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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!

Sat Oct 21 2017

Computational study of phase engineered transition metal dichalcogenides heterostructures

Author(s): Jiachen Ma, Qiaoxuan Zhang, Jie Yang, Shenyan Feng, Ming Lei, Ruge Quhe

Van der Waals heterostructures composed of the same transition metal dichalcogenide with different structural phases are promising for optimizing the electrical contacts in two-dimensional transistors. While most theoretical studies are focused on the “more stable” 1T′ or Td phase, experiments show that the metastable 1T phase can also form heterostructure with the 2H phase. However, the intrinsic properties of these phase engineered van der Waals heterostructures are not clear. By first–principles calculations, we investigate the structural and electronic properties of the MX2 (M=Mo, W; X=S, Se, Te) heterostructures composed by the semiconducting 2H and metallic 1T phases. A very small amount of electron transfers from 1T to 2H phases in the checked heterostructures, rendering the 2H phase undoped or only slightly N–doped. The existence of the Schottky barriers (0.39–0.94 eV) is found common in the free-standing 2H/1T MX2 heterostructures. In addition, the zero tunnel barrier heights in the checked heterostructures are advantageous for achieving high charge injection rate.

Computational Materials Science

Investigation on tensile properties of nanocrystalline titanium with ultra-small grain size

Author(s): Le Chang, Chang-Yu Zhou, Jian Li, Xiao-Hua He

Molecular dynamics was used to simulate tensile behavior of nanocrystalline titanium with ultra-small grain size ranging from 2.8 nm to 10.2 nm at the strain rate ranging from 108 s−1 to 1010 s−1. Three dimensional samples with random oriented grains containing no textures were developed by Voronoi tessellation. For all the samples, the yielding is solely controlled by GB mediated process. The inverse Hall-Petch relation between grain size and flow stress was found. The strain rate sensitivity shows increase with the decrease of grain size due to the enhancement of grain boundary (GB) mediated process. Meanwhile, it increases with the applied strain rate due to the local disorder around GBs. Remakeable grain coarsening observed in the 2.8 nm sample causes the slight increase of flow stress. Both GB mediated process and partial dislocation slips play an important role in the plastic deformation of nanocrystalline Ti. With the increase of grain size, rare twins initiated from GBs can be observed. From the size dependent dislocation density analysis, it is concluded that with the increase of grain size, dislocation related deformation contributes more to the plastic strain.

Computational Materials Science

Electronic structure, vibronic properties and enhanced magnetic anisotropy induced by tetragonal symmetry in ternary iron nitrides: A first-principles study

Author(s): Z.R. Li, W.B. Mi, H.L. Bai

Electronic structure, vibronic properties and magnetic anisotropy energy of ternary iron nitrides Mx Fe4− x N (x =1, 3) with 3d transition metal M are systematically studied using first-principles calculations. Different substitution elements and sites result in different lattice structure and exchange coupling. We found that the magnetic anisotropy energy in MFe3N and M 3FeN with tetragonal structure greatly improves, which is much larger than cubic Fe4N. Enhanced magnetic anisotropy is closely related to the tetragonal structure transition, and this result will help to further understand the mechanism of low-temperature anisotropic magnetoresistance of Fe4N. In consideration of the magnetism and stability, the implantation of Mn, Co, and Ni is more valuable in all of substitutions. MnFe3N energetically favors the tetragonal structure and has a perpendicular magnetic anisotropy, while Mn3FeN prefers to cubic structure and the total magnetic moment is enhanced. Co shows no strong preference for either lattice site but negative spin polarization appears, whereas Ni3FeN clearly favors tetragonal structure and possesses perpendicular magnetic anisotropy. Our results lay the groundwork for developing the application of Fe4N in spintronic devices and can stimulate further experimental research.

Computational Materials Science

Quantitative 3D phase field modelling of solidification using next-generation adaptive mesh refinement

Author(s): Michael Greenwood, K.N. Shampur, Nana Ofori-Opoku, Tatu Pinomaa, Lei Wang, Sebastian Gurevich, Nikolas Provatas

Phase field (PF) models are one of the most popular methods for simulating solidification microstructures due to their fundamental connections to the physics of phase transformations. However, these methods are numerically very stiff due to the multiple length scales in a solidifying material, from the nanoscopic solid-liquid interface, to dendritic structures on the order of hundreds of microns. While this problem can be greatly alleviated by thin-interface analytical treatments of the PF equations, additional numerical methods are required to explore experimentally relevant sample sizes and times scales. It was shown about 18years ago that the use of dynamic adaptive mesh refinement (AMR) can alleviate this problem by exploiting the simple fact that the majority of the solidification kinetics occur at the solid-liquid interface, which scales with a lower dimensionality than the embedding system itself. AMR methods, together with asymptotic analysis, nowadays provide one of the most efficient numerical strategies for self-consistent quantitative PF modelling of solidification microstructure processes. This paper highlights the latest developments in the AMR technique for 3D modelling of solidification using classical phase field equations. This includes a move away from finite element techniques to faster finite differencing through the use of dynamic mini-meshes which are each associated with each node of a 3D Octree data structure, and distributed MPI parallelism that uses a new communication algorithm to decompose a 3D domain into multiple adaptive meshes that are spawned on separate cores. The numerical technique is discussed, followed by demonstrations of the new AMR algorithm on select benchmark solidification problems, as well as some illustrations of multi-phase modelling using a recently developed multi-order parameter phase field model.

Computational Materials Science

Theoretical study of accumulation electron layers at ITO-dielectric interfaces and related nanostructures

Author(s): Liang Li, Hua Zhao, Jingwen Zhang

We present the transfer matrix method for accumulation electron layers at ITO-dielectric interfaces. Meanwhile, the optical transmission properties of related nanostructures are investigated in the visible frequencies. Our results show that the nanostructures possess band-stop property under corresponding conditions. It is observed that the performance of band-stop is critically dependent on the electron density and on the number of accumulation electron layers. In addition, the thickness of ITO layers and the condition of incident lights also influence the band-stop properties. Moreover, we find an optical switch behavior when the thickness of ITO layer is set as 5.0 nm. This work may provide means for designing tunable optical devices.

Computational Materials Science

Determination of JMAK dynamic recrystallization parameters through FEM optimization techniques

Author(s): Missam Irani, Mansoo Joun

A practical method for determining dynamic recrystallization (DRX) parameters, consisting of finite-element analysis, optimization techniques, and experimental studies, is presented. The method iteratively minimizes the error between target values and the corresponding finite-element solutions using a general-purpose optimization tool. An isothermal hot compression test of AA6060 aluminum alloy was used to investigate the validity of the method. Three main DRX parameters in the Johnson-Mehl-Avrami-Kolmogorov model were obtained using the method, and good agreement was observed between the optimized parameters and reference values from the literature. The effects of mesh size and the number of sample points on the accuracy of the responses were also studied to show the numerical characteristics of the method, revealing that the greater the number of elements and sample points, the more accurate and reliable the results. Parameters acquired using fewer elements also provided acceptable optimized parameters for predicting DRX grain size.

Computational Materials Science

The role of Cr on oxide formation in Ni-Cr alloys: A theoretical study

Author(s): Ki-Ha Hong, Jeoung Han Kim, Kunok Chang, Junhyun Kwon

Cr is a critical alloying element in Ni-based alloys. The role of Cr on the formation of oxide nuclei in Ni-Cr alloys is theoretically investigated with density functional theory. Our simulation indicates that Cr has strong binding energy with the nearest oxygen and that it enhances the stability of the metal-oxide nucleus. Therefore, oxide dispersion can be promoted by the incorporation of Cr into Ni alloys. Additionally, potential oxide candidates for the oxide strengthening of Ni-Cr alloys are theoretically explored. The findings, through an analysis of electronic structures, suggest that La and Y are most promising when attempting to realize stable oxide formation in Ni-Cr alloys and reveal that the metal used to create the primary oxides should have low electronegativity and a large atomic radius to create stable metal-oxide nuclei.

Computational Materials Science

Effects of alloying elements on the elastic properties of bcc Ti-X alloys from first-principles calculations

Author(s): Cassie Marker, Shun-Li Shang, Ji-Cheng Zhao, Zi-Kui Liu

Titanium alloys are great implant materials due to their mechanical properties and biocompatibility. However, a large difference in Young’s modulus between bone (∼10–40 GPa) and common implant materials (ie. Ti-6Al-4V alloy ∼110 GPa) leads to stress shielding and possible implant failure. The present work predicts the single crystal elastic stiffness coefficients (cij’s) for five binary systems with the body centered cubic lattice of Ti-X (X = Mo, Nb, Ta, Zr, Sn) using first-principles calculations based on Density Functional Theory. In addition, the polycrystalline aggregate properties of bulk modulus, shear modulus, Young’s modulus, and Poisson ratio are calculated. It is shown that the lower Young’s modulus of these Ti-alloys stems from the unstable bcc Ti with a negative value of (c11–c12). The data gathered from these efforts are compared with available experimental and other first-principles results in the literature, which set a foundation to design biocompatible Ti alloys for desired elastic properties.

Computational Materials Science

Atomic-scale dynamics and mechanical response of geopolymer binder under nanoindentation

Author(s): Mohammad Rafat Sadat, Stefan Bringuier, Krishna Muralidharan, George Frantziskonis, Lianyang Zhang

Using molecular dynamics simulations, the mechanical response of amorphous geopolymer binder (GB) under spherical nanoindentation was examined as a function of GB composition (Si/Al ratio), indenter size (radius of indenter) and loading rates. The observed hardness values were strongly dependent on the indenter size and loading rates. Specifically, the GB hardness increased with decreasing indenter size and increasing loading-rate. The indenter size effect and the effect of loading rate were related to the ease of rotation of the underlying Si and Al tetrahedra in conjunction with the breaking of bridging SiO and AlO bonds. Further, for a given indenter size, increasing the Si/Al ratio increased the hardness and Young’s modulus of the GB, which was correlated to higher strength of SiO bonds as compared to AlO bonds present in the GB.

Computational Materials Science

A hybrid optimization algorithm to explore atomic configurations of TiO2 nanoparticles

Author(s): Eric Inclan, David Geohegan, Mina Yoon

In this paper we present a hybrid algorithm comprised of differential evolution, coupled with the Broyden–Fletcher–Goldfarb–Shanno quasi-Newton optimization algorithm, for the purpose of identifying a broad range of (meta)stable Ti n O2 n nanoparticles, asan example system, described by Buckingham interatomic potential. The potential and its gradient are modified to be piece-wise continuous to enable use of these continuous-domain, unconstrained algorithms, thereby improving compatibility. To measure computational effectiveness a regression on known structures is used. This approach defines effectiveness as the ability of an algorithm to produce a set of structures whose energy distribution follows the regression as the number of Ti n O2 n increases such that the shape of the distribution is consistent with the algorithm’s stated goals. Our calculation demonstrates that the hybrid algorithm finds global minimum configurations more effectively than the differential evolution algorithms, widely employed in the field of materials science. Specifically, the hybrid algorithm is shown to reproduce the global minimum energy structures reported in the literature up to n =5, and retains good agreement with the regression up to n =25. For 25< n <100, where literature structures are unavailable, the hybrid effectively obtains structures that are in lower energies per TiO2 unit as the system size increases.

Computational Materials Science

Data assimilation for phase-field models based on the ensemble Kalman filter

Author(s): Kengo Sasaki, Akinori Yamanaka, Shin-ichi Ito, Hiromichi Nagao

We have developed a data assimilation (DA) methodology based on the ensemble Kalman filter (EnKF) for estimating unknown parameters involved in a phase-field model from observational/experimental data. The DA methodology based on Bayesian statistics is able to estimate parameters by incorporating observational/experimental data into the phase-field model and evaluate the uncertainty of the estimated parameters. In this paper, we apply the EnKF-based DA method to estimate the phase-field mobility for a phase-field simulation of the isothermal austenite-to-ferrite transformation in a Fe–C–Mn alloy. Our DA method is validated through numerical experiments called “twin experiments” to verify that the DA method can estimate a priori assumed-true phase-field mobility from synthetic observational data. The results of the twin experiments using various initial phase-field mobilities show that our DA methodology can successfully estimate the true phase-field mobility, even when the initial value largely deviates from the true value. Furthermore, our DA method reveals the sampling interval for observational data necessary to accurately estimate the parameter and its uncertainty.

Computational Materials Science

Dislocation-precipitate interaction map

Author(s): Amirreza Keyhani, Reza Roumina

The present research is the first attempt to systematically quantify the dislocation-precipitate interaction in terms of the applied shear stress, precipitate resistance, and required time to reach the critical state of dislocation-precipitate interaction when a dislocation line is about to pass through precipitates. A modified line dislocation dynamics is adopted to model the dislocation-precipitate interaction. In the present modeling approach utilizing three-dimensional dislocation dynamics simulations, thousands of data points on Cu are obtained accounting for various precipitate resistances, applied shear stresses, and precipitate spacing. A universal equation is found based on simulations to quantify dislocation-precipitate interactions in terms of the applied shear stress, precipitate resistance scale, and dislocation-precipitate interaction time. The dislocation-precipitate interaction time versus precipitate resistance and stress, referred to as “dislocation-precipitate interaction map,” determines the “pass” or “no-pass” state of the interaction. Using this map, we incorporate the dislocation-precipitate interaction time in a two-dimensional dislocation dynamics approach (DD) coupled with the finite element method (FEM). This framework, FEM-DD, is applied to model mechanical behavior of a free-standing copper thin film. Simulation results show a dual effect of the dislocation-precipitate interaction time on hardening behavior.

Computational Materials Science

Secondary creep stage behavior of copper-clad aluminum thin wires submitted to a moderate temperature level

Author(s): Antoine Gueydan, Eric Hug

This work focuses on the role of the microstructure on the creep behavior of thin copper-clad aluminum (CCA) wires. Creep tests were performed at 150°C on hard drawn and annealed CCA and on pure metals. It is shown that the Cu-Al interface of CCA ensures a mechanical resistance leading to lower creep velocities than for metals. Creep mechanisms are driven by aluminum at lower stresses and copper for higher stresses, independently of the physical nature of the interface.

Science and Engineering A

On the microstructure and mechanical properties of the AlCoCrCuNi high entropy alloy processed in the semi-solid state

Author(s): Ł. Rogal

After semi-solid processing the thixo-cast microstructure consisted of a unique composition of globular grains with a B2-ordered body centered cubic structure (BCC) within coherent nano-cuboidal precipitations (average size of 60nm). The grains contained near equiatomic concentrations of Al, Co, Cr and Ni. They were surrounded by a Cu-enriched homogeneously distributed solid solution with an L12-ordered structure and Co-Cr nano-precipitations. The complex microstructure was responsible for the high strength and moderate ductility at room temperature, which was confirmed by compression test results. The thixo-cast revealed a yield strength of 1580MPa, while its compression strength at 6% of strain reached 2070MPa at an average hardness of 581 HV. The obtained values of yield strength and hardness were about 20% higher than those of the ingot material after direct casting. The melting behavior, microstructure and mechanical properties obtained make the AlCoCrCuNi HEA a promising material for thixoforming.

Science and Engineering A

Revisiting local structural changes in GeO 2 glass at high pressure

Juncai Dong, Hurong Yao, Zhiying Guo, Quanjie Jia, Yan Wang, Pengfei An, Yu Gong, Yaxiang Liang and Dongliang Chen

Despite the great importance in fundamental and industrial fields, understanding structural changes for pressure-induced polyamorphism in network-forming glasses remains a formidable challenge. Here, we revisited the local structural transformations in GeO 2 glass up to 54 GPa using x-ray absorption fine structure (XAFS) spectroscopy via a combination diamond anvil cell and polycapillary half-lens. Three polyamorphic transitions can be clearly identified by XAFS structure refinement. First, a progressive increase of the nearest Ge–O distance and bond disorder to a maximum at ~5–16 GPa, in the same pressure region of previously observed tetrahedral-octahedral transformation. Second, a marked decrease of the nearest Ge–O distance at ~16–22.6 GPa but a slight increase at ~22.6–32.7 GPa, with a concomitant decrease of bond disorder. This stage can be related to a second-order-like transition from less dense to dense octahedral glass. Third, another decrease in the nearest ...

Journal of Physics Condensed Matter

Photonic surface waves on metamaterial interfaces

O Takayama, A A Bogdanov and A V Lavrinenko

A surface wave (SW) in optics is a light wave, which is supported at an interface of two dissimilar media and propagates along the interface with its field amplitude exponentially decaying away from the boundary. Research on surface waves has been flourishing in the last few decades due to their unique properties of surface sensitivity and field localization. These features have resulted in applications in nano-guiding, sensing, light-trapping and imaging based on near-field techniques, contributing to the establishment of nanophotonics as a field of research. Up to now, a wide variety of surface waves has been investigated in numerous material and structure settings. This article reviews the recent progress and development in the physics of SWs localized at metamaterial interfaces, as well as bulk media in order to provide broader perspectives on optical surface waves in general. For each type of surface wave, we discuss the material and structural platforms. We mainly focus on...

Journal of Physics Condensed Matter

Relaxation dynamics of a driven two-level system coupled to a Bose–Einstein condensate: application

Vadim M Kovalev and Wang-Kong Tse

We develop a microscopic theory for the relaxation dynamics of an optically pumped two-level system (TLS) coupled to a bath of weakly interacting Bose gas. Using Keldysh formalism and diagrammatic perturbation theory, expressions for the relaxation times of the TLS Rabi oscillations are derived when the boson bath is in the normal state and the Bose–Einstein condensate (BEC) state. We apply our general theory to consider an irradiated quantum dot coupled with a boson bath consisting of a two-dimensional dipolar exciton gas. When the bath is in the BEC regime, relaxation of the Rabi oscillations is due to both condensate and non-condensate fractions of the bath bosons for weak TLS-light coupling and pre dominantly due to the non-condensate fraction for strong TLS-light coupling. Our theory also shows that a phase transition of the bath from the normal to the BEC state strongly influences the relaxation rate of the TLS Rabi oscillations. The TLS relaxation rate is approximately in...

Journal of Physics Condensed Matter

A bird’s eye view on the flat and conic band world of the honeycomb and Kagome lattices: towards an

C Barreteau, F Ducastelle and T Mallah

We present a thorough tight-binding analysis of the band structure of a wide variety of lattices belonging to the class of honeycomb and Kagome systems including several mixed forms combining both lattices. The band structure of these systems are made of a combination of dispersive and flat bands. The dispersive bands possess Dirac cones (linear dispersion) at the six corners ( K points) of the Brillouin zone although in peculiar cases Dirac cones at the center of the zone {$(\Gamma$} point) appear. The flat bands can be of different nature. Most of them are tangent to the dispersive bands at the center of the zone but some, for symmetry reasons, do not hybridize with other states. The objective of our work is to provide an analysis of a wide class of so-called ligand-decorated honeycomb Kagome lattices that are observed in a 2D metal-organic framework where the ligand occu...

Journal of Physics Condensed Matter

Monovacancy paramagnetism in neutron-irradiated graphite probed by 13 C NMR

Z T Zhang, C Xu, D Dmytriieva, S Molatta, J Wosnitza, Y T Wang, M Helm, Shengqiang Zhou and H Kühne

We report on the magnetic properties of monovacancy defects in neutron-irradiated graphite, probed by 13 C nuclear magnetic resonance spectroscopy. The bulk paramagnetism of the defect moments is revealed by the temperature dependence of the NMR frequency shift and spectral linewidth, both of which follow a Curie behavior, in agreement with measurements of the macroscopic magnetization. Compared to pristine graphite, the fluctuating hyperfine fields generated by the defect moments lead to an enhancement of the 13 C nuclear spin-lattice relaxation rate {$1/T_{1}$} by about two orders of magnitude. With an applied magnetic field of 7.1 T, the temperature dependence of {$1/T_{1}$} below about 10 K can well be described by a thermally activated form, ##IMG##

Journal of Physics Condensed Matter

Strain engineering on transmission carriers of monolayer phosphorene

Wei Zhang, Feng Li, Junsong Hu, Ping Zhang, Jiuren Yin, Xianqiong Tang, Yong Jiang, Bozhao Wu and Yanhuai Ding

The effects of uniaxial strain on the structure, band gap and transmission carriers of monolayer phosphorene were investigated by first-principles calculations. The strain induced semiconductor-metal as well as direct–indirect transitions were studied in monolayer phosphorene. The position of CBM which belonged to indirect gap shifts along the direction of the applied strain. We have concluded the change rules of the carrier effective mass when plane strains are applied. In band structure, the sudden decrease of band gap or the new formation of CBM (VBM) causes the unexpected change in carrier effective mass. The effects of zigzag and armchair strain on the effective electron mass in phosphorene are different. The strain along zigzag direction has effects on the electrons effective mass along both zigzag and armchair direction. By contrast, armchair-direction strain seems to affect only on the free electron mass along zigzag direction. For the holes, the effective masses along z...

Journal of Physics Condensed Matter

Relaxation dynamics and polydispersivity associated with defects and ferroelectric correlations in

Asad M Iqbal, Ghulam Hassnain Jaffari, Mohsin Saleemi and Abdullah Ceylan

We present the frequency- and temperature-dependent dielectric response of Eu 1− x Ba x TiO 3 (0  ⩽   x   ⩽  0.5) in detail. Excluding grain boundary effects, four relaxation mechanisms were observed. Relaxation dynamics were observed to arise due to hopping conduction associated with defects, namely oxygen vacancies as well as Eu 3+ and Ti 3+ ions. Dielectric relaxation analysis led to the identification of Ti ions in two different environments with different relaxation rates in the overall EuTiO 3 perovskite structure. The emergence of another relaxation mechanism associated with ferroelectric order as a consequence of the formation of polar regions was also observed for higher Ba concentrations. The addition of Ba led to the identification of relaxation dynamics associated with hopping conduction between Eu ions, Ti ions (in the regions with and without oxygen vacancies) and with the formation of f...

Journal of Physics Condensed Matter

Bilayers of Ni 3 C 12 S 12 and Pt 3 C 12 S 12 : graphene-like 2D topological insulators tunable by

Orlando J Silveira, Érika N Lima and Hélio Chacham

In the present work we predict, through first-principles calculations, that bilayers of the recently synthesized Ni 3 {${\rm C}_{12}$} {${\rm S}_{12}$} and Pt 3 {${\rm C}_{12}$} {${\rm S}_{12}$} layered materials are topological insulators upon electron doping, and that their topological insulator properties can be modulated by the application of electric fields with magnitudes achievable in devices. The electronic structures of both bilayers are characterized by spin–orbit split graphene-like bands, with gap magnitudes that are three orders of magnitude larger t...

