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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!

Tue Aug 22 2017

Optical fingerprint of non-covalently functionalized transition metal dichalcogenides

Maja Feierabend, Ermin Malic, Andreas Knorr and Gunnar Berghäuser

Atomically thin transition metal dichalcogenides (TMDs) hold promising potential for applications in optoelectronics. Due to their direct band gap and the extraordinarily strong Coulomb interaction, TMDs exhibit efficient light-matter coupling and tightly bound excitons. Moreover, large spin orbit coupling in combination with circular dichroism allows for spin and valley selective optical excitation. As atomically thin materials, they are very sensitive to changes in the surrounding environment. This motivates a functionalization approach, where external molecules are adsorbed to the materials surface to tailor its optical properties. Here, we apply the density matrix theory to investigate the potential of non-covalently functionalized monolayer TMDs. Considering exemplary molecules with a strong dipole moment, we predict spectral redshifts and the appearance of an additional side peak in the absorption spectrum of functionalized TMDs. We show that the molecular characteristics,...

Journal of Physics Condensed Matter

Fermi-to-Bose crossover in a trapped quasi-2D gas of fermionic atoms

A V Turlapov and M Yu Kagan

The physics of many-body systems where particles are restricted to move in two spatial dimensions is challenging and even controversial: on one hand, neither long-range order nor Bose condensation may appear in infinite uniform 2D systems at finite temperature, on the other hand this does not prohibit superfluidity or superconductivity. Moreover, 2D superconductors, such as cuprates, are among the systems with the highest critical temperatures. Ultracold atoms are a platform for studying 2D physics. Unique from other physical systems, quantum statistics may be completely changed in an ultracold gas: an atomic Fermi gas may be smoothly crossed over into a gas of Bose molecules (or dimers) by tuning interatomic interactions. We review recent experiments where such crossover has been demonstrated, as well as critical phenomena in the Fermi-to-Bose crossover. We also present simple theoretical models describing the gas at different points of the crossover and compare the data to the...

Journal of Physics Condensed Matter

Spin-charge transformation of lattice fermion models: duality approach for diagrammatic simulation

Johan Carlström

I derive a dual description of lattice fermions, specifically focusing on the t - J and Hubbard models, that allow diagrammatic techniques to be employed efficiently in the strongly correlated regime, as well as for systems with a restricted Hilbert space. These constructions are based on spin-charge transformation, where the lattice fermions of the original model are mapped onto spins and spin-less fermions. This mapping can then be combined with Popov-Fedotov fermionisation, where the spins are mapped onto lattice fermions with imaginary chemical potential. The resulting models do not contain any large expansion parameters, even for strongly correlated systems. Also, they exhibit dramatically smaller corrections to the density matrix from nonlinear terms in the Hamiltonian. The combination of these two properties means that they can be addressed with diagrammatic methods, including simulation techniques based on stochastic sampling of diagrammatic expansions.

Journal of Physics Condensed Matter

Superconducting properties of nanostructured microhelices

Vladimir M Fomin, Roman O Rezaev, Evgenii A Levchenko, Daniel Grimm and Oliver G Schmidt

Superconducting micro- and nanohelices are proposed for the first time. A theoretical investigation of the superconducting state in the helical coils at the micro- and nanoscale is performed within the time-dependent Ginzburg–Landau approach. The pattern and number of vortices in a stationary distribution are determined by their confinement to the ultrathin helical coil and can therefore be efficiently controlled by the spiral stripe width and the spiral pitch distance for both dense and sparse coils. Quasi-degeneracy of vortex patterns is manifested in the helical coil when the number of vortices is incommensurable with the total number of half-turns. With increasing radius, superconducting helical coils provide a physical realization of a transition from the vortex pattern peculiar to an open tube to that of a planar stripe.

Journal of Physics Condensed Matter

Transparent p -CuI/ n -BaSnO 3− δ heterojunctions with a high rectification ratio

Jeong Hyuk Lee, Woong-Jhae Lee, Tai Hoon Kim, Takhee Lee, Seunghun Hong and Kee Hoon Kim

Transparent p -CuI/ n -BaSnO 3− δ heterojunction diodes were successfully fabricated by the thermal evaporation of a (1 1 1) oriented γ -phase CuI film on top of an epitaxial BaSnO 3– δ (0 0 1) film grown by the pulsed laser deposition. Upon the thickness of the CuI film being increased from 30 to 400 nm, the hole carrier density was systematically reduced from 6.0  ×  10 19 to 1.0  ×  10 19 cm −3 and the corresponding rectification ratio of the pn diode was proportionally enhanced from ~10 to ~10 6 . An energy band diagram exhibiting the type-II band alignment is proposed to describe the behavior of the heterojunction diode. A shift of a built-in potential caused by the hole carrier density change in the CuI film is attributed to the thickness-dependent rectification ratio. The best performing p -CuI/ n -BaSnO 3− δ diode exhibited a high current rectificat...

