiu Tang, Yuansheng Yang

Previous numerical inverse method to calculate composition-dependent interdiffusivity with one diffusion couple can encounter with low calculation efficiency due to the unavoidable diffusion simulation procedure. The present work applies the numerical inverse method to the integrated form of diffusion equation, degenerating the original inverse problem into a linear multi-objective optimization problem, which can be solved with dramatically increased calculation efficiency while maintaining the overall accuracy. The current method is verified in the fcc phase of the Al-Mg, Al-Mg-Zn, Al-Mg-Zn-Cu system as well as the intermetallic compounds of the Al-Mg system. The increment in computation efficiency is discussed.

Computational Materials Science

The effect of grain size on the deformation mechanisms and mechanical properties of polycrystalline TiN: A molecular dynamics study

Author(s): Huiling Jia, Xuejie Liu, Zhaoxi Li, Shiyang Sun, Mei Li

The uniaxial tensile mechanical properties of polycrystalline TiN with 14 different grain sizes measuring 2.0–5.8 nm were studied via molecular dynamics with the second-nearest-neighbour modified embedded-atom method (2NN MEAM). The results show that the grain size affects the movement mechanisms of the grains and grain boundaries, and the relationship between grain size and tensile yield strength. The direct and inverse Hall-Petch formula of TiN are given. The dislocation migration of grain boundaries is the main deformation mechanism when the grain size is larger than 3.2 nm. When grains are smaller than 3.2 nm, grain rotation and grain boundary sliding are the preferred deformation mechanisms, which cause an inverse Hall–Petch effect. Polycrystalline TiN is at its hardest when the grain size ranges from 3 to 4 nm. The results can serve as theoretical basis for further doping non-metallic elements with critical grain sizes in the grain boundary produce superhard TiN composites.

Computational Materials Science

Phase field modeling for cyclic phase transition of NiTi shape memory alloy single crystal with super-elasticity

Author(s): Xi Xie, Guozheng Kang, Qianhua Kan, Chao Yu, Qi Peng

From the Ginzburg-Landau’s theory and thermodynamics, a three-dimensional phase field model was developed to simulate the cyclic phase transition of the NiTi shape memory alloy (SMA) single crystal with super-elasticity. The phase transition from the austenite phase to martensite one and its reverse were numerically simulated by the developed phase field model considering the repeated tension-unloading. The simulations indicated the nucleation and evolution of the multi-variant martensite and austenite phases in a scale of single crystal during the cyclic phase transition. It is found that the NiTi SMA single crystal with super-elasticity exhibits a remarkable localized deformation, accompanied by a local softening feature. Moreover, the microstructure evolution of the NiTi single crystal during the cyclic deformation was investigated. It is shown that the super-elasticity of the NiTi single crystal degrades gradually until it reaches to a stable state after certain load cycles, and a progressive accumulation of remnant martensite phase occurs, simultaneously; the phase transition of the NiTi single crystal gradually alters from a local mode into a uniform one, which comes from the internal defects and their evolutions; a large number of remnant martensite phase occurs because the enriched defects can greatly prevent the occurrence of reverse phase transition.

Computational Materials Science

Structural and electronic properties of hydrogen doped Wurtzite ZnO

Author(s): Fahime Bustan Afruz, Majid Jafar Tafreshi, Mohammad Reza Mohammadizadeh, Mostafa Fazli

We have carried out comprehensive studies on the various types of hydrogen doping including, interstitial and substitutional (for oxygen) position (Hi, HO), HO + Hi, and oxygen vacancy plus interstitial hydrogen (VO + Hi) complexes using DFT + U calculations. The effect of Hi, HO, HO + Hi, and VO + Hi dopants on structural and electronic properties of Wurtzite ZnO at 0.020, 0.0625, and 0.125 nH/nZn concentrations, were studied. We found that cell parameters and volume of the supercells decrease in Hi, HO, and HO + Hi defects, whereas these values increase in the VO + Hi case. Bader analysis shows that H ion locates at V O + 2 in HO doping and it also places beside V O 0 in VO + Hi. Furthermore, the HO + Hi complex is a H2 molecule that is almost neutral, electrically inactive and is trapped in V O + 2 . So, it seems that this complex is an appropriate model for formation of H2 molecule at high temperature. ZnO band gap is slightly influenced by Hi in all concentrations while, HO has prominent effect on increment of it which is consistent with experimental results too. In addition, it is shown that growth of H-doped sample under O-poor condition leads to improved optical properties which is applicable in transparent conductive oxide devices. It is predicted that band gap of hydrogen doped samples will decrease by annealing due to trapping of HO by Hi and formation of H2 molecule. Finally, hydrogen beside oxygen vacancy can create two deep levels, decreasing the optical band gap and shifting absorption edge to lower energy, which leads to a red shift and possible interest in photocatalytic applications.

Computational Materials Science

Molecular dynamics simulation of cross-linked epoxy resin and its interaction energy with graphene under two typical force fields

Author(s): Yingying Sun, Lin Chen, Liu Cui, Yuwen Zhang, Xiaoze Du

A model of cross-linked epoxy system composed of bisphenol-A resin, 2,3,6-tetrahydro-3-methylphthalic anhydride curing agent, and 2,4,6-tris(dimethylaminomethyl)phenol accelerator was established and molecular dynamics simulations were performed to calculate the properties of the epoxy and its composites. The results show that the mean square displacement (MSD) and glass transition temperature (Tg ) calculated by Dreiding force field are always lower than that by PCFF force field, and the simulation results of Dreiding force field are better consistent with experiments. With the increasing simulation size, total MSD increases while Tg decreases slightly. The simulated systems with DGEBA more than 12 have Tg values similar to experiments. The molecular motion of epoxy system is also influenced by the cross-linking degree, and the presence of uncross-linked particles increases the total MSD. In the graphene/epoxy composites, interaction energy between modified monolayer graphene (MMG) sheets is larger than that between epoxy and graphene, indicating that MMG sheets tend to agglomerate when mixing with epoxy. Functionalization of graphene can reduce interaction energy between MMG sheets and increase that between epoxy resin and graphene, which is beneficial to the dispersion of graphene.

Computational Materials Science

Molecular dynamics simulation of the melting behavior of copper nanorod

Author(s): Jiacheng Zhang, Xinyun Wang, Yiying Zhu, Tielin Shi, Zirong Tang, Mo Li, Guanglan Liao

The melting behaviors of copper nanorod are studied by employing molecular dynamics simulations with the embedded atom method potential during a temperature elevation process from 200 to 1600 K. The melting point decreases with the diameter of the nanorod decreasing. Upon heating, copper nanorod shows thermal instability at two critical temperatures and undergoes two structural transitions: shape transition changing to a shorter and wider shape, and melting transition changing to a liquid sphere. The shape change of the nanorod is closely related to the atoms rearrangement, which results from releasing the stress in the inner layers of the nanorod motivated by heating. Besides, the nanorod with void defects goes through the same structural transition. This work will help deepen understanding of the thermodynamic evolution in copper nanorods.

Computational Materials Science

Simulation of the plastic response of Ti–6Al–4V thin sheet under different loading conditions using the viscoplastic self-consistent model

Author(s): Jesús Galán-López, Patricia Verleysen

In this article, the capacity of the Visco-Plastic Self-Consistent model (VPSC) as a constitutive model for the simulation of cold-rolled Ti–6Al–4V under a diverse set of loading conditions is investigated. The model uses information about the material crystallographic texture and grain morphology obtained with EBSD together with a grain constitutive model that is sensitive to strain rate and temperature. The ability of the model to capture the macroscopic response and underlying microstructural phenomena is critically assessed considering various sets of hardening parameters, obtained applying different fitting procedures to tensile experiments in different directions on the sheet plane. In order to validate the fitted parameters, the obtained model is applied to the simulation of experiments in a wide range of testing conditions, including uniaxial, plane strain and simple shear loading modes at strain rates ranging from 10 4 s 1 to 10 3 s 1 and temperatures between 10 ° C and 70 ° C . High strain rate experiments were performed using a Split Hopkinson Bar setup, with specimens designed to achieve the desired loading mode, while isothermal experiments were performed with the help of a fluid cell to keep a constant temperature. A good agreement between experimental and simulation results is obtained.

Science and Engineering A

Improving mechanical properties and yield asymmetry in high-speed extrudable Mg-1.1Al-0.24Ca (wt%) alloy by high Mn addition

Author(s): T. Nakata, C. Xu, R. Ajima, Y. Matsumoto, K. Shimizu, T.T. Sasaki, K. Hono, S. Kamado

The chemical composition of a Mg-1.1Al-0.33Ca-0.44Mn (wt%) alloy, AXM10304, has been optimized for high speed extrusion. The extrudability is substantially improved from 24m/min to 60m/min by slightly reducing the Ca content from 0.33wt% to 0.24wt%. Then, the effect of Mn content on the mechanical properties and microstructures of extruded Mg-1.1Al-0.24Ca based alloys were investigated. In a solution-treated condition, tensile yield stress is enhanced to 200MPa with increasing Mn content up to 0.68wt% and the compressive yield stress is also improved to 164MPa with increasing Mn content up to 1.0wt% due to grain size refinement. After an artificial aging, Guinier Preston zones were dispersed within the Mg matrix, and the tensile and compressive yield stresses are increased by 70MPa and 30MPa regardless of the Mn content. Consequently, the peak-aged 1.1Al-0.24Ca-1.0Mn alloy, AXM1021, exhibits high tensile and compressive yield stresses of 263MPa and 197MPa with a good yield asymmetry of 0.74 (a ratio of compressive yield stress to tensile one).

Science and Engineering A

Solution treatment of Ti-6Al-4V alloy produced by consolidating blended powder mixture using a powder compact extrusion route

Author(s): Ajit Pal Singh, Fei Yang, Rob Torrens, Brian Gabbitas

Titanium alloys are very sensitive to thermal history and different microstructures are obtained depending on their chemistry, processing route and post-processing heat treatment. The objective of this paper is to characterise solution-treated Ti-6Al-4V alloy rods from microstructural investigations using optical and transmission electron microscopy (TEM) in addition to measurements of chemical composition and levels of impurity oxygen. From the results, it is clear that the compositions and oxygen concentration of the different rods, which varied between 0.34–0.36wt%, was consistent from one extrusion to another. TEM analysis of the as-extruded material with a fine lamellar microstructure indicates that the severe deformation and attendant dynamic recrystallization during and after hot extrusion did not give rise to any undesirable features that can degrade the ductility. Solution treatment above the β transus and subsequent ageing causes grain growth with grains containing a metastable martensitic structure (only α’ phase was present), some retained β and a limited formation of acicular secondary α. The α+β quenched and aged treatment gives a lamellar type morphology, but at the α interfaces there is retained β with some secondary α and potentially some α’ phase. In terms of mechanical behaviour, the data from v-notch Charpy impact tests and non-standard micro-tensile testing suggests that both water quenched and aged microstructures give a higher yield strength (~1022–1033MPa) and micro-hardness (388 HV), while the fracture-related properties such as estimated plastic strain and impact toughness were between 5–6% and 13.7J respectively. Overall, the level of mechanical properties reported here is better than that for typical values reported in the literature for as-cast material after similar solution treatments.

Science and Engineering A

Porosity formation mechanisms and fatigue response in Al-Si-Mg alloys made by selective laser melting

Author(s): Kun V. Yang, Paul Rometsch, Tom Jarvis, Jeremy Rao, Sheng Cao, Chris Davies, Xinhua Wu

Defects in Al-7Si-Mg and Al-10Si-Mg alloys produced by selective laser melting are categorised into three types. The first type are large irregular-shaped defects with unmelted powder particles, formed due to a lack of fusion as a result of insufficient volumetric energy density. The second type are small round gas pores below 5µm in diameter, associated with high area energy density. These pores enlarge during solution heat treatment, but the enlargement is reduced significantly when the powder is pre-dried at 200°C for 16h under an argon atmosphere immediately before the build. The last type are large round keyhole type pores located at the base of melt pools. They can either form in contour scan regions, at the edges of core scans, or at island boundary overlap regions due to an excessive local energy density compared with the nominal energy density. Sub-surface porosity due to contour and core edge keyhole type defects can be more detrimental to the fatigue performance than net-shaped rough surfaces, but such sub-surface porosity can be minimised by either lowering the laser energy input for the contour scan and/or changing the way the laser turns between scan tracks.

Science and Engineering A

Effect of scanning strategies on residual stress and mechanical properties of Selective Laser Melted Ti6Al4V

Author(s): Haider Ali, Hassan Ghadbeigi, Kamran Mumtaz

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Currently post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility.

Science and Engineering A

A FEM study on mechanical behavior of cellular lattice materials based on combined elements

Author(s): Xiaoliang Geng, Liyang Ma, Chao Liu, Chen Zhao, ZhuFeng Yue

The mechanical behavior of AlSi10Mg alloy lattice materials with rhombic dodecahedron and BCC unit cells fabricated by selective laser melting was investigated in this paper. Tensile tests were carried out to obtain material properties, mechanical response and failure characteristic. The results show that rhombic dodecahedron and BCC cellular lattice have variant tensile strength and stiffness. Various fracture features are also observed to be related to its configuration. Scanning electron microscope images of fracture surface demonstrate the lattice material resulted in ductile fracture. Finite element models based on combined elements were established. Afterwards, numerical analysis was conducted by ABAQUS. The FE results which could be employed to make some useful predictions are consistent with the experimental results.

Science and Engineering A

In-situ investigation of the anisotropic mechanical properties of laser direct metal deposition Ti6Al4V alloy

Author(s): Junxia Lu, Ling Chang, Jin Wang, Lijun Sang, Shikai Wu, Yuefei Zhang

This study compares the microstructure and tensile properties of Ti6Al4V components fabricated by laser direct metal deposition (LDMD) additive manufacturing (AM) in the transverse and longitudinal directions. The results show anisotropic tensile properties with the transverse direction having high tensile and fracture strengths and the longitudinal direction having a high elongation and reduction of cross section. The anisotropic mechanical properties are attributed to the anisotropic microstructural distribution. The transverse tensile specimen is composed of short columnar prior-β grains which grow perpendicular to the tensile direction, and have a lamellar structure. Along the β grain boundary, αGB and large α colonies were identified. However, the longitudinal specimen shows that the long β structure is parallel to the tensile axis and that the microstructure is composed of basket-woven α phases with shorter α plates and smaller colony sizes compared with those in the transverse specimen. The fracture mechanism induced by the anisotropic microstructure along the transverse and longitudinal directions was compared by examining the fracture process in real-time using uniaxial in-situ scanning electron microscopy (SEM) tensile testing. The results show that shear fracture, which is caused by the vertical β grain boundaries and large α colonies with long α plates, occurs in the transverse specimen. The shear mode is the main reason behind the enhanced tensile strength and fracture strength due to the high resistance to microcrack propagation. However, in the longitudinal specimens, symmetric necking behavior due to the fine α grains resulted in uniform deformation of the grains on both sides of the grain boundaries, inducing greater elongation.

Science and Engineering A

The significant impact of pre-strain on the structure-mechanical properties relationship in cold-rolled medium manganese TRIP steel

Author(s): Z.C. Li, R.D.K. Misra, H. Ding, H.P. Li, Z.H. Cai

We elucidate here the impact of pre-strain on the structure on mechanical properties of cold-rolled Fe-0.2C-5.5Mn-1.0Al (wt%) transformation induced plasticity (TRIP) steel. The steel was intercritically hardened at 650°C and exhibited excellent combination of total elongation (TEL) of 45.2%, ultimate tensile strength (UTS) of 1100MPa, and UTS×TEL of 50.2GPa%. Pre-strain increased both yield strength (YS) and ultimate tensile strength (UTS), while it decreased total elongation (TEL). Product of strength and elongation (PSE) was similar with 10% pre-strain (50.4GPa%) and 0% pre-strain (50.2GPa%). However, the 10% pre-strain had lower yield-to-tensile ratio. The increase in YS is attributed to the increased dislocation density and transformed martensite, while the continuous increase in UTS was associated with increase in the fraction of transformed martensite and enhanced work-hardening rate. On the other hand, the increase in the average stability of retained austenite, increased density of dislocations in ferrite and increased volume fraction of transformed martensite induced by pre-strain led to decreased ductility. The desired mechanical properties can be obtained by optimizing the amount of pre-strain.

Science and Engineering A

Identifying the effects of heat treatment temperatures on the Ti50Ni45Cu5 alloy using dynamic mechanical analysis combined with microstructural analysis

Author(s): Albert Fabregat-Sanjuan, Francesc Gispert-Guirado, Francesc Ferrando, Silvia De la Flor

The properties of NiTiCu alloys strongly depend on their microstructure, which is greatly influenced by the temperature of the heat treatment (HT) carried out as a final step in their processing. This study investigates the effect of HT temperatures on the Ti50Ni45Cu5 alloy using dynamic mechanical analysis (DMA). To confirm the results obtained by the DMA as regards the effects of HT on the microstructure, they were checked against differential scanning calorimetry (DSC), synchrotron X-ray diffraction (SXRD), hardness measurement and stress-strain tests. By combining all these experimental techniques with the DMA measurements, the effects of the HT were determined and attributed to different factors: density dislocation morphology introduced by prior cold work, recovery and recrystallization processes with texture changes, grain growth, precipitation processes and wt% of retained martensite. The results showed that low HT temperatures (400 and 450°C) are not high enough to eliminate the effects of the cold work but do provide suitable mechanical properties and lower transformation temperatures. Medium temperatures (500–575°C) lead to higher transformation temperatures and maximum martensitic transformation capacity (ΔH and tan δ) because of the recovery and recrystallization process, but they also cause a reduction in the mechanical properties (slip stress). High temperatures (600 and 650°C) produce a reduction in transformation capacity and a huge decrease in the mechanical properties (slip stress and ultimate tensile stress).

Science and Engineering A

Deformation microstructures and strengthening mechanisms for the wire+arc additively manufactured Al-Mg4.5Mn alloy with inter-layer rolling

Author(s): Jianglong Gu, Xiaoshu Wang, Jing Bai, Jialuo Ding, Stewart Williams, Yuchun Zhai, Kun Liu

Applying inter-layer rolling to the wire+arc additively manufacturing (WAAM) process with increasing loads of 15kN, 30kN and 45kN, achieves excellent mechanical properties for 5087 (Al-Mg4.5-Mn) alloys. Compared with the as-deposited alloy, the average micro hardness, yield stress and ultimate tensile strength of 45kN rolled alloys reached to 107.2 HV, 240MPa and 344MPa, which were enhanced by 40%, 69% and 18.2%, respectively. Primary coarse grain structures were found to become greatly refined with an evident rolling texture after deformation. The strengthening mechanisms mainly are deformation strengthening, grain refinement, and solution strengthening. Meanwhile, the elongation of rolled alloys stays over 20%. The plasticity was not obviously diminished compared with the as-deposited alloy. This is two times greater than the commercial wrought Al-Mg alloy with similar composition. The excellent plasticity may be chiefly due to grain refinement, pores closure and reduction, and grain recrystallization during the WAAM re-heating process. The combination process of rolling deformation with WAAM deposition is an effective technique in refining microstructure and improving mechanical properties for AM aluminum alloys.

Science and Engineering A

Characterization of soldering alloy type Zn-In-Mg and the study of direct soldering of silicon and copper

Author(s): Roman Koleňák, Igor Kostolný, Martin Kusý

The aim of study was to characterize the Zn-In-Mg soldering alloy type and investigate the direct soldering of silicon and copper. The Zn-In-Mg solder type has a broad interval of melting, which depends on the indium content in the solder. Solder microstructure is formed of a matrix with pure Zn. The MgZn2 phases and solid solution (In) β-In are precipitated along the grain boundaries. Tensile strength attains values from 46 to 124MPa and is dependent on indium content. The bond with silicon is formed due to the reactions of active metals - In and Mg with the substrate surface. Diffraction analysis has also revealed the In13Mg7 phase. In spite of that, the bond formation with a copper substrate in not affected by In and Mg content. The bond is formed owing to interaction between the zinc from the solder and copper substrates. Two phases, namely CuZn4 and Cu5Zn8 were observed. The shear strength of Cu/Zn-In-Mg/Cu joints attains values from 56 to 62MPa and the shear strength of Si/Zn-In-Mg/Cu joint is within 34–42MPa.

Science and Engineering A

Deformation response of cube-on-cube and non-coherent twin interfaces in AgCu eutectic under dynamic plastic compression

Author(s): B.P. Eftink, N.A. Mara, O.T. Kingstedt, D. Safarik, S. Wang, J. Lambros, I.M. Robertson

Split-Hopkinson pressure bar dynamic compression experiments were conducted to determine the defect/interface interaction dependence on interface type, bilayer thickness and interface orientation with respect to the loading direction in the Ag-Cu eutectic system. Specifically, the deformation microstructure in alloys with either a cube-on-cube orientation relationship with { 111 } Ag || { 111 } Cu interface habit planes or a twin orientation relationship with { 3 ̅ 13 } Ag || { 1 ̅ 12 } Cu interface habit planes and with bilayer thicknesses of 500nm, 1.1µm and 2.2µm were probed using TEM. The deformation was carried by dislocation slip and in certain conditions, deformation twinning. The twinning response was dependent on loading orientation with respect to the interface plane, bilayer thickness, and interface type. Twinning was only observed when loading at orientations away from the growth direction and decreased in prevalence with decreasing bilayer thickness. Twinning in Cu was dependent on twinning partial dislocations being transmitted from Ag, which only occurred for cube-on-cube interfaces. Dislocation slip and deformation twin transfer across the interfaces is discussed in terms of the slip transfer conditions developed for grain boundaries in FCC alloys.