Journal of Physics Condensed Matter

Fri Oct 20 2017

Piezoelectricphotoluminescence effect in one-dimensional lead-free nanofibers

Author(s): Xiaoqiu Chen, Qing Wang, Xin Wu, Tao Wang, Yanxue Tang, Zhihua Duan, Dazhi Sun, Xiangyong Zhao, Feifei Wang, Wangzhou Shi

In present work, a multifunctional piezoelectric/photoluminescence effect, which originated from the combination of the piezoelectric properties and the photoluminescence effect, was realized in one-dimensional Er3+ doped lead-free BaTiO3 nanofibers prepared by a sol-gel based electrospinning method. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) were utilized to characterize the morphologies and phase structures. The effect of the host crystallization and rare-earth concentration on the structure and photoluminescence properties was studied. In addition, temperature-dependent PL spectrum exhibited a piezoelectric/photoluminescence coupling effect, characterized by the enhanced photoluminescence intensity around the ferroelectric-paraelectric phase transition temperature of BaTiO3.

Scripta Materialia

Partial dislocation in carbon-vacancy-ordered Nb12Al3C8

Author(s): Hui Zhang, Xiaohui Wang, Erdong Wu, Yanchun Zhou

Basal dislocations in nanolaminated machinable ternary carbides and nitrides (also known as MAX phases) have been postulated to be the result of shearing MA bonds, but have not been microscopically confirmed yet. This study on dislocations and stacking faults in C-vacancy-ordered Nb12Al3C8 demonstrates that the basal dislocation core lies between NbC slabs, verifying the shear of NbAl bonds. The formation of observed 1/3 01 1 ¯ 0 ⟩ type partial dislocations therefore does not give rise to stacking faults. The frequently observed stacking faults are special intergrowth structures of atomically thin NbC slabs embedded in the Nb12Al3C8 grain.

Scripta Materialia

Microstructure and tensile behavior of Fe-8Mn-6Al-0.2C low density steel

Author(s): Xuan Li, Renbo Song, Naipeng Zhou, Jiajia Li

The microstructure and tensile behavior of Fe-8Mn-6Al-0.2C low density steel were investigated in the present study. The results show that, after being solution treated at 900°C for 1h, the steel has a duplex microstructure consisting of ferrite and ~ 30% austenite, and it exhibits an excellent combination in mechanical properties, with a tensile strength of 846MPa and a total elongation of 32%, which were attributed to significant contribution of TRIP effect and three stages work hardening behavior. During tensile test, the work hardening behavior can be defined as three-stage characteristics, in which the stage 2(S2) was linked to the TRIP effect. In S2, with the increase of tensile strain, austenite volume fraction decreased while martensite volume fraction increased gradually. During this process, some austenite grains, positioned at grain boundaries among multiple ferrite grains, were the first ones to transform, while, some austenite grains, located between ferrite laths or embedded in large ferrite grains were often found to undergo rotations. The TRIP effect and the high dislocation density in ferrite and martensite significantly contributed to the excellent mechanical property. In addition, the volume fraction of small angle grain boundaries increased greatly, with a value of over 51% being observed at the true strain of 0.27. The steel possessed a density of 6.95g/cm3, which is about 11% lower than pure iron.

Science and Engineering A

Microstructure and mechanical properties of friction stir processed cast Eglin steel (ES-1)

Author(s): Vedavyas Tungala, Amit Arora, Bharat Gwalani, Rajiv S. Mishra, Raymond E. Brennan, Kyu C. Cho

Eglin steel, an ultra-high-strength steel developed for various demanding applications, is a low-cost alternative to 4340 steel, in which nickel and cobalt additions are replaced by higher tungsten additions, thereby achieving comparable strength and ductility. Friction stir processing (FSP) was carried out on this steel under two heat input conditions, which fell above the A3 transformation line. Microhardness values along the horizontal and vertical directions of the processed region cross section were reported for the lower heat input condition, and correlated with corresponding microstructures. A 3D heat transfer and material flow model was used to predict the peak temperature and cooling rates in these zones. Site-specific tensile tests of specimens extracted from the top to the bottom of the stir zone (SZ) showed ultimate tensile strength (UTS) greater than 2GPa, with a total elongation close to 10% at ~ 4mm from the top surface of SZ. Transmission electron microscopy (TEM) analysis of the high strength location showed microstructure consisting of nano-twinned martensite and nano-bainite laths of size ranging from 200nm to 300nm, and confirmed the existence of retained austenite. This mixed microstructure was comprised of finer aggregates of martensite, bainite and retained austenite, which were postulated to be responsible for the high strength and ductility combinations.

Science and Engineering A

Effects of microstructure and crystallography on mechanical properties of cold-rolled SAE1078 pearlitic steel

Author(s): Y. Liu, C.D. Yang, M. Liu, C.H. Wang, Y.C. Dai, X. Li, A.M. Russell, C.X. Zhang, Z.H. Zhang, G.H. Cao

The evolution of the microstructure and crystallography in SAE1078 pearlitic steel sheets under different cold-rolling reductions of up to 90% were quantified using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). The mechanical properties were determined by tensile testing at room temperature. TEM analysis showed that the pearlite structure was obviously refined with the interlamellar spacing decreasing to about 57nm at the rolling reduction of 90%. EBSD investigations indicated that the ferrite exhibited a {001} <110> texture in the 90% cold-rolled pearlitic steel. The dislocations were mainly concentrated during cold rolling between the 10% and 70% reduction ratios as the average kernel average misorientation (KAM) angle increased from 0.75° to 1.20°. XRD examination revealed that a transformation from bcc to bct crystal structure of ferrite occurred at 90% rolling reduction due to the supersaturation of carbon. Significant augmentation in the ultimate tensile strength during cold rolling results from the boundary, dislocation, and solid solution strengthening mechanisms.

Science and Engineering A

Coulometry and Calorimetry of Electric Double Layer Formation in Porous Electrodes

Author(s): Mathijs Janssen, Elian Griffioen, P. M. Biesheuvel, René van Roij, and Ben Erné

Two new experimental techniques that make coulometric and calorimetric measurements of porous electrodes are used to obtain the entropic contribution to the grand potential. The new results differ significantly from theoretical predictions.

Physical Review Letters

Effect of annealing on nanoindentation slips in a bulk metallic glass

Author(s): J. Patrick Coleman, Fanqiang Meng, Koichi Tsuchiya, James Beadsworth, Michael LeBlanc, Peter K. Liaw, Jonathan T. Uhl, Richard L. Weaver, and Karin A. Dahmen

We measure and analyze the statistics of nanoindentation pop-ins in a bulk metallic glass that has been annealed after being subjected to high pressure torsion. We argue that these pop-ins are avalanches of slip events. Larger slip avalanches are observed after annealing at higher temperatures. The ...

Physical Review B

Strain-induced improper ferroelectricity in Ruddlesden-Popper perovskite halides

Author(s): Yajun Zhang, M. P. K. Sahoo, Takahiro Shimada, Takayuki Kitamura, and Jie Wang

Activating multiple symmetry modes and promoting a strong coupling between different modes by strain are indispensable to stabilize a polar ferroelectric (FE) phase from a nonpolar perovskite. Herein, through first-principles calculations, we propose an undiscovered and general avenue to engineering...

Physical Review B

Charge transport by inverse micelles in non-polar media

Filip Strubbe and Kristiaan Neyts

Charged inverse micelles play an important role in the electrical charging and the electrodynamics of nonpolar colloidal dispersions relevant for applications such as electronic ink displays and liquid toner printing. This review examines the properties and the behavior of charged inverse micelles in microscale devices in the absence of colloidal particles. It is discussed how charge in nonpolar liquids is stabilized in inverse micelles and how conductivity depends on the inverse micelle size, water content and ionic impurities. Frequently used nonpolar surfactant systems are investigated with emphasis on aerosol-OT (AOT) and poly-isobutylene succinimide (PIBS) in dodecane. Charge generation in the bulk by disproportionation is studied from measurements of conductivity as a function of surfactant concentration and from generation currents in quasi steady-state. When a potential difference is applied, the steady-state situation can show electric field screening or complete charge...

Journal of Physics Condensed Matter

Halogenation of SiC for band-gap engineering and excitonic functionalization

L B Drissi, F Z Ramadan and S Lounis

The optical excitation spectra and excitonic resonances are investigated in systematically functionalized SiC with Fluorine and/or Chlorine utilizing density functional theory in combination with many-body perturbation theory. The latter is required for a realistic description of the energy band-gaps as well as for the theoretical realization of excitons. Structural, electronic and optical properties are scrutinized and show the high stability of the predicted two-dimensional materials. Their realization in laboratory is thus possible. Large band-gaps of the order of 4 eV are found in the so-called GW approximation, with the occurrence of bright excitons, optically active in the four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV depending on the decoration choice and in one case, a dark exciton is foreseen to exist in the fully chlorinated SiC. The wide variety of opto-electronic properties suggest halogenated SiC as interesting materials with potent...

Journal of Physics Condensed Matter

Assessing the performance of self-consistent hybrid functional for band gap calculation in oxide

Jiangang He and Cesare Franchini

In this paper we assess the predictive power of the self-consistent hybrid functional scPBE0 in calculating the band gap of oxide semiconductors. The computational procedure is based on the self-consistent evaluation of the mixing parameter α by means of an iterative calculation of the static dielectric constant using the perturbation expansion after discretization method and making use of the relation {$ \newcommand{\e}{{\rm e}} \alpha = 1/\epsilon_{\infty}$} . Our materials dataset is formed by 30 compounds covering a wide range of band gaps and dielectric properties, and includes materials with a wide spectrum of applications such as thermoelectrics, photocatalysis, photovoltaics, transparent conducting oxides, and refractory materials. Our results show that the scPBE0 functional provides better band gaps than the non self-consistent hybrids PBE0 and HSE06, but scPBE0 do...

Journal of Physics Condensed Matter

Study on the regulation of focal adesions and cortical actin by matrix nanotopography in 3D

Jingjing Han, Keng-Hui Lin and Lock Yue Chew

Matrix nanotopography plays an important role in regulating cell behaviors by providing spatial as well as mechanical cues for cells to sense. It has been proposed that nanoscale topography is possible to modulate the tensions which direct the formation of cytoskeleton and the organization of the membrane receptor within the cell, which in turn regulate intracellular mechanical and biochemical signaling. With current studies on this topic being performed mainly in 2D platforms, the question on how nanotopography can influence cell bahaviors in 3D environments has yet to be addressed. In this paper, we explored this question by placing cells in 3D hollow spherical polydimethylsiloxane scaffolds. After culturing rat embryonic fibroblast cells in two kinds of scaffold, one with smooth surface and the other with numerous nano-spikes, we observed that cells in the smooth scaffold have more anchoring sites and more focal adhesions than in the etched scaffold. Moreover, we found the pr...

Journal of Physics Condensed Matter

Large-scale calculations of thermoelectric transport coefficients: a case study of γ -graphyne with

Jinghua Liang, Huijun Liu, Dengdong Fan and Peiheng Jiang

Defects such as vacancies and impurities could have profound effects on the transport properties of thermoelectric materials. However, it is usually quite difficult to directly calculate the thermoelectric properties of defect-containing systems via first-principles methods since a very large supercell is required. In this work, based on the linear response theory and the kernel polynomial method, we present an efficient approach that can help to calculate the thermoelectric transport coefficients of a large system containing millions of atoms at arbitrary chemical potential and temperature. As a prototype example, we consider dilute vacancies and hydrogen impurities in a large-scale γ -graphyne sheet and discuss their effects on the thermoelectric transport properties.

Journal of Physics Condensed Matter

Phase transitions and thermal entanglement of the distorted Ising–Heisenberg spin chain: topology of

Hamid Arian Zad and Nerses Ananikian

We consider a symmetric spin-1/2 Ising-XXZ double sawtooth spin ladder obtained from distorting a spin chain, with the XXZ interaction between the interstitial Heisenberg dimers (which are connected to the spins based on the legs via an Ising-type interaction), the Ising coupling between nearest-neighbor spins of the legs and rungs spins, respectively, and additional cyclic four-spin exchange (ring exchange) in the square plaquette of each block. The presented analysis supplemented by results of the exact solution of the model with infinite periodic boundary implies a rich ground state phase diagram. As well as the quantum phase transitions, the characteristics of some of the thermodynamic parameters such as heat capacity, magnetization and magnetic susceptibility are investigated. We prove here that among the considered thermodynamic and thermal parameters, solely heat capacity is sensitive versus the changes of the cyclic four-spin exchange interaction. By using the heat capac...

Journal of Physics Condensed Matter

Diffusion of interstitials in metallic systems, illustration of a complex study case: aluminum

Matthieu David and Damien Connétable

While diffusion mechanisms of interstitial elements in fcc systems are generally well-known, especially in the case of H atoms, we show in this work that even in the case of a simple metallic system (aluminum), the diffusion of interstitials exhibits a wide variety of paths and mechanisms that depend on the specie. We used an approach based on first-principles calculations associated with kinetic Monte-Carlo simulations and a multi-state diffusion formalism to compute the diffusion coefficients of five interstitial elements: hydrogen, boron, carbon, nitrogen and oxygen. For instance, at the atomic scale, whilst we find that C atoms prefer to be located in octahedral sites (labeled o ) rather than in tetrahedral positions (labeled t ), we find one additional stable position in the lattice ( M ). The diffusion through these three stable positions are thus studied in detail. In the case of B atoms, for which the tetrahedral site is found unstable, the diffusion pat...

Journal of Physics Condensed Matter

Evidence for the confinement of magnetic monopoles in quantum spin ice

P M Sarte, A A Aczel, G Ehlers, C Stock, B D Gaulin, C Mauws, M B Stone, S Calder, S E Nagler, J W Hollett, H D Zhou, J S Gardner, J P Attfield and C R Wiebe

Magnetic monopoles are hypothesised elementary particles connected by Dirac strings that behave like infinitely thin solenoids (Dirac 1931 Proc. R. Soc . A 133 60). Despite decades of searching, free magnetic monopoles and their Dirac strings have eluded experimental detection, although there is substantial evidence for deconfined magnetic monopole quasiparticles in spin ice materials (Castelnovo et al 2008 Nature 326 411). Here we report the detection of a hierarchy of unequally-spaced magnetic excitations via high resolution inelastic neutron spectroscopic measurements on the quantum spin ice candidate {${\rm Pr}_{2}$} {${\rm Sn}_{2}$} {${\rm O}_{7}$}

Journal of Physics Condensed Matter

Optical properties of graphene nanocones under electric and magnetic fields

P Ulloa, M Pacheco and A Latgé

Here we present a theoretical study of the optical properties of graphene nanocones tuned by external electric and magnetic fields. We investigate the effects of the size and topology of the carbon nanostructures on the density of states and on the electro- and magneto-absorption of linearly polarized electromagnetic radiation in different nanocone geometries. We find that the electric field induces changes in the electric charge distribution mainly at the cone edges. In the infrared range the absorption coefficient shows a peculiar dependence on the electric field (magnitude and direction) and on the photon polarization for all investigated structures. Our results suggest that the electric field may be used to control the electric charge at the apex and for a selective light absorption. The presence of an axial magnetic field induces new features in the nanocone density of states due to the induced localization effects. For high fields the density of states exhibits a sequence ...

Journal of Physics Condensed Matter

Diverse carrier mobility of monolayer BNC x : a combined density functional theory and Boltzmann

Tao Wu, Kaiming Deng, Weiqiao Deng and Ruifeng Lu

BNC x monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNC x ( x   =  1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC 2 -1 is a material with very small band gap (0.02 eV) among all the structures while other BNC x monolayers are semiconductors with band gap ranging from 0.51 eV to 1.32 eV. The carrier mobility of BNC x varies considerably from tens to millions of cm 2 V −1 s −1 . For BNC 2 -1, the hole mobility and electron mobility along both x and y directions can reach 10 5 orders of magnitude, which is...

Journal of Physics Condensed Matter

Squeezed Dirac and topological magnons in a bosonic honeycomb optical lattice

S A Owerre and J Nsofini

Quantum information storage using charge-neutral quasiparticles is expected to play a crucial role in the future of quantum computers. In this regard, magnons or collective spin-wave excitations in solid-state materials are promising candidates in the future of quantum computing. Here, we study the quantum squeezing of Dirac and topological magnons in a bosonic honeycomb optical lattice with spin–orbit interaction by utilizing the mapping to quantum spin- {$1/2$} XYZ Heisenberg model on the honeycomb lattice with discrete Z 2 symmetry and a Dzyaloshinskii–Moriya interaction. We show that the squeezed magnons can be controlled by the Z 2 anisotropy and demonstrate how the noise in the system is periodically modified in the ferromagnetic and antiferromagnetic phases of the model. Our results also apply to solid-state honeycomb (anti)ferromagnetic insulators.

Journal of Physics Condensed Matter

Dipole-switch induced modification of the emissive response of carbon nanotubes

M Glaeske, P Bluemmel, S Juergensen, A Setaro and S Reich

The interaction of carbon nanotubes with the molecular dipole switch spiropyran is expected to affect the optical response of the tubes. Until now, the need of anchor groups to immobilize the switches on the tubes has hindered the experimental observation of the effects of switching on the emission behavior of the tubes. Here we present spiropyran-carbon nanotube complexes obtained by micelle swelling. This method does not require any anchor nor sophisticated chemistry to warrant close tube-switch proximity. For the first time, we observe the shifts predicted theoretically and their effect on the tubes’ excitation and emission energies.

Journal of Physics Condensed Matter

Possible instability of the Fermi sea against surface plasma oscillations

Hai-Yao Deng

We derive a generic formalism for studying the energy conversion processes in bounded metals. Using this formalism we show that in the collision-less limit the Fermi sea of metals should experience an instability against surface plasma oscillations, which opens for the latter an intrinsic self-amplification channel. The origin of the instability is clarified as arising from novel effects resulting from the translation symetry breaking due to the very presence of surface. The amplification rate of this channel is analytically evaluated on the basis of energy conservation and the effects of losses are discussed. In particular, the unique role played by the surface in energy conversion is unveiled. In contrast with common wisdom and in line with observations, Landau damping is shown as always overcompensated and therefore poses no serious issues in sub-wavelength plasmonics.

Journal of Physics Condensed Matter

Electronic and magnetic properties of multiferroic ScFeO 3 available from diffraction experiments

S W Lovesey and D D Khalyavin

Electronic and magnetic properties of ferric ions (3 d 5 ) in multiferroic ScFeO 3 are puzzling, in part because they are different from the only other multiferroic known to possess the same polar chemical structure, BiFeO 3 . Open questions about ScFeO 3 can be addressed by confronting observations with results for G-type antiferromagnetism allowed by the lithium niobate (LiNbO 3 )-like parent R3c structure. Calculated structure factors for resonant x-ray diffraction include all charge-like quadrupoles allowed by symmetry, and if experimental results for ScFeO 3 subsequently imply they are different from zero then ferric ions cannot be in the high-spin 6 S state. The same type of experiment can reveal the moment direction in the G-type antiferromagnetism, according to our calculations, and thereby contribute to understanding magnetic anisotropy. Furthermore, structure factors for magnetic neutron diffract...

Journal of Physics Condensed Matter

Effect of hydrostatic pressure on the superconducting properties of quasi-1D superconductor K 2 Cr 3

J P Sun, Y Y Jiao, C L Yang, W Wu, C J Yi, B S Wang, Y G Shi, J L Luo, Y Uwatoko and J-G Cheng

K 2 Cr 3 As 3 is a newly discovered quasi-1D superconductor with a T c   =  6.1 K and an upper critical field µ 0 H c2 (0)  ≈  40 T three times larger than the Pauli paramagnetic limit µ 0 H p that is suggestive of a spin-triplet Cooper pairing. In this paper, we have investigated the effects of hydrostatic pressure on its T c and µ 0 H c2 by measuring the ac magnetic susceptibility χ' ( T ) under magnetic fields at various hydrostatic pressures up to 7.5 GPa. The major findings include: (1) T c is suppressed gradually to below 2 K at 7.5 GPa; (2) the estimated µ 0 H c2 (0) decreases dramatically to below µ 0 H p above ~2 GPa and becomes slight lower than the orbital limiting field ##IMG## {${{\mu}...}

Journal of Physics Condensed Matter

Anomalous thermal conductance of graphyne under lower temperature

Xue-Kun Chen, Jun Liu, Dan Du and Ke-Qiu Chen

The thermal transport properties of graphyne are investigated via equilibrium molecular dynamics (EMD) simulations and non-equilibrium Green’s function (NEGF) method. It is found that the room-temperature thermal conductivity of graphyne is 93% lower than that of graphene with a similar size and decreases steeply with increasing the number of acetylenic linkages, which agrees with the results obtained by NEGF method qualitatively. Lattice dynamics calculations reveal that these phenomena can be attributed to the reduction of both phonon group velocities and phonon lifetimes in graphyne at low-frequency region. However, when the temperature is less than 30 K, the thermal conductance of graphyne exceeds that of graphene. Moreover, the anomalous thermal conductance behavior is not sensitive to the system lateral size. The underlying mechanisms for such phenomena are elaborated by the normal mode analysis.

Journal of Physics Condensed Matter

Thu Oct 19 2017

New environmentally friendly Ba-Fe-O thermoelectric material by flexible laser floating zone processing

Author(s): N.M. Ferreira, F.M. Costa, A.V. Kovalevsky, M.A. Madre, M.A. Torres, J.C. Diez, A. Sotelo

Ceramics with nominal BaFe12Ox composition were processed through laser floating zone technique from 25 to 200mm/h growth rates. These processing conditions were found to promote microstructural differences between the outer and inner parts of the rods, being more pronounced at higher growth rates. Along with the higher microstructural inhomogeneity, the samples grown at 100 and 200mm/h display reasonable n-type thermoelectric properties provided by BaFe18Ox phase formation. Their highest power factor measured at 800°C is comparable to the best observed so far in oxide ceramic materials, with additional advantage of high abundance and low costs of Fe2O3, and BaCO3 precursors.

Scripta Materialia

Stability of vacancy-type defect clusters in Ni based on first-principles and molecular dynamics simulations

Author(s): Shijun Zhao, Yanwen Zhang, William J. Weber

Using first-principles calculations based on density-functional theory, the energetics of different vacancy-type defects, including voids, stacking fault tetrahedra (SFT) and vacancy loops, in Ni are investigated. It is found that voids are more stable than SFT at 0K, which is also the case after taking into account the volumetric strains. By carrying out ab initio molecular dynamics simulations at temperatures up to 1000K, direct transformations from vacancy loops and voids into SFT are observed. Our results suggest the importance of temperature effects in determining thermodynamic stability of vacancy clusters in face-centered cubic metals.

Scripta Materialia

The effect of chromium and cobalt segregation at dislocations on nickel-based superalloys

Author(s): Paraskevas Kontis, Zhuangming Li, David M. Collins, Jonathan Cormier, Dierk Raabe, Baptiste Gault

The segregation of solutes at dislocations in a polycrystalline and a single crystal nickel-based superalloy is studied. Our observations confirm the often assumed but yet unproven diffusion along dislocations via pipe diffusion. Direct observation and quantitative, near-atomic scale segregation of chromium and cobalt at dislocations within γ' precipitates and at interfacial dislocations leading to the partial or complete dissolution of γ' precipitates at elevated temperatures is presented. Our results allow us to elucidate the physical mechanism by which pipe diffusion initiates the undesirable dissolution of γ' precipitates.