Journal of Physics Condensed Matter

Phase transitions of the dimerized Kane–Mele model with/without strong interaction

Tao Du, Yue-Xun Li, Yan Li, He-Lin Lu and Hui Zhang

The dimerized Kane–Mele model with/without strong interaction is studied using analytical methods. The boundary of the topological phase transition of the model without strong interaction is obtained. Our results show that the occurrence of the transition only depends on dimerized parameter {$\alpha $} . From the one-particle spectrum, we obtain the completed phase diagram including the quantum spin Hall state and the topologically trivial insulator. Then, using different mean field methods, we investigate the Mott transition and the magnetic transition of the strongly correlated dimerized Kane–Mele model. In the region between the two transitions, the topological Mott insulator with characteristics of Mott insulators and topological phases may be the most interesting phase. In this work, the effects of hopping anisotropy and Hubbard interaction U on the boundaries of the t...

Journal of Physics Condensed Matter

Statistics of twinning in strained ferroelastics

Xiangdong Ding, Oktay Aktas, Xiaofei Wang, Suzhi Li, Ziyuan Zhao, Libo Zhang, Xiaomei He, Turab Lookman, Avadh Saxena and Jun Sun

In this review, we show that the evolution of the microstructure and kinetics of ferroelastic crystals under external shear can be explored by computer simulations of 2D model materials. We find that the nucleation and propagation of twin boundaries in ferroelastics depend sensitively on temperature. In the plastic regime, the evolution of the ferroelastic microstructure under strain deformation maintains a stick-and-slip mechanism in all temperature regimes, whereas the dynamic behavior changes dramatically from power-law statistics at low temperature to a Kohlrausch law at intermediate temperature, and then to a Vogel–Fulcher law at high temperature. In the yield regime, the distribution of jerk energies follows power-law statistics in all temperature regimes for a large range of strain rates. The non-spanning avalanches in the yield regime follow a parabolic temporal profile. The changes of twin pattern and twin boundaries density represent an important step towards domain bo...

Journal of Physics Condensed Matter

Application of the Lany–Zunger polaron correction for calculating surface charge trapping

Meisam Farzalipour Tabriz, Bálint Aradi, Thomas Frauenheim and Peter Deák

Defect calculations, using density functional theory in a local or semi-local approximation, in transition metal oxides are severely handicapped by the electron self-interaction error. The underestimation of the band gap may cause incorrect occupation of defect states and wrong formation energies, and the underestimated localization of the states disfavors the formation of small polarons. These problems can be avoided by using higher level approximations (GW or a correctly chosen hybrid functional), but those methods are computationally too expensive to be used for calculating surface defects in a periodic slab model. Lany and Zunger have suggested a convenient (low-cost) solution for solving the band-gap and charge delocalization problem, by applying a correction scheme to the standard local or semi-local approximations. Most importantly, the linearity of the total energy as a function of the fractional occupation numbers, is restored, leading to the fulfillment of the generali...

Journal of Physics Condensed Matter

A new ternary magnetically ordered heavy fermion compound Pr 2 Rh 3 Ge: magnetic, electronic and

M Falkowski and A M Strydom

The results of the magnetic, electron transport, heat capacity and heat conduction measurements on the new rhombohedral ternary compound Pr 2 Rh 3 Ge have been investigated. The synthesized polycrystalline compound was found to crystallize in the ternary ordered variant of the cubic Laves phase {${\rm MgCu}_{2}$} -type of structure with the space group R {$\overline{3}$} m , as previously reported. Pr 2 Rh 3 Ge exhibits a ferromagnetic behaviour below {$T_{\rm C} = 8.5$} K, which was found to be unstable in low applied magnetic fields, revealing characteristics usually attributed to the long-range order. In the entire paramagnetic region elect...