Science and Engineering A

Evolution mechanism of dislocation boundary and characteristic micro-structure of commercial pure titanium during the projectile impact

Author(s): Tongbo Wang, Bolong Li, Yingchao Li, Mian Li, Zuoren Nie

Dislocation boundary response and characteristic micro-structure were investigated in commercial pure titanium during the penetration impact and dynamic compression. The initial dislocation boundary induced by cold rolling can strengthen the adiabatic shearing sensitivity of commercial pure titanium during the penetration impact, which is consistent with the mechanical response and micro-structure during the dynamic compression. The compression with a loading direction vertical to the initial geometrically necessary boundaries (GNBs) results in the decrease in the spacing between GNBs, while the compression with a loading direction parallel to the initial GNBs attributes to the generation of new dislocation boundary crossed with the initial one. Then, a model of distribution and morphology of dislocation boundary around crater is proposed.

Science and Engineering A

The effects of the initial microstructure on microstructural evolution, mechanical properties and reversed austenite stability of intercritically annealed Fe-6.1Mn-1.5Si-0.12C steel

Author(s): Shu Yan, Xianghua Liu, Taosha Liang, Yang Zhao

A typical medium Mn steel with nominal chemical composition of Fe-6.1Mn-1.47Si-0.12C (wt%) was intercritically annealed from different initial microstructures, i.e., cold-rolled martensite and as-quenched martensite (hereinafter referred as CR and AQ sample, respectively). The CR sample is mainly composed of equiaxed sub-micron grains owing to nearly full recrystallization of deformed martensitic matrix, while the AQ sample still presents martensite lath structure in general. Reversed austenite grains in the CR sample are almost entirely granular. In the AQ samples, there are two kinds of microscopic morphology of reversed austenite, i.e., acicular and granular austenite, and the nucleation and growth of them are analyzed individually. Additionally, Reversed austenite fraction is not affected by initial microstructure irrespective of deformed and as-quenched martensite. Tensile strength and yield strength of the CR sample are about 80MPa and 166MPa higher than that of the AQ sample, respectively, but the PSE value of both CR and AQ sample is a little higher than 30GPa%. Metastable austenite in the CR sample is more sensitive to the increasing strain, i.e., lower mechanical stability, which can be attributed to the higher nucleation rate of strain-induced martensitic transformation.

Science and Engineering A

Strain rate dependence of tensile strength and ductility of nano and ultrafine grained coppers

Author(s): Yue Jiang, Jiangjiang Hu, Zhonghao Jiang, Jianshe Lian, Cuie Wen

An ultrafine grained Cu with a wide distribution of grain size and an average grain size of d = 110nm was prepared by electric brush-plating technique. Tensile properties, microstructure and deformation surface morphologies of this ultrafine grained Cu were compared with two results on an electric brush-plated nanocrystalline Cu ( d = 59nm) and an electrodeposited ultrafine grained Cu ( d = 200nm) previously prepared by our group. Based on these comparisons, the strain rate dependences of strength and ductility of these three materials and underlying mechanisms were studied. It was revealed that the pronounced strain rate sensitivity (larger m value) of these three materials is a result of the competition between the dislocation and grain boundary mediated deformations. The grain boundary mediated deformation plays a very important role in controlling the tensile ductility of these three materials, which affects the deformation accommodation by promoting different modes of strain localization and hence leads to the completely different strain rate dependences of tensile ductility (the elongations decrease, remains unchangeable and increase with increasing strain rate).

Science and Engineering A

Thermo-mechanical processing, microstructure and mechanical properties of TiZrB alloy

Author(s): Chaoqun Xia, Xing Zhang, Shuguang Liu, Bohan Chen, Chunlin Tan, Xinyu Zhang, Mingzhen Ma, Riping Liu

This work was aimed in the relationships investigation among the microstructure, the tensile properties and the thermo-mechanical processing of a novel Ti-25Zr-1B (wt%) alloy. The alloy was evaluated under different conditions (as-cast, hot-rolled and hot-rolled-annealed). The XRD diagrams demonstrated that all specimens were completely consisted of the α and TiB phases. The results of EDS indicated that the TiB phase dissolved a certain amount of Zr, which was in agreement with the existence of TiB peaks shifting towards lower diffraction angles. The as-cast Ti-25Zr-1B alloy microstructure and the size, the morphology, the orientation, and the distribution of the TiB particles could be significantly altered by the hot-rolling process. The hot-rolling process provided an ultimate strength increase of at least 7 pct and the elongation-to-failure increase of 135 pct relatively to the as-cast sample. In addition, the annealing heat treatment could effectively modify the mechanical properties of the hot-rolled sample. The strength decreased and the elongation-to-failure increased as the annealing temperature increased, while the temperature was below 800°C. In contrast, as the annealing temperature increased to 900°C, the alloy demonstrated appreciably lower elongation-to-failure and higher strength compared to the annealed form at 800°C. The mechanical properties variation of the Ti-25Zr-1B alloy series was ascribed to the microstructure evolution of the matrix alloy as well as to the morphology and volume fraction changes of the TiB particles.

Science and Engineering A

Cryogenic temperature toughening and strengthening due to gradient phase structure

Author(s): Zhiwei Ma, Yang Ren, Runguang Li, Yan-Dong Wang, Lingling Zhou, Xiaolei Wu, Yujie Wei, Huajian Gao

Cold embrittlement is one of the primary concerns challenging the usage of steels in infrastructures like pipelines and ocean platforms. This challenge is also compounded by the limited selection of materials for application in a cold and corrosive environment. Inspired by recent progresses in developing gradient structured materials with extraordinary properties, here we report a class of stainless steels with gradient phase structures achieving a superb combination of strength (1753MPa) and tensile ductility (>25%) at the cryogenic temperature of 77K. A set of cylindrical steel samples acquire a graded mixture of hard martensitic and soft austenitic phases through pre-torsion, which results in an optimal stress partition in the material - the hard martensitic structures showing a positive density gradient from core to edge carry higher stress near the edge, while the soft austenitic phase showing a negative density gradient from core to edge serves to retain substantial tensile ductility. The phase-transformation at low temperature in gradient structures and the resulted work-hardening could be adopted to enhance the ductility and strength of widely used engineering materials for their applications in harsh environment.

Science and Engineering A

Stability of the structure and properties of an ultrafine-grained Cr-Ni steel irradiated with neutrons in nuclear reactor core conditions

Author(s): V.K. Shamardin, M.M. Abramova, T.M. Bulanova, A.A. Karsakov, A.E. Fedoseev, A.V. Obukhov, R.Z. Valiev, I.V. Alexandrov, G.I. Raab, N.A. Enikeev

An ultrafine-grained (UFG) austenite Cr-Ni stainless steel was produced by equal-channel angular pressing. The UFG samples were irradiated in a research nuclear reactor up to damaging doses of 12 and 15dpa at 350°C and 450°C, respectively. The post-irradiation examination featured in this work covers microstructural and mechanical property changes at test temperatures of 20–650°C. Microstructural studies showed that the concentration of Frank loops was considerably lower in the UFG steel compared to in the coarse-grained material. It was also found that irradiation resulted in precipitation of α-phase in triple junctions of the UFG steel. The post-irradiation examination of mechanical properties testified that the UFG steel exhibited lower irradiation hardening and higher strength compared to the coarse-grained material in the entire testing temperatures range, and improved ductility when tested at 500–650°C. In summary, we testified that microstructure refinement of an austenite stainless steel down to UFG scale led to enhancement of its resistance to neutron irradiation at elevated temperatures.

Science and Engineering A

An investigation of the limits of grain refinement after processing by a combination of severe plastic deformation techniques: A comparison of Al and Mg alloys

Author(s): Shima Sabbaghianrad, Seyed Alireza Torbati-Sarraf, Terence G. Langdon

An Al-7075 aluminum alloy and a ZK60 magnesium alloy were processed by a combination of equal-channel angular pressing (ECAP) for 4 passes and high-pressure torsion (HPT) through total numbers of up to 20 turns. Processing by ECAP and HPT were performed at 473K and room temperature, respectively. Mechanical testing showed an increase in the hardness value of the Al-7075 alloy after a combination of ECAP and HPT whereas in the ZK60 alloy the hardness was reduced. Microstructural images of the Al-7075 alloy revealed very significant grain refinement after a combination of ECAP + HPT compared to the individual processing techniques. By contrast, and consistent with the hardness measurements, the average grain size of the ZK60 alloy was larger after processing by the two-step SPD technique. These results are examined and an explanation is presented based on the available microstructural evidence.

Science and Engineering A

Microstructures and mechanical properties of TixNbMoTaW refractory high-entropy alloys

Author(s): Z.D. Han, H.W. Luan, X. Liu, N. Chen, X.Y. Li, Y. Shao, K.F. Yao

Refractory high-entropy alloys (RHEAs) are newly developed candidate materials for high-temperature applications. Among the existing RHEAs, NbMoTaW RHEA possesses the best mechanical properties with combined high strength, excellent thermal stability and softening resistance at elevated temperatures. However, the NbMoTaW RHEA is quite brittle at room temperature, which would restrict its application as structural material. Here, Ti x NbMoTaW RHEAs were developed by alloying Ti in the NbMoTaW RHEA. It shows that the room temperature ductility of the RHEAs increases from 1.9% of the NbMoTaW RHEA to 11.5% of the TiNbMoTaW RHEA, and the yield strength increases from 996MPa of the NbMoTaW RHEA to 1455MPa of the TiNbMoTaW RHEA. In addition, the Ti x NbMoTaW RHEAs keep stable single BCC structure up to their melt points. The present result indicates that Ti addition could effectively enhance both the ductility and strength of the NbMoTaW RHEA. The combined performance of superior mechanical properties and high thermal stability of the Ti x NbMoTaW RHEAs promises them an important role in engineering applications.

Science and Engineering A

Compositionally gradient Ti-Ta metal-metal composite with ultra-high strength

Author(s): Shenghang Xu, Yong Liu, Chao Yang, Henglv Zhao, Bin Liu, Jianbo Li, Min Song

Conventional lamellar-structured metallic composites usually have discontinuous interfaces with limited diffusion in between two metals. Here, for the first time, we fabricated a bulk Ti-Ta metal-metal composite containing Ti-enriched, Ta-enriched and obvious interfacial diffusional zones by using powder metallurgy technique. After hot swaging, all the three zones are lamellar-like along the radial direction. The metal-metal composite possesses ultra-high strength (σUTS = 1570MPa) and low elastic modulus. It was found that the high strength was mainly caused by precipitation strengthening of fine α phase inside the Ti-enriched zones and fine grain-size in the Ta-enriched zones. Furthermore, the stress-induced martensitic transformation inside the diffusional zone can further strengthen the interface. The bonding force of the lattice structure was reduced by the solution of Ta element, and thus, leading to the low modulus. This exceptional microstructure provides a novel approach to obtain ultra-high strength and low modulus Ti-based biomaterials.

Science and Engineering A

Modification and toughening of 3D needled CSiC composite by deformable MAX phase-based matrix

Author(s): Yuzhao Ma, Xiaowei Yin, Xiaomeng Fan, Panfei Ju, Xiaolin Dang

3D needled (3DN) C/SiC fabricated through CVI has promising mechanical properties, though the low fiber content in the short-cut fiber layer results in large pores, which is hard to be filled and influences its mechanical properties. To solve this problem, this paper focuses on short-cut fiber layer of 3DN C/SiC preform and in-situ formation of MAX phase-based matrix with a joint process of slurry impregnation (SI) and reactive melt infiltration (RMI). The ratio of MAX phase to brittle phases in the matrix is adjusted through controlling the fabrication process. With the increase of MAX phase content, the advantages of the “strong” MAX phase with weak interface gradually emerge. The modification of MAX phase not only enriches toughening mechanism, but also elevates the bearing structure of 3DN C/SiC from one strong ply sandwiching three weak plies into three strong plies sandwiching one weak ply. Finally, 3DN C/SiC containing 15 vol.% Ti3Si(Al)C2 showed excellent mechanical properties, with flexural strength and fracture toughness up to 440 ± 17MPa and 15.9 ± 1.4MPa·m1/2, respectively.

Science and Engineering A

Non-inclusion induced crack initiation in multiphase high-strength steel during very high cycle fatigue

Author(s): P. Zhao, Z. Liu, R.D.K. Misra, F. Du, C. Zhang, Z.G. Yang, E. Yan

During very high cycle fatigue (VHCF) crack can initiate at non-inclusion site in the interior of Mn-Si-Cr bainite/martensite (B/M) multiphase steels. The characteristics of non-inclusion induced crack initiation in B/M multiphase steels and the relationship between the size of granular bright facet (GBF) and VHCF limit were studied through ultrasonic fatigue testing, in terms of fracture surface analysis, crack initiation analysis, and failure mechanism. The current study reveals that GBF area became smaller with the increase of VHCF limit, i.e. the steel with higher fatigue-endurance strength exhibited smaller GBF area in the case of steel characterized by non-inclusion crack initiation in the VHCF regime. The relationship between GBF area, fatigue life, and applied stress was consistent with the following equation area GBF = 29.41LogN f − 0.2916 σ a + 47.51. The study provides an understanding of non-inclusion induced crack initiation in VHCF and a guidance for steel fatigue design.

Science and Engineering A

Effects of deformation temperature on second-phase particles and mechanical properties of 2219 Al-Cu alloy

Author(s): Hailin He, Youping Yi, Shiquan Huang, Yuxun Zhang

In this study, an age-hardenable 2219 Al–Cu alloy was severely deformed by multidirectional forging (MDF) at 360–510°C, followed by solution treatment and T8 aging treatment. The evolution of the second-phase Al2Cu particles and the mechanical properties of the T8-aged samples were examined. The results indicated that a higher volume fraction and a more disperse distribution of the fragmented coarse particles were obtained for the sample deformed at low temperatures. The fragmented coarse particles were still difficult to sufficiently dissolve in the Al matrix in the subsequent solution treatment and could act as crack initiation sites, thereby reducing the mechanical properties of the alloy. With increasing the temperature of MDF, the number of coarse particles was gradually decreased, and the particles exhibited a more spheroidized shape after MDF. Correspondingly, more Cu atoms were dissolved in the Al matrix in the subsequent solution treatment, and the uniformity and density of the precipitates after T8 aging were improved. Thus, the mechanical properties of the T8-aged forgings were improved with increasing the temperature of MDF. The optimal mechanical properties were obtained when the deformation temperature was 510°C, with ultimate tensile strength of 431.2MPa, yield strength of 341.3MPa, and elongation of 6.5% at room temperature.

Science and Engineering A

Compression creep of copper under electric current studied by a spark plasma sintering (SPS) apparatus

Author(s): Barak Ratzker, Maxim Sokol, Sergey Kalabukhov, Nachum Frage

A spark plasma sintering (SPS) apparatus was successfully employed to perform uniaxial compressive creep tests on pure copper under a stress of 30MPa in the 400–600°C temperature range. By utilizing two different configurations, the creep experiments were conducted without or with a low-density pulsed direct electric current (~ 6–7A/mm2) passing through the samples. It was found that under the influence of the applied electric current, the creep rate increased significantly, while the extent of the effect diminished with temperature. The apparent activation energy for creep with applied current decreased from 110 to 66kJ/mol. This was attributed to the effect of current on the thermally-activated process and dislocations motion. No distinct evidence that the electric current affects the microstructure was observed.

Science and Engineering A

Effects of post weld heat treatment (PWHT) on mechanical properties of C-Mn weld metal: Experimental observation and microstructure-based simulation

Author(s): Chengze Zhang, Shihua Yang, Baoming Gong, Caiyan Deng, Dongpo Wang

The instrumented indentation test and the microstructure-based finite element analysis were used to study the effect of post weld heat treatment (PWHT) on the mechanical properties of C-Mn weld metal, i.e. the individual microstructures, the heterogeneous deformation distribution and the plastic strain localization under tensile loading. It is found that the PWHT significantly influences the strength of weld metal by changing the strength of individual microstructures: the grain boundary ferrite is softened after PWHT regardless of the holding temperatures; the acicular ferrite is softened after treating at 400°C and 700°C, whereas hardened after treating at 600°C; the variation of the mechanical properties is due to the reduction of dislocation and the precipitation of carbonitride. As a result, the strength variation between grain boundary ferrite and acicular ferrite after the PWHT leads to the distinct distribution of plastic strain localization in weld metal and the critical fracture strain of weld metal.

Science and Engineering A

Dynamic restoration and deformation heterogeneity during hot deformation of a duplex-structure TC21 titanium alloy

Author(s): Ke Wang, Mingyu Wu, Zhibing Yan, Dongrong Li, Renlong Xin, Qing Liu

The coexistence of equiaxed α, lamellar α and β phase in a duplex-structure α/β titanium alloy not only benefits the balance between strength and ductility, but also arouses a complicated microstructure evolution during hot deformation. The research objective herein is first to investigate the dynamic restoration mechanism, including dynamic recovery (DRC) and dynamic recrystallization (DRX), of both α and β phases during hot deformation of a duplex-structure TC21 titanium alloy. The results show that, after deformation, the geometric morphologies of lamellar α and β phase change more apparently than equiaxed α. Based on the analysis on the distribution and frequency of misorientation angle, it reveals that DRC and continuous DRX (CDRX) occur in all of equiaxed α, lamellar α and β phase, and the increasing deformation temperature restrains the occurrence of CDRX. Synthetically, the correlation between geometric morphology and dynamic restoration is well established. Meanwhile, the heterogeneous degree of DRX indicates a morphology dependent deformation heterogeneity between equiaxed and lamellar α, and between different local regions in β phase which is closely related to the distribution of equiaxed and lamellar α. The further discussion reveals that the morphology dependent deformation heterogeneity is fundamentally attributed to the different orientation relationship (OR) between β phase and equiaxed/lamellar α. This new find enriches the understanding on the deformation heterogeneity of titanium alloy, which was mainly attributed to the difference in crystallographic orientations in past decades.

Science and Engineering A

Microstructural evolution, coarsening behavior of precipitates and mechanical properties of boron bearing steel 25CrMoNbB during tempering

Author(s): Yaxu Zheng, Fuming Wang, Changrong Li, Yongliang Li, Jin Cheng

In order to optimize the tempering process of boron bearing steel 25CrMoNbB, the effect of tempering temperature and holding time on martensitic microstructure, precipitation and mechanical properties has been studied. Thermo-Calc software was used to carry out thermodynamic calculation of the equilibrium precipitation. The precipitates growth behaviors were simulated using the program package diffusion-controlled transformations (DICTRA). Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAD) and energy dispersive spectrometer (EDS) were used to analyze microstructure and precipitates. B distribution was observed by secondary ion mass spectroscopy (SIMS). Electron back-scattered diffraction (EBSD) and TEM were used to analyze the martensitic microstructure evolution during tempering. The results showed that the B-bearing precipitates had a significant effect on the impact toughness and little effect on the tensile strength. The B atoms segregating at prior austenite grain boundaries (PAG) during austenitizing process promoted precipitation of M2B during tempering. The M2B precipitates mainly contained Fe, Mo, Cr and B elements and significantly reduced the toughness. Therefore, B content in 25CrMoNbB steel should be controlled below 20ppm. In addition, the tempering temperature and holding time should also be optimized to reduce the detrimental effect of M2B on the impact toughness. In as-tempered condition, besides M2B precipitates, M23C6, M7C3, and MC carbides were identified. M7C3 and MC carbides tended to precipitate in the matrix, whereas M23C6 tended to precipitate at PAG and sub-grain boundaries. Although carbides were coarsening with increasing tempering temperature or holding time, the coarsening of martensitic blocks and laths has reduced their detrimental effect on the toughness.

Science and Engineering A

Modelling and simulation of diffusion driven pore formation in martensitic steels during creep

Author(s): M.R. Ahmadi, B. Sonderegger, S.D. Yadav, M.C. Poletti

A model has been developed to describe the nucleation and growth of creep pores in crystalline material under service conditions. The nucleation model is based on Becker-Döring (BD) nucleation theory using Helmholtz free energy, while for the growth of pores, a vacancy flux model towards nucleated or existing pores is utilized. The whole model is able to describe nucleation and growth rates of pores in the matrix (homogeneous nucleation), at grain boundaries, at triple and at quadruple grain boundary points as well as at particles/inclusions. Nucleation and growth rates of pores in creep process are considered to be a function of external and internal stress due to the residual stresses, working temperature, local microstructure (nucleation and growth of particles), nucleation sites, interfacial energy of grain boundaries and phase boundary energies, diffusion rates in different paths, and pore geometry. Interrupted creep tests are performed for 9Cr-1Mo martensitic (ASME Gr.91) steels under 66MPa uniaxial creep loading at 650°C to track the pore evolution after 0, 2189, 4009, 5272 and 8030h. The model results are then compared to experimental findings in terms of mean pore size, and volume fraction. The model has good prediction and description power of the physical phenomena.