Scripta Materialia

Si-bronze to 304 stainless steel GTA weld fusion zone microstructure and mechanical properties

Author(s): N. Switzner, H. Queiroz, J. Duerst, Z. Yu

Si-bronze and 304 stainless steel were joined using gas tungsten arc (GTA) welding with two different welding (filler) rods matching the base metals. Cross-sections of the welds were analyzed using light optical microscopy (LOM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and hardness testing. Mechanical testing was performed for welds from each of the two filler metals using cross-weld tensile bars. For (1) the Si-bronze filler weld, partial mixing resulted in (a) copper-based regions with enhanced hardness due to iron-based islands and (b) iron-based regions with elevated hardness due to Si enrichment. Joint strength was ~ 90% that of the base Si-bronze strength of 340MPa, and elongation was ~ 48%, as compared to the base Si-bronze elongation of 55%. For (2) the stainless steel filler rod (a) primary austenite solidification and (b) micro-fissures were identified in the fusion zone. Thermodynamics calculation software was used to model solidification mode, demonstrating that the Cu (contributed by the Si-bronze) stabilized primary austenite solidification, which led to solidification micro-cracking or micro-fissuring. Micro-fissuring caused low ductility in the transverse weld tensile specimens. Fractography was performed to characterize the fracture mechanisms. For the stainless filler weld, a Cu film formed in interdendritic spaces between primary austenite grains.

Science and Engineering A

In-situ synthesis of graphene nanosheets coated copper for preparing reinforced aluminum matrix composites

Author(s): Xinghai Liu, Jiajun Li, Junwei Sha, Enzuo Liu, Qunying Li, Chunnian He, Chunsheng Shi, Naiqin Zhao

Graphene nanosheets (GNS) have attracted lots of attention as an ideal reinforcement in Al matrix composites. However, there are still challenges limiting the performance of the composites, such as the homogenous distribution of GNS in Al matrices, the structural integrity of GNS after ball-milling, and the interfacial bonding between GNS and Al matrices. This work presents a new approach to grow GNS on Al powders at 600°C by in-situ chemical vapor deposition (CVD) process, using methane as the carbon source, and Cu as the catalyst. The GNS coated Cu reinforced Al matrix bulk composites (Cu@GNS/Al) from the composite powders was fabricated by powder metallurgy (PM) technique. It was found that Cu2+ from CuCl2 can easily anchor onto the surfaces of Al particles uniformly though a nanoscale-disperse-method in alcohol, enabling the homogenously dispersion of Cu particles in Al matrices after being reduced from Cu2+ in hydrogen. With the assistance of Cu particles, network-liked GNS was successfully synthesized in-situ and strongly bonded with Al, resulting in a good mechanical performance of the composites. It is indicated that the tensile strength of the composites was improved by ~ 200% compared to the unreinforced pure Al. The strengthening mechanism was mainly ascribed to the load transfer of GNS, precipitation strengthening of Al2Cu compounds after 400°C annealing and dislocation strengthening.

Science and Engineering A

Deformation mechanism and ductile fracture behavior in high strength high ductility nanoultrafine grained Fe-17Cr-6Ni austenitic steel

Author(s): Chengshuai Lei, Xiaolin Li, Xiangtao Deng, Zhaodong Wang, Guodong Wang

Tensile deformation and ductile fracture behavior of nano/ultrafine grained (Nano/UFG) steel were studied in this investigation. The results indicate that deformation induced martensite transformation (DIMT) is the active deformation mechanism in coarse grained (CG) steel because of the low stacking fault energy. With grain size decreasing to Nano/UFG scale, deformation twining (DT) and DIMT become both the active deformation mechanism. This is because with grain size decreasing to a critical value (several hundred nanometers in this paper), the critical stress for nucleating a partial dislocation becomes smaller than that for nucleating a perfect dislocation, leading to the nucleation of deformation twins. Lüders deformation also contributes greatly to the plastic deformation in Nano/UFG steel. High fractions of low angle grain boundaries and DIMT during tensile process are believed to be responsible for Lüders deformation. Decreasing grain size to nano/ultrafine grain scale also greatly influences the fracture behavior. The voids in Nano/UFG steel are much smaller than that in CG steel. This is because high fraction of grain boundaries provide enough nucleation sites for microvoids in Nano/UFG steel. Besides, deformation twining leads to the formation of line-up of voids in Nano/UFG steel.

Science and Engineering A

Microstructure and mechanical properties of stainless steel CX manufactured by Direct Metal Laser Sintering

Author(s): Hamed Asgari, Mohsen Mohammadi

In the present paper, the microstructural evolution and tensile properties of additively manufactured stainless steel CX were investigated. Using scanning electron microscope (SEM), several powder particle morphologies were identified in the stainless s steel CX feedstock powder where the spherical morphology was found to be the dominant one. In addition, X-ray diffraction (XRD) technique detected austenite and martensite phases in both stainless steel CX powder and as-built sample, whereas no carbide peak appeared on the XRD patterns. Moreover, lath or needle-like martensite phase was observed in the microstructure of the as-built sample. The level of porosity was very low in the as-built sample, indicating the manufacturing of a nearly fully dense sample. Furthermore, a high ultimate tensile strength together with a good elongation to fracture was obtained for the horizontally-built stainless steel CX sample. Finally, examination of the fracture surfaces after tensile tests confirmed the ductile failure mode of the samples, in which the pull-out of the scan tracks and coalescence of the voids resulted in the tear and final rupture. This study demonstrates the successful additive manufacturing of stainless steel CX with outstanding tensile properties.

Science and Engineering A

Cr-induced morphology change of primary Mn-rich phase in Al-Si-Cu-Mn heat resistant aluminum alloys and its contribution to high temperature strength

Author(s): Guangjin Li, Hengcheng Liao, Xiaojing Suo, Yunyi Tang, Uday S. Dixit, Pavel Petrov

Effect of Cr addition on the microstructure and mechanical properties of Al-12wt%Si-3.5wt%Cu-2wt%Mn heat-resistant alloy were studied by OM, XRD and SEM microstructure observation and tension test at different temperature. Results indicate that Cr addition can obviously change the morphology of the primary Mn-rich phase in Al-12wt%Si-3.5wt. Cu-2wt%Mn, from slender rods to dendrites first and then to star-type particles. XRD and SEM-EDS results suggest that no new Cr-rich phase is formed in the studied alloy, and Cr is mainly dissolved in the Mn-rich phase, besides partially in the matrix. Addition of Cr can significantly increases strength and elongation at room temperature and high temperature. When Cr addition level is at 1.0%, the strength at 350°C reaches to the highest value of 106MPa, about 28% higher than that without addition of Cr. In the alloys with or without Cr addition, the internal cracks originate from and propagate along the primary Mn-rich phase during tension at 350°C.

Science and Engineering A

Anisotropic thermal expansion in flexible materials

Author(s): Carl P. Romao

A definition of the Grüneisen parameters for anisotropic materials is derived based on the response of phonon frequencies to uniaxial stress perturbations. This Grüneisen model relates the thermal expansion in a given direction (${α}_{ii}$) to one element of the elastic compliance tensor, which corr...

Physical Review B

Soft phonon mode dynamics in Aurivillius-type structures

Author(s): Deepam Maurya, Ali Charkhesht, Sanjeev K. Nayak, Fu-Chang Sun, Deepu George, Abhijit Pramanick, Min-Gyu Kang, Hyun-Cheol Song, Marshall M. Alexander, Djamila Lou, Giti A. Khodaparast, S. P. Alpay, N. Q. Vinh, and Shashank Priya

We report the dynamics of soft phonon modes and their role toward various structural transformations in Aurivillius materials by employing terahertz frequency-domain spectroscopy, atomic pair distribution function analysis, and first-principles calculations. We have chosen $\mathrm{B}{\mathrm{i}}_{4...

Physical Review B

Self-compensation in phosphorus-doped CdTe

Author(s): Mauricio A. Flores, Walter Orellana, and Eduardo Menéndez-Proupin

We investigate the self-compensation mechanism in phosphorus-doped CdTe. The formation energies, charge transition levels, and defect states of several P-related point defects susceptible to cause self-compensation are addressed by first-principles calculations. Moreover, we assess the influence of ...

Physical Review B

Magnetic diversity in stable and metastable structures of CrAs

Author(s): Busheng Wang, Qing Lu, Yanfeng Ge, Kaicheng Zhang, Wenhui Xie, Wu-Ming Liu, and Yong Liu

We present results of electronic structure calculations of the bulk properties of CrAs. The crystalline structures of CrAs are investigated by the use of ab initio calculations with an unbiased swarm structural search under ambient and high pressure. Both the double helimagnetic and nonmagnetic phas...

Physical Review B

Local strain heterogeneity and elastic relaxation dynamics associated with relaxor behavior in the single-crystal perovskite $\mathrm{Pb}(\mathrm{I}{\mathrm{n}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}\text{−}\mathrm{PbZr}{\mathrm{O}}_{3}\text{−}\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{−}\mathrm{PbTi}{\mathrm{O}}_{3}$

Author(s): Wenhui He, Michael A. Carpenter, Giulio I. Lampronti, Qiang Li, and Qingfeng Yan

Recently, $\mathrm{Pb}(\mathrm{I}{\mathrm{n}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}\text{−}\mathrm{PbZr}{\mathrm{O}}_{3}\text{−}\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{−}\mathrm{PbTi}{\mathrm{O}}_{3}$ (PIN-PZ-PMN-PT) relaxor single crys...

Physical Review B

Wed Oct 18 2017

High strain rate deformation of ARMOX 500T and effects on texture development using neutron diffraction techniques and SHPB testing

Author(s): Michael Saleh, Muhammad M. Kariem, Vladimir Luzin, Karl Toppler, Huijun Li, Dong Ruan

The authors evaluated the crystallographic texture, defined as the distribution of orientation of crystals (or grains), to gauge the deformation and microstructural evolution of ARMOX 500T armour plates at elevated strain rates. Using neutron diffraction, the authors examined a number of specimens deformed at room temperature and high strain rates and contrasted these with equivalent samples deformed quasi-statically. Since crystallographic texture can play a part in the armour's ballistic response the authors were able to observe a rate dependent textural development, with the strengthening of the rolling α-fibre. The hot rolling process used in the manufacture of these steels leads to a through thickness texture variation that leads to an asymmetric transitional texture in the strain regime (1–2%) but with increased strain a symmetric texture develops irrespective of the strain rate, albeit with different intensities. By extending the testing program the authors were also able to deduce the strength parameters for the Johnson-Cook model through split Hopkinson pressure bar testing at high strain rates (1000–3000s−1) and elevated temperatures (20–600°C). The results, when compared with existing literature, show deviations in the strain rate sensitives of the tested specimens and, subsequently, variations in the computed flow stress parameters.

Science and Engineering A

Contribution of extension twinning to plastic strain at low stress stage deformation of a Mg-3Al-1Zn alloy

Author(s): P. Chen, B. Li, D. Culbertson, Y. Jiang

Extruded magnesium alloys present a typical sigmoidal shaped stress-strain curve when compressed along the extrusion direction due to the activation of { 10 1 ̅ 2 } 10 1 ̅ 1 ̅ extension twinning. Quantitative measurements of the contribution of twinning to plastic strain at the low stress stage deformation are incomplete in the literature. In this work, we carefully measured the evolution of twin volume fraction during low stress stage plastic deformation of an extruded Mg-3Al-1Zn alloy by electron backscatter diffraction, and calculated the contribution of twinning to the plastic strain. Our results show that, at the very beginning of yielding, basal dislocation slip is the main contributor to the plastic strain, but twin nucleation is already activated. The contribution from twinning rapidly increases as the plastic strain increases. After the plastic strain exceeds 1.0%, extension twinning contributes 80–90% of the plastic deformation until twinning is saturated.

Science and Engineering A

New generation of eutectic Al-Ni casting alloys for elevated temperature services

Author(s): C. Suwanpreecha, P. Pandee, U. Patakham, C. Limmaneevichitr

Al-Ni alloys are promising aluminum alloys for high-temperature applications because of their many good properties partly due to the high stability of the Al3Ni eutectic fibers at elevated temperature. However, the matrix of eutectic Al-Ni is weak. Therefore, the alloying elements are important not only for improving the mechanical properties but also for preserving the stability of the fibers. Eutectic Al-Ni alloys with Sc additions are studied through the microstructure, resultant hardness after elevated temperature exposure, high-temperature tensile properties, fracture behavior, and TEM micrographs. The results reveal that the addition of Sc up to 0.4wt% does not significantly change the grain sizes and the morphology of the eutectic phase. However, the hardness increases with increasing amount of Sc and remains high when the temperature reaches 300°C, which is a result of precipitation hardening of Al3Sc precipitates. The hardness then decreases because of coarsening of the precipitates. The tensile properties at high temperature of Al-6Ni alloys with Sc additions are better than those of Al-6Ni alloy without Sc addition. This can also be explained by the occurrence of Al3Sc precipitates. The fracture behavior of the alloys has the same trend as the tensile properties. Therefore, Al-6Ni-0.4Sc is a new candidate for high-temperature engineering applications.

Science and Engineering A

Glassy selenium at high pressure: Le Chatelier's principle still works

Author(s): V. V. Brazhkin and O. B. Tsiok

Selenium is the only easily vitrified elementary substance. Numerous experimental studies of glassy Se ($g$-Se) at high pressures show a large spread in the data on the compressibility and electrical resistivity of $g$-Se. Furthermore, H. Liu et al. [Proc. Natl. Acad. Sci. USA 105, 13229 (2008)] hav...

Physical Review B

Tue Oct 17 2017

Sequential bottom-up assembly of mechanically stabilized synthetic cells by microfluidics

Marian Weiss, Johannes Patrick Frohnmayer, Lucia Theresa Benk, Barbara Haller, Jan-Willi Janiesch, Thomas Heitkamp, Michael Börsch, Rafael B. Lira, Rumiana Dimova, Reinhard Lipowsky, Eberhard Bodenschatz, Jean-Christophe Baret, Tanja Vidakovic-Koch, Kai Sundmacher, Ilia Platzman & Joachim P. Spatz

A microfluidics method to generate giant, copolymer-stabilized liposomes is presented. These vesicles are functionalized with distinct proteins to recapitulate cellular processes.

Nature

X-ray photoelectron spectroscopy analysis as a tool to assess factors influencing magnetic anisotropy type in CoMgO system with gold interlayer

Author(s): Iraida N. Demchenko, Yevgen Syryanyy, Yevgen Melikhov, Laurent Nittler, Leszek Gladczuk, Kinga Lasek, Luca Cozzarini, Matteo Dalmiglio, Andrea Goldoni, Pavlo Konstantynov, Maryna Chernyshova

X-ray photoelectron spectroscopy (XPS) studies of Au/Co/Au(0.3nm)/MgO and Au/Co/MgO systems were conducted in order to monitor the electronic structure modification at Co/MgO interface with/without gold interlayer. A detailed analysis of Co 2p states revealed that the amount of minor oxygen contribution at Co/MgO interface decreased after the Au interlayer was added. The obtained XPS results together with density functional theory (DFT) allowed explanation of the increase of surface anisotropy energy in the sample with the gold interlayer in terms of (i) noble and transitional metal d-d orbital hybridization; (ii) interfacial Co 3d and O 2p; and (iii) interface imperfection.

Scripta Materialia

Enhanced barocaloric effect produced by hydrostatic pressure-induced martensitic transformation for Ni44.6Co5.5Mn35.5In14.4 Heusler alloy

Author(s): Xijia He, Shengxian Wei, Yanru Kang, Yuanlei Zhang, Yiming Cao, Kun Xu, Zhe Li, Chao Jing

We report effect of hydrostatic pressure on martensitic transformation in polycrystalline Ni44.6Co5.5Mn35.5In14.4. With applying pressure, the transformation moves to high temperatures at a rate of 4.4K/kbar. Utilizing an indirect method, the calculated maximum isothermal entropy changes achieve ~14.3J/kg K at 2.76kbar and ~15.6J/kg K at 5.98kbar, which respectively output the values of ~5K and ~6K for the adiabatic temperature change near room temperature. After deducting the average hysteretic loss (~152J/kg), the effective refrigeration capacities still reach ~38J/kg at 2.76kbar and ~247J/kg at 5.98kbar, performing an enhanced barocaloric effect.

Scripta Materialia

In-situ TEM study of dislocation emission associated with austenite growth

Author(s): Juan Du, Frédéric Mompiou, Wen-Zheng Zhang

The emission of dislocations from the tip of a newly transformed austenite lath, with a near Pitsch orientation relationship with the ferrite matrix, was observed at 760°C in a duplex stainless steel, using in-situ transmission electron microscopy. The dynamics of dislocation loops with [111]b/2 Burgers vector were carefully analyzed. An estimation of stress concentration at the tip was made using dislocations as stress probes. These real-time observations verify directly for the first time that dislocation activity assists the growth of austenite precipitates, and provide quantitative data for revealing the stress field generated by interface migration.

Scripta Materialia

Low temperature sintering of Na1+xZr2SixP3−xO12 by the addition of Na3BO3

Author(s): Kenji Suzuki, Kousuke Noi, Akitoshi Hayashi, Masahiro Tatsumisago

Na1+ x Zr2Si x P3 x O12 (NASICON)-based solid electrolytes were prepared by liquid phase sintering with a Na3BO3 additive to lower the NASICON sintering temperature. NASICON was sintered at 700°C with 9.1wt% Na3BO3. The obtained NASICON ceramic exhibited high room-temperature conductivity of 1×104 Scm1, which were three orders of magnitude higher than that of a Na3BO3-free sample. SEM observation of this ceramic revealed that neck growth among NASICON grains proceeded reasonably and that sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic microstructure is expected to contribute to the high conductivity of the NASICON-Na3BO3 ceramic.

Scripta Materialia

Origin of low temperature toughness in a 12Cr-10Ni martensitic precipitation hardenable stainless steel

Author(s): C.R. Anoop, Aditya Prakash, S.V.S. Narayana Murty, Indradev Samajdar

Though ductile to brittle transition (DBT) is a typical feature of body centered cubic materials, the present 12Cr-10Ni precipitation hardened martensitic stainless steel exhibited excellent low temperature impact toughness. This was, however, dependant on the aging temperature. 250°C aging led to higher toughness both at room temperature and at −196°C (77K). Specimens aged at 400 and 500°C, on the other hand, displayed significantly lower sub-zero impact properties. Though martensite packet size, and size distribution, were identical between the two ageing treatments; there was clear evidence of second phase coarsening: from very fine precipitates of less than ~ 10nm to relatively coarser second phase of ~ 5–25nm range. It is suggested that precipitate coarsening and associated loss of coherency are the limiting factors to the DBT performance of this important class of material.

Science and Engineering A

Method to decode stress-strain diagrams to identify the structure-strength relationships in aged aluminum alloys

Author(s): S. Saimoto, M.A. Singh, M.R. Langille, J. Levesque, K. Inal, M. Niewczas, A.R. Woll

Leading-edge infrastructure renovation is driven by innovations in materials processing to optimize desired properties. The immediate challenge for the metal industry is the development of effective and efficient materials quality-control practices. At present, mechanical and sheet forming properties are assessed using standard tensile tests to measure yield and ultimate tensile strengths, as well as the total elongation to failure. The resulting stress-strain data is used as a “pass-fail” test to ensure that the product meets industry specifications. We report a new method of constitutive relation analyses (CRA) that can extract fundamental information regarding the underlying crystalline mechanisms of deformation and failure from standard stress-strain data. The CRA method is applied to industrial extrusion product made from AA6063 aluminum alloy to demonstrate how this type of forensic examination can be used to direct changes to thermo-mechanical processing to optimize desired material properties. The CRA prediction of point-defect generation and nano-void formation leading to ductile failure during plastic flow was validated by small angle X-ray scattering (SAXS) studies.

Science and Engineering A

The effect of residual stresses and strain reversal on the fracture toughness of TiAl alloys

Author(s): Fritz Appel, Jonathan D.H. Paul, Peter Staron, Michael Oehring, Otmar Kolednik, Jozef Predan, Franz Dieter Fischer

The effect of local deformation on the fracture behaviour of TiAl alloys was investigated. Roller indentations impressed parallel to the crack plane significantly improve the fracture toughness. The residual strains present in the indentation zone were characterized by X-ray diffraction and modelled using finite element (FE) calculations. The experimentally observed macrostrains exhibit remarkable crystallographic anisotropies and are unequally shared between the major alloy constituents. The mechanisms behind the observed toughening have been discussed in terms of the residual strains and factors improving the crack tip plasticity. With regard to intended high-temperature applications, the temperature retention of the toughening effect was studied.

Science and Engineering A

Effect of additives and heat treatment on the tensile properties of 354 alloy at 25°C and 250°C

Author(s): L. Alyaldin, E.M. Elgallad, A.M. Samuel, H.W. Doty, S. Valtierra, F.H. Samuel

The present study was performed on 354 alloy containing Al-9% Si-1.8% Cu-0.5% Mg. Six alloys were prepared using 0.2wt% Ti grain-refined 354 alloy, comprising alloy R (354 + 0.25% Zr) considered as the base or reference alloy, and five others, viz., alloys S (354 + 0.25% Zr + 2% Ni), T (354 + 0.25% Zr + 4% Ni), U (354 + 0.25% Zr + 0.75% Mn), V (354 + 0.25% Zr + 0.75% Mn + 2% Ni), and Z (354 + 0.25% Zr + 0.15% Sc). The tensile data showed that the UTS and percent elongation values of R, S, T, U, V and Z alloys increased in the one-step solution heat-treated (SHT 1) condition compared to the as-cast case. The multi-step solution heat treatment (SHT 2) displayed higher tensile properties than those achieved with SHT 1 treatment. The use of the T62 treatment, incorporating the SHT 2, allows for maximum dissolution of the copper phases in the multiple stages of solution treatment, resulting in the greatest improvement in both UTS and YS. Without stabilization, T6 and T62 treatments provide the best improvements in both UTS and YS values of all alloys. The best tensile properties of alloys tested at room temperature after stabilization at 250°C for 200h are obtained with the T6 heat treatment. After T62 treatment, Alloy U showed the maximum increase in UTS and YS values. Addition of Zr, Ni, Mn and Sc to 354 alloys improves the high temperature tensile properties. Alloys S and U perform better when tested under high temperature conditions, after one hour stabilization at 250°C. After 200h stabilization at 250°C, however, the strength of the T6-treated alloys is reduced considerably, while the ductility is increased, with alloy R showing the highest percent elongation, ~ 19%, which is attributed to the precipitation of a high density of coarse Al2Cu phase particles during the prolonged stabilization treatment at 250°C.