Journal of Physics Condensed Matter

Mon Aug 21 2017

Exploring the microstructural evolution of Hf-Ti: From nanometallic multilayers to nanostructures

Author(s): J. Sebastian Riano, Andrea M. Hodge

Microstructural changes in nanocrystalline materials are driven by their high grain boundary density and greater mobility at elevated temperatures. For instance, segregation, precipitation, and grain growth become active as the temperature increases. The potential of nanometallic multilayers as a route to synthesize nanostructures is explored examining the microstructural evolution of Hf-Ti nanometallic multilayers through differential scanning calorimetry and characterization at different temperatures. Thermodynamic and kinetic calculations were performed to understand the observed transitions. This study presents a guide to microstructural changes in nanometallic multilayers which can be used to develop new nanocrystalline systems.

Scripta Materialia

Multi-functionality of carbon nanotubes reinforced 3mol% yttria stabilized zirconia structural biocomposites

Author(s): Rubia Hassan, Ambreen Nisar, S. Ariharan, Fahad Alam, Anil Kumar, Kantesh Balani

The effect of the addition of carbon nanotubes (CNT) on the densification, microstructural, mechanical, wear performance and biocompatibility of 3mol% yttria stabilized zirconia (3YSZ) have been investigated in the present study. The retention of high temperature phases (tetragonal and cubic) was found to increase with CNT addition in 3YSZ. Enhancement in the mechanical properties (hardness and elastic modulus) with CNT reinforcement is attributed to the refinement of grain size, enhanced densification and homogeneous distribution of CNT (up to 6vol%). Correspondingly, fracture toughness increased from 5MPam1/2 (in 3YSZ) to 10.1MPam1/2 with CNT reinforcement (for 12vol%) through energy dissipating mechanisms such as crack branching, CNT bridging, and crack-deflection, evidenced by the fractographs of 3YSZ-CNT composites. The toughening contribution with CNT reinforcement is observed to dominate over that of tm transformation up to 38 times, evaluated using a modified fractal model. However, this effectiveness decreased with CNT agglomeration. The physical damage in the bacterial cell is observed to increase with CNT addition. The present findings promote 3YSZ-CNT as an optimal structural and antibacterial multifunctional biomaterial.

Science and Engineering A

Microstructure evolution and strengthening mechanism study of Mg-Li alloys during deformation and heat treatment

Author(s): Yan Tang, Qichi Le, Weitao Jia, Li Fu, Xuan Liu, Jianzhong Cui

Microstructural evolution, textural development and strengthening mechanism of α, α+β and β structure Mg-Li alloys during extruding and annealing process were investigated. It showed that, with increasing Li content, the α-Mg matrix was transformed gradually to the β-Li matrix. In the three alloys, for α-Mg structure, the c-axis of close-packed plane of α-Mg was inclined perpendicular to the extrusion direction after extruding. However, for β-Li structure, the c-axis of close-packed plane of β-Li was inclined parallel to the extrusion direction after extruding. During annealing, a new predominance of {11−20} recrystallization texture was observed in α-Mg matrix. However, no recrystallization texture was achieved in β-Li matrix. Additionally, a solid solution strengthening during annealing could be observed apparently in the β-Li matrix. To study and quantify the strengthening behavior, the relationship models between ultimate tensile strength (UTS), yield strength (YS), hardness (HV) and AlLi volume fraction (VF) were defined using mathematical modeling and regression analysis. It showed that the effect of VF on UTS was much more than that of others. In LAZ832-0.2Y, the fitting models for UTS on VF and UTS on YS revealed the relationship of quadratic function. However, in LAZ1132-0.2Y, the fitting models for them exhibited the linear relationship.

Science and Engineering A

Superplasticity of a dual-phase-dominated Mg-Li-Al-Zn-Sr alloy processed by multidirectional forging and rolling