Science and Engineering A

Equation of Motion for a Grain Boundary

Author(s): Luchan Zhang, Jian Han, Yang Xiang, and David J. Srolovitz

Grain boundary (GB) migration controls many forms of microstructural evolution in polycrystalline materials. Recent theory, simulations, and experiments demonstrate that GB migration is controlled by the motion of discrete line defects or disconnections. We present a continuum equation of motion for...

Physical Review Letters

Temperature hysteresis of the order-disorder transition in carbon-supersaturated α-Fe

Author(s): P. Maugis, F. Danoix, H. Zapolsky, S. Cazottes, and M. Gouné

Although many works have been devoted to the order-disorder transition in carbon-supersaturated α-Fe, all seem to have overlooked the temperature hysteresis phenomenon occurring around the critical temperature. It is shown, from a mean-field model based on the elasticity theory of point defects, tha...

Physical Review B

Tue Dec 12 2017

enhanced methane uptake

Tian Tian, Zhixin Zeng, Diana Vulpe, Mirian E. Casco, Giorgio Divitini, Paul A. Midgley, Joaquin Silvestre-Albero, Jin-Chong Tan, Peyman Z. Moghadam & David Fairen-Jimenez

Using a sol–gel process, monoliths of metal–organic frameworks

Nature

nitride

T. Q. P. Vuong, S. Liu, A. Van der Lee, R. Cuscó, L. Artús, T. Michel, P. Valvin, J. H. Edgar, G. Cassabois & B. Gil

Isotope engineering in hexagonal boron nitride can affect its vibrational, electronic

Nature

Safe and recyclable lithium-ion capacitors using sacrificial organic lithium salt

P. Jeżowski, O. Crosnier, E. Deunf, P. Poizot, F. Béguin & T. Brousse

Strategies to incorporate a lithium-cation source in lithium-ion capacitors have so far proved challenging. A sacrificial organic lithium salt is now shown to irreversibly provide lithium cations to a graphite electrode during the initial operando charging step without any negative effects.

Nature

boron nitride

Alexander J. Giles, Siyuan Dai, Igor Vurgaftman, Timothy Hoffman, Song Liu, Lucas Lindsay, Chase T. Ellis, Nathanael Assefa, Ioannis Chatzakis, Thomas L. Reinecke, Joseph G. Tischler, Michael M. Fogler, J. H. Edgar, D. N. Basov & Joshua D. Caldwell

Isotopic enrichment in hexagonal boron nitride is shown to enhance the propagation

Nature

Classical and quantum calculations of the temperature dependence of the free energy of argon

Author(s): Wenwu Xu, Andrew P. Horsfield, David Wearing, Peter D. Lee

The free energy is central to statistical mechanics and thermodynamics, and its accurate calculation via. computational modelling is important for a large number of applications, especially when its experimental value is hard to obtain. Several established and general methods for calculating the Helmholtz free energy across different length scales, including continuum, atomistic and quantum mechanical, are compared and analyzed. A computational approach is then proposed to calculate the temperature dependences of internal energy and absolute Helmholtz free energy for solid and liquid phases with the coupling of thermodynamic integration (TI) and harmonic approximation calculations from both classical molecular dynamics (MD) and density functional theory (DFT). We use the Lennard-Jones system as an example (i.e. argon) for the demonstration of the approach. It is observed that the free energy transits smoothly from being describable by the harmonic approximation to including anharmonic effects at a transition temperature around 0.56 Tm ; below this temperature, the quantum behavior of atoms is important. At higher temperatures (T > 0.56 Tm ), the TI and harmonic approximation results for the Helmholtz free energy functions become increasingly divergent with the increase of temperature. This work demonstrates that a multiscale approach employing TI, MD, and DFT can provide accurate calculations of the temperature dependence of absolute Helmholtz free energy for both solid and liquid phases.

Computational Materials Science

Structural Origin of Enhanced Dynamics at the Surface of a Glassy Alloy

Author(s): Gang Sun, Shibu Saw, Ian Douglass, and Peter Harrowell

The enhancement of mobility at the surface of an amorphous alloy is studied using a combination of molecular dynamic simulations and normal mode analysis of the nonuniform distribution of Debye-Waller factors. The increased mobility at the surface is found to be associated with the appearance of Arr...

Physical Review Letters

Elastic moduli of a Brownian colloidal glass former

S Fritschi and M Fuchs

The static, dynamic and flow-dependent shear moduli of a binary mixture of Brownian hard disks are studied by an event-driven molecular dynamics simulation. Thereby, the emergence of rigidity close to the glass transition encoded in the static shear modulus {$G_\infty$} is accessed by three methods. Results from shear stress auto-correlation functions, elastic dispersion relations, and the elastic response to strain deformations upon the start-up of shear flow are compared. This enables one to sample the time-dependent shear modulus {$G(t)$} consistently over several decades in time. By that a very precise specification of the glass transition point and of {$G_\infty$} is feasible. Pr...

Journal of Physics Condensed Matter

Surface development of a brazing alloy during heat treatment–a comparison between UHV and APXPS

L Rullik, N Johansson, F Bertram, J Evertsson, T Stenqvist and E Lundgren

In an attempt to bridge the pressure gap, APXPS was used to follow the surface development of an aluminum brazing sheet during heating in an ambient oxygen-pressure mimicking the environment of an industrial brazing furnace. The studied aluminum alloy brazing sheet is a composite material consisting of two aluminum alloy standards whose surface is covered with a native aluminum oxide film. To emphasize the necessity of studies of this system in ambient sample environments it is compared to measurements in UHV. Changes in thickness and composition of the surface oxide were followed after heating to 300 °C, 400 °C, and 500 °C. The two sets presented in this paper show that the surface development strongly depends on the environment the sample is heated in.

Journal of Physics Condensed Matter

Synthesis, structure and magnetism of the new S   =  1 kagome magnet NH 4 Ni 2.5 V 2 O 7 (OH) 2 ⋅H 2

E T Connolly, P Reeves, D Boldrin and A S Wills

Kagome antiferromagnets (KAFMs) have long been known to host exotic electronic states due to their strong geometric frustration, including the quantum spin liquid state in {$S=\frac{1}{2}$} systems. Away from that limit, S   =  1 KAFMs are also predicted to host unconventional ground states such as spin nematic phases, but a paucity of studies on known model materials has restricted progress. Here, we present the crystal structure and preliminary magnetization measurements on the newly synthesized S   =  1 KAFM, NH 4 Ni 2.5 V 2 O 7 (OH) {$_2\cdot$} H 2 O, which has the three-fold symmetry of the kagome lattice but significant site depletion, with  ∼ {$77\%$}

Journal of Physics Condensed Matter

Temperature dependent infrared nano-imaging of La 0.67 Sr 0.33 MnO 3 thin film

Peng Xu, T J Huffman, In Hae Kwak, Amlan Biswas and M M Qazilbash

We investigate the temperature dependence of infrared properties at nanometer length scales in La 0.67 Sr 0.33 MnO 3 (LSMO) thin film with a thickness of 47 unit cells grown on SrTiO 3 substrate. The infrared nano-imaging experiments were performed using a near-field optical microscope in conjunction with a variable temperature heating stage. The near-field infrared data is consistent with the bulk of the LSMO film undergoing the thermally-driven non-percolative second-order transition from a metallic, ferromagnetic phase to an insulating, paramagnetic phase. We find persistent infrared contrast on the nanoscale that is independent of temperature and which we attribute to two novel phases with different conductivities coexisting in the vicinity of the film-substrate interface. These two coexisting phases at the film-substrate interface do not undergo the metal–insulator transition (MIT) and hence are different from the metallic, ferromagneti...

Journal of Physics Condensed Matter

Sat Dec 9 2017

Atomic and electronic basis for solutes strengthened (010) anti-phase boundary of L12 Co3(Al, TM): A comprehensive first-principles study

Author(s): William Yi Wang, Fei Xue, Ying Zhang, Shun-Li Shang, Yi Wang, Kristopher A. Darling, Laszlo J. Kecskes, Jinshan Li, Xidong Hui, Qiang Feng, Zi-Kui Liu

The crystallographic and electronic structures of (010) APB of L12 Co3Al0.75TM0.25 are studied by high-resolution transmission electron microscopy and first-principles calculations. Effects of solute atoms (TM = Cr, Hf, Mo, Ni, Re, Ru, Ta, Ti, W and Y) on the formation energy, lattice parameters/distortion, magnetism, and bonding strength of the (010) APB in Co3Al0.75TM0.25 are obtained from first-principles calculations. Comparing to the equilibrium volume of Co3Al, it is found that the volume change of the Co3Al0.75TM0.25 with and without the presence of APB increases linearly with the volume of the corresponding FCC elements, indicating the contribution of the solute atoms on lattice distortion of bulk and (010) APB. Particularly, the strong dependence of the APB energy on the composition is comprehensively discussed together with the available experimental and theoretical data in the literature. The negative (010) APB energy indicates that the formation of (010) APB could stabilize the ordered L12 (or the FCC-lattice) Co3Al, and the local L12 → D022 phase transformation can occur. The physical natures of lattice distortions caused by the fault layers of APB and the solute atoms are characterized by bonding charge density. It is found that the solute atoms, occupying Al site of L12 phase and its (010) APB, increase the local bonding strength along (010) through the electron redistribution during forming the chemical bonds with Co, revealing an intrinsic solid-solution strengthening mechanism. This work provides an insight into the atomic and electronic basis for solid-solution strengthening mechanism of L12 Co3Al0.75TM0.25.

Acta Materialia

Prediction of ceramic fracture with normal distribution pertinent to grain size

Author(s): Chunguo Zhang, Xiaozhi Hu, Tim Sercombe, Qingbin Li, Zhimin Wu, Pengmin Lu

Fracture of brittle ceramics initiated from shallow surface cracks comparable to their average grain sizes (G) can fluctuate significantly. Such fluctuations can contain crucial information on the inherent relations between the average grain size G and bulk ceramic properties such as the tensile strength f t and fracture toughness K IC. It was proposed in this study that the characteristic crack a*ch = 0.25(K IC/f t)2 = constant × G, inspired by observations of strength distributions with different a*ch/G ratios. It was found that normal distributions with the smallest standard deviation exist around a*ch = (2.5–3.5) × G, based on quasi-brittle fracture results of four different ceramics with G from 2 to 20 μm and shallow surface cracks from 100 nm to 650 μm. Using the average value of the relative characteristic crack a*ch/G ≈ 3, the mean and standard deviation (σ) were determined by normal distributions for both the tensile strength ft and fracture toughness K IC. Quasi-brittle fracture of those fine-grained ceramics based on the mean values and standard deviations was thus predicted. The upper and lower bounds with 96% reliability (±2σ) specified by the normal distributions covered nearly all experimental data ranging from the strength-controlled to toughness-controlled asymptotic limits, and quasi-brittle fracture between the two. With the knowledge of the average grain size G, the tensile strength f t becomes the sole parameter required to describe the entire fracture range.

Acta Materialia

The Third Law of Thermodynamics: Phase equilibria and phase diagrams at low temperatures

Author(s): David E. Laughlin, William A. Soffa

Great progress has been made over the recent decades in the application of computational thermodynamics (Calphad) and theoretical methodologies (CVM) including so-called first principles approaches to modeling thermodynamic properties and the calculation of phase diagrams of materials. The aim of this paper is to call attention to considerations of the THIRD LAW OF THERMODYNAMICS when evaluating these results when applied to low temperature phase equilibria. In this effort we call attention to the essential content of the modern version of this third principle of thermodynamics using an historical and pedagogical approach. An appreciation of the constraints of the THIRD LAW is shown to be valuable in projecting possible low temperature phase fields and boundaries and predicting thermodynamically consistent phase diagram configurations as T→0 K. The ideas of Simon regarding aspects or subsystems are shown to be of paramount importance in assessing the thermodynamic properties of materials at low temperatures.

Acta Materialia

Microstructural effects on effective piezoelectric responses of textured PMN-PT ceramics

Author(s): Chen Ming, Tiannan Yang, Kun Luan, Lei Chen, Liang Wang, Jiangtao Zeng, Yongxiang Li, Wenqing Zhang, Long-Qing Chen

The effective piezoelectric properties of [001]c fiber textured Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ceramics were studied by phase-field modeling. The effects of microstructures such as texture, grain shape, grain boundaries, residual pores and heterogeneous growth templates were investigated. It was found that the degree of texture plays a dominant role in determining the properties. The pores, heterogeneous templates and grain boundaries reduce the properties significantly at high degrees of texture with the effect diminishing at decreasing degrees of texture. The presence of heterogeneous templates leads to a more significant reduction in the properties than pores although the piezoelectric coefficients of pores are zero. The shape of grains has a weak effect at all degrees of texture. By utilizing the experimentally measured microstructural parameters in the calculations and comparing the computed properties with the corresponding measurements, we showed that the low performance of sintered textured PMN-PT ceramics ( d 33 ∼1000 pC/N) relative to single crystals ( d 33 ∼2800 pC/N) is mainly due to the insufficiently high degree of texture even with Lotgering factors up to 0.9, while the influences of other microstructures are weak.

Acta Materialia

Composition and automated crystal orientation mapping of rapid solidification products in hypoeutectic Al-4 at.%Cu alloys

Author(s): K.W. Zweiacker, Can Liu, M.A. Gordillo, J.T. McKeown, G.H. Campbell, J.M.K. Wiezorek

Rapid solidification can produce metastable phases and unusual microstructure modifications in multi-component alloys during additive manufacturing or laser beam welding. Composition and phase mapping by transmission electron microscopy have been used here to characterize the morphologically distinct zones developing in hypoeutectic Al-4 at.% Cu alloy after pulsed laser melting for different crystal growth rate regimes. Deviations of the compositions of the alloy phases from equilibrium predictions and unique orientation relationships between the solidification transformation products have been determined. Specifically, for the columnar growth zone at solidification rates of 0.8 m s 1 < v < v a = 1.8 m s 1 , two distinct orientation relationships were established between the concomitantly forming non-equilibrium phases, supersaturated α-Al solid solution and the discontinuously distributed α-Al2Cu-based θ′-phase, which can be described as {110}θ {001}α, [001]θ [110]α and {001}θ {001}α, [100]θ [100]α. These orientation relationships permit formation of coherent interphase interfaces with low interfacial free energy. This endows a kinetic advantage to the thermodynamically less stable θ′-Al2Cu phase relative to the more stable equilibrium θ-Al2Cu phase during formation of the morphologically modified eutectic of the columnar growth zone grains, since repeated nucleation is required to establish the discontinuous distribution of θ′-Al2Cu phase.

Acta Materialia

Investigating nano-precipitation in a V-containing HSLA steel using small angle neutron scattering

Author(s): Y.Q. Wang, S.J. Clark, V. Janik, R.K. Heenan, D. Alba Venero, K. Yan, D.G. McCartney, S. Sridhar, P.D. Lee

Interphase precipitation (IPP) of nanoscale carbides in a vanadium-containing high-strength low-alloy steel has been investigated. Small angle neutron scattering (SANS) and transmission electron microscopy (TEM) were employed to characterize the precipitates and their size distributions in Fe-0.047C-0.2V-1.6Mn (in wt.%) alloy samples which had been austenitized, isothermally transformed at 700 °C for between 3 and 600 min and water quenched. TEM confirms that, following heat treatment, rows of vanadium-containing nanoscale interphase precipitates were present. Model-independent analysis of the nuclear SANS signal and model fitting calculations, using oblate spheroid and disc-shapes, were performed. The major axis diameter increased from 18 nm after 3 min to 35 nm after 600 min. Precipitate volume percent increased from 0.09 to 0.22 vol% over the same period and number density fell from 2 × 1021 to 5 × 1020 m−3. A limited number of measurements of precipitate maximum diameters from TEM images showed the mean value increased from 8 nm after 5 min to 28 nm after 600 min which is in reasonable agreement with the SANS data.

Acta Materialia

Ab initio dynamical stability of tungsten at high pressures and high temperatures

Author(s): Huai-Yong Zhang, Zhen-Wei Niu, Ling-Cang Cai, Xiang-Rong Chen, Feng Xi

Considering the phonon-phonon interactions, the temperature-dependent phonon dispersion curves of bcc and fcc W are derived at high pressures. Both the fcc and bcc phases are dynamically stable in a wide range of pressures and temperatures. The bcc-fcc phase boundary is obtained by comparing the Gibbs free energy in their dynamically stable regions and the bcc-fcc phase boundary is located above 1200 GPa in the temperature range 300–8000 K.

Computational Materials Science

Structural transformations in $(1\text{−}x){\mathbf{Na}}_{0.5}{\mathbf{Bi}}_{0.5}{\mathbf{TiO}}_{3}\text{−}x{\mathbf{BaTiO}}_{3}$ single crystals studied by Raman spectroscopy

Author(s): G. de la Flor, T. Malcherek, S. Gorfman, and B. Mihailova

Hard-mode Raman spectroscopy was applied to analyze the temperature-induced transformation processes in perovskite-type $({\mathrm{ABO}}_{3})$ single crystals of $(1\text{−}x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}\text{−}x{\mathrm{BaTiO}}_{3}$ (NBT-$x\mathrm{BT})$ in a wide tempera...

Physical Review B

Atomic structure of a decagonal Al-Pd-Mn phase

Author(s): Marek Mihalkovič, Johannes Roth, and Hans-Rainer Trebin

We present a detailed structure solution for the $16\text{−}Å$ decagonal quasicrystal in the Al-Pd-Mn system by means of cluster decoration and ab initio energy minimization. It is based on structure models of the $ɛ$ and other approximant phases. The $ɛ$ phases can be represented as subsets of a he...

Physical Review B

Thermal annealing and pressure effects on BaFe 2− x Co x As 2 single crystals

Dongwon Shin, Soon-Gil Jung, G Prathiba, Soonbeom Seo, Ki-Young Choi, Kee Hoon Kim and Tuson Park

We investigate the pressure and thermal annealing effects on BaFe 2− x Co x As 2 (Co-Ba122) single crystals with x   =  0.1 and 0.17 via electrical transport measurements. The thermal annealing treatment not only enhances the superconducting transition temperature ( T c ) from 9.6 to 12.7 K for x   =  0.1 and from 18.1 to 21.0 K for x   =  0.17, but also increases the antiferromagnetic transition temperature ( T N ). Simultaneous enhancement of T c and T N by the thermal annealing treatment indicates that thermal annealing could substantially improve the quality of the Co-doped Ba122 samples. Interestingly, T c of the Co-Ba122 compounds shows a scaling behavior with a linear dependence on the resistivity value at 290 K, irrespective of tuning parameters such as chemical doping, pressure, and thermal annealing. These results not only ...

Journal of Physics Condensed Matter

Varieties of charge distributions in coat proteins of ssRNA+  viruses

Anže Lošdorfer Božič and Rudolf Podgornik

A major part of the interactions involved in the assembly and stability of icosahedral, positive-sense single-stranded RNA (ssRNA+) viruses is electrostatic in nature , as can be inferred from the strong pH - and salt-dependence of their assembly phase diagrams. Electrostatic interactions do not act only between the capsid coat proteins (CPs), but just as often provide a significant contribution to the interactions of the CPs with the genomic RNA, mediated to a large extent by positively charged, flexible N-terminal tails of the CPs. In this work, we provide two clear and complementary definitions of an N-terminal tail of a protein, and use them to extract the tail sequences of a large number of CPs of ssRNA+  viruses. We examine the pH -dependent interplay of charge on both tails and CPs alike, and show that—in contrast to the charge on the CPs—the net positive charge on the N-tails persists even to very basic pH values. In addition, we note a limit to ...

Journal of Physics Condensed Matter

Collapse in two good solvents, swelling in two poor solvents: defying the laws of polymer

Debashish Mukherji, Carlos M Marques and Kurt Kremer

In this work we discuss two mirror but distinct phenomena of polymer paradoxical properties in mixed solvents: co-non-solvency and co-solvency. When a polymer collapses in a mixture of two miscible good solvents the phenomenon is known as co-non-solvency, while co-solvency is a phenomenon that is associated with the swelling of a polymer in poor solvent mixtures. A typical example of co-non-solvency is provided by poly( N -isopropylacrylamide) in aqueous alcohol, while poly(methyl methacrylate) in aqueous alcohol shows co-solvency. We discuss these two phenomena to compare their microscopic origins and show that both can be understood within generic universal concepts. A broad range of polymers is therefore expected to exhibit these phenomena where specific chemical details play a lesser role than the appropriate combination of interactions between the trio of molecular components.