Science and Engineering A

Submolecular Resolution by Variation of the Inelastic Electron Tunneling Spectroscopy Amplitude and its Relation to the AFM/STM Signal

Author(s): Bruno de la Torre, Martin Švec, Giuseppe Foti, Ondřej Krejčí, Prokop Hapala, Aran Garcia-Lekue, Thomas Frederiksen, Radek Zbořil, Andres Arnau, Héctor Vázquez, and Pavel Jelínek

Here we show scanning tunneling microscopy (STM), noncontact atomic force microscopy (AFM), and inelastic electron tunneling spectroscopy (IETS) measurements on an organic molecule with a CO-terminated tip at 5 K. The high-resolution contrast observed simultaneously in all channels unambiguously dem...

Physical Review Letters

Entropic Comparison of Atomic-Resolution Electron Tomography of Crystals and Amorphous Materials

Author(s): S. M. Collins, R. K. Leary, P. A. Midgley, R. Tovey, M. Benning, C.-B. Schönlieb, P. Rez, and M. M. J. Treacy

Electron tomography bears promise for widespread determination of the three-dimensional arrangement of atoms in solids. However, it remains unclear whether methods successful for crystals are optimal for amorphous solids. Here, we explore the relative difficulty encountered in atomic-resolution tomo...

Physical Review Letters

Phase stability, ordering tendencies, and magnetism in single-phase fcc Au-Fe nanoalloys

Author(s): I. A. Zhuravlev, S. V. Barabash, J. M. An, and K. D. Belashchenko

Bulk Au-Fe alloys separate into Au-based fcc and Fe-based bcc phases, but ${\mathrm{L}1}_{0}$ and ${\mathrm{L}1}_{2}$ orderings were reported in single-phase Au-Fe nanoparticles. Motivated by these observations, we study the structural and ordering energetics in this alloy by combining density funct...

Physical Review B

Pressure-induced anomalous enhancement of insulating state and isosymmetric structural transition in quasi-one-dimensional $\mathrm{Ti}{\mathrm{S}}_{3}$

Author(s): Chao An, Pengchao Lu, Xuliang Chen, Yonghui Zhou, Juefei Wu, Ying Zhou, Changyong Park, Chuanchuan Gu, Bowen Zhang, Yifang Yuan, Jian Sun, and Zhaorong Yang

We present in situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on quasi-one-dimensional single-crystal $\mathrm{Ti}{\mathrm{S}}_{3}$ up to 29.9–39.0 GPa in diamond-anvil cells, coupled with first-principles calculations. Counterintuitively, the conductive ...

Physical Review B

Hydrogen-rich scandium compounds at high pressures

Author(s): Kazutaka Abe

Scandium hydrides at high pressures have been investigated by using ab initio density functional calculations. Although the stable scandium hydride so far known to have the highest content rate of hydrogen is ${\mathrm{ScH}}_{3}$, other more hydrogen-rich compounds are found to be possible at high p...

Physical Review B

Mon Oct 16 2017

Multiscale modeling of the anisotropic electrical conductivity of architectured and nanostructured Cu-Nb composite wires and experimental comparison

Author(s): T. Gu, J.-R. Medy, F. Volpi, O. Castelnau, S. Forest, E. Hervé-Luanco, F. Lecouturier, H. Proudhon, P.-O. Renault, L. Thilly

Nanostructured and architectured copper niobium composite wires are excellent candidates for the generation of intense pulsed magnetic fields (> 90T) as they combine both high electrical conductivity and high strength. Multi-scaled Cu-Nb wires can be fabricated by accumulative drawing and bundling (a severe plastic deformation technique), leading to a multiscale, architectured and nanostructured microstructure providing a unique set of properties. This work presents a comprehensive multiscale study to predict the anisotropic effective electrical conductivity based on material nanostructure and architecture. Two homogenization methods are applied: a mean-field theory and a full-field approach. The size effect associated with the microstructure refinement is taken into account in the definition of the conductivity of each component in the composites. The multiscale character of the material is then accounted for through an iterative process. Both methods show excellent agreement with each other. The results are further compared, for the first time, with experimental data obtained by the four-point probe technique, and also show excellent agreement. Finally, the qualitative and quantitative understanding provided by these models demonstrates that the microstructure of Cu-Nb wires has a significant effect on the electrical conductivity.

Acta Materialia

Sun Oct 15 2017

An interatomic potential for the Li-Co-O ternary system

Author(s): Eunkoo Lee, Kwang-Ryeol Lee, Byeong-Joo Lee

Although large-scale atomistic simulations provide useful insights into various material phenomena, such studies on LiCoO2, which is the most widely used cathode material for lithium ion batteries (LIBs), have rarely been undertaken due to difficulties in developing adequate interatomic potentials. In this study, an interatomic potential (2NNMEAM + Qeq) for the Li-Co-O ternary system is developed to carry out molecular dynamics (MD) simulation studies on lithium cobalt oxides. Potential parameters are optimized so that the potential can successfully reproduce fundamental materials properties (structural, elastic, thermodynamic and migration properties) of various compounds of sub-binary and lithium cobalt ternary oxide systems. Through MD simulations, we investigate lithium diffusion properties (activation energy for lithium migration and diffusion coefficient) in layered Li1− x CoO2 (0 ≤ x ≤ 0.5) of various lithium vacancy concentrations. We find that the lithium vacancy concentration has a significant influence on the activation energy for lithium diffusion and the lithium diffusion coefficient in the Li1− x CoO2 cathode. The developed potential can be further utilized for atomistic simulation studies on other materials phenomena (phase transitions, defect formation, lithiation/delithiation, etc.) in LIB cathode materials.

Computational Materials Science

Deformation twinning and dislocation processes in nanotwinned copper by molecular dynamics simulations

Author(s): Xing Zhao, Cheng Lu, Anh Kiet Tieu, Lihua Zhan, Minghui Huang, Lihong Su, Linqing Pei, Liang Zhang

Nanotwinned materials exhibit a combination of high strength and good ductility which is attributed to the interactions between dislocations and twin boundaries. But no attempt has been made to explore the possibility for deformation twinning in nanotwinned face-centered cubic materials. Here we use large scale molecular dynamics simulations to elucidate the mechanical behaviour of nanotwinned Cu. We demonstrate that deformation twinning plays an important role in the deformation of nanotwinned Cu with specific twin orientations, in addition to conventional dislocation slip. Deformation twins are formed through the glide of Shockley partials on adjacent {1  1  1} slip planes and two twinning mechanisms are identified based on the arrangement of Shockley partials. The first mechanism involves the successive motion of Shockley partials of different types, named as double-Shockley partials, which forms unstable thin twin plates. The second process involves the successive passage of the same twinning dislocations on neighbouring slip planes, which forms stable deformation twins along one primary twinning system or symmetric twinning systems. The dislocation processes involved in the dislocation-twin reactions are analysed at atomic level. The orientation dependence of deformation twinning is discussed and compared with available experimental results.

Computational Materials Science

Competitive adsorption of gases dissolved in transformer oil on Co-doped ZnO (0001) surface

Author(s): Gongwei Xiao, Weigen Chen, Shangyi Peng, Chutian Yu, Zikai Jiang

The use of gas sensors to monitor gases dissolved in transformer oil is of great value to prevent safety issues of power grid. As a part of developing high performance gas sensor, the deeply sensing mechanism in material aspect study has great guiding significance. In this paper, a new surface adsorption model is proposed which provides an approach to achieve the improvement of the gas-sensing performance of Co-doped ZnO sensor theoretically. Utilizing first-principle density functional theory, the adsorption properties of six main characteristic fault gases (C2H2, CO2, CH4, C2H4, CO, and H2) on Co-doped ZnO (0001) surface were systemically investigated which indicated that H2 and CH4 molecules were physically adsorbed, whereas CO, CO2, C2H2, and C2H4 were chemisorbed. In addition, using the as-prepared Co-doped ZnO material, the gas-sensing response for these fault gases were experimentally measured, which matches well with the theoretical analysis. This work reveals the gas-sensing performance of Co-doped ZnO composites in microscopic. It lays the foundation for the preparation of gas sensors to serve for monitoring the condition of power transformers.

Computational Materials Science

Displacement thresholds and knock-on cross sections for hydrogenated h-BN monolayers

Author(s): Rajesh Kumar, Avinash Parashar, Pierre Mertiny

In the present article, the authors initially studied the effects of hydrogenation on the displacement threshold energies (E d) of B and N atoms in h-BN nanosheets with the help of reaction force field (ReaxFF) based molecular dynamics simulations. Subsequently, the E d values estimated for B, N and H atoms were used for predicting the knock-on cross section for these atoms against electron irradiation. The results show that in most of the cases the displacement threshold value deteriorates with hydrogenation. The redistribution of charge among different atoms with the increase in B-N bond length plays a significant role in deciding the displacement thresholds for B and N atoms. It was predicted with the help of obtained E d values that in a lower energy range the H atoms have a higher knock-on cross-section than B and N atoms against electron irradiation. The results from this study lead the authors to recommend cautious use of transmission electron microscopy for the characterization of hydrogenated h-BN nanosheets.

Computational Materials Science

Development of a parallel adaptive multigrid algorithm for solving the multi-scale thermal-solute 3D phase-field problems

Author(s): Jun Wu, Zhipeng Guo, Chao Luo

A parallel adaptive multigrid algorithm was developed to solve the coupled thermal-solute phase field equations so that the multi-scale difficulty of the problem when both thermal and solute fields were presented could be resolved. Comparing with the explicit method, it was showed that the proposed algorithm even converged when the time step was enlarged to be 4 orders of magnitude larger, and combined with Para-AMR algorithm [1] the computation efficiency could be improved by about 4–5 orders of magnitude with little accuracy compromised, when a much higher and realistic Lewis number was used, e.g. Le =10,000. With this numerical capability, 3D phase field simulations on dendrite growth in a much larger scale, in particular under multi-scale thermal-solute conditions, could be performed in a much more sensible manner with moderate amount of computing resources. Dendrite growth simulations with Le varying from 1 to 10,000 in 3D were carried out for the first time, and the result showed that variation of Le led to a great difference of both tip velocity and tip radius, which was similar to the 2D case reported by [2].

Computational Materials Science

Pressure-dependent structural, elastic, electronic and vibrational studies of Ba2InMO6 (M=Ta, Nb) from first principles

Author(s): Shaobo Zhang, Liwei Shi

First-principles calculations have been performed to study the structural, elastic, electronic and lattice dynamical properties of Ba2InTaO6 and Ba2InNbO6 double perovskites under hydrostatic pressure. Pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velocities, as well as Debye temperature are observed, without any softening behaviors. Both Ba2InTaO6 and Ba2InNbO6 have a quasi-direct band-gap character at zero pressure, and their fundamental energy gaps vary nonlinearly with increasing pressure. Moreover, the evolution of Roman-active and infrared-active optical phonon frequencies with pressure is analyzed. A pressure-induced softness of low-frequency T 1 g silent modes around the Γ point suggests that Ba2InTaO6 and Ba2InNbO6 becomes dynamically instable beyond a certain critical pressure.

Computational Materials Science

High-speed collision of copper nanoparticles with aluminum surface: Inclined impact, interaction with roughness and multiple impact

Author(s): Victor V. Pogorelko, Vasiliy S. Krasnikov, Alexander E. Mayer

Using molecular dynamic simulation, we investigate the influence of the angle of incidence and surface roughness at the high-speed collision of copper nanoparticles with aluminum surface. Also, special attention is devoted to the consideration of several successive collisions. Nanoparticles with a diameter of 16.2 nm and a collision speed of 500 and 1000 m/s are considered. An increase in the angle of incidence of the nanoparticle leads to a lower defect structure in the surface layer of the substrate and reduces the adhesion of the deposited particle to the substrate. The presence of a roughness on the surface always leads to decrease in the total length of dislocations remaining in the substrate after collision; this decrease is due to the escape of some dislocations on the lateral surface of the protrusions on the substrate. The depth of the modified layer formed by several successive collisions of nanoparticles with the aluminum surface does not exceed the size of the nanoparticle. In the case of successive collisions of nanoparticles, the use of collision speed of 1000 m/s is optimal, since it ensures a minimum porosity of the formed copper layer. The hardness of the formed copper layer at successive collisions of nanoparticles is below the hardness of a copper single crystal by 30–45%. At the same time, the copper layer has good adhesion properties, which are comparable with the adhesion properties of copper atoms to a copper single crystal.

Computational Materials Science

Proton conduction of fuel cell polymer membranes: Molecular dynamics simulation

Author(s): Zhonghao Rao, Chenyang Zheng, Fan Geng

Fuel cell polymer membrane as the core of the proton exchange membrane fuel cell (PEMFC) plays an important role in maintaining high intrinsic proton conductivity and insulating electrode. To investigate the effect of crosslinking formation on proton conduction, the proton mobility and ion conduction were calculated and analyzed by using molecular dynamics (MD) simulation. As the crosslinking number increased, the proton diffusion coefficients increased at first and then decreased. The results indicated that the formation of crosslinked bonds was beneficial for opening new channels to enhance the proton conductivity. But too many crosslinking also can decrease proton diffusivity due to the blocking along the chain backbone. Besides, the proton conduction was better at 350 K than at 300 K. It can be concluded that temperature also had a significant impact on diffusivity and conductivity.

Computational Materials Science

Sat Oct 14 2017

In-situ strength of individual silicon particles within an aluminium casting alloy

Author(s): M.G. Mueller, G. Žagar, A. Mortensen

Measurements of local strength are performed in-situ on individual silicon particles that constitute the second phase of aluminium alloy A356. Particles are shaped using Focused Ion Beam (FIB) milling such that, upon the application of a compressive force on the particle, a volume of material unaffected by FIB milling is subjected to bending. Silicon particles in this commercial aluminium casting alloy are shown to be capable of locally sustaining tensile stresses as high as 16 GPa, i.e., approaching theoretical strength. The reason why such strengths are not reached by most alloy Si particles is shown to be the presence of specific surface defects, the effect of which is assessed. The most deleterious defects are interfaces between merged silicon crystals; therefore, eliminating these might lead to significantly enhanced strength and ductility in this widely-used casting alloy family.

Acta Materialia

Size effects on intergranular crack growth mechanisms in ultrathin nanocrystalline gold free-standing films

Author(s): Ehsan Hosseinian, Saurabh Gupta, Olivier N. Pierron, Marc Legros

This study investigated the combined effects of thickness (30 vs 100 nm) and average grain size (40 vs 70 nm for the thicker films) on the crack propagation mechanisms in ultrathin nanocrystalline gold microbeams, using a microelectromechanical system device to perform in situ transmission electron microscope (TEM) tensile experiments. Monotonic tensile tests of the two types of microbeams show similar strength levels (∼400 MPa) and ductility (∼2%). However, the thicker specimens exhibit a much more ductile behavior under repeated stress relaxation experiments, which the in situ TEM experiments revealed to be related to differences in intergranular crack propagation mechanisms. The governing crack growth process is in both cases dominated by grain boundary dislocation activities leading to grain boundary sliding. For the thinner specimens, secondary nanocracks are generated (as a result of grain boundary sliding) ahead of the main crack and coalesce together. Instead, secondary nanocracks do not form ahead of the main crack for the thicker specimens; the main crack extends as a result of sustained grain boundary sliding at the crack tip.

Acta Materialia

Computation of entropies and phase equilibria in refractory V-Nb-Mo-Ta-W high-entropy alloys

Author(s): Yi Wang, Ming Yan, Qiang Zhu, Williams Yi Wang, Yidong Wu, Xidong Hui, Richard Otis, Shun-Li Shang, Zi-Kui Liu, Long-Qing Chen

We have applied the first-principles phonon method to the refractory V-Nb-Mo-Ta-W high-entropy alloys (HEAs) to predict the major phase separations in the temperature-compositional space and hence the associated entropy changes within the systems, taking into account vibrational, electronic, and configurational contributions to the total entropy. The first-principles calculations covered 178 phases ranging from pure elements, the ordered B2, B32, B23, B22, hR8, hR7, tI6, C15, and D03 binary phases, two ordered MoNbTaW quaternary phases, and the partially disordered and completely disordered bcc phases. By sorting their relative phase stabilities with the Dantzig's simplex minimization algorithm, the possibilities of phase separation for the refractory quaternary and quinary HEAs were thermodynamically found in the temperature range of 500–907 K.

Acta Materialia

Hydrogen embrittlement revealed via novel in situ fracture experiments using notched micro-cantilever specimens

Author(s): Yun Deng, Afrooz Barnoush

The susceptibility of the FeAl intermetallic alloy to hydrogen-assisted cracking was investigated by in situ fracture experiments using notched micrometre-sized specimens using an Environmental Scanning Electron Microscope (ESEM). The notched beams were loaded under two different environmental conditions: one in high vacuum (5 × 10−4 Pa) to avoid hydrogen effects and one under a certain water vapor pressure (450 Pa) to promote hydrogen uptake. The fracture behaviour on a non-ASTM-standard micro-sized specimen was successfully studied by the experimental approach, and the microstructure of the whole crack area was analysed by Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscopy (TEM) techniques. Three crack growth stages were observed in all the specimens: i) elastic regime, ii) notch blunting and micro-crack formation; and iii) stable crack growth. We observed an accelerated crack propagation rate in specimens under hydrogen exposure. The hydrogen embrittlement phenomenon was found to occur because of the strong hydrogen-dislocation interactions. The combined effect of hydrogen-enhanced dislocation nucleation and hydrogen-restricted dislocation mobility is responsible for the hydrogen-enhanced cracking behaviour.

Acta Materialia

In-situ observations of single micro-particle impact bonding

Author(s): Mostafa Hassani-Gangaraj, David Veysset, Keith A. Nelson, Christopher A. Schuh

We study supersonic impact of individual metallic microparticles on metallic substrates, that is, the unit process of materials buildup in cold spray coatings/additive manufacturing. We resolve the moment of impact bonding through real-time observations of single particle impacts with micron-scale and nanosecond-level resolution. We offer the first in-situ observation of a material-dependent threshold velocity, above which the particle undergoes an impact-induced jet-like material ejection and adheres to the substrate. We report direct measurements of critical velocities for structural metals, which unlike in nozzle experiments, are not affected by process-related complexities obscuring particles' kinetic and thermal histories.

Scripta Materialia

Influence of hot isostatic pressing on the performance of aluminum alloy fabricated by ultrasonic additive manufacturing

Author(s): M.N. Gussev, N. Sridharan, Z. Thompson, K.A. Terrani, S.S. Babu

Ultrasonic additive manufacturing (UAM) is a solid-state manufacturing technique employing principles of ultrasonic welding coupled with mechanized tape layering to fabricate fully functional parts. However, UAM-fabricated parts often exhibit a reduction in strength when loaded normal to the welding interfaces (Z-direction). Here, the effect of hot isostatic pressing (HIP) on UAM builds of aluminum alloy was explored. Tensile testing and microstructure characterization were conducted; it was established that HIP eliminated the brittle Z-direction fracture and improved the strength and ductility of the Z-direction specimens. HIP eliminated voids and produced recrystallized structure; however, welding interfaces survived the HIP treatment.

Scripta Materialia

Deformation twinning in response to cracking in Al: An in situ TEM and molecular dynamics study

Author(s): Zongde Kou, Yanqing Yang, Lixia Yang, Wei Zhang, Bin Huang, Xian Luo

In situ tension tests in a transmission electron microscope and molecular dynamics simulations were carried out on pure Al, revealing a localized twinning-responded cracking mechanism. The dynamic twinning at crack tip was suggested to impose dual effects on fracture. On one hand, the intrinsic brittleness of coherent twin boundary (CTB) and the dislocation activities along it could facilitate cracking; on the other, twinning in Al could impede cracking mainly by effectively releasing the concentrated stress at crack tip. The competition between twin growth and CTB brittleness under stress was responsible for the behaving of crack.

Scripta Materialia

Fourier-based spectral method solution to finite strain crystal plasticity with free surfaces

Author(s): Tias Maiti, Philip Eisenlohr

A plastically dilatational material model is proposed that enables to simulate the mechanical response of non-compact geometries (containing free surfaces) by means of established spectral methods without any particular adaptations, i.e. in combination with arbitrary constitutive laws describing the remainder of the simulated geometry and under mixed boundary conditions. The versatility of this material model and more accurate representation of empty space in comparison to an isotropic elastic model employing low stiffness is demonstrated for the cases of void growth under biaxial extension and grain-scale deformation behavior of an oligocrystalline dogbone-shaped aluminum sample under uniaxial tension.

Scripta Materialia

Band engineering and tuning thermoelectric transport properties of p-type Bi0.52Sb1.48Te3 by Pb doping for low-temperature power generation

Author(s): Kwanlae Kim, Gwansik Kim, Hwijong Lee, Kyu Hyoung Lee, Wooyoung Lee

Herein, we report the results of a systematic study on the effect of Pb doping on the thermoelectric transport properties of p-type BiSbTe alloys to validate its potential applications for low-temperature power generation. The maximum power factor (~4.4mWm1 K2) at 300K was obtained using 0.31at.% Pb-doped Bi0.52Sb1.48Te3 and was found to originate from an enlarged density of states effective mass as a result of the band engineering effect. The maximum efficiency of thermoelectric power generation (η max ) could be enhanced by 150% at ΔT =220K when the Pb concentration was optimized.

Scripta Materialia

Quantification of strain localisation in a bimodal two-phase titanium alloy

Author(s): D. Lunt, X. Xu, T. Busolo, J. Quinta da Fonseca, M. Preuss

High Resolution Digital Image Correlation in combination with orientation imaging microscopy has been applied to compare quantitatively strain localisation in Ti-6Al-4V alloy with a bimodal microstructure subjected to two different ageing treatments, i.e. above and below the α2 solvus temperature. Interestingly, the most pronounced strain heterogeneity was observed in the secondary-α regions for the sample heat-treated to promote α2 formation. The high local strain was associated with intense slip bands within single long secondary-α laths with low levels of neighbouring strain, which is likely to have a significant impact on the low-cycle fatigue performance.

Scripta Materialia

Atomistic study on the super-elasticity of single crystal bulk NiTi shape memory alloy under adiabatic condition

Author(s): Bing Wang, Guozheng Kang, Qianhua Kan, Wenping Wu, Kun Zhou, Chao Yu

The temperature-induced phase transition and the super-elasticity (from the stress-induced phase transition) of equiatomic single crystal bulk NiTi shape memory alloys are investigated by the molecular dynamics method. By the simulation to the thermo-mechanical response of the single crystal NiTi alloy along the 〈001〉B2 under the compression/unloading and an adiabatic condition, the temperature change and the nucleation and growth of martensite transformation during the compression/unloading are discussed. The simulated results of molecular dynamics show that the single crystal bulk NiTi shape memory alloy exhibits a significant temperature change during the martensite transformation and its reverse under an adiabatic condition; moreover, a localized instability occurs apparently in the process of martensite transformation, which is closely related to the nucleation and growth rates of martensite phase; finally the effect of model size and strain rate on the thermo-mechanical response of the single crystal bulk NiTi alloy is also discussed, and no instability is observed in the simulated stress-strain curves if the model size is relatively larger, e.g., 8V0 and 13.824V0.