Author(s): Furong Cao, Guoqiang Xue, Guangming Xu

The microstructures, mechanical properties, deformation mechanism and cavitation growth of Mg-10.2Li-2.1Al-2.23Zn-0.2Sr alloy subjected to multidirectional forging and rolling (MDFR) were studied to examine the deformability of the Mg-Li alloy. X-ray diffraction (XRD) results confirm the existence of α (Mg) and β (Li) phases and Mg17Al12, Al4Sr and LiMgAl2 intermetallic compounds. Studies of the microstructures reveal that a thin banded-grained microstructure with a grain size less than 3.75µm is obtained via MDFR and annealing at 523K for 1h. There were significant grain refinements. The maximum elongation to failure was 712.1%, and this was obtained in the current alloy at 623K with a strain rate of 1.67 × 10−3 s−1. The strain rate sensitivity exponent and the activation energy for deformation were estimated to be 0.884 and 91.9kJ/mol, respectively. This indicates that the rate-controlling deformation mechanism under the aforementioned condition is grain boundary sliding controlled by lattice diffusion. Experimental cavity observations show the existence of cavity interlinkage or stringers. A new plasticity-controlled cavity growth rate equation considering cavity interlinkage was established, and a cavity growth diagram was constructed. The diagram prediction is consistent with the experimental results. In addition, fractographs show that the intergranular fracture is a ductile fracture mechanism. At room temperature, the ultimate tensile strength of 242MPa and the elongation of 23.59% were obtained in the present alloy.

Science and Engineering A

Investigating shear band interaction in metallic glasses by adjacent nanoindentation

Author(s): Hu Huang, Jiwang Yan

The plastic deformation of metallic glass (MG) at room temperature progresses mainly through shear banding. Although many previous studies have addressed the behaviors of the shear band (SB), there is still some debate regarding the SB processes, particularly its reactivation and interaction. Hence, it is essential to further understand the SB processes in MGs. In this paper, a new method via adjacent nanoindentation was introduced to investigate the SB interaction, which makes it easy to distinguish between the pre-existing and newly generated SBs. Nanoindentation was performed on Zr-based MG under various spatial intervals, and the resultant residual morphologies and serrated flows were comparatively analyzed. With a change in the interval, various SB behaviors, such as straight SBs, SB reactivation, stoppage, intersection, narrowing, and suppression were clearly observed in the interaction region. The evolution of the SBs indicated that the main carrier of the plastic deformation in the interaction region changed between the newly generated SBs and the pre-existing SBs. Corresponding to the strong SB interaction, serrated flows in the load-depth (P-h) curves were also found to be promoted. These findings are expected to enhance our understanding of SB processes, which is meaningful for controlling the SBs and tuning the surface plasticity of MGs.

Science and Engineering A

Superplastic deformation behavior of Ti-55 alloy without and with 0.1wt%H addition

Author(s): Xifeng Li, Nannan Chen, Jun Chen, Qiongfeng Mei, Li Wan, Chunli Jia, He Liu

The superplastic deformation behaviors of as-received Ti-55 alloy and hydrogenated alloy with 0.1wt%H were studied. The results indicate γ hydride readily decomposes and hydrogen easily escapes under vacuum or argon-protected condition near the starting temperature of α + β / β transformation. Compared with as-received alloy, the optimum superplastic temperature of hydrogenated alloy is reduced by about 125℃. Larger decrease of elongation in hydrogenated alloy appears if the deformation temperature and initial strain rate deviate from optimal ones. More proportion of β phase distributes uniformly around α phase grain boundaries and prominently inhabits the formation of voids in hydrogenated alloy during superplastic deformation. Hydrogen addition can lead to the improvement of discontinuous dynamic recrystallization mechanism and deformation compatibility of grain boundaries, as well as the reduction of dislocation pile-ups and tangles.

Science and Engineering A

Asymmetric distribution of microstructure and impact toughness in stir zone during friction stir processed a high strength pipeline steel

Author(s): G.M. Xie, H.B. Cui, Z.A. Luo, R.D.K. Misra, G.D. Wang

A high strength pipeline steel was successfully friction stir processed at high, medium and low heat input processing parameters. The microstructural evolution on advancing side and retreating side of the stir zone was studied using a combination of optical microscopy, scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy. The microstructural observations suggested that the prior austenite grain size, bainite morphology, texture components and impact toughness were distributed asymmetrically on the advancing side and retreating side at high and medium heat input. This was caused by asymmetric distribution and change in temperature and strain rate. While, the entire stir zone at low heat input was characterized by uniform microstructure and excellent impact toughness. Therefore, the microstructure and impact toughness on the advancing side and retreating side was closely related to processing parameters.