Journal of Physics Condensed Matter

Cascaded longitudinal stimulated Raman scattering and the frequency doubling process of potassium

Zhixin Wu, Zhengping Wang, Hongkai Ren, Hongwei Qi, Lisong Zhang, Ying Zhou, Qingtian Gu, Xun Sun, Dawei Hu and Xinguang Xu

Cascaded longitudinal stimulated Raman scattering (LSRS) and the frequency doubling process are reported for the first time. When potassium dihydrogen phosphate (KDP) crystals are used as the type I and type II frequency doublers of picosecond, focused 1064 nm laser pulses, strong LSRS effects are observed. Three new laser spectrum lines appeared successively, i.e. at 558.9, 588.9 and 622.1 nm, and were excited by the frequency doubling laser at 532 nm. Second- and third-order nonlinear optical frequency conversions were achieved in a single KDP crystal. The near-infrared light was thus converted into a new spectra laser spanning the green-to-red spectral range.

Journal of Physics Condensed Matter

Retardation effects on the dispersion and propagation of plasmons in metallic nanoparticle chains

Charles A Downing, Eros Mariani and Guillaume Weick

We consider a chain of regularly-spaced spherical metallic nanoparticles, where each particle supports three degenerate localized surface plasmons. Due to the dipolar interaction between the nanoparticles, the localized plasmons couple to form extended collective modes. Using an open quantum system approach in which the collective plasmons are interacting with vacuum electromagnetic modes and which, importantly, readily incorporates retardation via the light-matter coupling, we analytically evaluate the resulting radiative frequency shifts of the plasmonic bandstructure. For subwavelength-sized nanoparticles, our analytical treatment provides an excellent quantitative agreement with the results stemming from laborious numerical calculations based on fully-retarded solutions to Maxwell’s equations. Indeed, the explicit expressions for the plasmonic spectrum which we provide showcase how including retardation gives rise to a logarithmic singularity in the bandstructure of transver...

Journal of Physics Condensed Matter

Fri Dec 8 2017

Metastable phase transformation and deformation twinning induced hardening-stiffening mechanism in compression of silicon nanoparticles

Author(s): Yu Hong, Ning Zhang, Mohsen Asle Zaeem

The compressive mechanical responses of silicon nanoparticles with respect to crystallographic orientations are investigated by atomistic simulations. Superelastic and abrupt hardening-stiffening behaviors are revealed in [110]-, [111]- and [112]-oriented nanoparticles. The obtained hardness values of these particles are in good agreement with the experimental results. In particular, [111]-oriented particle is extremely hard since its hardness (∼33.7 GPa) is almost three times greater than that of the bulk silicon (∼12 GPa). To understand the underlying deformation mechanisms, metastable phase transformation is detected in these particles. Deformation twinning of the metastable phase accounts for the early hardening-stiffening behavior observed in [110]-oriented particle. The twin phase then coalescences and undergoes compression to resist further deformation, and leads to the subsequent re-hardening and re-stiffening events. The same metastable phase is also detected to form in [111]- and [112]-oriented particles. The compression of such metastable phase is responsible for their hardening-stiffening behavior. In contrast, the crystal lattice of diamond cubic silicon is merely elastically deformed when compressing along [100] direction. Throughout the simulations, no perfect tetragonal β-tin silicon phase formed due to the deconfinement status of nanoparticle comparing to the bulk silicon. A size effect on hardness of silicon nanoparticles, i.e., “smaller is harder”, is also revealed.

Acta Materialia

Sample-size-dependent surface dislocation nucleation in nanoscale crystals

Author(s): Qing-Jie Li, Bin Xu, Shotaro Hara, Ju Li, Evan Ma

The finite-temperature mechanical strength of nanoscale pristine metals at laboratory strain rates may be controlled by surface dislocation nucleation, which was hypothesized to be only weakly dependent on the sample size. Previous studies on surface dislocation nucleation investigated factors such as surface steps, oxidation layers and surface diffusion, while the role of surface stresses and sample size remains unclear. Here we perform systematic atomistic calculations on the activation free energy barriers of surface dislocation nucleation in sub-50 nm nanowires. The results demonstrate that surface stresses significantly influence the activation processes of surface dislocation nucleation. This renders the strength strongly dependent on sample size; whether it is “smaller is stronger” or “smaller is weaker” depends on the combined effects of surface stress and applied axial stress, which can be universally explained in terms of the local maximum resolved shear stress. A linear relation between the activation entropy and activation enthalpy (Meyer-Neldel compensation rule) was found to work well across a range of stresses and sample sizes.

Acta Materialia

Dose rate dependence of Cr precipitation in an ion-irradiated Fe18Cr alloy

Author(s): Elaina R. Reese, Nathan Almirall, Takuya Yamamoto, Scott Tumey, G. Robert Odette, Emmanuelle A. Marquis

Precipitation of α′ in FeCr alloys under neutron irradiation is thermodynamically driven while being accelerated by radiation-enhanced diffusion. However, similar alloys under ion irradiation at high dose rates (>104 dpa/s) fail to exhibit α′ precipitation. Here, the microstructure of an Fe18Cr alloy under ion or neutron-irradiation at 300°C at dose rates from ~107 to 104 dpa/s was analyzed by atom probe tomography. The steady-state composition content of the clusters depends on the ion irradiation dose and dose rate, confirming the contribution of ballistic mixing in diluting the Cr concentration in non-equilibrium α′ precipitates.

Scripta Materialia

On the kinetic and equilibrium shapes of icosahedral Al71Pd19Mn10 quasicrystals

Author(s): Nancy Senabulya, Xianghui Xiao, Insung Han, Ashwin J. Shahani

The dynamics of growth and relaxation of icosahedral single quasicrystals in a liquid phase were investigated using in situ synchrotron-based X-ray tomography. Our 4D studies (i.e., space- and time-resolved) provide direct evidence that indicates the growth process of an Al71Pd19Mn10 quasicrystal is governed predominantly by bulk transport rather than attachment kinetics. This work is in agreement with theoretical predictions, which show that the pentagonal dodecahedron is not the minimum energy structure in Al-Pd-Mn icosahedral quasicrystals, but merely a growth shape characterized by non-zero anisotropic velocity. This transient shape transforms into a truncated dodecahedral Archimedian polyhedron once equilibrium has been attained.

Scripta Materialia

Significant coercivity enhancement of hot deformed NdFeB magnets by doping Ce-containing (PrNdCe)70Cu30 alloys powders

Author(s): H.W. Chang, Y.I. Lee, P.H. Liao, W.C. Chang

Coercivity enhancement of hot-deformed NdFeB magnets made from commercially available NdFeB powders doped with Ce-containing alloys is demonstrated. By doping (Pr71Nd27Ce2)70Cu30 powders, where Pr71Nd27Ce2 is a commercial mischmetal, the hot-deformed magnets exhibit significant enhancement of coercivity from 15.0kOe to 19.0kOe, and it can reach 20.1kOe by doping higher Ce-content (Pr71Nd9Ce20)70Cu30 powders, yet sustains high energy product. The magnetic isolation effect with Ce-containing phase in the grain boundary and the microstructure refinement lead to coercivity enhancement. This study provides an economic way to enhance coercivity of hot deformed NdFeB magnets without using heavy rare earth.

Scripta Materialia

A spatial decomposition parallel algorithm for a concurrent atomistic-continuum simulator and its preliminary applications

Author(s): Hao Chen, Shuozhi Xu, Weixuan Li, Rigelesaiyin Ji, Thanh Phan, Liming Xiong

This paper presents the development of a spatial decomposition parallel algorithm and its implementation into a concurrent atomistic-continuum (CAC) method simulator for multiscale modeling of dislocations in metallic materials. The scalability and parallel efficiency of the parallelized CAC are tested using up to 512 processors. With a modest computational resource, a single crystalline f.c.c. sample containing 10.6 billion atoms is modeled using only 4 , 809 , 108 finite elements in a CAC model at a fraction of the cost of full molecular dynamics (MD). The simulation demonstrates a nearly ideal scalability of the newly parallelized CAC simulator. The parallel efficiency of the newly parallelized CAC is shown to be higher than 90 % when using 512 processors in the high performance computing cluster at Iowa State University. This parallel efficiency is comparable to the state-of-the-art atomistic simulator. Moreover, the newly parallelized CAC simulator employing a uniform coarse mesh is capable of capturing important atomistic features of dislocations, including dislocation nucleation, migration, stacking faults as well as the formation of Lomer-Cottrell locks, in a billion-atom system. The spatial decomposition-based parallelization algorithm developed in this work is general and can be transferable to many other existing concurrent multiscale simulation tools.

Computational Materials Science

Generalization of the Fourier-spectral Eyre scheme for the phase-field equations: Application to self-assembly dynamics in materials

Author(s): G. Demange, M. Chamaillard, H. Zapolsky, M. Lavrskyi, A. Vaugeois, L. Lunéville, D. Simeone, R. Patte

Self-assembly is one of the most promising ways to develop novel materials with high performance. Thanks to the flexibility in treating of the topological changes in systems, the phase-field approach has emerged as a method of choice to study this phenomenon at mesoscale. In recent years, the phase-field equations have also been upgraded to incorporate the atomic scale effects. In this work, we propose an efficient numerical method based on the generalization of the Fourier-spectral Eyre’s scheme, to simulate the dynamics of self-assembly, using the phase-field model at different time and length scales. To show its versatility, the method is explicitly implemented, and numerically tested, for three phase-field models describing patterned structure in systems: the modified Cahn-Hilliard equation for irradiation induced patterned microstructures at mesoscale, and the Phase-field Crystal and Continuous Atomic Density Functional methods, to study the formation of complex crystal structures at atomic scale.

Computational Materials Science

Strengthening effects of alloying elements W and Re on Ni3Al: A first-principles study

Author(s): Wei Gong, Wenyue Zhao, Naihua Miao, Jian Zhou, Zhimei Sun, Shusuo Li, Shengkai Gong

The structural stabilities, mechanical and thermodynamic properties of pure and alloyed Ni3Al systems have been investigated systematically, based on density functional theory and Debye-Grüneisen model. Close attentions are paid to alloying elements W and Re by studying the strengthening effects of single-alloying element W, Re, Mo, Ta, Ru and co-alloying elements WRe, WMo, ReMo, WTa, ReTa, WRu, ReRu, which substitute the Al site and the nearest neighboring Al-Al sites, respectively. The calculated formation energies and elastic constants show that all the alloyed Ni3Al are thermodynamically and mechanically stable. It is found that alloying elements W has similar effects on the mechanical and thermodynamic properties of Ni3Al to those of Re, which suggests a possibility of replacing Re with W in Ni-based single crystal superalloys. In addition, co-alloyings with different elements have no distinct synergistic enhancement but simple combined effect on the mechanical properties of Ni3Al. The enhanced chemical bondings between the alloying atoms W/Re and the neighboring host atoms Ni are considered to be the main strengthening mechanisms in W/Re alloyed Ni3Al systems.

Computational Materials Science

Hot deformation characteristics and microstructure evolution of Hastelloy C-276

Author(s): Nitesh Raj Jaladurgam, Anand K. Kanjarla

High temperature deformation characteristics of Hastelloy C-276 was investigated using compression tests at elevated temperature ranging from 900°C to 1200°C and strain rates covering quasi-static to quasi-dynamic regions (0.001s−1-10s−1) to a final true strain 0.69 (50% Engineering strain). The flow curves at all strain rates and high temperatures (1100°C, 1200°C) exhibit a peak stress, confirming the dynamic recrystallization (DRX) phenomena. To predict and establish the safe hot workability window, processing maps were evaluated based on the dynamic material model and plotted for strain of 0.65. Based on the detailed microstructure analysis and the identification of the active softening mechanisms, the optimal hot workability window was found to be 1200°C and 0.001s−1. Bulk texture analysis revealed presence of fiber textures < 011 > and < 001 > parallel to deformation axis (ND), which were prevalent at high temperature and low strain rate conditions. Using Arrhenius model, the apparent hot working activation energy ( Q HW ) was determined as 474kJ/mol. Zener-Holloman parameter was calculated to gauge the deformation resistance and found to have a linear relationship with peak flow stress at given deformation conditions.

Science and Engineering A

Ductility and formability of three high-Mn TWIP steels in quasi-static and high-speed tensile and Erichsen tests

Author(s): A.S. Hamada, A. Kisko, A. Khosravifard, M.A. Hassan, L.P. Karjalainen, D. Porter

The ductility and formability properties of three high-Mn TWIP steels were investigated under quasi-static and high-speed deformation conditions. The ductility was evaluated from conventional and Hopkinson split-bar tensile tests at 1250s−1 and the stretch formability was evaluated using Erichsen tests made with a special high-speed electro-hydraulic forming machine at about 1000s−1. The data were related to microstructural features revealed using electron backscatter diffraction and X-ray diffraction. Furthermore, the stacking fault energy (SFE) was estimated using a thermodynamic approach. It was found that the 0.6C-22Mn and 0.2C-21Mn-0.23N steels (compositions in wt%) with SFEs of 23–24mJ/m2 exhibited good elongation and a large Erichsen index at both low and high strain rates. These were attributed to intensive mechanical twinning though partly replaced by dislocation slip in deformation bands in the high-speed tests. However, it was noticed that the high-speed stretching failure of these TWIP steels occurred in the uniform elongation range without diffuse necking. In the austenitic - ferritic 21Mn-3Al-3Si steel strain-induced martensite was formed, but the ferrite phase seemed to impair formability.

Science and Engineering A

Influence of extruding temperature and heat treatment process on microstructure and mechanical properties of three structures containing Mg-Li alloy bars

Author(s): Yan Tang, Qichi Le, R.D.K. Misra, Guanqiao Su, Jianzhong Cui

The Mg-Li alloy bars with density of 1.38g/cm3 as well as ultimate tensile strength of 329.2MPa were fabricated. Simultaneously, an industrial process with convenience and low cost was developed. In the cast alloys, hcp-Mg was transformed gradually into bcc-Li with increasing Li content. During extruding at different temperatures, the dynamic recrystallization (DRX) in Mg-8Li-5Al-2Zn-0.5Y was weaker than Mg-5Li-5Al-2Zn-0.5Y and Mg-11.4Li-5Al-2Zn-0.5Y due to the mutual limitation between hcp-Mg (α) and bcc-Li (β). In the annealing process, abnormal grain growth in Mg-11.4Li-5Al-2Zn-0.5Y was found at 573K, and a kind of regular dendritic structure was discovered in the coarse grains at 593K. Additionally, a sort of solid solution strengthening behavior in bcc-Li was observed and enhanced remarkably the alloy strength (increased by ~160MPa in maximum). Through controlling cooling velocity, a precipitation behavior could be achieved in the alloys. Through mathematical modeling with high accuracy, the relationships among microstructural evolution and alloy properties in the solid solution and precipitation processes were quantitatively analyzed and determined.

Science and Engineering A

Emergence of Non-Abelian Magnetic Monopoles in a Quantum Impurity Problem

Author(s): E. Yakaboylu, A. Deuchert, and M. Lemeshko

Recently, it was shown that molecules rotating in superfluid helium can be described in terms of the angulon quasiparticles [Phys. Rev. Lett. 118, 095301 (2017)]. Here, we demonstrate that in the experimentally realized regime the angulon can be seen as a point charge on a two-sphere interacting wit...

Physical Review Letters

Domain-size effects on the dynamics of a charge density wave in $1\mathrm{T}\text{−}{\mathrm{TaS}}_{2}$

Author(s): G. Lantz, C. Laulhé, S. Ravy, M. Kubli, M. Savoini, K. Tasca, E. Abreu, V. Esposito, M. Porer, A. Ciavardini, L. Cario, J. Rittmann, P. Beaud, and S. L. Johnson

The phase diagram of 1T-TaS2 involves many different solid-state phases, including several different kinds of charge-density-wave (CDW) phase both commensurate and incommensurate with the underlying lattice. Recent experiments have shown that it is possible to drive transitions between these phases with intense femtosecond pulses of light. This work focuses on the light-driven transition from the nearly commensurate CDW phase to the incommensurate phase, using time-resolved x-ray diffraction. The authors find that the new incommensurate domains grow in a self-similar way. They also employ a double-pump method to study the dynamics and stability of the novel incommensurate phase during growth.

Physical Review B

Angular-dependent magnetoresistance study in Ca 0.73 La 0.27 FeAs 2 : a ‘ parent ’ compound of

Xiangzhuo Xing, Chunqiang Xu, Zhanfeng Li, Jiajia Feng, Nan Zhou, Yufeng Zhang, Yue Sun, Wei Zhou, Xiaofeng Xu and Zhixiang Shi

We report a study of angular-dependent magnetoresistance (AMR) with the magnetic field rotated in the plane perpendicular to the current on a Ca 0.73 La 0.27 FeAs 2 single crystal, which is regarded as a ‘ parent ’ compound of 112-type iron pnictide superconductors. A pronounced AMR with twofold symmetry is observed, signifying the highly anisotropic Fermi surface. By further analyzing the AMR data, we find that the Fermi surface above the structural/antiferromagnetic (AFM) transition ( T s / T N ) is quasi-two-dimensional (quasi-2D), as revealed by the 2D scaling behavior of the AMR, Δ ρ / ρ (0) ( H , θ )  =  Δ ρ / ρ (0) ( µ 0 H cos θ ), θ being the magnetic field angle with respect to the c axis. While such 2D scaling becomes invalid at temperatures below T s / T N , the three-dimensional (3D) scaling approac...

Journal of Physics Condensed Matter

Thermodynamics of two-impurity Anderson model with Dzyaloshinskii–Moriya interaction

Liang Chen, Bo Hu and Rong-Sheng Han

In this work, we use the numerical renormalization group (NRG) theory to study the thermodynamics of the two-impurity Anderson model. Two different methods are used to estimate the effect of Dzyaloshiskii–Moriya (DM) interaction on the variation of the Kondo temperature. When the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction is vanishing, the two different estimations give different tendencies. If we use the peak of the specific heat to identify the variation of the Kondo temperature versus the DM interaction, we get an almost linear function. However, if we use the low temperature universal curve of the impurity entropy, we get a quadratic function. These results indicate that previous debates about the influence of spin–orbit coupling on the Kondo temperature may come from the different definitions of the Kondo temperature. When the RKKY interaction is ferromagnetic, there are two stages of Kondo screening. Both estimations demonstrate that the second stage of the Kondo tem...

Journal of Physics Condensed Matter

Evidence for confinement induced phase separation in ethanol–water mixture: a positron annihilation

T Muthulakshmi, D Dutta, Priya Maheshwari and P K Pujari

We report an experimental evidence for the phase separation of ethanol–water mixture confined in mesoporous silica with different pore size using positron annihilation lifetime spectroscopy (PALS). A bulk-like liquid in the core of the pore and a distinct interfacial region near the pore surface have been identified based on ortho -positronium lifetime components. The lifetime corresponding to the core liquid shows similar behavior to the bulk liquid mixture while the interfacial lifetime shows an abrupt rise within a particular range of ethanol concentration depending on the pore size. This abrupt increase is attributed to the appearance of excess free-volume near the interfacial region. The excess free-volume is originated due to microphase separation of confined ethanol–water primarily at the vicinity of the pore wall. We envisage that probing free-volume changes at the interface using PALS is a sensitive way to investigate microphase separation under nanoconfinement.

Journal of Physics Condensed Matter

Dark field differential dynamic microscopy enables accurate characterization of the

Roberto Cerbino, Davide Piotti, Marco Buscaglia and Fabio Giavazzi

Micro- and nanoscale objects with anisotropic shape are key components of a variety of biological systems and inert complex materials, and represent fundamental building blocks of novel self-assembly strategies. The time scale of their thermal motion is set by their translational and rotational diffusion coefficients, whose measurement may become difficult for relatively large particles with small optical contrast. Here we show that dark field differential dynamic microscopy is the ideal tool for probing the roto-translational Brownian motion of anisotropic shaped particles. We demonstrate our approach by successful application to aqueous dispersions of non-motile bacteria and of colloidal aggregates of spherical particles.

Journal of Physics Condensed Matter

Negative differential mobility and trapping in active matter systems

C Reichhardt and C J O Reichhardt

Using simulations, we examine the average velocity as a function of applied drift force for active matter particles moving through a random obstacle array. We find that for low drift force, there is an initial flow regime where the mobility increases linearly with drive, while for higher drift forces a regime of negative differential mobility appears in which the velocity decreases with increasing drive due to the trapping of active particles behind obstacles. A fully clogged regime exists at very high drift forces when all the particles are permanently trapped behind obstacles. We find for increasing activity that the overall mobility is nonmonotonic, with an enhancement of the mobility for small levels of activity and a decrease in mobility for large activity levels. We show how these effects evolve as a function of disk and obstacle density, active run length, drift force, and motor force.

Journal of Physics Condensed Matter

Possible half-metallicity and variable range hopping transport in Sb-substituted Fe 2 TiSn Heusler

S Chaudhuri, P A Bhobe and A K Nigam

The investigation of the magnetotransport properties on {${\rm Fe}_{2}$} {${\rm TiSn}_{1-x}$} Sb x with 0 {$\leqslant x \leqslant$} 0.6 are presented in this paper. The substitution of Sb in place of Sn decreases the anti-site disorder as evident from x-ray diffraction patterns as well as from transport properties measurement. The much-disputed upturn in low temperature electrical resistivity of {${\rm Fe}_{2}$} TiSn has been demonstrated to be a result of weak localization induced by anti-site disorder. With increased Sb substitution (⩾25%) the metallic transport behav...