Computational Materials Science

Effects of cold pre-forging on microstructure and tensile properties of extruded AZ80 alloy

Author(s): Ye Jin Kim, Sang-Hoon Kim, Jong Un Lee, Jae Ok Choi, Ha Sik Kim, Young Min Kim, Yongjin Kim, Sung Hyuk Park

This study investigates the effects of cold pre-forging (CPF) on the extrusion load during hot extrusion and on the microstructure and mechanical properties of extruded AZ80 alloy. For this purpose, a billet is forged to 10% strain at room temperature and then subjected to indirect extrusion at 200°C. The CPF process causes the formation of a large amount of deformation twins and an increase in the dislocation density of the billet. The cold-forged AZ80 billet exhibits a higher extrusion load during extrusion than an unforged AZ80 billet, owing to the occurrence of twin-boundary hardening and enhanced strain-hardening effects in the former. The initial twins introduced by the CPF process promote dynamic recrystallization behavior during extrusion because they act as nucleation sites for recrystallization. As a result, the CPF process increases the area fraction of fine recrystallized grains and the microstructural homogeneity of the extruded alloy. The AZ80 alloy extruded after CPF shows a higher tensile strength and ductility than the AZ80 alloy extruded without CPF. The improvement in strength is attributed mainly to the decrease in the average grain size caused by an increased recrystallization fraction. The increased ductility is due to the reduced area fraction of coarse unrecrystallized grains, in which microcracks occur during tensile plastic deformation.

Science and Engineering A

Temperature-dependent strain hardening, precipitation and deformation-induced microstructure evolution in AA 6061

Author(s): Johannes Kreyca, Ernst Kozeschnik

The effect of β ′′ precipitation on strain hardening and yield stress evolution in an A6061 aluminium alloy is studied experimentally and through thermokinetic computer simulation. Samples were deformed by compression at temperatures from 25 to 500°C to strains of about 0.4 and three different precipitation states. Simulations on the thermal stability of β ′′ are conducted on an equivalent model Al-Mg-Si alloy resulting in an explanation for the experimentally observed softening at intermediate temperatures. EBSD micrographs confirm that different dislocation storage and annihilation mechanisms are operative at low and high temperatures. The low temperature microstructure correlates very well with the subgrain structures typically observed during stage IV strain hardening.

Science and Engineering A

Fabrication and energy absorption properties of titanium foam with CaCl2 as a space holder

Author(s): B. Xie, Y.Z. Fan, T.Z. Mu, B. Deng

This study employed calcium chloride (CaCl2) as a space holder to fabricate titanium foam via the sintering and dissolution process (SDP). The effects of the space holder content on the porosity and compression mechanical properties were investigated. The microstructure and composition of the titanium foam samples were analyzed with scanning electron microscopy (SEM) and X-ray diffraction (XRD). Compression tests used a universal testing machine. The results indicated the successful preparation of titanium foam using CaCl2 as the space holder. The porosities of the titanium foam were 71–89%, and the cells were 1.0–3.0mm. The porosities of the titanium foam increased following the addition of the space holder. The plateau stress of the titanium foam, which was 18.1–105.6MPa and obeyed the Gibson-Ashby models, decreased with an increase in porosity. The energy absorption capacity of the titanium foam was 11.2–55.6MJ/m3 when the strain was 50%. The maximum ideal energy absorption efficiency was about 0.78. Titanium foam with 71–88% porosity exhibited the potential for energy absorption applications.

Science and Engineering A

Experimental investigation of the fatigue crack propagation in a closed-cell aluminum alloy foam

Author(s): Xueling Fan, Modi Zhao, Tiejun Wang

Fatigue crack propagation in a closed-cell aluminum alloy foam is experimentally investigated in this work. A series of fatigue tests are conducted to obtain the a-N curves (i.e. crack length vs loading cycle number) and da/dN-∆K curves (fatigue crack propagation rate vs stress intensity). Statistical analysis is carried out to assess the stochastic and scattering characteristics in the fatigue crack propagation of the foam, based on which a statistical fatigue crack propagation model is developed. Furthermore, the fracture surfaces of the foam specimens are characterized, as well as the fatigue fracture morphologies. It is demonstrated that the structural heterogeneity inherent in the material and the difference in the cell structures between different specimens account for the great scatter of the fatigue crack propagation in the close-cell aluminum alloy foam.

Science and Engineering A

Evaluating the flow properties of a magnesium ZK60 alloy processed by high-pressure torsion: A comparison of two different miniature testing techniques

Author(s): Seyed Alireza Torbati-Sarraf, Reza Alizadeh, Reza Mahmudi, Terence G. Langdon

A ZK60 magnesium alloy was processed through 5 turns of high-pressure torsion (HPT) at room temperature under an applied pressure of 2.0GPa to produce a reasonably homogeneous ultrafine microstructure with a grain size of ~ 700nm. The potential superplastic behavior of this alloy was investigated by measuring the strain rate sensitivity using two different procedures of miniature tensile testing and miniature shear punch testing (SPT). The tensile experiments were conducted at initial strain rates of 3.0 × 10−5 to 1.0 × 10−1 s−1 and the SPT was performed at shear strain rates from 3.3 × 10−3 to 3.3 × 10−1 s−1 at temperatures of 473 and 523K. It is shown that the strain rate sensitivity index, m, has a maximum value of ~ 0.5 at intermediate strain rates in both tensile testing and SPT and there was a maximum elongation to failure of 940% in the tensile testing. The results demonstrate that tensile testing and SPT are both effective procedures in indicating the potential for achieving superplasticity.

Science and Engineering A

In-situ deformation mechanism and orientation effects in sintered 2D boron nitride nanosheets

Author(s): Archana Loganathan, Amit Sharma, Chris Rudolf, Cheng Zhang, Pranjal Nautiyal, Satyam Suwas, Benjamin Boesl, Arvind Agarwal

Two-dimensional hexagonal Boron nitride nanosheets (BNNS) are consolidated as a monolithic pellet by spark plasma sintering at 1650°C with a pressure of 50MPa without any sintering aid. X-ray diffraction study confirmed the h-BN structure after sintering of BNNS along with B2O3 and B4C as trace impurities. The preferred orientation of the basal plane (0002) perpendicular to the direction of pressure is observed on the top surface of the pellet after consolidation. High load (several hundred N) in-situ indentation studies inside SEM are carried out in the cross-section and on the top surface of the sintered BNNS pellet to understand the orientation effect on the deformation behavior. Total energy dissipation during indentation along the top surface was 50% greater than the cross-section. Predominant deformation mechanisms observed on the top surface indentation are compression of the layered sheets, delamination and BNNS pile-up. For the cross-section direction, the key deformation mechanisms are cracking and fracturing with an insignificant delamination. The dominant deformation mechanisms in sintered BNNS pellet are directly related to the preferred orientation of the crystallographic planes which was validated with TEM and crystallographic texture studies. Along the top surface, there is no active slip system. However, twinning was observed. In the cross-section, active pyramidal slip system resulted in a plastic deformation. Additionally, the weak van der Waals forces between the layers caused crack propagation along the cross-section.

Science and Engineering A

Microstructure–mechanical property correlation in oxide dispersion strengthened 18Cr ferritic steel

Author(s): M. Nagini, R. Vijay, Koteswararao V. Rajulapati, A.V. Reddy, G. Sundararajan

The tensile deformation of Oxide Dispersion Strengthened 18Cr ferritic steels (henceforth designated as ODS–18Cr steel) was studied over a temperature range of 298–1073K. At each temperature, the influence of microstructure (grain size and dispersoid size) which could be refined progressively by increasing the milling time over the range 1–6h on strength was also investigated. Oxide free 18Cr steel (NODS) provided the baseline data as compared to ODS–18Cr steel. At all the test temperatures, the flow stress of ODS–18Cr steels increased with increasing milling time or equivalently with refinement of the grains and dispersoids. The decrease in flow stress with increasing tensile test temperature was marginal up to 673K. Beyond 673K, the flow stress decreased rapidly. Enhanced strength of ODS steels when compared to NODS steel is due to the formation of ultra–fine grained structure along with fine dispersion of complex Y–Ti–O oxide particles. The concomitant roles of the grain size related strengthening and dispersion strengthening due to oxide particles in the strengthening of ODS–18Cr steels at all test temperatures were rationalized using root mean square superposition model.

Science and Engineering A

Creep behaviors of hot compressed Mg-4Y-2Nd-0.2Zn-0.5Zr alloy with and without aging

Author(s): Zhengwu Fang, Qinghuan Huo, Jing Wang, Zhenyu Xiao, Duxiu Zhang, Weiying Huang, Mingchun Zhao, Xuyue Yang

Tensile creep behaviors of a hot compressed Mg-4Y-2Nd-0.2Zn-0.5Zr alloy with aging (the AA sample) and without aging (the AS sample) are studied at 473, 493 and 523K under a wide range of applied stresses. The creep mechanism of the AS sample is dislocation viscous gliding. For the AA sample, the creep mechanism is cross-slip during the early creep stage and dislocation climb and pyramidal slip during the late. Furthermore, the precipitates can be dissolved into the matrix and restarted during the subsequent creep stages. At 473 and 493K, the AA sample has a better creep resistance than the AS sample, due to the hindering effect of the precipitates on dislocation movement. At 523K and higher stress levels, the precipitates have limited hindering effect on dislocation movement in the AA sample while the kink bands continue to serve as the strengthening mechanism in the AS sample. The creep resistance of the AA sample becomes lower than that of the AS sample. It is recommended that the AA sample be used in the applications where their service temperature is lower than the pre-aging temperature while the AS sample be used where service temperature is higher than the pre-aging temperature.

Science and Engineering A

Superplastic deformation behaviour and microstructure evolution of near-α Ti-Al-Mn alloy

Author(s): A.V. Mikhaylovskaya, A.O. Mosleh, A.D. Kotov, J.S. Kwame, T. Pourcelot, I.S. Golovin, V.K. Portnoy

Superplastic deformation behaviour of conventional sheets of a near-α titanium alloy (Ti-2.5Al-1.8Mn) was studied by a step-by-step decrease of the strain rate and constant strain rate tests in a temperature range of 790–915°C. The research found that superplastic deformation is possible in a temperature range of 815–890°С and a constant strain rate range of 2 × 10−4 to 1 × 10−3 s−1 with elongation above 300% and m-index above 0.4. Also, the research identified the optimum superplastic temperature range of 815–850°C and constant strain rate of 4 × 10−4 s−1 which provide a maximum elongation of 600–650%. Strain hardening is accelerated by dynamic grain growth at high temperatures of 865 and 890°С. High dislocation activity is observed at superplastic flow in α-phase. Constitutive modelling of superplastic deformation behaviour is performed, and possible deformation mechanisms are discussed. It is suggested that grain boundary sliding between the α-grains is accommodated by a dislocation slip/creep mechanism.

Science and Engineering A

Fractographic analysis of anisotropic deformation behavior after tensile testing of pipeline steels at elevated temperatures

Author(s): T.R. Jacobs, D.K. Matlock, K.O. Findley

Changes in anisotropic deformation behavior at elevated temperatures in three American Petroleum Institute ×70 pipeline steels were assessed using fractographic analysis of tensile specimens. An anisotropy parameter was developed to quantify the change in anisotropic deformation with increasing tensile test temperature. A minimum in the degree of anisotropic deformation was observed at 300°C, which corresponded directly with maxima in strength and ductility at the same temperature. Observed changes in anisotropic deformation at elevated temperatures suggest that dynamic strain aging preferentially strengthens the steel in the through-thickness direction indicating that contributions of dynamic strain aging depend on crystallographic texture.

Science and Engineering A

Effect of Ti addition on the microstructure and mechanical properties of cast Mg-Gd-Y-Zn alloys

Author(s): Peng Cheng, Yuhong Zhao, Ruopeng Lu, Hua Hou, Zhiqiang Bu, Feng Yan

The effect of Ti addition on the microstructure and mechanical properties of cast Mg-Gd-Y-Zn alloys was investigated. Ti exhibited a refinement effect comparable to that of Zr in the as-cast condition. With the increase of Ti content, the phenomenon of grain refinement and homogenization was more obvious, especially for solution-treated alloys. The finest grains were obtained with 0.8wt% Ti and were composed of α-Mg and refined Mg5(Gd, Y, Zn) in the as-cast structure. Zr and Ti could be found along the grain boundaries. After solid solution treatment, Zr was still concentrated in the vicinity of the grain boundaries, while Ti was uniformly dispersed in the α-Mg matrix. The Ti-refined alloys exhibited superior grain size stability and mechanical properties. In addition, the mechanical properties of the considered alloys were improved due to the transition from Mg5(Gd, Y, Zn) to long-period stacking ordered (LPSO) phase during the heat treatment. Subsequent aging at 220℃ resulted in remarkable precipitation hardening and consequently the strength was significantly improved. The Ti-refined alloy exhibited higher ultimate tensile strength and elongation than the alloy containing the same amount of Zr, with an increase of 20MPa and 29%, respectively.

Science and Engineering A

Simple Theory for the Dynamics of Mean-Field-Like Models of Glass-Forming Fluids

Author(s): Grzegorz Szamel

We propose a simple theory for the dynamics of model glass-forming fluids, which should be solvable using a mean-field-like approach. The theory is based on transparent physical assumptions, which can be tested in computer simulations. The theory predicts an ergodicity-breaking transition that is id...

Physical Review Letters

Temperature-dependent elasticity of $\mathrm{Pb}[{(\mathrm{M}{\mathrm{g}}_{0.33}\mathrm{N}{\mathrm{b}}_{0.67})}_{1\text{−}x}\mathrm{T}{\mathrm{i}}_{x}]{\mathrm{O}}_{3}$

Author(s): Sumudu Tennakoon, Joseph Gladden, Mainak Mookherjee, Tiglet Besara, and Theo Siegrist

Relaxor ferroelectric materials, such as $\mathrm{Pb}[{(\mathrm{M}{\mathrm{g}}_{0.33}\mathrm{N}{\mathrm{b}}_{0.67})}_{1\text{−}x}\mathrm{T}{\mathrm{i}}_{x}]{\mathrm{O}}_{3}$ (PMN-PT) with generic stoichiometry, undergo a ferroelectric-to-paraelectric phase transition as a function of temperature. Th...

Physical Review B

Perturbation and asymptotic solutions of energy localization of impurity modes in a one-dimensional anharmonic chain

Author(s): Xuan-Lin Chen, Gang-Bei Zhu, Ze-Hui Jiang, and Yan-Qiang Yang

A 1D anharmonic chain with a single impurity particle is used to study the vibrational energy localization. Numerical and asymptotic solutions for the symmetric anharmonic localized mode are both presented. The numerical results reveal that the energy localization strengthens with decreasing impurit...

Physical Review B

Confinement sensitivity in quantum dot singlet–triplet relaxation

C J Wesslén and E Lindroth

Spin–orbit mediated phonon relaxation in a two-dimensional quantum dot is investigated using different confining potentials. Elliptical harmonic oscillator and cylindrical well results are compared to each other in the case of a two-electron GaAs quantum dot subjected to a tilted magnetic field. The lowest energy set of two-body singlet and triplet states are calculated including spin–orbit and magnetic effects. These are used to calculate the phonon induced transition rate from the excited triplet to the ground state singlet for magnetic fields up to where the states cross. The roll of the cubic Dresselhaus effect, which is found to be much more important than previously assumed, and the positioning of ‘spin hot-spots’ are discussed and relaxation rates for a few different systems are exhibited.

Journal of Physics Condensed Matter

Hybrid-DFT  +  V w method for band structure calculation of semiconducting transition metal

Viktor Ivády, Adam Gali and Igor A Abrikosov

Hybrid functionals’ non-local exchange-correlation potential contains a derivative discontinuity that improves on standard semi-local density functional theory (DFT) band gaps. Moreover, by careful parameterization, hybrid functionals can provide self-interaction reduced description of selected states. On the other hand, the uniform description of all the electronic states of a given system is a known drawback of these functionals that causes varying accuracy in the description of states with different degrees of localization. This limitation can be remedied by the orbital dependent exact exchange extension of hybrid functionals; the hybrid-DFT  +  V w method (Ivády et al 2014 Phys. Rev . B 90 035146). Based on the analogy of quasi-particle equations and hybrid-DFT single particle equations, here we demonstrate that parameters of hybrid-DFT  +  V w functional can be determined from approximate theoretical quasi-particle s...

Journal of Physics Condensed Matter

Homogeneous crystal nucleation in polymers

C Schick, R Androsch and J W P Schmelzer

The pathway of crystal nucleation significantly influences the structure and properties of semi-crystalline polymers. Crystal nucleation is normally heterogeneous at low supercooling, and homogeneous at high supercooling, of the polymer melt. Homogeneous nucleation in bulk polymers has been, so far, hardly accessible experimentally, and was even doubted to occur at all. This topical review summarizes experimental findings on homogeneous crystal nucleation in polymers. Recently developed fast scanning calorimetry, with cooling and heating rates up to 10 6 K s −1 , allows for detailed investigations of nucleation near and even below the glass transition temperature, including analysis of nuclei stability. As for other materials, the maximum homogeneous nucleation rate for polymers is located close to the glass transition temperature. In the experiments discussed here, it is shown that polymer nucleation is homogeneous at such temperatures. Homogeneous nucleation...

Journal of Physics Condensed Matter

Hidden symmetries in N -layer dielectric stacks

Haihao Liu, M Shoufie Ukhtary and Riichiro Saito

The optical properties of a multilayer system with arbitrary N layers of dielectric media are investigated. Each layer is one of two dielectric media, with a thickness one-quarter the wavelength of light in that medium, corresponding to a central frequency f 0 . Using the transfer matrix method, the transmittance T is calculated for all possible 2 N sequences for small N . Unexpectedly, it is found that instead of 2 N different values of T at f 0 ( T 0 ), there are only {$(N/2+1)$} discrete values of T 0 , for even N , and ( N + 1) for odd N . We explain this high degeneracy in T 0 values by finding symmetry operations on the sequences that do not change T 0 . Analytical formulae were derived for ...

Journal of Physics Condensed Matter

Evidence for thermal activation in the glassy dynamics of insulating granular aluminum conductance

T Grenet and J Delahaye

Insulating granular aluminum is one of the proto-typical disordered insulators whose low temperature electrical conductance exhibits ubiquitous non-equilibrium phenomena. These include slow responses to temperature or gate voltage changes, characteristic field effect anomalies and ageing phenomena typical of a glass. In this system the influence of temperature on the glassy dynamics has remained elusive. A similar situation was met in insulating indium oxide and it was concluded that in high carrier density Anderson insulators, electronic slow relaxations essentially proceed via activationless processes. In this work we experimentally demonstrate that thermal effects do play a role and that the slow dynamics in granular aluminum is subject to thermal activation. We show how its signatures can be revealed and activation energy distributions can be extracted, providing a promising grasp on the nature of the microscopic mechanism at work in glassy Anderson insulators. We explain wh...

Journal of Physics Condensed Matter

Anomalous cyclotron mass dependence on the magnetic field and Berry’s phase in (Cd 1− x − y Zn x Mn

V S Zakhvalinskii, T B Nikulicheva, E Lähderanta, M A Shakhov, E A Nikitovskaya and S V Taran

Shubnikov–de Haas (SdH) effect and magnetoresistance measurements of single crystals of diluted II–V magnetic semiconductors (Cd 1− x − y Zn x Mn y ) 3 As 2 ( x   +   y   =  0.4, y   =  0.04 and 0.08) are investigated in the temperature range T   =  4.2 ÷ 300 K and in transverse magnetic field B   =  0 ÷ 25 T. The values of the cyclotron mass m c , the effective g -factor g *, and the Dingle temperature T D are defined. In one of the samples ( y   =  0.04) a strong dependence of the cyclotron mass on the magnetic field m c ( B )  =   m c (0)  +   αB is observed. The value of a phase shift close to β   =  0.5 indicates the presence of Berry phase and 3D Dirac fermions in a single crystals of (Cd 1− x − y Zn x Mn y ) 3...

Journal of Physics Condensed Matter

Fri Oct 13 2017

Interfacial thermal resistance between few-layer MoS2 and silica substrates: A molecular dynamics study

Author(s): Hamid Farahani, Ali Rajabpour, Mansour Khanaki, Ali Reyhani

Interfacial thermal resistance (ITR) between molybdenum disulfide (MoS2) and crystalline or amorphous silica as a substrate was studied using molecular dynamics (MD) simulation. To do so, pump–probe method, which is a MD technique inspired by an experimental method, was employed. The effects of substrate type, temperature, number of layers of MoS2 and van der Waals (vdW) coupling strength on ITR between MoS2 and its silica substrates were explored. It was observed that, obtained ITR values for crystalline or amorphous silica substrate were close to one another. Our findings showed that, by increasing the temperature from 200 K to 400 K, ITR between a single-layer MoS2 and its crystalline or amorphous silica substrate decreases by about 20%, which might be due to better phonons couplings at the interface in higher temperatures. We also showed that, ITR between multilayer MoS2 and crystalline or amorphous silica substrate does not differ by increasing the number of layers of MoS2. It has been found that, by increasing Lennard-Jones coupling strength from 0.5 to 2, ITR between a single-layer MoS2 and crystalline/amorphous silica substrate decreases by around 80%, showing better phonons couplings at the interface between the two structures.

Computational Materials Science

Phase-dependent mechanical properties of two-dimensional silica films: A molecular dynamics study

Author(s): Jin Zhang

The recently discovered two-dimensional (2D) silica films can be grown in both crystalline phase and vitreous phase. Using molecular dynamics simulations, we show that these two phases of 2D silica possess vastly different mechanical properties. It is shown that the Young’s modulus of crystalline 2D silica is 54% greater than that of its vitreous counterpart. The vitreous 2D silica fails via the ductile fracture with serrated stress-strain curves, while its crystalline counterpart exhibits the abrupt brittle rupture. Both the fracture strength and fracture strain of crystalline 2D silica are significantly larger than those of its vitreous counterpart. Fracture mechanics theory is utilized to give some explanations to the different fracture properties of these two phases. Moreover, 2D silica mixed by crystalline phase and vitreous phase is also studied. Owing to the different mechanical properties of crystalline and vitreous phases, the elastic and fracture properties of this mixed-phase 2D silica are strongly dependent on its crystallinity. Specifically, we find that the Young’s modulus of the mixed-phase 2D silica, ranging between the values of its crystalline and vitreous counterparts, increases as the degree of crystallinity increases. Meanwhile, with the increase of the degree of crystallinity the fracture strain of the mixed-phase 2D silica is found to decrease, making its value smaller than that of the purely crystalline and vitreous 2D silica.