Science and Engineering A

Studies on tensile behaviour and microstructural evolution of UFG Mg-4Zn-4Gd alloy processed through hot rolling

Author(s): Raviraj Verma, A. Srinivasan, R. Jayaganthan, S.K. Nath, Sunkulp Goel

The effect of rolling at elevated temperature on mechanical behaviour of bulk Mg-4Zn-4Gd alloy was investigated. The alloy was subjected to solution treatment at 673K for 24h, followed by flat and cross rolling at elevated temperature (723K). The microstructure of ultrafine grains (less than ~1µm size) was successfully achieved from solutionized sample with grain size ~78µm after the rolling process, which has significantly influenced the mechanical behaviour of the deformed Mg alloy. The yield strength (YS), ultimate tensile strength (UTS), elongation-to-failure (%E) and hardness of a rolled specimen have enhanced maximum up to the value of 276.3 ± 13MPa, 341.3 ± 7MPa, ~26.1% and 74.2 ± 2HV, respectively. The improvement in mechanical properties of processed Mg alloys due to the combined effect of grain refinement and secondary phase (W-phase) distribution into the Mg matrix upon the repetitive rolling process. In addition, the improvement in tensile properties is explained by correlating the dislocation density (ρ), scanning electron (SEM) and transmission electron (TEM) micrographs. The developed intrinsic property of rolled specimens is substantiated by tensile fractography obtained from SEM equipped with energy dispersive X-ray spectroscopy (EDS). Various other microstructural features were also revealed by TEM and X-ray diffraction (XRD) analysis. The anisotropy of various rolled Mg-4Zn-4Gd specimens was also elucidated on the basis of tensile results.

Science and Engineering A

Comprehensive analysis on the effects of different stress states on the bainitic transformation

Author(s): Mingxing Zhou, Guang Xu, Haijiang Hu, Qing Yuan, Junyu Tian

The bainitic transformation with various stress states was investigated by thermal simulation, scanning electron microscope and X-ray diffraction. The results show that as the stress increases, the promotion degree of stress on bainitic transformation increases fast firstly, then increases slowly and finally decreases. There are two critical stress values. The first one corresponds to the yield strength of parent austenite and the second one corresponds to the maximum promotion degree of stress. A simple model is proposed to describe this phenomenon. In addition, the effects of compressive stress and tensile stress on microstructure and the kinetics of bainitic transformation are similar. Moreover, even a small stress such as 30MPa can slightly promote bainitic transformation. Finally, like bainitic transformation without stress, bainitic transformation with stress is an incomplete-reaction.

Science and Engineering A

Discontinuous yielding behavior and microstructure evolution during hot deformation of TC11 alloy

Author(s): L. Jing, R.D. Fu, Y.P. Wang, L. Qiu, B. Yan

Discontinuous yielding behavior of TC11 alloy was found during hot compression tests at temperatures of 1000–1050°C and the strain rate of 10s−1. The flow stress showed a rapid drop and subsequent oscillation after peak stress before true strain of 0.15. It was proved to be the result of the stress induced phase transformation (αβ), which introduced softening effect for the generation of new β phase. Moreover, the magnitude of the stress drop increased with the increasing of the fraction of stress induced phase transformation. In addition, these newly generated β phase were further deformed by subsequent compression strain. As a result, the phase transformation of βα″ was found in final quenched microstructures.

Science and Engineering A

Influence of dwell time on fatigue crack propagation in Alloy 718 laser welds

Author(s): Anand H.S. Iyer, Krystyna Stiller, Gunnar Leijon, Henrik C.M. Andersson-Östling, Magnus Hörnqvist Colliander

The introduction of welded assemblies in aerospace components aid in weight reduction, but also lead to an increased risk of defects. It is therefore important to analyze the high temperature crack growth resistance of such welds. The results from high temperature cyclic and dwell-fatigue testing of surface flawed Alloy 718 welds are presented here. An increasing temperature and application of a dwell time accelerate the crack growth and increase interaction with secondary phases. During cyclic loading at 550°C, there is little interaction with the microstructure during transgranular propagation, but the application of dwell times results in a mixture of transgranular propagation and intergranular cracking of boundaries between different dendrites. At 650°C, mixed intergranular and transgranular mode of crack growth is seen under both cyclic and dwell conditions. However, during dwell-fatigue the interfaces between the secondary arms of the same dendrite are also weakened, leading to an interfacial type of crack growth also in the intergranular parts.