Journal of Physics Condensed Matter

Quantum modeling of ultrafast photoinduced charge separation

Carlo Andrea Rozzi, Filippo Troiani and Ivano Tavernelli

Phenomena involving electron transfer are ubiquitous in nature, photosynthesis and enzymes or protein activity being prominent examples. Their deep understanding thus represents a mandatory scientific goal. Moreover, controlling the separation of photogenerated charges is a crucial prerequisite in many applicative contexts, including quantum electronics, photo-electrochemical water splitting, photocatalytic dye degradation, and energy conversion. In particular, photoinduced charge separation is the pivotal step driving the storage of sun light into electrical or chemical energy. If properly mastered, these processes may also allow us to achieve a better command of information storage at the nanoscale, as required for the development of molecular electronics, optical switching, or quantum technologies, amongst others. In this Topical Review we survey recent progress in the understanding of ultrafast charge separation from photoexcited states. We report the state-of-the-art...

Journal of Physics Condensed Matter

Wed Dec 6 2017

Lattice-alignment mechanism of SiGe on Sapphire

Author(s): Hyun Jung Kim, Adam Duzik, Sang H. Choi

Heteroepitaxy of silicon germanium (SiGe) prepared on a sapphire substrate (Al2O3) requires scrupulous attention to growth conditions. Previous work was used a substrate temperature of 890°C to grow a SiGe (111) film on the trigonal sapphire (0001) substrate without twin defects. Although the growth conditions were effective for the formation of single crystal film, how the formation of SiGe at the interface of sapphire was not experimentally defined with the order of atomic arrangement. This work presents high resolution transmission electron microscope (TEM) images of the SiGe/Al2O3 interface to show the SiGe/Al2O3 interface bonding for heteroepitaxy mechanism. The first two monolayers of the SiGe are Si-rich as this match with the surface oxygen lattice of the Al2O3 substrate. After the Ge composition increases, the monolayer spacing also increased while maintaining the cubic crystal structure. These results highlight the importance of a cleanliness of sapphire substrate, the Al2O3 termination for SiGe growth, and the cubic structure deformation of SiGe for heteroepitaxy. From the essential understanding of the SiGe/Al2O3 interface and growth mechanism, both low temperature SiGe heteroepitaxy and the III-V or II-VI semiconductor epitaxy are possible.

Acta Materialia

Impact of morphology and phase composition on mechanical properties of α-structured (Cr,Al)2O3(Al,Cr,X)2O3 multilayers

Author(s): C.M. Koller, A. Kirnbauer, S. Kolozsvári, J. Ramm, P.H. Mayrhofer

The impact of thin Cr-rich (Cr,Al)2O3 layers on the stabilisation of Al-rich (Al,Cr)2O3, (Al,Cr,Fe)2O3, (Al,Cr,Si)2O3, and (Al,Cr,B)2O3 coatings in the hexagonal corundum-type (α) structure through a multilayer architecture was investigated. We show that even for typically fine-grained cubic-dominated Si- or B-alloyed Al-rich (Al,Cr)2O3 coatings, the α phase can be stabilised through a multilayer architecture with α-structured Cr-rich (Cr0.78Al0.22)2O3 seed layers. Using (Cr,Al)2O3/(Al,Cr,B)2O3 multilayers with larger bilayer periods we show that efficiency of such stabilisation depends on the bilayer period. However, the mechanical properties of our multilayers are next to the overall α phase content also significantly influenced by their growth morphology.

Scripta Materialia

Anisotropic Superattenuation of Capillary Waves on Driven Glass Interfaces

Author(s): Bruno Bresson, Coralie Brun, Xavier Buet, Yong Chen, Matteo Ciccotti, Jérôme Gâteau, Greg Jasion, Marco N. Petrovich, Francesco Poletti, David J. Richardson, Seyed Reza Sandoghchi, Gilles Tessier, Botond Tyukodi, and Damien Vandembroucq

Metrological atomic force microscopy measurements are performed on the silica glass interfaces of photonic band-gap fibers and hollow capillaries. The freezing of attenuated out-of-equilibrium capillary waves during the drawing process is shown to result in a reduced surface roughness. The roughness...

Physical Review Letters

Semiconductor of spinons: from Ising band insulator to orthogonal band insulator

T Farajollahpour and S A Jafari

We use the ionic Hubbard model to study the effects of strong correlations on a two-dimensional semiconductor. The spectral gap in the limit where on-site interactions are zero is set by the staggered ionic potential, while in the strong interaction limit it is set by the Hubbard U . Combining mean field solutions of the slave spin and slave rotor methods, we propose two interesting gapped phases in between: (i) the insulating phase before the Mott phase can be viewed as gapping a non-Fermi liquid state of spinons by the staggered ionic potential. The quasi-particles of underlying spinons are orthogonal to physical electrons, giving rise to the ‘ARPES-dark’ state where the ARPES gap will be larger than the optical and thermal gap. (ii) The Ising insulator corresponding to ordered phase of the Ising variable is characterized by single-particle excitations whose dispersion is controlled by Ising-like temperature and field dependences. The temperature can be conveniently empl...

Journal of Physics Condensed Matter

Anomalous water dynamics at surfaces and interfaces: synergistic effects of confinement and surface

Rajib Biswas and Biman Bagchi

In nature, water is often found in contact with surfaces that are extended on the scale of molecule size but small on a macroscopic scale. Examples include lipid bilayers and reverse micelles as well as biomolecules like proteins, DNA and zeolites, to name a few. While the presence of surfaces and interfaces interrupts the continuous hydrogen bond network of liquid water, confinement on a mesoscopic scale introduces new features. Even when extended on a molecular scale, natural and biological surfaces often have features (like charge, hydrophobicity) that vary on the scale of the molecular diameter of water. As a result, many new and exotic features, which are not seen in the bulk, appear in the dynamics of water close to the surface. These different behaviors bear the signature of both water–surface interactions and of confinement. In other words, the altered properties are the result of the synergistic effects of surface–water interactions and confinement. Ultrafast spectrosco...

Journal of Physics Condensed Matter

45° sign switching of effective exchange bias due to competing anisotropies in fully epitaxial Co 3

T Hajiri, T Yoshida, M Filianina, S Jaiswal, B Borie, H Asano, H Zabel and M Kläui

We report an unusual angular-dependent exchange bias effect in ferromagnet/antiferromagnet bilayers, where both ferromagnet and antiferromagnet are epitaxially grown. Numerical model calculations predict an approximately 45° period for the sign switching of the exchange-bias field, depending on the ratio between magnetocrystalline anisotropy and exchange-coupling constant. The switching of the sign is indicative of a competition between a fourfold magnetocrystalline anisotropy of the ferromagnet and a unidirectional anisotropy field of the exchange coupling. This predicted unusual angular-dependent exchange bias and its magnetization switching process are confirmed by measurements on fully epitaxial Co 3 FeN/MnN bilayers by longitudinal and transverse magneto-optic Kerr effect magnetometry. These results provide a deeper understanding of the exchange coupling phenomena in fully epitaxial bilayers with tailored materials and open up a complex switching energy landscape e...

Journal of Physics Condensed Matter

Tue Dec 5 2017

Sub-nanometre channels embedded in two-dimensional materials

Yimo Han, Ming-Yang Li, Gang-Seob Jung, Mark A. Marsalis, Zhao Qin, Markus J. Buehler, Lain-Jong Li & David A. Muller

Coherent 1D MoS2 channels, which are free of dangling bonds and dislocations, can be fabricated in monolayer WSe2, driven by dislocations at the interface of the two materials.

Nature

Topological order and thermal equilibrium in polariton condensates

Davide Caputo, Dario Ballarini, Galbadrakh Dagvadorj, Carlos Sánchez Muñoz, Milena De Giorgi, Lorenzo Dominici, Kenneth West, Loren N. Pfeiffer, Giuseppe Gigli, Fabrice P. Laussy, Marzena H. Szymańska & Daniele Sanvitto

The Berezinskii–Kosterlitz–Thouless transition is observed in a 2D gas of exciton-polaritons, through measurements of the first-order coherence both in space and time.

Nature

Control of piezoelectricity in amino acids by supramolecular packing

Sarah Guerin, Aimee Stapleton, Drahomir Chovan, Rabah Mouras, Matthew Gleeson, Cian McKeown, Mohamed Radzi Noor, Christophe Silien, Fernando M. F. Rhen, Andrei L. Kholkin, Ning Liu, Tewfik Soulimane, Syed A. M. Tofail & Damien Thompson

Proceeding from quantum mechanical predictions, a high shear piezoelectric constant of 178 pm V−1 was measured for the amino acid crystal beta glycine. This originates from the efficient packing of the molecules of the amino acid.

Nature

Melting Curve and Liquid Structure of Nitrogen Probed by X-ray Diffraction to 120 GPa

Author(s): Gunnar Weck, Frédéric Datchi, Gaston Garbarino, Sandra Ninet, Jean-Antoine Queyroux, Thomas Plisson, Mohamed Mezouar, and Paul Loubeyre

Synchrotron x-ray diffraction measurements of nitrogen are performed up to 120 GPa to determine the melting curve and the structural changes of the solid and liquid phases along it. The melting temperature exhibits a monotonic increase up to the triple point where the epsilon molecular solid, the cu...

Physical Review Letters

Contrasting behavior of covalent and molecular carbon allotropes exposed to extreme ultraviolet and soft x-ray free-electron laser radiation

Author(s): M. Toufarová, V. Hájková, J. Chalupský, T. Burian, J. Vacík, V. Vorlíček, L. Vyšín, J. Gaudin, N. Medvedev, B. Ziaja, M. Nagasono, M. Yabashi, R. Sobierajski, J. Krzywinski, H. Sinn, M. Störmer, K. Koláček, K. Tiedtke, S. Toleikis, and L. Juha

All carbon materials, e.g., amorphous carbon (a-C) coatings and ${\mathrm{C}}_{60}$ fullerene thin films, play an important role in short-wavelength free-electron laser (FEL) research motivated by FEL optics development and prospective nanotechnology applications. Responses of a-C and ${\mathrm{C}}_...

Physical Review B

Engineered superlattices with crossover from decoupled to synthetic ferromagnetic behavior

Rajesh V Chopdekar, Vivek K Malik, Alexander M Kane, Apurva Mehta, Elke Arenholz and Yayoi Takamura

The extent of interfacial charge transfer and the resulting impact on magnetic interactions were investigated as a function of sublayer thickness in La 0.7 Sr 0.3 MnO 3 /La 0.7 Sr 0.3 CoO 3 ferromagnetic superlattices. Element-specific soft x-ray magnetic spectroscopy reveals that the electronic structure is altered within 5–6 unit cells of the chemical interface, and can lead to a synthetic ferromagnet with strong magnetic coupling between the sublayers. The saturation magnetization and coercivity depends sensitively on the sublayer thickness due to the length scale of this interfacial effect. For larger sublayer thicknesses, the La 0.7 Sr 0.3 MnO 3 and La 0.7 Sr 0.3 CoO 3 sublayers are magnetically decoupled, displaying two independent magnetic transitions with little sublayer thickness dependence. These results demonstrate how interfacial phenomena at perovskite oxid...

Journal of Physics Condensed Matter

Phase diagrams of ferroelectric nanocrystals strained by an elastic matrix

A I Nikitchenko, A V Azovtsev and N A Pertsev

Ferroelectric crystallites embedded into a dielectric matrix experience temperature-dependent elastic strains caused by differences in the thermal expansion of the crystallites and the matrix. Owing to the electrostriction, these lattice strains may affect polarization states of ferroelectric inclusions significantly, making them different from those of a stress-free bulk crystal. Here, using a nonlinear thermodynamic theory, we study the mechanical effect of elastic matrix on the phase states of embedded single-domain ferroelectric nanocrystals. Their equilibrium polarization states are determined by minimizing a special thermodynamic potential that describes the energetics of an ellipsoidal ferroelectric inclusion surrounded by a linear elastic medium. To demonstrate the stability ranges of such states for a given material combination, we construct a phase diagram, where the inclusion’s shape anisotropy and temperature are used as two parameters. The ‘shape-temperature’ phase ...

Journal of Physics Condensed Matter

Annealing cycles and the self-organization of functionalized colloids

Cristóvão S Dias, Nuno A M Araújo and Margarida M Telo da Gama

The self-assembly of functionalized (patchy) particles with directional interactions into target structures is still a challenge, despite the significant experimental advances in their synthesis. Self-assembly pathways are typically characterized by high energy barriers that hinder access to stable (equilibrium) structures. A possible strategy to tackle this challenge is to perform annealing cycles. By periodically switching on and off the inter-particle bonds, one expects to smooth-out the kinetic pathways and favor the assembly of targeted structures. Preliminary results have shown that the efficiency of annealing cycles depends strongly on their frequency. Here, we study numerically how this frequency-dependence scales with the strength of the directional interactions (size of the patch σ ). We use analytical arguments to show that the scaling results from the statistics of a random walk in configurational space.

Journal of Physics Condensed Matter

Density functional study of carbon vacancies in titanium carbide

Mikael Råsander, Håkan W Hugosson and Anna Delin

It is well established that TiC contains carbon vacancies not only in carbon-deficient environments but also in carbon-rich environments. We have performed density functional calculations of the vacancy formation energy in TiC for C- as well as Ti-rich conditions using several different approximations to the exchange-correlation functional, and also carefully considering the nature and thermodynamics of the carbon reference state, as well as the effect of varying growth conditions. We find that the formation of carbon vacancies is clearly favorable under Ti-rich conditions, whereas it is slightly energetically unfavorable under C-rich conditions. Furthermore, we find that the relaxations of the atoms close to the vacancy site are rather long-ranged, and that these relaxations contribute significantly to the stabilization of the vacancy. Since carbon vacancies in TiC are also experimentally observed in carbon-rich environments, we conclude that kinetics may play an important role...

Journal of Physics Condensed Matter

Mon Dec 4 2017

Enhanced magnetostriction of Fe81Ga19 by approaching an instable phase boundary

Author(s): Nasir Rahman, Junming Gou, Xiaolian Liu, Tianyu Ma, Mi Yan

A novel principle of approaching an instable phase boundary (IPB) is developed for achieving large magnetostriction in Fe-Ga alloys. Unlike the usual approach by quenching at high temperature, the present Fe81Ga19 alloy was quenched near the decomposition phase boundary (773K) between A2 and (A2+L12) after slow cooling to approach the equilibrium condition. The obtained magnetostriction is 120ppm, 71.4% higher than that of the sample quenched at high temperature (1273K). The enhanced magnetostriction is due to the lattice softening by quenching near IPB, which may add important insight into developing high-performance magnetostrictive materials.

Scripta Materialia

Single-roll angular-rolling: A new continuous severe plastic deformation process for metal sheets

Author(s): Hak Hyeon Lee, Jae Ik Yoon, Hyoung Seop Kim

In this paper, a new continuous severe plastic deformation (SPD) process for metal sheets called single-roll angular-rolling (SRAR) is introduced. The SRAR process achieves maximized deformation homogeneity of metal sheets by combining circumferential shear deformation with channel-angular shear deformation. The grain refinement and mechanical properties were investigated experimentally in relation to the number of repetitive SRAR passes. The finite element method was used to demonstrate that the SRAR process provides highly uniform SPD by strengthening the less deformed region that inevitably occurs near the lower part of the workpiece during the channel-angular deformation processes.

Scripta Materialia

First-principles studies on the electronic and optical properties of Fe-doped potassium dihydrogen phosphate crystal

Author(s): Yongqiang Liu, Xiangcao Li, Jian Wu, Baoan Liu, Dongsheng Geng, Yanning Zhang

In this work, we performed first-principles calculations on the electronic structure and optical properties of the perfect and Fe-doped potassium dihydrogen phosphate (KDP). The partial substitution of P in perfect KDP with Fe (Fe-KDP) induced defect states in the band gap of Fe-KDP, narrowing its band gap to 3.8 eV (the corresponding optical absorption wavelength is 355 nm). We also see the impurity peaks induced by the Fe-doped point defects in dielectric function, absorb spectrum, L(ω), k(ω) and R(ω) curves. All of the results can provide the good basis for deeply understanding the optical properties of the KDP crystal.

Computational Materials Science

Theoretical study of the oxygen impurity doped Ta5N6

Author(s): Jiajia Wang, Jinghua Jiang, Jianqing Chen, Yuhua Li, Aibin Ma

The oxygen impurity is a common defect in tantalum nitrides. Experiments reveal that the concentration of oxygen impurity in Ta5N6 is as low as 0.2%, which is much lower than that in other tantalum nitrides such as Ta3N5 and Ta2N3. In this study, the density functional theory calculations were performed to unravel the cause of low oxygen impurity concentrations in Ta5N6. By calculating the elastic moduli, cohesive energies, defect formation energies and surface energies, we found that doping with high concentration oxygen impurities into Ta5N6 was thermodynamically and mechanically unfavorable. Electronic structure calculations revealed that the electron donated from oxygen impurity was localized, providing one proper reason for the difficulty of doping oxygen impurity into Ta5N6.

Computational Materials Science

Strain induced phase transformation in zirconium thin films

Author(s): Zahabul Islam, Aman Haque

While high temperature is known to transform zirconium from the hexagonal closed pack (hcp) to the body centered cubic (bcc) phase, there is little or no evidence of mechanical strain induced transformation to the face centered cubic (fcc) phase in the literature. We performed molecular dynamics simulation to show irreversible hcp to fcc phase transformation in zirconium, triggered at about 14.6% tensile strain. The transformation mechanism depends on the crystallographic direction of loading. Gliding of Shockley partial dislocations on prism plane { 1 0 1 ¯ 0 } is suggested facilitate this transformation. Nudged elastic band (NEB) theory was used to estimate the transformation energy barrier, which was observed to decrease with any increase in temperature.

Computational Materials Science

Tight binding parametrization of few-layer black phosphorus from first-principles calculations

Author(s): Marcos G. Menezes, Rodrigo B. Capaz

We employ a tight-binding parametrization based on the Slater Koster model in order to fit the band structures of single-layer, bilayer and bulk black phosphorus obtained from first-principles calculations. We find that our model, which includes 9 or 17 parameters depending on whether overlap is included or not, reproduces quite well the ab initio band structures over a wide energy range, especially the occupied bands. We also find that the Inclusion of overlap parameters improves the quality of the fit for the conduction bands. On the other hand, hopping and on-site energies are consistent throughout the different systems, which is an indication that our model is suitable for calculations on multilayer black phosphorus and more complex situations in which first-principles calculations become prohibitive, such as disordered systems and heterostructures with a large lattice mismatch. We also discuss the limitations of the model and how the fit procedure can be improved for a more accurate description of bands in the vicinity of the Fermi energy.

Computational Materials Science

Bending rigidity of two-dimensional titanium carbide (MXene) nanoribbons: A molecular dynamics study

Author(s): Vadym N. Borysiuk, Vadym N. Mochalin, Yury Gogotsi

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) were predicted to possess high mechanical properties, similar to their bulk counterparts – refractory carbides and nitrides, that represent some of the hardest materials. Bending rigidity is one of the most important and poorly understood mechanical characteristics of MXenes distinguishing them from many other single-atom thick 2D materials. We present results of in silico study of bending deformation of nanoribbons of three different 2D titanium carbides (Ti2C, Ti3C2 and Ti4C3). Dynamical behavior of the samples under applied bending load was simulated via classical molecular dynamics. The central deflection and bending rigidity of the MXene nanoribbons as a function of applied force were calculated. Calculated bending rigidity of the Ti2C nanoribbon is 5.21 eV at small deflections and nonlinearly increases at larger deflections, reaching the maximum magnitude of 12.79 eV before the onset of disintegration.

Computational Materials Science

Electrochemical Pourbaix diagrams of NiTi alloys from first-principles calculations and experimental aqueous states

Author(s): Rui Ding, Jia-Xiang Shang, Fu-He Wang, Yue Chen

Pourbaix diagrams (electrode potential-pH diagrams) for pure metals systems are studied completely, whereas researches on diagrams for binary or ternary alloys systems are lagging behind. In this work, we provide a complete procedure for simulations of Pourbaix diagrams of binary and multielement systems. With this scheme, Pourbaix diagrams of NiTi alloys with different Ni/Ti molar ratios in aqueous solutions at 25 °C have been constructed based on first-principles chemical potentials of solids and experimental chemical potentials of ions. The corrosion behaviors of different NiTi alloys including NiTi, Ti2Ni, and Ni3Ti are discussed and compared based on the obtained Pourbaix diagrams. The corrosion resistance of Ti2Ni is comparable to or even better than that of NiTi while the corrosion resistance of Ni3Ti is worse than that of NiTi. In addition, the electrochemical corrosion behaviors of NiTi alloys under Ti-rich and Ni-rich conditions are also investigated; it is found that the precipitations of Ti2Ni in Ti-rich NiTi alloys and Ni3Ti in Ni-rich NiTi alloys deteriorate the pitting corrosion resistance.