Computational Materials Science

Ab initio GGA+U investigations of the electronic properties and magnetic orderings in Mn, Gd doped ZBWZ structural CdSe

Author(s): Bo Kong, Ti-Xian Zeng, Yong-Gang Wu, Zhi-Jian Fu, Zhu-Wen Zhou

In the paper, the electronic properties and magnetic orderings in the Mn/Gd doped zinc-blende (ZB) and wurtzite (WZ) structural CdSe are investigated extensively using first-principles calculations. We find that the infiltration of Mn ions in the ZB/WZ structural CdSe will lead to the antiferromagnetic ordering; while the infiltration of Gd ions will lead to the ferromagnetic ordering. Moreover, the antiferromagnetic/ferromagnetic coupling interactions between the magnetic Mn/Gd ions will become more and more strong with decreasing Mn-Mn/Gd-Gd distance. Particularly, the electronic properties in the Mn mono-doping case (spin-polarized) and in the Mn double-doping case (ferromagnetic) are almost same, and they tend to be the magnetic/ferromagnetic semiconductor via the comparison from the different calculations; as a whole, the Mn doped ZB/WZ structural CdSe system tends to present the antiferromagnetic semiconductor character. In addition, the band gap in the Mn doped ZB/WZ structural CdSe decreases with increasing Mn concentration using GGA+U calculations. In contrast to the case of the Mn doped ZB/WZ CdSe: (i) the Gd doped ZB/WZ CdSe in the mono-doping case displays the magnetic semiconductor character; but the Gd doped ZB/WZ structural CdSe in the double-doping case presents the n-type ferromagnetic semiconductor, and the robust ferromagnetism is obtained in the system irregardless of the crystal structure and the distance between the magnetic ions; (ii) the band gap in the Gd doped ZB/WZ structural CdSe increases with increasing Gd concentration, Burstein-Mott-effect might be major reason.

Computational Materials Science

Light impurity atoms as the probes for the electronic structures of actinide dioxides

Author(s): Bingyun Ao, Ruizhi Qiu, Guangfeng Zhang, Zhen Pu, Xiaolin Wang, Peng Shi

First-principles density functional theory are used to calculate the formation energies of ten light impurities X (X: H, He, Li, Be, B, C, N, O, F and Ne) in seven actinide dioxides AnO2 (An: Th, Pa, U, Np, Pu, Am and Cm), in order to elucidate the relative stability of X and to obtain some trends of impurities behaviors. The Hubbard parameter U is used to describe the strongly correlated electron behavior of An 5f electrons. The results indicate that the formation energies of X significantly depend on the properties of AnO2 and X. For X at the octahedral interstitial sites of AnO2, F is the only energetically favorable impurity for all AnO2, owing to its strong oxidability; H in PaO2, O in PaO2 and UO2, Li in PuO2, AmO2 and CmO2, Be in AmO2 and CmO2 are also energetically favorable. The oxidability or reductivity of X and the delocalization → localization transition of 5f electrons across actinide series can account for the trends of the behaviors of X in AnO2. Particularly, H, a very typical amphoteric element, is chosen to illustrate its difference existence states in AnO2. H prefers to occupy the octahedral interstitial sites of early AnO2 or form hydroxyl group in the later AnO2.

Computational Materials Science

Layered heterostructures based on graphene, hexagonal zinc oxide and molybdenum disulfide: Modeling of geometry and electronic properties

Author(s): Alexander G. Kvashnin, Pavel B. Sorokin, Leonid A. Chernozatonskii

Here we present a comprehensive investigation of novel composite layered structures based on graphene, molybdenum disulfide (MoS2) monolayers and hexagonal zinc oxide (ZnO), which display promising optical and electronic properties for photovoltaic applications. Theoretical study of the atomic structure, optical and electronic properties of proposed ZnO/MoS2 and G/MoS2/ZnO/G nanostructures were carried out. We show that making the G/MoS2/ZnO/G heterostructure leads to high doping ratio of graphene layers and zero band gap which allows a conclusion of the possibility of using such structures in photovoltaic applications, due to broad energetic region of high electronic density of states.

Computational Materials Science

Dipolar Crystals: The Crucial Role of the Clinohexagonal Prism Phase

Author(s): Ludovic Spiteri and René Messina

We report a new phase called clinohexagonal prism (CHP) that accounts for all the ground states of dipolar hard spheres prepared at any density. This phase merely consists of an oblique prismatic lattice with a hexagonal base. Our calculations show that at intermediate densities, a special close pac...

Physical Review Letters

Polarized Neutron Study of Ni-Mn-Ga Alloys: Site-Specific Spin Density Affected by Martensitic Transformation

Author(s): P. Lázpita, J. M. Barandiarán, J. Gutiérrez, C. Mondelli, A. Sozinov, and V. A. Chernenko

Polarized neutron scattering has been used to obtain the magnetic moment at specific crystallographic sites of the austenitic and martensitic phases of two nonstoichiometric Ni-Mn-Ga single crystals with close composition. These alloys have been chosen because they exhibit different structures in th...

Physical Review Letters

Thu Oct 12 2017

Investigation on temporal evolution of the grain refinement in copper under high strain rate loading via in-situ synchrotron measurement and predictive modeling

Author(s): Pooja Nitin Shah, Yung C. Shin, Tao Sun

Synchrotron X-rays are integrated with a modified Kolsky tension bar to conduct in situ characterization of the grain refinement mechanism operating during the dynamic deformation of metals. Copper with an initial average grain size of 36 μm is refined to 6.3 μm when loaded at a constant high strain rate of 1200 s−1. Synchrotron measurements revealed the temporal evolution of the grain refinement mechanism in terms of the initiation and rate of refinement throughout the loading test. A multiscale finite element based recrystallization model has been developed to predict the grain size evolution occurring during the dynamic deformation process. The model accurately predicts the initiation and temporal evolution of the refinement phenomenon with a predicted final average grain size of 2.4 μm.

Acta Materialia

Alloying effects on the microstructure and mechanical properties of nanocrystalline Cu-based alloyed thin films: Miscible Cu-Ti vs immiscible Cu-Mo

Author(s): J.Y. Zhang, J.T. Zhao, X.G. Li, Y.Q. Wang, K. Wu, G. Liu, J. Sun

Tuning the microstructure to optimize the mechanical performance of nanocrystalline Cu thin films via the alloying strategy is quite important for their application in microdevices. In this work, we prepared nanocrystalline miscible Cu-Ti and immiscible Cu-Mo alloyed thin films to investigate alloying effects on the microstructure and mechanical properties of Cu thin films in terms of mixing enthalpies. It is found that the dopants of both Ti and Mo can notably refine the grains, and in particular promote the formation of nanotwins below a critical content of solute, beyond which the formation of nanotwins is notably suppressed. The nonmonotonic solute concentration-dependent twinning behavior observed in Cu-Ti and Cu-Mo alloyed thin films is explained by the coupling effects between grain size and grain boundary segregated dopants that affects the stimulated slip process of partials. The increased hardness of both Cu-Ti and Cu-Mo systems with increasing the solute contents are quantitatively explained by combining several strengthening mechanisms, including solid solution strengthening, grain/twin boundary (GB/TB) strengthening, solute segregation-induced strengthening. It unexpectedly appears that with increasing the solute contents, the Cu-Ti system exhibits monotonically reduced positive strain rate sensitivity (SRS, m), whereas the Cu-Mo system manifests almost constant negative SRS. The fundamental difference in SRS m between Cu-Ti and Cu-Mo is rationalized in terms of the interactions between solute atomic clusters and dislocations based on the cross-core diffusion mechanism.

Acta Materialia

Intrinsic impact toughness of relatively high strength alloys

Author(s): Q.Q. Duan, R.T. Qu, P. Zhang, Z.J. Zhang, Z.F. Zhang

Although the Charpy impact test has been routine for decades to assess the ductile or brittle nature of materials, the impact toughness, which is strongly sample-thickness dependent, is not an intrinsic property. By re-examining the energy absorption during fracturing of relatively high strength alloys, here we find a remarkably good linear relation between the impact energies and the fracture surface areas of samples with different thickness, and the slope essentially renders the intrinsic impact toughness. The new findings, which also provide a scaling law to well predict the thickness effect on the traditional impact toughness, may have broad applications for precisely determining the ductile-to-brittle transition temperature of small-dimensional devices, selecting materials according to their toughness at the thickness in usage, and evaluating the intrinsic toughness of emerging high strength materials with limited achievable size.

Acta Materialia

Nanodomains in metalferroelectric 0–3 type composites: On the origin of the strong piezoelectric effect

Author(s): Mupeng Zheng, Yudong Hou, Mankang Zhu, Hui Yan

Outstanding piezoelectric properties were obtained in Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3/6vol.% Ag (PZN–PZT/Ag) metal/ferroelectric 0–3 type composites. A combination of X-ray diffractometer (XRD) and high resolution transmission electron microscopy (HRTEM) revealed that the PZN–PZT/Ag ferroelectric composite presents a novel intragranular structure. It is suggested that the uniform distribution of nano‑silver particles in a PZN–PZT matrix was the origination of nanodomains in PZN–PZT/Ag composites, further leading to the strong piezoelectric effect. We believe that our discovery here will be helpful in the development of new piezoelectric composites with outstanding properties.

Scripta Materialia

Crystallography and interfacial structure in a directionally solidified Al2O3Y3Al5O12ZrO2 eutectic crystal

Author(s): Xu Wang, Yujie Zhong, Qian Sun, Yiran Li, Wen Zhang, Dongqing Qi, Dong Wang, Bailing Jiang

The single crystal of Al2O3/Y3Al5O12/ZrO2 ternary eutectic was prepared by an optical floating zone furnace. The crystallography and interfacial structure of the directionally solidified ternary eutectic were investigated by means of electron backscattered diffraction and transmission electron microscopy. We found that the preferred crystallographic orientation of the Al2O3/Y3Al5O12/ZrO2 eutectic crystal was { 11 2 ¯ 0 }〈 1 1 ¯ 00 〉Al2O3 ||{001}〈001〉Y3Al5O12 ||{001}〈001〉ZrO2. Most of the tiny ZrO2 locates dispersively between Al2O3 and Y3Al5O12. It can be concluded that interfacial energy between Al2O3-Y3Al5O12-ZrO2 played an important role in deciding the preferred crystallographic orientations of the Al2O3/Y3Al5O12/ZrO2 eutectic and the precipitation behavior of the ZrO2 phase.

Scripta Materialia

Dynamic interaction between grain boundary and stacking fault tetrahedron

Author(s): Liang Zhang, Cheng Lu, Kiet Tieu, Yasushi Shibuta

We utilize molecular dynamics simulations to investigate the dynamic interaction between the grain boundary (GB) and the stacking fault tetrahedron (SFT) in bicrystal copper. The grain boundary can migrate itself under the shear strain and can serve as a sink to remove SFT. The sink efficiency of grain boundaries is sensitive to their structural characteristics. The high-angle GBs can show a great ability to remove SFT even at an extreme low temperature, while the increase of temperature can facilitate the annihilation of SFT at the low-angle GBs. This study reveals a new possible GB-mediated damage healing mechanism of irradiated materials.

Scripta Materialia

Orientation dependent plasticity of metallic amorphous-crystalline interface

Author(s): Ehsan Alishahi, Chuang Deng

In this study, the influence of crystalline orientation on the plasticity of metallic amorphous-crystalline interface (ACI) has been investigated in a model system of crystalline Cu-amorphous CuZr multilayers by molecular dynamics simulations. It is found that the ACIs with various crystalline orientations all show similar structure with a gradual transition from crystalline to amorphous. As a result, the energy of ACIs shows only a weak dependence on the crystalline orientation. Additionally, the ACI shows no stress concentration or area of high energy than the rest of the materials. Those findings are in contrast to other types of planar defects such as grain boundaries or free surfaces. However, the strength and the yielding behavior vary significantly among the different amorphus-crystalline (A-C) multilayers. Specifically, it is found that the yielding mechanism, e.g., through dislocation nucleation in the crystalline Cu or through shear localization in the amorphous CuZr layer, is mainly determined by the crystalline orientation. What is more, the simulations on ACIs with polycrystalline Cu suggest that the plasticity of A-C multilayers strongly depends on the texture orientation of the columnar grains in the Cu layer, which can be used to explain some of the experimental observations on similar materials.

Computational Materials Science

A hierarchical thermo-mechanical multi-scale technique for modeling of edge dislocations in nano-crystalline structures

Author(s): M. Jahanshahi, A.R. Khoei, N. Heidarzadeh, N. Jafarian

In this paper, a hierarchical multi-scale technique is developed to investigate the thermo-mechanical behavior of nano-crystalline structures in the presence of edge dislocations. The primary edge dislocations are generated by proper adjustment of atomic positions to resemble discrete dislocations. The interatomic potential used to perform atomistic simulation is based on the Finnis-Sinclair embedded-atom method as many-body potential and, the Nose-Hoover thermostat is employed to control the effect of temperature. The strain energy density function is obtained for various representative volume elements under biaxial and shear loadings by fitting a fourth order polynomial in the atomistic level. The material elastic constants are calculated by evaluating the second derivatives of the total potential energy per unit volume with respect to strain components. The evolutions of yield stress, elastic constants and bulk modulus are derived for nano-crystals of magnesium with hcp atomic structure containing different number of edge dislocations at various temperature levels. In order to provide a relation between various quantities in nano-scale level to their counterparts in macro-scale level, the material properties obtained from molecular dynamics simulations are transferred to the Gauss points of finite element mesh using the calculated strain energy function. The numerical results clearly demonstrate the behavior of material in the presence of edge dislocations at various temperatures.

Computational Materials Science

Effect of carbide interlayers on the microstructure and properties of graphene-nanoplatelet-reinforced copper matrix composites

Author(s): Xiaoyang Si, Mian Li, Fanyan Chen, Per Eklund, Jianming Xue, Feng Huang, Shiyu Du, Qing Huang

Copper matrix composites reinforced with carbide-coated graphene nanoplatelets (GNPs) were investigated in order to understand the role of the interlayers on the thermal, electrical, mechanical and electro-tribological properties of the composites. The TiC or VC coatings were formed in situ on the two sides of GNPs through a controllable reaction in molten salts. Compared with bare GNPs composites, the bonding between the GNPs and copper was improved. Accordingly, the tensile strength and the fracture elongation of Cu/GNPs composites with an interlayer were enhanced by strengthened interfacial bonding. Furthermore, the wear resistance of Cu/GNPs composites was remarkably improved.

Science and Engineering A

Study of age hardening in a Mg–2.2wt%Nd alloy by in situ synchrotron X-ray diffraction and mechanical tests

Author(s): Bijin Zhou, Leyun Wang, Bin Chen, Yiwang Jia, Wen Wen, Dejiang Li, Da Shu, Peipeng Jin, Xiaoqin Zeng, Wenjiang Ding

Microstructure evolution of a supersaturated solid solution Mg–2.2wt%Nd alloy during aging at 240°C for 6h (stage I), at 295°C for 3h (stage II), and finally at 320°C for 1h (stage III) was studied by in situ synchrotron X-Ray diffraction. In stage I, diffraction from β 1 (Mg3Nd, FCC) precipitates was observed after ~ 15min. Evolution of the volume, size, and aspect ratio of β 1 with the aging time were estimated from peak intensity and peak width analysis. In stage II, diffraction from β (Mg12Nd, tetragonal) precipitates was observed after ~ 36min while β 1 remained in the material. In stage III, the volume of β 1 reduced substantially in relative to β. The identified precipitation sequence from the in situ aging experiment is supported by ex situ electron microscopy observations. β′, which had not been detected by synchrotron X-rays, was additionally found using transmission electron microscopy in the material by the end of aging stage I. Tension and hardness tests using specimens with different aging conditions were further performed to understand how different precipitates affect the strength and ductility of the Mg–Nd alloy.

Science and Engineering A

Characterization and compressive properties of NiMg hybrid foams

Author(s): Jiaan Liu, Shouquan Shi, Zhaobin Zheng, Kuo Huang, Yuying Yan

In this study, Ni/Mg hybrid foams were fabricated by depositing electroless Ni-P coatings on open-cell Mg foams. The microstructure, composition and phases of Ni-P coatings were observed and analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively. The compressive properties of the Ni/Mg hybrid foams were evaluated by compressive tests. The results show that the compressive strength, specific strength and energy absorption capacity of open-cell Mg foams are improved by electroless plating. However, compared with open-cell Mg foams, the Ni/Mg hybrid foams exhibit more brittleness characteristics. It was found that the different compressive properties between the Mg foams and Ni/Mg hybrid foams were attributed to the diverse failure mechanisms confirmed by fractography observation.

Science and Engineering A

Relationship between unusual high-temperature fatigue crack growth threshold behavior in superalloys and sudden failure mode transitions

Author(s): J. Telesman, T.M. Smith, T.P. Gabb, A.J. Ring

An investigation of high temperature cyclic fatigue crack growth (FCG) threshold behavior of two advanced nickel disk alloys was conducted. The focus of the study was the unusual crossover effect in the near-threshold region of these type of alloys where conditions which produce higher crack growth rates in the Paris regime, produce higher resistance to crack growth in the near threshold regime. It was shown that this crossover effect is associated with a sudden change in the fatigue failure mode from a predominant transgranular mode in the Paris regime to fully intergranular mode in the threshold fatigue crack growth region. This type of a sudden change in the fracture mechanisms has not been previously reported and is surprising considering that intergranular failure is typically associated with faster crack growth rates and not the slow FCG rates of the near-threshold regime. By characterizing this behavior as a function of test temperature, environment and cyclic frequency, it was determined that both the crossover effect and the onset of intergranular failure are caused by environmentally driven mechanisms which have not as yet been fully identified. A plausible explanation for the observed behavior is proposed.

Science and Engineering A

Fine-grained structure and recrystallization at ambient temperature for pure magnesium subjected to large cold plastic deformation

Author(s): X.M. Chen, L.T. Li, W.Z. Chen, W.C. Zhang, L.X. Zhang, Y.D. Qiao, E.D. Wang

The cold drawing of magnesium wires has constantly been an obstacle in industrial manufacture due to the lack of sufficient independent slip system at ambient temperature and the frequent intermediate annealing gives rise to regrettably low production efficiency. Aiming at the technique challenge, a feasible and effective method of multi-pass cumulative cold drawing with small deformation per pass was utilized in this study and a wire of 0.625mm in diameter was obtained out of primitive Φ2.0mm material with the accomplishment of about 2.3 in cumulative strain (90% in area reduction), the efficiency of which was dramatically improved. The microstructure, texture and mechanical properties with the variation of cumulative area reduction were investigated. It can be apparently observed that continuous dynamic recrystallization had occurred during cold drawing. The outcomes are displayed as follows: the transition emerged from low angle grain boundary to high angle grain boundary; the increased fraction of undeformed/recrystallized structure first descended then ascended; the deformed structure initially appeared yet disappeared quickly. All of above owned an almost same critical point at around 23% cumulative area reduction. One of the most remarkable consequences of the deformation was the degeneration of the average grain size steadily from 11.8 to 3.1µm. 〈10-10〉//DD fiber component was gradually transformed from the primitive {0002}//DD fiber texture and this deformation texture was continuously strengthened. The enhanced mechanical properties achieved uncommonly high strength of 214.5MPa in yield strength and 256.9MPa in ultimate tensile strength due to grain refinement arising from continuous dynamic recrystallization and intense fiber texture strengthening introduced during cold drawing.

Science and Engineering A

Effect of Mn and Al contents on hot ductility of high alloy Fe-xMn-C-yAl austenite TWIP steels

Author(s): Hongbo Liu, Jianhua Liu, Bowei Wu, Yaozu Shen, Yang He, Hao Ding, Xiaofeng Su

Effect of Mn (14.94,18.21, and 23.6wt%) and Al (0.002,0.75, and 1.47wt%) contents on hot ductility of five high alloy Fe-xMn-C-yAl austenitic Twinning induced plasticity (TWIP) steels were investigated by Gleeble-3500 thermo-mechanical simulator in the temperature range 700–1200℃ under a constant strain rate of 3 × 10−3 s−1. The results indicated that the hot ductility of different Mn-containing TWIP steels are not appreciable with all the reduction of area (RAs) values lower than 30%, and RAs would be further decreased as the Mn content increased. The matrix of TWIP steel is inhomogeneous with severe Mn microsegregation in the interdendritic zone. Moreover, the C microsegregation ratio increases from 0.85 to 1.16, 0.76–1.22, to 0.74–1.32 when Mn concentration increases from 14.94wt%, 18.21wt%, to 23.6wt%, respectively. Additionally, the microstructure and the true stress-true strain curves suggested that dynamic recrystallization (DRX) took place in 14.94wt% Mn bearing TWIP steel, while the fraction of DRX grains decreased dramatically with increasing Mn content. Therefore, it is inferred that the high Mn addition inhibited DRX, together with the acceleration effect of C microsegregation by Mn addition should be the most predominant factor of the hot ductility loss with Mn content increases in TWIP steels. On the other hand, Al addition to TWIP steels resulted in a dramatic increase of AlN particles content. The AlN particle accounted for nearly 64% of the total precipitate content for the 1.59wt% Al containing TWIP steel. Compared with Al-free TWIP steel, the excessive number of fine AlN particles in the 1.59wt% Al containing steel effectively pinned the austenite grain boundaries, which inhibited the occurrence of DRX and simultaneously promote grain boundary sliding, resulting in the deterioration of hot ductility.

Science and Engineering A

Modeling of the thermal and mechanical properties of clay ceramics incorporating organic additives

Author(s): P.M. Nigay, R. Sani, T. Cutard, A. Nzihou

This paper presents the results of a combined experimental and theoretical study on a clay ceramic used for building applications. The thermal and mechanical properties of the clay ceramic were improved by addition of organic additives. The organic additives consisted of Olive Stone Flour (OSF), with round-shape particles of 55µm, and Wheat Straw (WS), with 877µm particles in the form of fibers. It was found that the combustion of OSF and WS resulted in a porosity formation during the firing process. The morphology of these pores corresponded to that of the organic additives. Therefore, the addition of small OSF particles decreased the median pore size of the clay ceramic. It improved the mechanical strength of the clay ceramic by 12% for an 8wt% addition of OSF. On the other hand, the WS fibers increased the clay ceramic anisotropy. This resulted in a 41% improvement of the thermal conductivity using an 8wt% addition of WS. Finally, a model was developed from these experimental results to predict the thermal conductivity and the mechanical strength of the clay ceramic with other organic additives. They were predicted from the parameters of the organic additives (i.e., true density, swelling degree, particle size distribution, particle shape factor). The predictions indicated that the thermal conductivity is improved by 50% with a 25% improvement of the mechanical strength using small organic fibers, which is a step forward in the development of fired clay bricks that can be used for both insulation and structure purposes in building applications.

Science and Engineering A

Microstructure, elevated-temperature mechanical properties and creep resistance of dispersoid-strengthened Al-Mn-Mg 3xxx alloys with varying Mg and Si contents

Author(s): Zhen Li, Zhan Zhang, X.-Grant Chen

In the present work, the effects of magnesium and silicon addition on microstructure, elevated-temperature yield strength and creep resistance of Al-Mn-Mg 3xxx alloys were investigated. The microstructure evolution under as-cast and heat-treated conditions was quantitatively evaluated by optical and electron microscopy. Results revealed that both magnesium and silicon had an important influence on the distribution and volume fraction of precipitated dispersoids in 3xxx alloys. Without Mg or Si addition, dispersoids could hardly form during the precipitation heat treatment; hence, the alloys free of Mg or Si possessed low yield strength and creep resistance at elevated temperature. A significant improvement in elevated-temperature yield strength and creep resistance was obtained over a wide range of Mg (0.5–1.5wt%) and Si (0.25–1wt%) content studied due to the precipitation of a large number of dispersoids. The best combination of yield strength and creep resistance at 300 was obtained by the alloy containing 1.0wt% Mg and 0.25wt% Si with the maximum volume fraction of dispersoids and the minimum volume fraction of dispersoid free zone. The effects of dispersoid strengthening, solid solution strengthening and grain size on yield strength and creep resistance were discussed based on experimental results.

Science and Engineering A

Dislocation theory-based cohesive model for microstructurally short fatigue crack growth

Author(s): Shardul Panwar, Veera Sundararaghavan

A continuous representation of dislocations is used to represent a mode-II crack and the associated plastic zone. In the original formulation of dislocation theory, the friction stress that opposes the motion of the dislocations is represented by a constant stress. In our new formulation, we embed a cohesive zone in the plastic region in front of a crack tip by representing the friction stress as a function of the crack displacement. This allows cohesive zone models (obtainable from a lower scale simulation, such as molecular dynamics) to be integrated into a dislocation theory-based model, for the first time, to predict short crack growth. The details of this new formulation are shown for the two cases: the crack and the associated plastic zone inside a grain, and the crack and the associated plastic zone tip at the grain boundary. The main features of this new model are discussed along with an experimental comparison to the case of microstructurally short fatigue crack growth across two grains in a Ni-based CMSX-4 alloy.

Science and Engineering A

Spatiotemporal dynamics of the spin transition in $[\mathrm{Fe}{(\mathrm{HB}{(\mathrm{tz})}_{3})}_{2}]$ single crystals

Author(s): Karl Ridier, Sylvain Rat, Helena J. Shepherd, Lionel Salmon, William Nicolazzi, Gábor Molnár, and Azzedine Bousseksou

The spatiotemporal dynamics of the spin transition have been thoroughly investigated in single crystals of the mononuclear spin-crossover (SCO) complex $[\mathrm{Fe}(\mathrm{HB}(\mathrm{tz}{)}_{3}{)}_{2}]$ ($\mathrm{tz}=1,2$,4-triazol-1-yl) by optical microscopy. This compound exhibits an abrupt spi...

Physical Review B

Tip-enhanced bulk photovoltaic effect

Author(s): B. Sturman and E. Podivilov

Using the conventional macroscopic description of the bulk photovoltaic effect we analyze the light-induced currents and electric fields arising in the optical configuration with a continuous bottom electrode and a small circular top electrode. This scheme is relevant to recent experiments on the ti...

Physical Review B

Neutron diffraction study of the inverse spinels ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$

Author(s): S. Thota, M. Reehuis, A. Maljuk, A. Hoser, J.-U. Hoffmann, B. Weise, A. Waske, M. Krautz, D. C. Joshi, S. Nayak, S. Ghosh, P. Suresh, K. Dasari, S. Wurmehl, O. Prokhnenko, and B. Büchner

We report a detailed single-crystal and powder neutron diffraction study of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$ between the temperature 1.6 and 80 K to probe the spin structure in the ground state. For both compounds the strongest magnetic intensity was ob...

Physical Review B

Synthesis and stability of hydrogen iodide at high pressures

Author(s): Jack Binns, Xiao-Di Liu, Philip Dalladay-Simpson, Veronika Afonina, Eugene Gregoryanz, and Ross T. Howie

Through high-pressure Raman spectroscopy and x-ray diffraction experiments, we have investigated the formation, stability field, and structure of hydrogen iodide (HI). Hydrogen iodide is synthesized by the reaction of molecular hydrogen and iodine at room temperature and at a pressure of 0.2 GPa. Up...

Physical Review B

Influence of point defects on the near edge structure of hexagonal boron nitride

Author(s): Nicholas L. McDougall, Jim G. Partridge, Rebecca J. Nicholls, Salvy P. Russo, and Dougal G. McCulloch

Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor with applications including gate insulation layers in graphene transistors, far-ultraviolet light emitting devices and as hydrogen storage media. Due to its complex microstructure, defects in hBN are challenging to identify. Here, we com...

Physical Review B

Wed Oct 11 2017

{101¯1} Twin boundary structures in a Mg–Gd alloy

Author(s): Y.M. Zhu, S.W. Xu, J.F. Nie

In this work, we report our direct observations of atomic structures of both end and broad interfaces of { 10 1 ¯ 1 } twin boundary (TB) in a deformed and annealed Mg–Gd solid solution single phase alloy using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The end interface is an asymmetric tilt boundary decorated by a periodic array of clusters comprising ordered Gd-rich columns. The broad interface consists of coherent { 10 1 ¯ 1 } twin boundaries that are disconnected by various steps and basal/pyramidal (BPy) facets, which all contain characteristic Gd segregation. The twin steps, including S1/1, S2/2, S3/2, S5/4, S7/6 and S7/7, are observed and classified in terms of their heights, orientations, and Burgers vectors. The S1/1 and S2/2 steps are not associated with any misfit dislocations, but S1/1 is always associated with an I1 stacking fault, which is free of Gd segregation, in the twin crystal. The S3/2, S5/4, S7/6 and S7/7 steps contain misfit dislocations but no stacking faults. The BPy facets, observed for the first time in the { 10 1 ¯ 1 } TB of Mg alloys, exhibit different widths and orientations, and they are associated with elastic strain arising from in-plane mismatch and plane disclination. A double-segregation-layer structure is also observed, with the coherent { 10 1 ¯ 1 } 1} TB lying on one of these two layers. These observations are discussed in terms of existing crystallographic models.

Acta Materialia

In situ dynamic TEM characterization of unsteady crystallization during laser processing of amorphous germanium

Author(s): Garth C. Egan, Tian T. Li, John D. Roehling, Joseph T. McKeown, Geoffrey H. Campbell

The unsteady propagation mechanism for the crystallization of amorphous germanium (a-Ge) was studied with in situ movie-mode dynamic transmission electron microscopy (MM-DTEM). Short laser pulses were used to heat sputter-deposited a-Ge films and the resulting crystallization process was imaged with up to 16 sequential 50 ns long electron pulses separated by a controlled delay that was varied between 0.5 and 5 μs The unsteady crystallization in the radial, net-growth direction was observed to occur at a decreasing rate of ∼1.5–0.2 m/s through a mechanism involving the formation of discrete ∼1.1 μm wide bands that grew with velocities of 9–12 m/s perpendicular to the radial direction and along the perimeter of the crystallized area. The crystallization rate and resulting microstructure were consistent with a liquid-mediated growth mechanism, which suggests that locally the band front reaches the amorphous melting temperature of Ge. A mechanism based on the notion of a critical temperature is proposed to explain the unsteady, banded behavior.

Acta Materialia

In situ studies on irradiation resistance of nanoporous Au through temperature-jump tests

Author(s): Jin Li, C. Fan, Q. Li, H. Wang, X. Zhang

Nanoporous materials have great potentials to alleviate irradiation-induced damage due to their giant surface-to-volume ratio. Previous in situ irradiation study on nanoporous Au at room temperature has shown the shrinkage of nanopores due to the absorption of irradiation-induced defects, and the shrinkage rate is pore-size-dependent. In this follow-up temperature-dependent study, we show that both defect density and nanopores evolve with irradiation temperature. Higher temperature results in lower defect density and reduced shrinkage rate of nanopores. The sink strength of nanopores as a function of temperature is estimated. Moreover, nanoporous Au exhibits significantly enhanced swelling resistance compared to coarse-grained Au. Potential mechanisms for temperature dependent irradiation resistance of nanoporous metals are discussed.

Acta Materialia

Shock wave compaction and sintering of mechanically alloyed CoCrFeMnNi high-entropy alloy powders

Author(s): Dami Yim, Wooyeol Kim, S. Praveen, Min Ji Jang, Jae Wung Bae, Jongun Moon, Eunbin Kim, Soon-Jik Hong, Hyoung Seop Kim

In this study, mechanically alloyed CoCrFeMnNi high-entropy alloy (HEA) powders were compacted using static and shock wave compaction methods followed by pressureless sintering. The microstructural evolution and the mechanical properties were analyzed using optical microscopy, scanning electron microscopy, finite element method simulations, and tensile tests. The alloy consists of an FCC phase with a minor amount of ZrO2 in the as-milled and sintered condition. The presence of ZrO2 is due to the contamination during milling, and it led to the formation of composite microstructure after sintering. The static compaction of the alloyed powders resulted in an increase in compaction density (~ 85 to 88%) with the increasing pressure (1–3GPa), and the shock wave compaction of the alloyed powders resulted in the high relative density (~ 95%) with relatively fine and isolated pores. After sintering, almost full densification (~ 99.5%) with smaller grain size and better mechanical properties was achieved in the shock wave compacted specimens as compared to the sintering of static compacted specimens. The sintered shock wave compacted specimen exhibited high yield strength of ~ 630MPa and uniform strain distributions.

Science and Engineering A

Experimental evaluation of the plane stress fracture toughness for ultra-fine grained aluminum specimens prepared by accumulative roll bonding process

Author(s): D. Rahmatabadi, R. Hashemi, B. Mohammadi, T. Shojaee

In this research, the plane stress fracture toughness of ultra-fine grained aluminum specimens produced through accumulative roll bonding (ARB) process was investigated for the first time. The specimens were produced successfully by the ARB process up to 7 cycles with the amount of 50% thickness reduction in each cycle at room temperature without using lubricant. The fracture toughness was evaluated for the annealed and different ARB cycles using ASTM E561 standard and compact tension specimens. Additionally, mechanical properties, tensile fracture surfaces and crystallite size of ultra-fine grained aluminum ARBed specimens were evaluated by uniaxial tensile tests, microhardness measurements, scanning electron microscopy and X-ray diffraction. By increasing the number of the ARB cycles, fracture toughness was increased and the maximum value of this parameter was achieved in the last cycle, which was approximately 25.4 MPam1/2 that it increased by 155% higher than the annealed specimen. Results of X-ray diffraction demonstrated that by increasing the number of the ARB cycles, crystallite size decreased so that it reached 175nm for the 7th cycle ARBed specimen from 1341nm for annealed samples. Furthermore, by increasing the number of the ARB cycles up to the 7th cycle, tensile strength and microhardness of ultra-fine grained aluminum increased to 232MPa and 51VHN, respectively. At first, the value of elongation decreased and then increased. The SEM results showed that ductile fracture mode with large dimples occurring in the annealed specimen, changed to shear ductile fracture with elongated sophomoric shear and fine dimples after the ARB process.

Science and Engineering A

Simultaneous dynamic characterization of charge and structural motion during ferroelectric switching

Author(s): C. Kwamen, M. Rössle, M. Reinhardt, W. Leitenberger, F. Zamponi, M. Alexe, and M. Bargheer

Monitoring structural changes in ferroelectric thin films during electric field induced polarization switching is important for a full microscopic understanding of the coupled motion of charges, atoms, and domain walls in ferroelectric nanostructures. We combine standard ferroelectric test sequences...

Physical Review B

Tue Oct 10 2017

Liquid metal–organic frameworks

Romain Gaillac, Pluton Pullumbi, Kevin A. Beyer, Karena W. Chapman, David A. Keen, Thomas D. Bennett & François-Xavier Coudert

The structure and porosity of a zinc imidazolate framework above the melting point was studied with neutron and X-ray scattering, and molecular dynamics. The porosity and local bonding of the framework persist even in the liquid phase.

Nature

Circulating tumour DNA methylation markers for diagnosis and prognosis of hepatocellular carcinoma

Rui-hua Xu, Wei Wei, Michal Krawczyk, Wenqiu Wang, Huiyan Luo, Ken Flagg, Shaohua Yi, William Shi, Qingli Quan, Kang Li, Lianghong Zheng, Heng Zhang, Bennett A. Caughey, Qi Zhao, Jiayi Hou, Runze Zhang, Yanxin Xu, Huimin Cai, Gen Li, Rui Hou, Zheng Zhong, Danni Lin, Xin Fu, Jie Zhu, Yaou Duan, Meixing Yu, Binwu Ying, Wengeng Zhang, Juan Wang, Edward Zhang, Charlotte Zhang, Oulan Li, Rongping Guo, Hannah Carter, Jian-kang Zhu, Xiaoke Hao & Kang Zhang

Circulating tumour DNA is used as a biomarker for cancer diagnosis. Here, the authors identified a DNA methylation biomarker for hepatocellular carcinoma and developed diagnostic and prognostic models to predict specificity and survival of patients.

Nature

Continuous-wave infrared optical gain and amplified spontaneous emission at ultralow threshold by colloidal HgTe quantum dots

Pieter Geiregat, Arjan J. Houtepen, Laxmi Kishore Sagar, Ivan Infante, Felipe Zapata, Valeriia Grigel, Guy Allan, Christophe Delerue, Dries Van Thourhout & Zeger Hens

Stimulated emission under continuous-wave excitation from mercury telluride quantum dots at very low thresholds (compatible with electrical injection) is achieved by exploiting surface traps that render the quantum dots into four-level systems.

Nature

Two-stage cracking of metallic bi-layers on polymer substrates under tension

Author(s): B. Putz, C. May-Miller, V. Matl, B. Völker, D.M. Többens, C. Semprimoschnig, M.J. Cordill

This study demonstrates two-stage cracking of an Inconel‑silver film system on a flexible Teflon substrate under uniaxial tension. In situ fragmentation experiments revealed that primary fracture of the brittle 30nm Inconel overcoat induced brittle, secondary fracture of the normally ductile 150nm silver base layer. Good correlation exists between the ratios of primary and secondary saturation crack spacing and individual layer thicknesses. Two-stage cracking was confirmed by cross-sectional analysis and explained by different steady state energy release rates of the individual layers. The results further illustrate how brittle layers are detrimental to the mechanical behaviour of polymer-supported thin film multilayer structures.

Scripta Materialia

Fabricating geometrically-complex B4C ceramic components by robocasting and pressureless spark plasma sintering

Author(s): Siamak Eqtesadi, Azadeh Motealleh, Fidel H. Perera, Pedro Miranda, Antonia Pajares, Rune Wendelbo, Fernando Guiberteau, Angel L. Ortiz

Robocasting and pressureless spark plasma sintering are combined for the first time to fabricate geometrically-complex B4C components. It is shown that robocasting allows B4C green pieces to be printed with near-net shape from inks with suitable rheological properties, and that subsequent pressureless spark plasma sintering permits an ultrafast, energy-efficient, solid-state densification that yields B4C parts with adequate mechanical properties. Furthermore, the usefulness of cold-isostatic pressing to improve the densification of the pieces is evaluated, and the benefits of robocasting over conventional dry powder compaction are identified. Finally, the scalability for the production of large B4C pieces is discussed.

Scripta Materialia

Cooperative phenomena in spin crossover systems

Author(s): Alexander I. Nesterov, Yuri S. Orlov, Sergey G. Ovchinnikov, and Sergey V. Nikolaev

Within the framework of a realistic multiband $p−d$ model, we derived an effective Hamiltonian to describe the exchange interaction effects near the spin crossover in magnetic Mott-Hubbard insulators under pressure. It is shown that the single-ion mechanism of spin crossover under the change in the ...

Physical Review B

Electric field driven evolution of topological domain structure in hexagonal manganites

Author(s): K. L. Yang, Y. Zhang, S. H. Zheng, L. Lin, Z. B. Yan, J.-M. Liu, and S.-W. Cheong

Controlling and manipulating the topological state represents an important topic in condensed matters for both fundamental researches and applications. In this work, we focus on the evolution of a real-space topological domain structure in hexagonal manganites driven by electric field, using the ana...

Physical Review B

Mon Oct 9 2017

Designing disordered hyperuniform two-phase materials with novel physical properties

Author(s): D. Chen, S. Torquato

Heterogeneous materials consisting of different phases are ideally suited to achieve a broad spectrum of desirable bulk physical properties by combining the best features of the constituents through the strategic spatial arrangement of the different phases. Disordered hyperuniform heterogeneous materials are new, exotic amorphous matter that behave like crystals in the manner in which they suppress volume-fraction fluctuations at large length scales, and yet are isotropic with no Bragg peaks. In this paper, we formulate for the first time a Fourier-space numerical construction procedure to design at will a wide class of disordered hyperuniform two-phase materials with prescribed spectral densities, which enables one to tune the degree and length scales at which this suppression occurs. We demonstrate that the anomalous suppression of volume-fraction fluctuations in such two-phase materials endow them with novel and often optimal transport and electromagnetic properties. Specifically, we construct a family of phase-inversion-symmetric materials with variable topological connectedness properties that remarkably achieves a well-known explicit formula for the effective electrical (thermal) conductivity. Moreover, we design disordered stealthy hyperuniform dispersion that possesses nearly optimal effective conductivity while being statistically isotropic. Interestingly, all of our designed materials are transparent to electromagnetic radiation for certain wavelengths, which is a common attribute of all hyperuniform materials. Our constructed materials can be readily realized by 3D printing and lithographic technologies. We expect that our designs will be potentially useful for energy-saving materials, batteries and aerospace applications.

Acta Materialia

Molecular design of confined organic network hybrids with controlled deformation rate sensitivity and moisture resistance

Author(s): Yichuan Ding, Qiran Xiao, Reinhold H. Dauskardt

We demonstrate molecular design strategies for engineering the deformation rate sensitivity and fracture resistance of organic-inorganic hybrid films in moist environment. Hybrids with intimate mixing of inorganic and organic molecular networks were synthesized with an epoxy-functionalized silane, (3-glycidoxypropyl) trimethoxysilane and an acetate-stabilized zirconium alkoxide, tetra-n-propoxyzirconium. The highly confined non-hydrolysable organic molecular network connectivity was systematically manipulated by tuning the epoxy ring opening polymerization reaction and the incorporation of carbon bridges of selected lengths. By investigating the corresponding time-dependent crack growth in moist environments, new insights into the fundamental molecular-scale relaxation and cracking mechanisms of the hybrids are provided. These processes were found to be impacted by the confined organic network connectivity which results in significant changes in the deformation rate sensitivity and fracture resistance. With increasing non-hydrolysable organic network connectivity, mechanical behavior that varied from almost perfectly elastic to increasingly viscoelastic could be obtained in a controlled fashion. The related resistance to cracking in moist environments was found to be significantly improved. These findings provide a basis for the rational design of functional hybrids with precisely defined mechanical properties.

Acta Materialia

Metastable solidification of hypereutectic Co2Si-CoSi composition: Microstructural studies and in-situ observations

Author(s): Yeqing Wang, Jianrong Gao, Matthias Kolbe, Andrew (Chih-Pin) Chuang, Yang Ren, Douglas Matson

Metastable solidification of undercooled Co60Si40 melts was investigated by microstructural studies and in-situ high-energy X-ray diffraction. Five solidification paths were identified. Three of them were observed at low undercoolings, which show uncoupled and coupled growth of stable β-Co2Si and CoSi compounds. The other paths were observed at high undercoolings, which show peritectic and primary crystallization of a metastable Co5Si3 compound. The β-Co2Si and Co5Si3 compounds crystallize into a hexagonal crystal structure and experience solid-state decomposition. Microstructure formation depends on solidification path. The coupled and uncoupled growth of the stable compounds produces a regular lamellar eutectic structure and an anomalous eutectic structure, respectively. The crystallization and solid-state decomposition of the metastable Co5Si3 compound brings about a fine-grained two-phase mixture, which represents another type of anomalous eutectic structure. The results provide proof of two rare mechanisms of anomalous eutectic formation and shed light onto metastable phase relations in the undercooled region of the CoSi system.

Acta Materialia

The role of W on the thermal stability of nanocrystalline NiTiWx thin films

Author(s): Aslan Ahadi, Arvind R. Kalidindi, Junpei Sakurai, Yoshitaka Matsushita, Koichi Tsuchiya, Christopher A. Schuh

This paper investigates the effect of minority W additions on the thermal stability of nanocrystalline NiTi thin films. The films were produced in an amorphous state, and the addition of W was found to increase the activation energy for crystallization and lead to finer grain sizes after crystallization. In the crystallized films, W was observed to both segregate to the NiTi grain boundaries and to phase separate into fine precipitates; together these effects contribute to the stability of the nanocrystalline state up to 1200 °C. Using in-situ transmission electron microscopy, grain growth was observed concomitantly with coarsening of W precipitates, indicating that a primary mechanism for stability is Zener pinning by the W precipitates. At the same temperature where coarsening begins to occur rapidly, grain boundaries were also observed to undergo a transformation to thick, amorphous complexions. Monte Carlo simulations showed that W segregation to grain boundaries increases with temperature, which contributes to an increased rate of coarsening and loss of stability against grain growth.

Acta Materialia

On the characterisation of antisite defects and ordering in off-stoichiometric Fe2VAl-based Heusler compounds by X-ray anomalous diffraction

Author(s): Camille van der Rest, Alain Schmitz, Pascal J. Jacques

Compounds based on Fe2VAl are good candidates for low grade heat harvesting owing to the thermoelectric effect. However, it is claimed that their thermoelectric properties are badly influenced by antisite defects, especially at higher temperatures. The present study investigates order-disorder transitions in Fe2VAl ternary Heusler compounds. An inherent problem of these compounds is the close atomic numbers of Fe and V, leading to similar x-ray atomic scattering factors. Hence, the D03 and L21 structures, corresponding to Fe-V antisite defects, are hardly distinguishable by X-ray diffraction. In this work, anomalous scattering and neutron diffraction were successfully combined with differential scanning calorimetry to highlight the order-disorder transitions in Fe2VAl-based compounds. A model has been developed to quantify the ordering parameters. From these results, specific heat-treatments were defined to promote the formation of the L21 ordered phase.