Computational Materials Science

Formability of a wrought Mg alloy evaluated by impression testing

Author(s): Walid Mohamed, Srikant Gollapudi, Indrajit Charit, K. Linga Murty

This study is focused on furthering our understanding of the different factors that influence the formability of Magnesium alloys. Towards this end, formability studies were undertaken on a wrought Mg-2Zn-1Mn (ZM21) alloy. In contrast to conventional formability studies, the impression testing method was adopted here to evaluate the formability parameter, B, at temperatures ranging from 298 to 473K. The variation of B of ZM21 with temperature and its rather limited values were discussed in the light of different deformation mechanisms such as activation of twinning, <c+a> slip, grain boundary sliding (GBS) and dynamic recrystallization (DRX). It was found that the material characteristics such as grain size, texture and testing conditions such as temperature and strain rate, were key determinants of the mechanism of plastic deformation. A by-product of this analysis was the observation of an interesting correlation between the Zener-Hollomon parameter, Z, and the ability of Mg alloys to undergo DRX.

Science and Engineering A

Characterization of precipitates in an Al-Zn-Mg alloy processed by ECAP and subsequent annealing

Author(s): Mohamed A. Afifi, Ying Chun Wang, Pedro Henrique R. Pereira, Yangwei Wang, Shukui Li, Yi Huang, Terence G. Langdon

Experiments were conducted to examine the influence of equal-channel angular pressing (ECAP) and post-ECAP annealing on the microstructures of an Al-Zn-Mg alloy. The results show that precipitates, mainly of the η′, η (MgZn2), T (Al20Cu2Mn3) and E (Al18Mg3Cr2) phases, are fragmented to fine spherical precipitates during ECAP processing for 4 and 8 passes. After post-ECAP annealing at 393 and 473K for 20h, precipitates with larger sizes lie primarily along the grain boundaries and finer particles are evenly distributed within the grains. Increasing the numbers of ECAP passes from 4 to 8 leads to an increase in the volume fraction of the finer precipitates in the ECAP-processed and annealed alloy. After 4 passes and heat treatment at 473K, the precipitates are slightly larger compared with the alloy processed under identical conditions and annealed at 393K. Nevertheless, significant coarsening is evident after processing for 8 passes and increasing the annealing temperature from 393 to 473K. Different types of precipitates are effective in impeding grain growth during the post-ECAP annealing even at 473K for 20h. In addition, η precipitates form within the T and E phases after both ECAP and post-ECAP annealing.

Science and Engineering A

Microstructure and mechanical properties of ultrafine grained Cu-0.8wt%C alloy with a bimodal microstructure produced by powder metallurgy techniques

Author(s): Wenjing Wang, Wei Zeng, Chenguang Li, Jiamiao Liang, Deliang Zhang

Bulk ultrafine grained Cu-0.8wt%C alloy samples were fabricated by spark plasma sintering (SPS) of a nanocrystalline Cu-0.8wt%C alloy powder prepared by high energy mechanical milling. The SPS temperature was 800°C. The SPSed samples exhibited a bimodal microstructure consisting of ultrafine and coarse Cu grains (average grain sizes: ~95nm and ~1µm respectively) and a yield strength of 483MPa, but underwent a premature fracture at a stress of 511MPa with a low elongation to fracture of only 0.3%. It is established that the premature fracture and low tensile ductility are caused by the high flow stress associated with grain boundary strengthening and reaching the strength of IPBs before reaching its maximum. Hot extrusion of the SPSed sample at 800°C caused substantial growth of the ultrafine Cu grains and enhancement of the strength of IPBs, leading to a clearly lower tensile yield strength of 353MPa, but mature fracture with an ultimate tensile strength of 428MPa and a significantly improved elongation to fracture of 6.5%.

Science and Engineering A

Testing of Ni-base superalloy single crystals with circular notched miniature tensile creep (CNMTC) specimens

Author(s): L. Cao, D. Bürger, P. Wollgramm, K. Neuking, G. Eggeler

The present work introduces a novel circular notched miniature tensile creep (CNMTC) specimen which is used to study the influence of notches on creep and of multiaxial stress states on microstructural evolution in Ni-based single crystal (SX) superalloys. It is briefly discussed how mild circular notches affect the stress state in the notch root of a tensile bar during elastic loading. Then the stress redistribution under creep conditions is calculated using the finite element method (FEM), assuming isotropic material behavior. Two series of interrupted creep experiments with the Ni-based single crystal superalloy ERBO1 (CMSX-4 type) were then performed at 950°C, with flat uniaxial miniature tensile creep (FUMTC) and CNMTC <100> specimens, respectively. The evolution of cavities and microcracks in both types of specimens was carefully analyzed after 81, 169, and 306h. In the uniaxial experiments, a growth of cast pores and the formation of new creep cavities were observed. These degradation processes were much less pronounced in the circular notched specimens. The results of the present work are discussed in the light of previous findings on the influence of multiaxial stress states on creep in single crystal superalloys.

Science and Engineering A

Investigating of the tensile mechanical properties of structural steels at high strain rates

Author(s): A.R. Khalifeh, A.Dehghan Banaraki, H. Danesh Manesh, M.Dehghan Banaraki

St37 and St52 structural steel plates were tested in uniaxial tension at room temperature over various strain rates ranging from 0.001/s to 0.1/s. The yield stress, flow stress and fracture behavior of steels were analyzed. It was found that the strain rate has a strong effect on the tensile mechanical properties of St37 steel, while St52 has a less sensitive strain rate and that the yield strength of both steels exhibits a higher strain sensitivity rate than the other mechanical properties. An increase in the loading rate from 0.001/s to 0.1/s led to a %30 increase in the lower yield strength of St37 steel and an increase of %6 for St52. The equations were derived to express the yield stress behavior with the strain rate. The ductile dimple fracture was observed in static and dynamic conditions; however, increasing the strain rate resulted in a pronounced cleavage-type fracture in both steels. The St37 fracture strain decreased considerably by increasing the strain rate.

Science and Engineering A

Ultrarapid formation of multi-phase reinforced joints of hypereutectic Al-Si alloys via an ultrasound-induced liquid phase method using Sn-51In interlayer

Author(s): Lin Zhu, Qian Wang, Lei Shi, Xin Zhang, Tianhao Yang, Jiuchun Yan, Xiaoyu Zhou, Shengyong Chen

Ultrarapid formation of hypereutectic Al-Si alloys /Sn-51In/ hypereutectic Al-Si alloys joints was achieved within 0.2s via an ultrasound-induced liquid phase method at 180°C in air. Ultrasonic vibration accelerated the element inter-diffusion between aluminium and indium to induce liquid phase. The joints consisted of Si particles, an Al-In solid solution and intermetallic compounds. The migration behaviour of Si particles, which were regarded as tracers, under the influence of ultrasonic vibration was used to investigate the joint formation. As the ultrasonic vibration time increased, the shear strength of the joints increased. The existence of Sn in the interlayer considerably affected the joint shear strength by forming intermetallic compounds.

Science and Engineering A

Dissimilar friction stir lap welding of magnesium to aluminum using plasma electrolytic oxidation interlayer

Author(s): Y. Gao, Y. Morisada, H. Fujii, J. Liao

Joining the die-cast non-combustible magnesium alloy AMX602 to the die-cast aluminum alloy ADC12 via friction stir lap welding (FSLW) was investigated. The aluminum alloy on the top can more easily form a sound dissimilar joint than that on the bottom. Strong dissimilar joints were achieved after the magnesium alloy was subjected to a plasma electrolytic oxidation treatment. After the welding process, the plasma electrolytic oxidation interlayer was stirred and flowed into the aluminum alloy side. The growth of the intermetallic compound was restrained by the plasma electrolytic oxidation interlayer by reducing the reaction time of the magnesium alloy and aluminum alloy.

Science and Engineering A

Hot corrosion and low cycle fatigue of a Cr2AlC-coated superalloy

Author(s): J.L. Smialek, J.A. Nesbitt, T.P. Gabb, A. Garg, R.A. Miller

Low temperature Type II hot corrosion is a serious problem for low cycle fatigue (LCF) failure of advanced turbine disk alloys operating at increased temperatures. Accordingly, the present effort studied 15–20µm corrosion resistant Cr2AlC sputter coatings on Low Solvus High Refractory (LSHR) disk alloy LCF test specimens. These were cycled to failure at 840/−430MPa and 0.33Hz, after 500h oxidation and 50h of Mg-Na2SO4 hot salt corrosion, all at 760°C. The coating successfully prevented hot corrosion pitting that was responsible for a 90% decrease in uncoated LCF specimens. However, fractography identified unintentional 15–30µm deep defects produced by grit blast surface preparation of coated samples. These acted as failure origins and introduced anomalous life reduction for some coated test specimens. Furthermore, the presence and growth of an undesirable Cr7C3 second phase diminished protectiveness by promoting internal oxidation and embrittlement of the coating.

Science and Engineering A

Partially-recrystallized, Nb-alloyed TWIP steels with a superior strength-ductility balance

Author(s): Hojun Gwon, Jin-Kyung Kim, Bian Jian, Hardy Mohrbacher, Taejin Song, Sung-Kyu Kim, Bruno C. De Cooman

We investigated the effect of Nb micro-alloying in the range of 0.01 to 0.l wt% on the microstructures and mechanical properties of Fe17Mn0.6C1.5Al (wt%) TWIP steel. EBSD analysis shows that the Nb addition retards recrystallization in both the hot-rolled steels and cold-rolled and annealed steels. The Nb addition in the cold-rolled and annealed TWIP steel leads to an increase in yield strength. This phenomenon is due to a combined effect of precipitation hardening and a low degree of recrystallization. Recovery annealing of the cold-rolled TWIP steels at 650°C results in a good combination of yield strength and ductility. The steels containing 0.01wt% and 0.025wt% of Nb show a superior combination of UTS×TE exceeding 40,000MPa∙% and yield strength higher than 800MPa. The design of TWIP steels utilizing both precipitation hardening and partial recrystallization opens a way to develop steels with a superior combination of yield strength and ductility.

Science and Engineering A

Microstructural and mechanical properties of a beta-type titanium alloy joint fabricated by friction stir welding

Author(s): Huihong Liu, Hidetoshi Fujii

Friction stir welding (FSW) was performed on β-type Ti-15V-3Cr-3Al-3Sn alloy (Ti-15-3, mass%) plates to fabricate a sound weld joint. The microstructural and mechanical properties of the weld joint were systematically investigated in order to understand the response of the β-type titanium alloys to the FSW. The results show that a defect-free Ti-15-3 alloy weld joint can be successfully fabricated by the FSW. In the thermo-mechanically affected zone (TMAZ), the microstructural evolution is predominantly driven by discontinuous dynamic recrystallization because of the low stacking fault energy of the alloy that results in a low dislocation mobility; while in the stir zone (SZ) it is mainly promoted by continuous dynamic recrystallization due to the high temperature and high strain achieved in the SZ which improve the dislocation mobility. The material flow field within the SZ shows an incline of ~ 10° towards the advancing side (AS) around the welding direction (WD). The base material and the weld-associated zones (TMAZ and SZ) show comparable mechanical properties, which is due to the competitive effects of the dislocation density, grain diameter and grain orientation. This homogeneous mechanical property distribution makes the Ti-15-3 alloy joint preferred for industrial applications.

Science and Engineering A

Crystallographic texture and lattice strain evolution during tensile load of swaged brass

Author(s): Nowfal Al-Hamdany, Heinz-Günter Brokmeier, Weimin Gan

Evolutions of texture and lattice strain of swaged brass samples were investigated by neutron diffraction at STRESS-SPEC under tensile deformation using a unique tension/compression rig. The two phased sample BS1 (61% α-brass and 39% β-brass) became 100% α-brass after 400°C annealing (sample BS2). The starting texture of the as-received material BS1 was the typical <111>, <200> double fiber. This texture develops firstly by in-situ tension to a moderate strengthening. After annealing (BS2) the <111> fiber survives with surprisingly high strength and develops by in-situ tension a very strong <111> fiber of 39 mrd. Line broadening and lattice strain behaviour shows the development of the elastic strain and plastic strain.

Science and Engineering A

Effects of caliber rolling on microstructure and mechanical properties in twinning-induced plasticity (TWIP) steel

Author(s): Joong-Ki Hwang

The effect of caliber rolling on microstructure and mechanical properties of Fe-Mn-Al-C twinning-induced plasticity (TWIP) steel has been investigated to find alternative methods of wire drawing process using the numerical simulation, electron backscatter diffraction (EBSD) techniques, transmission electron microscopy (TEM), and hardness test. Behavior of twinning, texture, and effective strain was different with areas of rolled wire due to the difference in stress state and strain. The center area had maximum twin density, low angle boundary (LAB), effective strain, and hardness; whereas the surface area had minimum values. In comparison with wire drawing process, caliber rolling process imposed higher stain with slightly uniform distribution on wire, indicating that caliber rolling can manufacture high strength wires more effectively. For instance, after the area reduction of 22% by caliber rolling, the tensile strength was 47% higher and hardness inhomogeneity factor was 15% lower as compared to samples processed by wire drawing. In other words, caliber rolling process was suitable to make high strength materials deformed and hardened by twinning mechanism such as TWIP steels due to the characteristics of imposing severe strain with multi-direction, multi-pass with different shape at each pass, and alternating the loading direction between passes. Therefore, caliber rolling process can be a strong candidate in replacement of wire drawing process, especially TWIP steels.

Science and Engineering A

Microstructure evolution in Alloy 617 B after a long-term creep and thermal aging at 700°C

Author(s): Magdalena Speicher, Florian Kauffmann, Jae-Hyeok Shim, Mahesh Chandran

Changes in the microstructure of heat-resistant materials may influence their long-term behaviour. For this reason, materials chosen for high temperature-based applications, e.g. advanced ultra-supercritical (A-USC) steam power plants, must exhibit long-term microstructure stability. Therefore, changes in the microstructure of frequently used materials must be determined and correlated with their creep behaviour to assure a reliable operation of components. In this work, a long-term study investigated the microstructure of a creep rupture specimen made of a nickel-based alloy 617 B. The creep tests were carried out at 700°C for up to 45,148h. By using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) the virgin, thermal-loaded and creep states were characterized. The precipitate location, number and size have been determined. The experiment results were compared, discussed and correlated to the creep and failure behaviour. Furthermore, the long-term precipitation kinetics of Alloy 617 B was simulated, which considered its thermal history, using the MatCalc software. The simulated precipitates fraction and size were compared with the experimental data obtained in this study. The comparison between the experimental and simulation results demonstrated comparable gamma prime (γ') phases. The simulation results of the carbides identified at the grain boundaries were satisfactory. However, the size of the intragranular fine carbides M23C6 was not reproduced correctly.

Science and Engineering A

Improved microstructural homogeneity and mechanical property of medium manganese steel with Mn segregation banding by alternating lath matrix

Author(s): Juhua Liang, Zhengzhi Zhao, Di Tang, Nan Ye, Shufeng Yang, Weining Liu

The influence of Mn segregation banding on the microstructure homogeneity and mechanical property of medium manganese system steels with both equiaxed and lath-like microstructures has been investigated by adding the pre-quenching treatment before the intercritical austenitization and subsequent quenching & partitioning process (IQP). Both the crack observation and KAM characterization in these two types of microstructure intuitively indicate different micro-cracking resistance and in-grain strain accommodation of intercritical ferrite during the deformation. The excessive inhomogeneous microstructure and blocky retained austenite with low stability accelerate the failure of the material with Mn segregation banding. The fine lath-like ferrites with great contact ratio to the surrounding martensites improve the microstructural homogeneity under pre-quenching process. The alternative arrangement of lath-like soft ferrite and hard martensite results in optimal mechanical properties of material with Mn segregation banding.

Science and Engineering A

Microstructural evolution of adiabatic shear bands in pure copper during impact at high strain rates

Author(s): Solomon Boakye-Yiadom, Nabil Bassim

A systematic study was conducted on copper of commercial purity (99.9% purity) to elucidate the sequence of events that leads to the evolution of adiabatic shear bands and the structure of the evolved shear bands after strain localization during deformation at high strain rates and large strains. A direct impact Hopkinson Pressure Bar was used to deform different specimens at increasing impact momentum and strain rate followed by microstructural characterization using metallographic techniques and transmission electron microscopy. It was observed that sequential occurrence of emergence of dislocations from grain and twin boundaries, formation of dislocation cell structures and substructures with varying cell sizes and cell walls, dislocation-nucleation controlled softening and extensive micro twinning characterized the structure of the evolved adiabatic shear bands as a function of impact momentum and strain rate. The dislocation cell structures and substructures were typically made up of high-density dislocation walls surrounding low-density dislocation cell interiors. The microhardness distribution within the evolved shear bands increased up to a peak value at a critical impact momentum and strain rate (≥ 45kgm/s and ≥6827s−1). Above this threshold, microhardness of the regions within the evolved shear bands decreased because of the occurrence of softening. However, the first specimen that exhibited softening had high-density dislocation cell walls surrounding dislocation-free cell interiors with no observed recrystallized grains. Despite the onset of softening both within and outside the shear bands, the regions within the shear bands were always harder than the regions outside the shear bands. The shear bands that exhibited significant softening were clad with vast distribution of microtwins in addition to evolved refined grains and sub-grains. It is discussed that thermally activated dislocation processes as a result of the rise in temperature during impact, dynamic recovery and dynamic recrystallization do not necessarily result in strain localization in the impacted copper specimens because their effects are observed in the structure of the evolved shear bands at a latter stage when strain localization had already occurred.

Science and Engineering A

Effects of solutioning and ageing treatments on properties of Inconel-713C nickel-based superalloy under creep loading

Author(s): Mahboobeh Azadi, Armin Marbout, Sama Safarloo, Mohammad Azadi, Mehdi Shariat, Mohammad Hassan Rizi

As nickel-based superalloys have been widely used in turbine blades of turbo-chargers in automobile industries, this article has presented the creep behavior of the Inconel-713C nickel-based superalloy. For this objective, forced-controlled creep testing has been performed at the temperature of 850°C and under the stress of 585.5MPa. Then, the effect of the solutioning process on creep (time-dependent) properties of the superalloy was investigated. Optical and scanning electron microscopies were utilized to study the material microstructure, before and after creep testing. The X-ray diffraction (XRD) spectrometry was also used to detect different phases in the superalloy. Results showed that solutioning at 1200°C for 1h had a lowering effect on the creep rupture time of the Inconel-713C superalloy, as the mean size of the γ′ particles crystallite was about 6.8nm. When the superalloy was aged for 16h at 930°C, an insignificant effect with respect to the as-cast sample could be observed, due to the precipitation of NbC carbides (at grain boundaries) and the coarsening behavior of γ′ particles. Consequently, different microstructures led to different creep fracture mechanisms for this alloy. The decomposition of MC carbides to M23C6 was also observed for all samples after creep testing.

Science and Engineering A

{101¯2}−{101¯2} double tensile twinning in a Mg-3Al-1Zn alloy sheet during cyclic deformation

Author(s): Li Tan, Xiyan Zhang, Ting Xia, Qi Sun, Guangjie Huang, Renlong Xin, Qing Liu

The typical fracture morphology of the Mg-3Al-1Zn alloy after cyclic deformation was investigated using optical microscopy (OM), and an electron back-scatter diffraction (EBSD). A novel finding of this study is that large number of stripe-like laminas were found within the primary { 10 1 ¯ 2 } tensile twins in the crack initiation and crack propagation regions. It is shown that these laminas primarily result from { 10 1 ¯ 2 } { 10 1 ¯ 2 } twinning during cyclic deformation. According to the results and combined with theoretical analysis, the formation of { 10 1 ¯ 2 } { 10 1 ¯ 2 } twins are largely contributing to the local strain accommodation caused by different twin variants.

Science and Engineering A

Cyclic deformation and microcrack initiation during stress controlled high cycle fatigue of a titanium alloy

Author(s): Changsheng Tan, Qiaoyan Sun, Lin Xiao, Yongqing Zhao, Jun Sun

Cyclic plastic deformation, slip characteristics and crack nucleation in Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) with different morphologies of equiaxed and lamellar α phase were systematically analyzed during high-cycle fatigue. The heterogeneous plastic deformation could take place within different α morphologies during high-cycle fatigue even though the cyclic stress amplitude is much less than yield strength. Slip is the dominant deformation mode in the equiaxed primary α, while the slip and ( 10 1 ¯ 1 ) deformation twin are prevalent in the primary α lath. Interactions between slip, twin and interface result in ledges at the primary α lath interface. The relationship between cyclic slip irreversibility, accumulated irreversible strain, and fatigue life is established. A critical parameter, accumulated irreversible strain per area in the crack initiation region (region I), was calculated to be (8.1 ± 2) × 10−4 μm−2 for initiating fatigue crack. Fatigue cracks will nucleate when the accumulated irreversible strain exceeds the critical value. The primary α lath is the dominant site for crack initiation. The cracks initiate and propagate in interface and slip band, and easily connect each other in the primary α lath. In comparison, most cracks lie within an individual or occupy several equiaxed α phases and often cease in front of the phase boundary, which delays the connection of microcracks. It indicates that the primary α lath is more detrimental than the equiaxed primary α phase during high cycle fatigue.