Acta Materialia

Thermodynamic instability of a nanocrystalline, single-phase TiZrNbHfTa alloy and its impact on the mechanical properties

Author(s): B. Schuh, B. Völker, J. Todt, N. Schell, L. Perrière, J. Li, J.P. Couzinié, A. Hohenwarter

An equiatomic, single-phase TiZrNbHfTa high-entropy alloy was subjected to high-pressure torsion, leading to a grain size below 100 nm. Introducing a nanocrystalline microstructure to the material should help to accelerate a possible phase decomposition of the material by having a high amount of fast diffusion pathways and possible nucleation sites in the form of grain boundaries. In order to test the materials thermodynamic stability the nanocrystalline high-entropy alloy. was subjected to various heat treatments for temperatures between 300 °C and 1100 °C. Isochronal heat treatments (1 h) resulted in a hardness increase from 420 HV1 for the as-processed state to 530 HV1 for an annealing temperature of 500 °C, while for temperatures of 700 °C and higher a softening compared to the as-processed state occurred. In order to clarify this unexpected annealing response, analysis of selected microstructural states was performed utilizing electron microscopy, x-ray diffraction as well as mechanical testing to gain further information on microstructure-property relationships. Complementary, thermodynamic simulations were performed via the Calphad approach and compared to the experimental results. A phase decomposition of the originally equimolar single-phase high-entropy alloy into a NbTa-rich body-centered cubic phase and ZrHf-rich phases, which occurred in two different crystal structures depending on the annealing temperature, was the main reason for the property changes. The obtained results not only give valuable new insights into the phase stability of the TiZrNbHfTa alloy, but also demonstrate the impact of the newly forming phases in regards to mechanical properties and its implication for a possible practical application of this alloy.

Acta Materialia

Surface-oxidized, freeze-cast cobalt foams: Microstructure, mechanical properties and electrochemical performance

Author(s): Hyeji Park, Hoon-Hwe Cho, Kyungbae Kim, Kicheol Hong, Jae-Hun Kim, Heeman Choe, David C. Dunand

Cobalt with anisotropic open porosity is fabricated by directional solidification of aqueous slurries of nanometric Co3O4 powder where ice dendrites push powders into aligned interdendritic spaces, followed by ice sublimation, reduction of the oxide to metallic Co powders, and sintering of these Co powders into parallel lamellae. As the Co3O4 powder slurry fraction decreases (from 10 to 4 vol%), Co lamellae width in the final foam also decreases (from 93 to 8 μm) while foam porosity increases (from 66 to 85%). A drop in solidification temperature (from −10 to −50 °C) decreases porosity (from 77 to 63%) and lamellae width (from 11 to 5 μm) at a constant 8 vol% slurry fraction. Finally, with increasing sintering time (for −10 °C solidification temperature and 8% slurry fraction), Co foam porosity decreases (from 77 to 68%) and lamella width strongly increases (from 10 to 59 μm), consistent with sintering-induced coalescence of lamellae. The Co foams exhibit high strength but relatively low stiffness as compared to simple theoretical models, consistent with internal Co lamella buckling. A uniform Co oxide layer is grown by oxidation to create an active coating on the Co lamellae useful for lithium-ion storage. A coin-cell test carried out on the oxidized Co foam demonstrates a capacity (1283 mAhg−1) almost twice that of a control oxidized Co foil anode, owing to its considerably larger surface area. Finite-element analysis is used to compute stresses and plastic strain evolutions during the lithiation process to understand the effect of oxide layer thickness and roughness, and micropores within the Co lamellae.

Acta Materialia

Grain boundary mediated plasticity: The role of grain boundary atomic structure and thermal activation

Author(s): D. Terentyev, A. Bakaev, A. Serra, F. Pavia, K.L. Baker, N. Anento

The interaction of dislocation pile-ups with several tilt grain boundaries (GB) is studied in copper by using a hybrid continuum-atomistic approach. The effects of temperature, pile-up intensity and GB structure on absorption and transmission of slip as a function of local stress state are explored. By considering several high-angle GBs with different misorientation angles, we demonstrate that GB atomic structure primarily defines its ability to accommodate incoming pile-up dislocations, thus limiting the direct transmission of pile-ups through the interface.

Scripta Materialia

Enhanced red emission from FeSi co-doped ZnO nano-particles

Author(s): Gaurav Bajpai, Tulika Srivastava, Faizan Husian, Sunil Kumar, Sajal Biring, Somaditya Sen

Structural, optical, and photoluminescence properties of Zn0.95Si0.05xFexO (0x0.05) nano-crystallites prepared by sol-gel process has been investigated thoroughly revealing enhanced red emission in co-doped samples originating from oxygen interstitials. Greater ionic charge and smaller size of the substituent ions helps generate more interstitials forming defect states within the bandgap. The correlation between oxygen interstitials, i.e. defect states corresponding to the doping concentrations of Si and Fe ions and the enhanced red emissions from the samples has been established unequivocally following a quantitative approach by analyzing the photoluminescence spectra contributed from the individual defect levels.

Scripta Materialia

Evolution of tension twinning in single crystal Ti under compressive uniaxial strain conditions

Author(s): Sunil Rawat, Nilanjan Mitra

We perform molecular dynamics simulations to study the evolution of tension twinning in single crystal Ti under uniaxial strain conditions. The objective is to understand the evolution trends of twin volume fraction as well as number of twins and express them in terms of the individual twin dynamics. The compressive strain applied along 2 1 ¯ 1 ¯ 0 and 0 1 1 ¯ 0 directions leads to the activation of { 1 0 1 ¯ 2 } twin variants. We find that the activation of twin variants follows the Schmid criterion. However, they do not activate at the same time even with equal Schmid factor indicating the stochastic nature of the twin variant activation. For the case where four variants activate (loading along 2 1 ¯ 1 ¯ 0 direction), high nucleation events occur compared to the case where only two variants activate (loading along 0 1 1 ¯ 0 direction). The activated variants in each case do not evolve with same rate even with equal Schmid factor and a clear dominance of twin variants is observed. For the case where only two variants activate, average twin growth rate and overall twin volume fraction are high compared to the case where four variants activate. A correlation between the number of activated variants and the overall twin volume fraction is observed. The size distribution of the nucleated twins follows the power law function. The expressions provided for overall number of twins and overall twin volume fraction can serve as a basis to develop the physics-based twin evolution laws to model twinning at higher length scales.

Computational Materials Science

Magnetic and electronic transport properties of some tunnel junctions with AgBr symmetry-filter barriers

Author(s): P. Vlaic, E. Burzo, K. Carva

A new prototype tunnel junction with AgBr based barrier is proposed in this study. Electronic and magnetic properties of Fe/AgBr/Fe (001) heterostructures are investigated on the basis of first-principles and ballistic conductance calculations. The interlayer exchange coupling and electronic transport properties are analyzed as functions of the AgBr barrier thickness. The results show that Fe magnetism at Fe/AgBr (001) interfaces is enhanced as compared with the bulk material. A very strong direct tunnelling of Fe Δ1 states is observed. The minority-spin channel is characterized by resonant tunnelling of interfacial states that are forming at Fe/AgBr (001) interfaces. Spin-polarization of the tunnelling current is positive and can exceed 99%. Tunnelling magnetoresistance ratios of about 8000% are evidenced for the junctions with clean interfaces. Fe/AgBr (001) coherent interfaces could act as nearly ideal symmetry-filters. In the presence of interfacial disorder the tunnelling magnetoresistance ratios decrease dramatically. The exchange coupling between Fe electrodes oscillates as a function of the barrier thickness.

Computational Materials Science

Investigation and comparison of the large-signal characteristics and dynamical parameters of silicene and germanene nanoribbon interconnects

Author(s): Serhan Yamacli

One dimensional materials such as nanotubes and nanoribbons are studied widely in the literature for their peculiar properties. In particular, zigzag graphene and silicene nanoribbons constitute an active research area for the possibility to use them as interconnects. Recently, it is exposed that asymmetric zigzag germanene nanoribbons also show similar metallic characteristics. In this work, the electronic transport properties and dynamical parameters of zigzag silicene and germanene nanoribbons (ZZ-SiNR and ZZ-GeNR) are investigated and compared. First-principles quantum mechanical simulations using density functional theory in conjunction with non-equilibrium Green’s function formalism are utilized to extract the current-voltage behaviours and Fermi velocities of equivalent ZZ-SiNR and ZZ-GeNR samples. The results show that ZZ-SiNR and ZZ-GeNR samples show similar current-voltage characteristics with the ZZ-GeNR having slightly higher conductance. Then, dynamical parameters namely kinetic inductance and quantum capacitance values are obtained. The obtained values show that the kinetic inductance and quantum capacitance of the ZZ-GeNR are lower than those of ZZ-SiNR. It is concluded that both ZZ-SiNR and ZZ-GeNR can be used as nanoscale interconnects with the ZZ-GeNR having slightly higher linearity and lower kinetic inductance and quantum capacitance compared to its SiNR counterpart.

Computational Materials Science

Effects of post-welding heat treatment on microstructure and mechanical properties of welding joint of new Ni-Fe based superalloy with Haynes 282 filler metal

Author(s): Hongfei Yin, Yimin Gao, Yuefeng Gu

The gas tungsten arc welding process was employed to weld a new wrought Ni-Fe based superalloy with the Haynes 282 filler metal, which was designed for advanced ultra-supercritical boiler tubes operated beyond 700°C. In this study, the evolution of microstructure and mechanical properties of the welding joint prior to and following the post-welding heat treatment (such as annealing treatment at 1010°C for 2h and 788°C for 8h) were investigated. The microstructure stability and hardness were also studied following 500h, 1000h and 3000h at the service temperature (750°C). An epitaxial growth of the grains near the fusion line was observed, whereas the partition coefficients and solidification temperature range during the thermodynamic state of equilibrium were calculated in the weld metal zone. The welding heat affected zone of the as-welded welding joint was the weakest zone. In contrast, following the post-welding heat treatment, the segregation was significantly eliminated in the weld metal zone. Also, the mechanical properties of the welding heat affected zone were restored to the property levels of the base metal, with the weld strength reduction factor of 4%. Therefore, the effects of post-welding heat treatment on the microstructure and properties of welding joint were also investigated. Following 3000h thermal exposure at 750°C, the coarsening rates of the spherical γ′ precipitates of both the as-welded and post-welding heat treatment samples were consistent with the Lifshitz-Slyozov-Wagner (LSW) model. In addition, the microstructure stability of the welding joint was excellent, without any topologically closed packed phase precipitation. The Haynes 282 filler metal could be a potential candidate material for the welding of this new Ni-Fe based superalloy.

Science and Engineering A

Influence of microstructure and pre-straining on the bake hardening response for ferrite-martensite dual-phase steels of different grades

Author(s): Dengpeng Ji, Mei Zhang, Delong Zhu, Siwei Luo, Lin Li

This work aims to investigate the effects of pre-straining and microstructure on bake hardening (BH) response of dual phase (DP) steels. Four kinds of different grades of commercial DP steels, DP340/600, DP420/780, DP500/780 and DP550/980, were pre-strained in tension to 0.5%, 1%, 2%, 4%, 6% and 8%, then baked at 170°C for 20min followed by restraining, or just baked at 170, 190, 210, 230, 250°C for 20min and at 250°C for 40min followed by straining without pre-straining. Electron backscattered diffraction (EBSD) measurements and temperature-dependent internal friction measurements were conducted to characterize geometrically necessary dislocations (GNDs) and analyze solute carbon content and interactions between point defects and dislocations in DP steels. The results show that for all grades of DP steels investigated, BH values increase to peak values at pre-straining ranging from 0.5% to 2% and then decline with further pre-straining. At BH condition of 170°C/20min, peak BH values with pre-straining are 33MPa for DP340/600, 34MPa for DP420/780, 78MPa for DP500/780 and 90MPa for DP550/980 respectively. Pre-straining can cause increase in tensile strength and decrease in total elongation after baking, especially for DP500/780 and DP550/980. DP steels without pre-straining can reach very high BH0 values by applying either higher temperature or longer holding time. Solute carbon content in ferrite controls the speed of BH response in DP steels. Microstructures of higher volume fraction of martensite, smaller martensite islands and smaller ferrite grains can produce higher BH values.

Science and Engineering A

Assessment of microstructure and shear strength for low melting point tin-free alloys on Cu

Author(s): Chih-Hao Chen, Subhendu Chakroborty, Boon-Ho Lee, Hsiang-Chuan Chen, Chang-Meng Wang, Albert T. Wu

This study explores and compares two new alloys, namely 50In-50Bi and 17Sn-26In-57Bi (wt%), with melting peak temperatures of 95 and 82°C, which are substantially lower than the melting points of lead-free solders commonly used for low-temperature applications. The microstructures and intermetallic compounds (IMCs) of the new alloys, which were reflowed on Cu substrates, were studied. Notable Bi3In5 and BiIn2 compounds were formed in the reflowed 50In-50Bi alloy. The IMCs detected at the interfaces of 50In-50Bi and 17Sn-26In-57Bi were Cu11(In, Bi)9 and Cu6(In, Sn)5, respectively. Shear tests were systematically performed on organic solderability preservative (OSP) Cu substrates with 300-μm openings. Investigation of the fracture surfaces of both alloys indicated that the 17Sn-26In-57Bi/OSP Cu system had the higher shear strength and the more ductile joints.

Science and Engineering A

Effect of recrystallization on hot deformation mechanism of TA15 titanium alloy under uniaxial tension and biaxial gas bulging conditions

Author(s): Kehuan Wang, Gang Liu, Ke Huang, Denis J. Politis, Liliang Wang

To investigate the effect of recrystallization on hot deformation mechanism of TA15 titanium alloy, rolled sheet and laser-welded tubes before and after recrystallization annealing were tested by uniaxial tensile tests and biaxial gas bulging respectively. The results show that both of the initial rolled sheet and the as-welded tube consisted of equiaxed α microstructure with high initial dislocation density. During uniaxial tensile tests, a quasi-steady flow state was achieved at 800°C, 0.001s−1, which demonstrated the best formability with a maximum elongation of 536%. Significant recrystallization occurred at the early stage of tensile deformation and the grain size was found to decrease first and then increase. Significant recrystallization also occurred at the early stage of the biaxial gas bulging of the as-welded tube. During the bulging of the annealed tube consisting of equiaxed α with low initial dislocation density, dynamic recrystallization (DRX) was the main microstructure evolution characteristic, but most of the DRX occurred at the middle-late stage. The relatively coarse equiaxed α and increasing fraction of low angle grain boundaries during deformation may restrict the occurrence of grain boundary sliding (GBS), which resulted in the relatively worse formability of the annealed tube. However, recrystallization which happened widely at the early stage, increased the fraction of high angle grain boundaries, refined the microstructure, promoted the occurrence of GBS and enhanced the formability of the initial rolled sheet and as-welded tube. It was also found that the stress state had no effect on the DRX mechanism for TA15 at 800°C, which was confirmed to be discontinuous dynamic recrystallization under both uniaxial tension and biaxial gas bulging conditions.

Science and Engineering A

Characterization and modeling of the mechanical behavior of high silicon ductile iron

Author(s): Keivan A. Kasvayee, Ehsan Ghassemali, Ingvar L. Svensson, Jakob Olofsson, Anders E.W. Jarfors

This paper investigates the effect of the solidification conditions and silicon content on the mechanical properties of ductile iron and presents empirical models for predicting the tensile behavior based on the microstructural characterizations. Two ductile iron grades of GJS-500-7 and GJS-500-14 were cast with silicon content of 2.36% and 3.71%, respectively. The cast geometry consisted of six plates with different thicknesses that provided different cooling rates during the solidification. Microstructure analysis, tensile and hardness tests were performed on the as-cast material. Tensile behavior was characterized by the Ludwigson equation. The tensile fracture surfaces were analyzed to quantify the fraction of porosity. The results showed that graphite content, graphite nodule count, ferrite fraction and yield strength were increased by increasing the silicon content. A higher silicon content resulted in lower work hardening exponent and strength coefficient on the Ludwigson equation. The results for 0.2% offset yield and the Ludwigson equation parameters were modeled based on microstructural characteristics, with influence of silicon content as the main contributing factor. The models were implemented into a casting process simulation to enable prediction of microstructure-based tensile behavior. A good agreement was obtained between measured and simulated tensile behavior, validating the predictions of simulation in cast components with similar microstructural characteristics.

Science and Engineering A

The effect of pre-annealing on defects, microstructure and recrystallization of ultra-fine grained Al produced by high-pressure torsion

Author(s): Mehrnoosh Naderi, Martin Peterlechner, Sergiy V. Divinski, Gerhard Wilde

The defect accumulation, thermal expansion, microhardness and microstructure are investigated for severely plastically deformed technically pure Al (Al-base alloy of the 1050 series) as a function of initial state and post-deformation annealing treatments. Coarse grained as-cast and heat-treated states are deformed via high-pressure torsion. The excess volume release upon subsequent annealing is measured by dilatometry employing a constant heating rate and two characteristic sub-stages are revealed. The corresponding microstructure changes are further investigated by transmission electron microscopy. A strong impact of the initial state on the microstructure and the properties of ultrafine grained Al is established. The dilatometric length changes of ultrafine grained Al under annealing are related to the formation of Fe-rich precipitates as well as to the annihilation of deformation-induced defects.

Science and Engineering A

Corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater

Author(s): Tianliang Zhao, Zhiyong Liu, Cuiwei Du, Chunduo Dai, Xiaogang Li, Bowei Zhang

In the present paper, the corrosion fatigue crack initiation and initial propagation mechanism of E690 steel in simulated seawater were studied by stress-controlled fatigue tests and a series of subsequent characterizations on the fracture surface, microstructure and secondary cracks. Results show that the corrosion fatigue crack initiation and initial propagation mechanism evolves with elevated peak stress level in simulated seawater. When peak stress is far below the proof stress, cracks preferentially initiate at the parent austenite grain boundaries (PAGBs) with 68.4% probability and at the ferrite lath boundaries (FLBs) with 31.6% probability. Meanwhile, the cracks also preferentially propagate along the PAGBs and FLBs. Upon the peak stress close to or above the proof stress, cracks turn to initiate from the emerging corrosion pits and propagate without zigzag detour but by splitting the ferrite laths which transversely block its propagation way.

Science and Engineering A

Exceptional mechanical properties of ultra-fine grain Mg-4Y-3RE alloy processed by ECAP

Author(s): Peter Minárik, Jozef Veselý, Robert Král, Jan Bohlen, Jiří Kubásek, Miloš Janeček, Jitka Stráská

Precipitation hardenable WE43 magnesium alloy containing yttrium and rare earth elements was processed by ECAP. Microstructure, phase composition, mechanical properties and texture of the processed material was investigated by several complementary techniques. Substantial precipitation during ECAP led to exceptional grain refinement with the resulting average grain size of ~ 340nm. The processed material exhibited previously unreported weak texture without a typical component usually observed in magnesium alloys processed similarly. The observed texture resulted from a massive particle-induced recrystallization during the processing through ECAP. The ultra-fine grain microstructure, the high density of Mg5RE particles and the specific texture resulted in the significant strengthening of the ECAPed material. The yield compression strength of ~ 427MPa was by 340% higher than that of the initial as-cast condition and by 210% higher than that of the peak age-hardened one.

Science and Engineering A

High damage-tolerance bio-inspired ZL205ASiC composites with a lamellar-interpenetrated structure

Author(s): Alateng Shaga, Ping Shen, Li-Guang Xiao, Rui-Fen Guo, Ya-Bing Liu, Qi-Chuan Jiang

Novel bio-inspired ZL205A/SiC composites with a lamellar-interpenetrated structure were successfully fabricated via gas-pressure infiltration of a commercial ZL205A alloy into freeze-cast porous SiC scaffolds. The influences of initial ceramic content (20, 30 and 40vol%) on the microstructure and damage-tolerance behavior of the resultant composites as well as their toughening mechanism were investigated. With an increase in the ceramic content in the composite, the flexural strength and fracture toughness in the longitudinal direction gradually decreased; whereas, in the transverse direction they showed a first increase and then decrease but their elastic modulus increased. The composites displayed significant damage-tolerance anisotropy, with higher strength and toughness in the longitudinal direction (parallel to the lamellae) than that in the transverse direction (perpendicular to the lamellae). In the longitudinal bending, the ZL205A/20vol%SiC composite exhibited the maximum flexural strength (760MPa) and the largest fracture toughness (33.0MPam1/2). The excellent damage tolerance of the composites was attributed to multiple toughening mechanisms such as plastic deformation in the matrix alloy, crack blunting, deflection and branching, and uncracked-ligament bridging of the ductile alloy layers. However, in the transverse bending, the crack propagated along the ceramic layer in the composites and thus greatly weakened their damage-tolerance capability.

Science and Engineering A

Effects of heat-treatment on the plastic anisotropy of extruded aluminium alloy AA6063

Author(s): M. Khadyko, C.D. Marioara, S. Dumoulin, T. Børvik, O.S. Hopperstad

The plastic anisotropy of aluminium alloys is known to depend not only on the crystallographic texture but also on the heat-treatment, and this effect has been studied on various alloys both experimentally and numerically. However, the 6000 series of aluminium alloys is not broadly represented in these studies. In this work, an extruded profile of the AA6063 alloy was investigated. Electron backscatter diffraction (EBSD) measurements revealed a strong cube crystallographic texture with a minor Goss component, which is typical for recrystallized aluminium alloys. The plastic anisotropy was studied by uniaxial tension tests in different material directions, using digital image correlation to measure the displacement field and thus to calculate the strain field. The tensile specimens were heat-treated to three different tempers: T6, T7 and O, in addition to the as-received T1 condition. Transmission electronic microscopy (TEM) was used to characterize the precipitate structure of the heat-treated material. A crystal plasticity finite element model of the tensile test was created and calibrated using some of the experimental data. The comparison of the experimental stress-strain curves, strain ratios and flow stress ratios with their simulated counterparts revealed that the crystallographic texture is dominating the anisotropy in all tempers. The accuracy of the CP-FEM predictions varies for different material orientations, and, in general, the simulated material exhibits a sharper anisotropy than the real material. The effect of the heat treatment on the anisotropy is found to be minor compared with the texture effect.

Science and Engineering A

Sat Oct 21 2017

Computational Materials Science
Science and Engineering A
Journal of Physics Condensed Matter

Fri Oct 20 2017

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

Thu Oct 19 2017

Scripta Materialia
Science and Engineering A
Physical Review Letters
Physical Review B

Wed Oct 18 2017

Science and Engineering A
Physical Review B

Tue Oct 17 2017

Nature
Scripta Materialia
Science and Engineering A
Physical Review Letters
Physical Review B

Mon Oct 16 2017

Acta Materialia

Sun Oct 15 2017

Computational Materials Science

Sat Oct 14 2017

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

Fri Oct 13 2017

Computational Materials Science
Physical Review Letters

Thu Oct 12 2017

Acta Materialia
Scripta Materialia
Computational Materials Science
Science and Engineering A
Physical Review B

Wed Oct 11 2017

Acta Materialia
Science and Engineering A
Physical Review B

Tue Oct 10 2017

Nature
Scripta Materialia
Physical Review B

Mon Oct 9 2017

Acta Materialia
Scripta Materialia
Computational Materials Science
Science and Engineering A