Science and Engineering A

Creep rupture behavior of Hastelloy C276-BNi2 brazed joint

Author(s): Yun Luo, Wenchun Jiang, Yucai Zhang, Muming Hao, Shan-Tung Tu

This paper studied the creep rupture behavior of Hastelloy C276-BNi2 brazed joint by experimental. Based on a series of creep rupture tests at 600°C under 110–250MPa, a relation between the stress and rupture time was achieved to evaluate the creep life of brazed joint. The results show that all the brazed joints were fractured in brazed seam immediately after the second creep stage. The creep model, fracture behavior and failure mechanism are dependent on the level of applied stress. The creep behavior of the brazed joint should be depicted by two-regime Norton (2RN) creep model rather than single Norton model. The creep damage tolerance factor was used to identify the cause of creep damage. From low to high stress level, the failure mode is changed from intergranular brittle to transgranular ductile fracture due to the increases of damage tolerance factor with applied stress increases.

Science and Engineering A

Multiple strengthening sources and adiabatic shear banding during high strain-rate deformation of AISI 321 austenitic stainless steel: Effects of grain size and strain rate

Author(s): A.A. Tiamiyu, A.G. Odeshi, J.A. Szpunar

The dynamic impact response of AISI 321 steel at strain rate of 4000, 5500, 6500 and 7500s−1 was investigated using the split Hopkinson pressure bar system. The alloy samples processed to have grain size of 0.24, 3, 13 and 34µm were studied. While the yield strength and hardness increases with decrease in grain size, strain hardening rate is comparable for all grain sizes. Microstructural evaluation of the impacted specimens using high-resolution electron backscattered diffraction (HR-EBSD) technique showed grain boundary strengthening, deformation twinning, deformation-induced martensitic transformation, dislocation multiplication during slip and precipitation of carbides that act as barriers to dislocation motion as additional source of strengthening. Slip and twinning were the dominant deformation mechanisms observed in the steel. Twinning, dislocation multiplication during slip and carbide precipitation contributed more strongly to strain-hardening in coarse-grained (CG) specimen while stain-induced martensite and grain boundary strengthening are the most beneficial to strengthening in the ultra-fine-grained (UFG) specimens. The temperature rise in the specimens during impact increases with strain rate. This slowed the kinetics of twinning, phase transformation and dislocation interaction especially in CG structure. Both XRD and HR-EBSD texture results confirmed the development of {110}||CD (CD: compression direction) texture in the austenite phase at the expense of {100}||CD and {111}||CD fibres. Thermomechanical instability leading to the formation of adiabatic shear band (ASB) occurred in the test specimens as they deformed at high strain rates. While the amount of deformation twinning and αʹ-martensite decreases towards the ASB, only the carbides and small fraction of αʹ-martensite are observed inside the ASB. Grain refinement via rotational dynamic recrystallization occurred within the ASB. The extent of grain refinement increased with increase in initial grain size of the test specimen.

Science and Engineering A

Microstructure evolution within adiabatic shear band in peak aged ZK60 magnesium alloy

Author(s): Lihong Jiang, Yang Yang, Zhen Wang, Haibo Hu

Microstructure evolution in adiabatic shear band (ASB) in the peak aged ZK60 magnesium alloy cylindrical tube specimen after explosive radial compression was systematically investigated. High strain rate compression tests were performed by means of the radial collapse of thick-walled cylinder technique to achieve nominal strain rates of about 104 s−1. The TEM results indicate that the elongated grains and deformed twins are the major characteristics in the boundary of the shear band. The central region in ASB was found to consists of ultrafine and equiaxed grains with a typical size of 100nm. And it was found that precipitates within ASB were significantly reduced, namely the precipitates instantaneous dissolution during adiabatic shearing. It is proposed that fine equiaxed grains within ASB are the result of rotational dynamic recrystallization during localization. The free energy difference between the precipitates and matrix provided a thermodynamic condition for the dissolution of precipitates. Diffusion rate increased due to high strain rate, high shear stress (large strain) and adiabatic temperature rise, which caused instantaneous dissolution of precipitates.

Science and Engineering A

Impact of the heating rate on the annealing behavior and resulting mechanical properties of UFG HSLA steel

Author(s): Jörn Niehuesbernd, Enrico Bruder, Clemens Müller

The formability of materials with ultrafine grained (UFG) microstructures produced by severe plastic deformation processes is often limited due to low strain hardening capabilities and strain localizations. Most commonly, heat treatments are used to regain the formability. However, conventional furnace heat treatments can lead to grain growth and significant losses in strength. One approach to address this issue is to increase the heating and cooling rates of the heat treatments. The present work focuses on the heating rate dependencies of the microstructure evolution and resulting mechanical properties of a UFG high strength low alloy (HSLA) steel. Electron backscatter diffraction (EBSD) measurements are used to characterize the microstructure and texture. Bending tests and uniaxial tensile tests are conducted to provide insight into the mechanical properties that are related to the obtained microstructures, with an emphasis on the tendency for strain localizations. The investigations reveal a distinct effect of the heating rate on the annealing behavior, i.e., continuous vs. discontinuous growth. Thus, resulting microstructures and mechanical properties are not only a function of time and temperature but also depend on the heating rate. In this context, the application of laser annealing is shown to be a suitable approach to impede strain localizations in the form of shear bands without sacrificing much of the strength of the UFG material obtained by severe plastic deformation.

Science and Engineering A

Fri Dec 1 2017

Enhanced thermoelectric properties of n-type direction solidified Bi2Te2.7Se0.3 alloys by manipulating its liquid state

Author(s): Bin Zhu, Zhong-Yue Huang, Xiao-Yu Wang, Yuan Yu, Na Gao, Fang-Qiu Zu

Liquid state manipulation (LSM) has been applied to prepare direction solidified Bi2Te2.7Se0.3 bulk alloys. The maintained strong texturing and increased effective mass benefits the power factor. Moreover, the lattice thermal conductivity in Bi2Te2.7Se0.3 reduces to 0.55Wm1 K1 at 490K after LSM. As a result, the largest ZT value of 1.10 is achieved for the sample experienced LSM, which is increased by 20% compared with that unexperienced LSM. The average ZT value for LSM sample rises from 0.80 to 1.02, which makes this material suitable for application in the wide temperature range.

Scripta Materialia

Effect of substrate on the structural transformation and optical properties of Zn1−xMgxO thin films grown by pulsed laser deposition

Author(s): F. Bellarmine, E. Senthil Kumar, M.S. Ramachandra Rao

Pulsed laser deposited MgxZn1xO films grown on sapphire substrates exhibited wurtzite hexagonal structure at lower Mg concentrations (x<0.20), whereas use of MgO substrates promotes cubic rocksalt structure. For x0.20, single-phase cubic ZnMgO thin films with 〈200〉 orientation were realized on MgO substrates. However, the films grown on sapphire substrates exhibited single-phase cubic structure with 〈111〉 orientation only when x0.4. We show that the optical bandgap of wurtzite ZnMgO films can be tuned from 3.27eV to 3.80eV. On the other hand cubic ZnMgO films exhibited larger bandgap values as high as 5.99eV.

Scripta Materialia

Improving hydrogen embrittlement resistance of Hadfield steel by thermo-mechanical flash-treatment

Author(s): Mahmoud Khedr, Wei Li, Xu Zhu, Pengwei Zhou, Shan Gao, Xuejun Jin

Nano-twins microstructures were introduced in a high carbon manganese steel by a novel flash thermo-mechanical treatment in order to achieve high resistance to hydrogen embrittlement. Nano-twinned grains decreased hydrogen diffusivity through the bulk material, although it absorbed more hydrogen than the twins-free specimens. Moreover, it was shown that after electrochemical hydrogen charging, the nano-twins microstructures can reduce dislocations glide during plastic deformation, resulting in forming of fine twin plates.

Science and Engineering A

Symmetry Protected Dynamical Symmetry in the Generalized Hubbard Models

Author(s): Jinlong Yu, Ning Sun, and Hui Zhai

In this Letter we present a theorem on the dynamics of the generalized Hubbard models. This theorem shows that the symmetry of the single-particle Hamiltonian can protect a kind of dynamical symmetry driven by the interactions. Here the dynamical symmetry refers to the phenomenon that time evolution...

Physical Review Letters

Bright x-rays reveal shifting deformation states and effects of the microstructure on the plastic deformation of crystalline materials

Author(s): A. J. Beaudoin, P. A. Shade, J. C. Schuren, T. J. Turner, C. Woodward, J. V. Bernier, S. F. Li, D. M. Dimiduk, P. Kenesei, and J.-S. Park

The plastic deformation of crystalline materials is usually modeled as smoothly progressing in space and time, yet modern studies show intermittency in the deformation dynamics of single-crystals arising from avalanche behavior of dislocation ensembles under uniform applied loads. However, once the ...

Physical Review B

Crystal growth and transport properties of Weyl semimetal TaAs

Raman Sankar, G Peramaiyan, I Panneer Muthuselvam, Suyang Xu, M Zahid Hasan and F C Chou

We report the single crystal growth and transport properties of a Weyl semimetal TaAs. Unsaturated large magnetoresistance of about 22 100% at 2 K and 9 T is observed. From the Hall measurement, carrier concentrations n   =  4.608  ×  10 24 m −3 and p   =  3.099  ×  10 24 m −3 , and mobilities µ p   =  2.502 m 2 V −1 s −1 and µ n   =  16.785 m 2 V −1 s −1 at 2 K are extracted. The de Haas–van Alphen oscillations at 2 K and 9 T suggest the presence of a Fermi surface, and the quantum electronic parameters such as effective cyclotron mass and Dingle temperature were obtained using Lifshitz–Kosevich fitting. Temperature dependent resistivity measurements at different static magnetic fields suggest the formation of an insulating gap in the Weyl semimetal TaAs. An angle-resolved photoemission spectroscopy study reveals Ferm...

Journal of Physics Condensed Matter

All-spinel oxide Josephson junctions for high-efficiency spin filtering

S Mesoraca, S Knudde, D C Leitao, S Cardoso and M G Blamire

Obtaining high efficiency spin filtering at room temperature using spinel ferromagnetic tunnel barriers has been hampered by the formation of antiphase boundaries due to their difference in lattice parameters between barrier and electrodes. In this work we demonstrate the use of LiTi 2 O 4 thin films as electrodes in an all-spinel oxide CoFe 2 O 4 -based spin filter devices. These structures show nearly perfect epitaxy maintained throughout the structure and so minimise the potential for APBs formation. The LiTi 2 O 4 in these devices is superconducting and so measurements at low temperature have been used to explore details of the tunnelling and Josephson junction behaviour.

Journal of Physics Condensed Matter

Prediction of phonon-mediated superconductivity in hole-doped black phosphorus

Yanqing Feng, Hongyi Sun, Junhui Sun, Zhibin Lu and Yong You

We study the conventional electron–phonon mediated superconducting properties of hole-doped black phosphorus by density functional calculations and get quite a large electron–phonon coupling (EPC) constant λ ~ 1.0 with transition temperature T C ~ 10 K, which is comparable to MgB 2 when holes are doped into the degenerate and nearly flat energy bands around the Fermi level. We predict that the softening of low-frequency {$B_{3{\rm g}}^{1}$} optical mode and its phonon displacement, which breaks the lattice nonsymmorphic symmetry of gliding plane and lifts the band double degeneracy, lead to a large EPC. These factors are favorable for BCS superconductivity.

Journal of Physics Condensed Matter

Thu Nov 30 2017

Low temperature and high strain rate superplastic flow in structural ceramics induced by strong electric-field

Author(s): Hidehiro Yoshida, Yamato Sasaki

High-strength structural ceramics with sub-micron grain sizes can exhibit superplasticity, but the superplasticity appears at high temperatures and low strain rates. Low temperature and high speed superplastic flow in structural ceramics was achieved by applying a strong electric field above a threshold value during deformation. The application of a direct-current field of 190V·cm1 led to superplastic deformation in Y2O3-stabilized tetragonal ZrO2 polycrystal with a total tensile elongation of >150% at 800°C and an initial strain rate of 2×103 s1. The field-activated plasticity was attributed to highly-accelerated self-diffusion induced not only by temperature rise but also by a field effect.

Scripta Materialia

Achieving superior ductility for laser solid formed extra low interstitial Ti-6Al-4V titanium alloy through equiaxial alpha microstructure

Author(s): Zhuang Zhao, Jing Chen, Hua Tan, Guohao Zhang, Xin Lin, Weidong Huang

The limited ductility of laser solid formed (LSFed) titanium alloy is a critical issue, which hinders their potential engineering applications. Here, we reported a largely improved ductility (25.1% total elongation) of LSFed extra low interstitial Ti-6Al-4V titanium alloy with a comparable tensile strength (>860MPa, international standards) using triple heat treatment. Superior ductility was mainly attributed to the gradual globularization of the α laths during subcritical annealing. The secondary α lamellar obtained by the solution treatment and aging was responsible for the tensile strength. The present findings provide significant guidance for fabricating LSFed titanium alloy having high ductility and good strength.

Scripta Materialia

Effects of carbon and nitrogen on precipitation and tensile behavior in 15Cr-15Mn-4Ni austenitic stainless steels

Author(s): Kyung-Shik Kim, Jee-Hyun Kang, Sung-Joon Kim

Precipitation behavior and tensile properties have been investigated for 15Cr-15Mn-4Ni austenitic steels containing 0.2wt% carbon and/or 0.2wt% nitrogen. During aging at 600–1000°C, precipitates such as σ, M23C6, and Cr2N were formed and time-temperature-precipitation diagrams were constructed. The type, size, and density of precipitates depended on the carbon and nitrogen contents. The nitrogen addition suppressed carbide formation while the effect of carbon addition on nitride precipitation was comparatively small. The evolution of tensile properties after aging is explained by the changes in solid solution strengthening and deformation modes caused by the precipitation. The precipitation of M23C6 slightly decreased yield strength and elongation while it increased ultimate tensile strength. On the other hand, the precipitation of Cr2N hardly affected tensile properties due to its low volume fraction.

Science and Engineering A

Quantifying Complexity in Quantum Phase Transitions via Mutual Information Complex Networks

Author(s): Marc Andrew Valdez, Daniel Jaschke, David L. Vargas, and Lincoln D. Carr

We quantify the emergent complexity of quantum states near quantum critical points on regular 1D lattices, via complex network measures based on quantum mutual information as the adjacency matrix, in direct analogy to quantifying the complexity of electroencephalogram or functional magnetic resonanc...

Physical Review Letters

Induced-Charge Enhancement of the Diffusion Potential in Membranes with Polarizable Nanopores

Author(s): I. I. Ryzhkov, D. V. Lebedev, V. S. Solodovnichenko, A. V. Shiverskiy, and M. M. Simunin

When a charged membrane separates two salt solutions of different concentrations, a potential difference appears due to interfacial Donnan equilibrium and the diffusion junction. Here, we report a new mechanism for the generation of a membrane potential in polarizable conductive membranes via an ind...

Physical Review Letters

Absence of single critical dose for the amorphization of quartz under ion irradiation

S Zhang, O H Pakarinen, M Backholm, F Djurabekova, K Nordlund, J Keinonen and T S Wang

In this work, we first simulated the amorphization of crystalline quartz under 50 keV {$^{23}$} Na ion irradiation with classical molecular dynamics (MD). We then used binary collision approximation algorithms to simulate the Rutherford backscattering spectrometry in channeling conditions (RBS-C) from these irradiated MD cells, and compared the RBS-C spectra with experiments. The simulated RBS-C results show an agreement with experiments in the evolution of amorphization as a function of dose, showing what appears to be (by this measure) full amorphization at about 2.2 eV⋅ {${\rm atom}^{-1}$} . We also applied other analysis methods, such as angular structure factor, Wigner–Seitz, coordination analysis and topological analysis, to analyze the structural evolution of the irradiated MD c...

Journal of Physics Condensed Matter

Site occupancy, composition and magnetic structure dependencies of martensitic transformation in Mn

Ashis Kundu and Subhradip Ghosh

A delicate balance between various factors such as site occupancy, composition and magnetic ordering seems to affect the stability of the martensitic phase in {${\rm Mn}_{2}$} {${\rm Ni}_{1+x}$} {${\rm Sn}_{1-x}$} . Using first-principles DFT calculations, we explore the impacts of each one of these factors on the martensitic stability of this system. Our results on total energies, magnetic moments and electronic structures upon changes in the composition, the magnetic configurations and the site occupancies show that the occupancies at the 4d sites in the inverse Heusler crystal structure play the most crucial role. The presence of Mn at the 4d sites originally occupied by Sn and its ...

Journal of Physics Condensed Matter

Temperature dependent structural evolution in liquid Ag 50 Ga 50 alloy

Y Su, X D Wang, Q Yu, Q P Cao, U Ruett, D X Zhang and J Z Jiang

The temperature dependence of atomic structural evolution in liquid Ag 50 Ga 50 alloy has been studied using an in situ high energy x-ray diffraction (XRD) experiment combined with first-principles molecular dynamics (FPMD) simulations. The experimental data show a reversible structural crossover at the temperature of about 1050 K. Changes in both electrical resistivity and absolute thermoelectric power at about 1100 K strongly support the XRD results. Additionally, FPMD simulations reveal the abnormal temperature dependent behavior of partial coordination number and atomic diffusivity at about 1200 K, elucidating that the partition experimentally observed changes in structure and properties could be linked with the repartition between Ag and Ga atoms in the liquid at around 1050–1200 K. This finding will trigger more studies on the structural evolution of noble-polyvalent metals in particular and metallic liquids in general.

Journal of Physics Condensed Matter

Aggregate frequency width, nuclear hyperfine coupling and Jahn–Teller effect of Cu 2+ impurity ion

M A Hosain, J-M Le Floch, J Krupka and M E Tobar

The impurity paramagnetic ion, {${\rm Cu^{2+}}$} substitutes Al in the {${\rm SrLaAlO_4}$} single crystal lattice, this results in a {${\rm CuO_6}$} elongated octahedron, and the resulting measured g -factors satisfy four-fold axes variation condition. The aggregate frequency width of the electron spin resonance with the required minimum level of impurity concentration has been evaluated in this single crystal {${\rm SrLaAlO_4}$} at 20 millikelvin. Measured parallel hyperfine constants, {$A_{\Vert {\rm Cu}}$}

Journal of Physics Condensed Matter

Magnetic behavior of metastable Fe films grown on Ir(1 1 1)

Alberto Calloni, Matteo Cozzi, Madan S Jagadeesh, Gianlorenzo Bussetti, Franco Ciccacci and Lamberto Duò

We investigated the growth of ultra-thin Fe films on Ir(1 1 1) by means of in situ low energy electron diffraction and spin-resolved photoemission techniques. We observe a (1  ×  1) diffraction pattern, characteristic of the fcc substrate, below four monolayers (ML). Then, a complex superstructure starts to develop, compatible with the formation of bcc -like Fe domains aligned with the substrate according to the Kourdjumov–Sachs orientation relationships. The analysis of the diffraction patterns reveals a progressive evolution towards a fully relaxed bcc lattice, characteristic of bulk Fe. Both photoemission (filled states) and inverse photoemission (empty states) results show characteristic features related to the contribution of the Fe layer, evolving towards those observed on the Fe (1 1 0) bcc surface. Spin resolution allows to detect a spectral polarization above 4 ML, corresponding to the formation of bcc Fe, which gradually increas...

Journal of Physics Condensed Matter

Current-induced instability of domain walls in cylindrical nanowires

Weiwei Wang, Zhaoyang Zhang, Ryan A Pepper, Congpu Mu, Yan Zhou and Hans Fangohr

We study the current-driven domain wall (DW) motion in cylindrical nanowires using micromagnetic simulations by implementing the Landau–Lifshitz–Gilbert equation with nonlocal spin-transfer torque in a finite difference micromagnetic package. We find that in the presence of DW, Gaussian wave packets (spin waves) will be generated when the charge current is suddenly applied to the system. This effect is excluded when using the local spin-transfer torque. The existence of spin waves emission indicates that transverse domain walls can not move arbitrarily fast in cylindrical nanowires although they are free from the Walker limit. We establish an upper velocity limit for DW motion by analyzing the stability of Gaussian wave packets using the local spin-transfer torque. Micromagnetic simulations show that the stable region obtained by using nonlocal spin-transfer torque is smaller than that by using its local counterpart. This limitation is essential for multiple DWs since the instab...

Journal of Physics Condensed Matter

Wed Nov 29 2017

Probabilistic design of a molybdenum-base alloy using a neural network

Author(s): B.D. Conduit, N.G. Jones, H.J. Stone, G.J. Conduit

An artificial intelligence tool is exploited to discover and characterize a new molybdenum-base alloy that is the most likely to simultaneously satisfy targets of cost, phase stability, precipitate content, yield stress, and hardness. Experimental testing demonstrates that the proposed alloy fulfills the computational predictions, and furthermore the physical properties exceed those of other commercially available Mo-base alloys for forging-die applications.

Scripta Materialia

Colossal permittivity in niobium doped BaTiO3 ceramics annealed in N2

Author(s): Pengrong Ren, Jiaojiao He, Xin Wang, Mingqiang Sun, Hu Zhang, Gaoyang Zhao

Nb5+ and/or In3+ doped BaTiO3 ceramics were prepared by a conventional solid-state method, followed by annealing in N2. At the room temperature and 1kHz, permittivity of Nb5+ doped BaTiO3 (BTNb) is up to 80,888 and its dielectric loss is as low as 0.03. The origin of the colossal permittivity in BTNb is ascribed to the giant defect dipoles (Ti4+ ·e′VO •• Ti4+ ·e′). Besides, (VBa VO ••) defect dipoles are also proved to be present in BTNb, which plays an important role on maintaining lower dielectric loss in a wide temperature region for BTNb.

Scripta Materialia

Fabrication of high aspect ratio gallium nitride nanostructures by photochemical etching for enhanced photocurrent and photoluminescence property

Author(s): Miao-Rong Zhang, Qing-Mei Jiang, Fei Hou, Zu-Gang Wang, Ge-Bo Pan

High aspect ratio gallium nitride (GaN) nanostructures were fabricated by photochemical etching using hydrofluoric acid as the etchant. Under the etching time of 20min, the surface pore density is estimated to be ~5.6×109 cm2, and the aspect ratio of the pore structure is as high as ~10: 1. The biggest photocurrent and strongest photoluminescence intensity of the etched GaN are ~3.2 and 9.7 times of that of the as-grown GaN, respectively, which demonstrates high aspect ratio GaN nanostructures have great potential for a series of photoelectric and optical devices.

Scripta Materialia

Effect of Cu doping on microstructure and thermoelectric properties of Bi2Te2.85Se0.15 bulk materials

Author(s): Zhi-Lei Wang, Tetsuhiko Onda, Zhong-Chun Chen

Cu was incorporated into Bi2Te2.85Se0.15 compound as a dopant, and the microstructure and thermoelectric properties of sintered CuxBi2Te2.85Se0.15 samples have been investigated. The Cu atoms intercalated into interstitial sites along the c-axis, which caused an increase in lattice constant c. The accommodation of Cu in Bi2Te2.85Se0.15 reached saturation at x=0.05 and the excessive Cu formed a copper telluride compound. The interstitial Cu resulted in decreases in carrier concentration and phonon thermal conductivity as well as an increase in carrier mobility. A maximum ZT value was obtained at x=0.05, which was enhanced for about 4 times compared to Cu-free sample.

Scripta Materialia

Bonding technology based on solid porous Ag for large area chips

Author(s): Chuantong Chen, Seungjun Noh, Hao Zhang, Chanyang Choe, Jinting Jiu, Shijo Nagao, Katsuaki Suganuma

A bonding technology is introduced by using surface polished porous Ag in die-attachment structure. Bonding strength did not change much as the chip size varied from 3×3mm2 to 15×15mm2. This confirms that the technology was not influenced by the chip size, and thus can be used in large area bonding. Bonding mechanism based on stress-induced migration was discussed with the three dimensional finite element analyses. Transmission electron microscopy (TEM) observation further confirmed that single crystal hillocks and Ag particles formed at the bonding interface, bridging the interface together.

Scripta Materialia

Investigation on laser welding of selective laser melted Ti-6Al-4V parts: Weldability, microstructure and mechanical properties

Author(s): Hanchen Yu, Fangzhi Li, Jingjing Yang, Jianjun Shao, Zemin Wang, Xiaoyan Zeng

In this study, laser welding was successfully used to join selective laser melted (SLMed) to SLMed and SLMed to wrought Ti-6Al-4V specimens. The microstructures, microhardnesses, tensile performances, fatigue lives and fatigue crack growth rates (FCGRs) of the welded specimens as well as SLMed specimens were investigated. It is found that the stress-relieved SLMed Ti-6Al-4V has a good laser weldability. The SLMed to SLMed and SLMed to wrought joints have similar microstructures (columnar prior β grain boundaries with inside acicular α′), microhardnesses (410 HV–450 HV), ultimate tensile strengths (UTSs) (~ 1200MPa), yield strengths (YSs) (~ 1080MPa) and FCGRs in the welding zones, which are also similar to those of SLMed specimens. But the fatigue lives of SLMed to SLMed and SLMed to wrought joints are lower than those of SLMed Ti-6Al-4V, and much lower than those of traditionally wrought annealed Ti-6Al-4V. Heat treatment on SLMed to SLMed joints decreases the microhardnesses (390 HV–410 HV), UTSs (1106MPa), YSs (1008MPa) and FCGRs in the welding zones, but shows no improvements on the fatigue lives. It can be concluded that microstructures significantly influence the microhardnesses, UTSs, YSs and FCGRs of the welded and SLMed specimens, but have very limited influences on the fatigue lives due to the existences of pores. Pores play a more decisive role in the fatigue lives, but have very limited influences on the UTSs, YSs and FCGRs.

Science and Engineering A

Microstructural evolution and mechanical behavior of W-Si-C multi-phase composite prepared by arc-melting

Author(s): Kejia Kang, Lianmeng Zhang, Guoqiang Luo, Jian Zhang, Rong Tu, Chuandong Wu, Qiang Shen

In this work, to improve the strain capacity of tungsten, β-SiC was added to form W-Si-C multi-phase composites composed of W, W2C, and W5Si3 using the arc-melting method. The relationship between microstructure and mechanical behavior was investigated, and a microstructural evolution mechanism was proposed. The grain size of the W was refined significantly from 1071.8 to 5.4µm as the SiC content increased. The micro-hardness (3.67–12.79GPa) and ultimate compressive strength (UCS) (0.90–2.29GPa) of the W-Si-C multi-phase composite obviously increased as the β-SiC content increased to 4wt% due to the grain refinement and segregation of the W2C and W5Si3 at the grain boundaries. The highest strain of UCS (21.9%) was obtained at 1wt% SiC, which is three times of that of pure W (7.4%) and is a result of the refined W grain size and the potentiation of W5Si3.

Science and Engineering A

Key criterion for achieving giant recovery strains in polycrystalline Fe-Mn-Si based shape memory alloys

Author(s): Huabei Peng, Gaixia Wang, Shanling Wang, Jie Chen, Ian MacLaren, Yuhua Wen

In this study, it is proposed that coarsening austenitic grains is a key criterion for achieving giant recovery strains in polycrystalline Fe-Mn-Si based shape memory alloys. In order to verify the hypothesis, the relationship between recovery strains and austenitic grain-sizes in cast and processed Fe-Mn-Si based shape memory alloys was investigated. The recovery strain of cast Fe-19Mn-5.5Si-9Cr-4.5Ni alloy with the coarse austenitic grains of 652µm reached 7.7% while the recovery strain of one with the relatively small austenitic grains of 382µm was only 5.4%. Moreover, a recovery strain of 5.9%, which is the highest previously published value for solution-treated processed Fe-Mn-Si based shape memory alloys, was obtained by coarsening the austenitic grains through only solution treatment at 1483K for 360min in a processed Fe-17Mn-5.5Si-9Cr-5.5Ni-0.12C alloy. However, its recovery strain was still 5.9% after thermo-mechanical treatment consisting of 10% tensile strain at room temperature and annealing at 1073K for 30min. This happens because annealing twins play a negative role, refining the austenitic grains, limiting the recovery strains to below 6%. In summary, coarse austenitic grains enable the achievement large recovery strains by two mechanisms. Firstly, the grains are bigger, and consequently there are fewer grain boundaries, and thus their suppressive effects of grain boundaries on stress-induced ε martensitic transformation is reduced. Secondly, coarse austenitic grains are advantageous to introduce ε martensite with single orientation and reduce the collisions of different martensite colonies, especially when the deformation strain is large. As such, the ceiling of recovery strains is dependent on the austenitic grain-sizes.

Science and Engineering A

Rate and temperature dependent deformation behavior of as-cast WE43 magnesium-rare earth alloy manufactured by direct-chill casting

Author(s): Mohammad Jahedi, Brandon A. McWilliams, Franklin R. Kellogg, Irene J. Beyerlein, Marko Knezevic

In this work, we study the deformation behavior of a direct chill cast WE43 Mg alloy. This material initially has equiaxed grains approximately 40µm in diameter and a random texture. The room temperature, quasi-static response exhibits little plastic anisotropy when evaluated parallel to and normal to the solidification direction and no initial yield tension-compression asymmetry. The deformation at room temperature is accompanied by significant basal texture development and formation of three types of deformation twins: { 10 1 ̅ 2 } 1 ̅ 011 , { 10 1 ̅ 1 } 10 12 ̅ , and { 11 2 ̅ 1 } 1 ̅ 1 ̅ 26 as well as double twins { 10 1 ̅ 1 } 10 12 ̅ - { 10 1 ̅ 2 } 1 ̅ 011 , although each in small amounts < 10% up to failure. We find that in the elevated 250°C ± 20°C regime, the material exhibits a negative strain rate sensitivity, with a decreasing flow stress as the strain rate increases. In most of the high-temperature, 250°C to 350°C, and high-strain-rate, 0.01/s to 10/s, tests, the material failed at moderate compression strains, 0.35–0.45. Subsequent fracture analyses find that the material fractures transgranularly by a typical shear fracture, often with the presence of additional arrested cracks. At elevated temperatures and under strain rates sufficiently low (i.e., 275°C and 0.01/s, 300°C and 0.1/s, 350°C and 1/s, 375°C and 10/s) or at a temperature of 400°C deformation conditions, the material exhibited pseudo-super plastic behavior, experiencing relatively high deformation strains (> 1.0 true strain) without fracturing.

Science and Engineering A

Microstructure and mechanical properties of as-cast and as-hot-rolled novel Mg-xSn-2.5Zn-2Al alloys (x = 2, 4wt%)

Author(s): Zhang-Zhi Shi, Jun-Yi Xu, Jing Yu, Xue-Feng Liu

Two novel Mg-Sn based wrought alloys are developed with good mechanical properties, which are Mg-2Sn-2.5Zn-2Al (TZA222) alloy and Mg-4Sn-2.5Zn-2Al (TZA422) alloy. The as-cast alloys with high elongations to failure over 10% and low tensile strengths just over 70MPa are suitable for hot deformation processing. After hot rolling at 350°C of 60% total reduction, the strengths of the alloys are significantly improved, meanwhile, their elongations to failure still remain at high level. The as-hot-rolled TZA422 alloy has better mechanical properties, of which the yield strength, ultimate tensile strength and elongation to failure are 196.9MPa, 285.5MPa and 20.7%, respectively. It has nano-sized Mg2Sn particles dispersed within fine grains with an average size of 10µm, while the main second phase is coarse Mg5Zn2Al2 in the as-cast TZA422 alloy. Both the as-hot-rolled TZA422 and TZA222 alloys have good combinations of strength and ductility, which are comparable to other as-hot-rolled Mg alloys with high performances.

Science and Engineering A

Effect of yttria inclusion on room temperature tensile properties of investment cast TiAl

Author(s): Bochao Lin, Renci Liu, Qing Jia, Yuyou Cui, Paul A. Withey, Rui Yang

Yttria is the best face coat material in the shell mold for the investment casting of γ-TiAl alloys. However, yttria inclusions may occur in the cast products and there is a lack of knowledge about the influence of yttria inclusions on the mechanical properties of TiAl. In the present study, the effect of yttria inclusions on room temperature tensile properties of two investment cast TiAl alloys, Ti-45Al-2Mn-2Nb (at%) + 0.08vol% TiB2 and Ti-46 Al-2Nb-2Cr-0.15B (at%), were studied. The results show that tensile failure tends to initiate from yttria inclusions, resulting in reduced plastic strain and ultimate strength. Yttria inclusions differ in location (surface /internal), size, and morphology (agglomerate /whole piece), those with larger sizes or located at the sample surface being more harmful. The tensile properties of inclusion containing TiAl parts can be estimated using the size and location determined by X-ray inspection. The critical size at which inclusions become detrimental to the mechanical properties of TiAl was discussed.

Science and Engineering A

High temperature stabilization of a nanostructured Cu-Y2O3 composite through microalloying with Ti

Author(s): Dengshan Zhou, Hongwei Geng, Wei Zeng, Dengqi Zheng, Hucheng Pan, Charlie Kong, Paul Munroe, Gang Sha, Challapalli Suryanarayana, Deliang Zhang

The effects of minor additions (0.2, 0.4 and 0.8wt%) of Ti on the thermal stability of both Cu nanograins and Y2O3 particles in nanostructured Cu-5vol%Y2O3 composite powder particles were investigated via 1h isochronal annealing at temperatures ranging from 300 to 1000°C. It was found that a small amount addition of 0.4wt%Ti effectively inhibits the coarsening of the Y2O3 particles during annealing at a very high homologous temperature of 0.87Tm, where Tm is the melting point of Cu, which, in turn, stabilizes Cu nanograins and retains the hardness value of the as-milled powder sample. However, this is in clear contrast with the significant decrease in hardness of the Ti-free and 0.2wt%Ti doped milled powder samples annealed at the same condition, resulting from the coarsening of the Y2O3 particles and growth of Cu nanograins. The energy dispersive X-ray spectrometry elemental analysis shows that the stabilizing mechanisms responsible for the improved thermal stability of the Y2O3 particles are associated with the chemical reactions between Ti, O and Y2O3.

Science and Engineering A

Vacancy effects on the mechanical behavior of B2-FeAl intermetallics

Author(s): M. Zamanzade, G. Hasemann, C. Motz, M. Krüger, A. Barnoush

The present work addresses the impact of quenched-in thermal vacancies on the mechanical properties of B2 iron aluminides. For this reason different binary Fe-Al alloys with different histories of heat treatment were indented at room temperature. The contributions of mono- and bi-vacancies in addition to triple defects on the elastic, plastic and elastic-plastic transition were evaluated using nanoindentation technique. In the samples containing bi-vacancies, the increase of the hardness was much more pronounced compared to the mono-vacancies.

Science and Engineering A

Microstructure modification and mechanical property improvement of reduced activation ferriticmartensitic steel by severe plastic deformation

Author(s): Xiaojie Jin, Shenghu Chen, Lijian Rong

9Cr2WVTa ferritic/martensitic steel was processed by cold-swaging and post-annealing in order to investigate the microstructure evolution and its effect on the mechanical properties. Optical microscopy, scanning electron microscopy with electron backscatter diffraction, and transmission electron microscopy were utilized for the microstructural characterization during the cold-swaging and post-annealing process, and the mechanical properties were determined by microhardness, tensile and creep tests. The results revealed that, nearly equiaxed ultrafine grains with the average size of ~ 330nm and fine dispersed carbides with the average length of ~ 50nm, were obtained after cold-swaging and post-annealing. In comparison with the initial normalized-tempered sample, effective grain boundary strengthening and dispersion strengthening in the post-annealed sample enhanced the strength at room and elevated temperature. The presence of fine dispersed carbides could retard the initiation and propagation of the cracks, leading to a better ductility in the post-annealed sample. During creep test, fine dispersed carbides in the post-annealed sample effectively slowed down grain boundary migration. The pinned grain boundaries and fine dispersed carbides in the post-annealed sample acted as obstacles to the motion of mobile dislocations, which resulted in the enhanced creep properties.

Science and Engineering A

Mechanical properties and deformation twinning behavior of as-cast CoCrFeMnNi high-entropy alloy at low and high temperatures

Author(s): Jeoung Han Kim, Ka Ram Lim, Jong Woo Won, Young Sang Na, Hyoung-Seop Kim

Tensile properties of an as-cast CoCrFeMnNi high-entropy alloy were investigated at various temperatures ranging from −160 to 1000°C. The tensile strength and ductility did not vary significantly with loading direction, despite the alloy’s strongly preferred crystallographic orientation. The impact toughness values of the as-cast high-entropy alloy were much higher than those of many traditional alloys, particularly at low temperatures. The mechanical properties of the as-cast high-entropy alloy were compared with those of wrought high-entropy alloy and noticeable differences between the two alloys were found. The maximum tensile ductility and three different strain hardening stages were observed at 500°C in the as-cast structure. Transmission electron microscopy observations demonstrated that the initiation of deformation twinning was very active even at 500°C. A simple calculation suggests that very large grains of the as-cast structure induced a reduction in twinning stress, retarding the onset of strain localization.

Science and Engineering A

A coarse-grained polarizable force field for the ionic liquid 1-butyl-3-methylimidazolium

Johannes Zeman, Frank Uhlig, Jens Smiatek and Christian Holm

We present a coarse-grained polarizable molecular dynamics force field for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF 6 ]). For the treatment of electronic polarizability, we employ the Drude model. Our results show that the new explicitly polarizable force field reproduces important static and dynamic properties such as mass density, enthalpy of vaporization, diffusion coefficients, or electrical conductivity in the relevant temperature range. In situations where an explicit treatment of electronic polarizability might be crucial, we expect the force field to be an improvement over non-polarizable models, while still profiting from the reduction of computational cost due to the coarse-grained representation.

Journal of Physics Condensed Matter

Tue Nov 28 2017

Chiral liquid crystal colloids

Ye Yuan, Angel Martinez, Bohdan Senyuk, Mykola Tasinkevych & Ivan I. Smalyukh

Colloidal chiral springs and helices are formed by light inside a nematic liquid crystal suspension, predefining the mesoscopic superstructures self-assembled in such systems.

Nature

Dislocation nucleation facilitated by atomic segregation

Lianfeng Zou, Chaoming Yang, Yinkai Lei, Dmitri Zakharov, Jörg M. K. Wiezorek, Dong Su, Qiyue Yin, Jonathan Li, Zhenyu Liu, Eric A. Stach, Judith C. Yang, Liang Qi, Guofeng Wang & Guangwen Zhou

In situ transmission electron microscopy combined with theory modelling reveals that surface segregation in CuAu solid solution generates misfit dislocations, providing atomistic mechanisms of dislocation nucleation and dynamics at heterointerfaces.

Nature

Plating and stripping calcium in an organic electrolyte

Da Wang, Xiangwen Gao, Yuhui Chen, Liyu Jin, Christian Kuss & Peter G. Bruce

Although multivalent cation batteries based on magnesium, calcium or aluminium are technologically attractive, the metal anode still represents a challenge. It is now demonstrated that significant quantities of calcium can be plated and stripped at room temperature with low polarization.

Nature

Ubiquitous formation of bulk Dirac cones and topological surface states from a single orbital manifold in transition-metal dichalcogenides

M. S. Bahramy, O. J. Clark, B.-J. Yang, J. Feng, L. Bawden, J. M. Riley, I. Marković, F. Mazzola, V. Sunko, D. Biswas, S. P. Cooil, M. Jorge, J. W. Wells, M. Leandersson, T. Balasubramanian, J. Fujii, I. Vobornik, J. E. Rault, T. K. Kim, M. Hoesch, K. Okawa, M. Asakawa, T. Sasagawa, T. Eknapakul, W. Meevasana & P. D. C. King

Type-I and type-II bulk Dirac cones and ladders of topological surface states are shown to form in six different transition-metal dichalcogenides.

Nature

Gradient residual strain and stress distributions in a high pressure torsion deformed iron disk revealed by high energy X-ray diffraction

Author(s): J. Todt, J. Keckes, G. Winter, P. Staron, A. Hohenwarter

High energy X-ray diffraction is used to investigate for the first time the distribution of residual X-ray elastic stresses inside a high pressure torsion (HPT) deformed iron disk with a diameter and thickness of ~30 and ~8mm, respectively. In the experiment, a dedicated conical slit system restricts the diffraction gauge volume in three dimensions to ~0.45mm3, which is then used to scan the bulk sample cross-section in a non-invasive manner. Pronounced residuals stress gradients with maximal tensile stresses of ~200MPa are observed along radial and tangential directions and are correlated to the deformation gradient arising from HPT.

Scripta Materialia

Thu Dec 14 2017

Acta Materialia
Physical Review B
Journal of Physics Condensed Matter

Wed Dec 13 2017

Computational Materials Science
Science and Engineering A
Physical Review Letters
Physical Review B

Tue Dec 12 2017

Nature
Computational Materials Science
Physical Review Letters
Physical Review B
Journal of Physics Condensed Matter

Sat Dec 9 2017

Acta Materialia
Computational Materials Science
Physical Review B
Journal of Physics Condensed Matter

Fri Dec 8 2017

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

Wed Dec 6 2017

Acta Materialia
Scripta Materialia
Physical Review Letters
Journal of Physics Condensed Matter

Tue Dec 5 2017

Nature
Physical Review Letters
Physical Review B
Journal of Physics Condensed Matter

Mon Dec 4 2017

Scripta Materialia
Computational Materials Science
Science and Engineering A

Fri Dec 1 2017

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

Thu Nov 30 2017

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

Wed Nov 29 2017

Scripta Materialia
Science and Engineering A
Journal of Physics Condensed Matter

Tue Nov 28 2017

Nature
Scripta Materialia