Science and Engineering A

New insights into the mechanism of cooling rate on the impact toughness of coarse grained heat affected zone from the aspect of variant selection

Author(s): X.L. Wang, Z.Q. Wang, L.L. Dong, C.J. Shang, X.P. Ma, S.V. Subramanian

In this article, the authors tried to establish a direct correlation between crystallographic variants and impact toughness, and provided some novel insights into the mechanism of cooling rate on the impact toughness of coarse grained heat affected zone (CGHAZ) of offshore engineering steel by means of electron back-scattering diffraction (EBSD) analysis. The results showed that variant selection becomes stronger with an decrease in the cooling rate, resulting in the decline of high angle grain boundaries (HAGBs) and thus lower the impact toughness. Moreover, the variation in impact toughness is mainly correlated to the crystallographic block size. The larger the block size, the lower the impact toughness. By visualizing the crystallographic features, it has been clarified that the transition from Bain zone grouping to close-packed plane grouping with the increase of cooling rate, while the corresponding microstructure changes from granular bainite to lath bainite. Furthermore, it has been found that ~ 25% (number fraction) reconstructed prior austenite grains in simulated CGHAZ present a twin-related structure (austenitic twin), which can enhance the variant selection and displays a negative effect on the formation of HAGBs.

Science and Engineering A

Effect of laser shock peening on the high-temperature fatigue performance of 7075 aluminum alloy

Author(s): J.T. Wang, Y.K. Zhang, J.F. Chen, J.Y. Zhou, K.Y. Luo, W.S. Tan, L.Y. Sun, Y.L. Lu

7075 aluminum alloy was subjected to intensive laser shock peening (LSP), and the effect of LSP on the microstructure and high-temperature fatigue properties of the alloy at various elevated temperatures was investigated. Microstructural characterization of the laser-shock-peened (LSPed) material was performed with scanning electron microscopy (SEM) and transmission electron microscopy. The LSPed sample exhibited an improved high-temperature fatigue performance. Its fatigue life increased by 110% at 150°C. Grain refinement, work hardening, and precipitates were detected through SEM. After LSP, significant changes in surface morphology in three stages of high-temperature fatigue were examined. Results suggested that the highly dense dislocation structure and high compressive residual stress induced by LSP significantly improved the high-temperature fatigue performance of the 7075 aluminum alloy.

Science and Engineering A

Sun Aug 20 2017

Crack propagation along grain boundaries and twins in Mg and Mg–0.3 at.%Y alloy during in-situ straining in transmission electron microscope

Author(s): Dudekula Althaf Basha, Hidetoshi Somekawa, Alok Singh

In-situ TEM imaging has been carried out on thin foils of pure Mg and Mg-0.3at.%Y alloy under tension, to study crack nucleation and propagation. During propagation along grain boundaries, twin nucleation occurred from the crack tip. In case of pure Mg, the crack did not propagate along the twin boundaries but continued along the grain boundaries. However, in case of Mg-0.3at.%Y alloy, in which the grain boundaries were segregated with yttrium, the crack propagated along newly formed twin boundaries nucleated at the crack tip. This directly demonstrates the effect of alloying on the fracture behavior.

Scripta Materialia

Insights into the radiation behavior of Ln2TiO5 (Ln=La-Y) from defect energetics

Author(s): Xiao Liu, Dongyan Yang, Chenguang Liu, Huan Liu, Shiyin Ji, Pengcheng Mu, Yiyuan Wu, Yuhong Li

First-principles calculations have been performed to study the structural properties and defect energetics of orthorhombic Ln2TiO5 (Ln=La, Pr, Nd, Sm, Gd, Dy and Y). The lattice parameters decrease with increasing lanthanide cation radius. The mean 〈Ln1-O〉 and 〈Ln2-O〉 bond lengths increase systematically by respectively 1.16% and 1.19% from Dy2TiO5 to La2TiO5. Meanwhile, mean 〈Ti-O〉 bond length increases slightly with increasing Ln cation radius from Y2TiO5 to La2TiO5. The calculated results of point defect formation energies suggest that Ln2-Ti antisite defect is the most energetically favorable defect type for Ln2TiO5. The trend of Ln2-Ti antisite defect formation energy of Ln2TiO5 is precisely consistent with the radiation resistance, as demonstrates by the critical amorphization temperature, which indicates that the formation of Ln2-Ti antisite defect may has a significant influence on the radiation behavior of Ln2TiO5.

Computational Materials Science

Strengthening behaviors of V and W modified Cr19 series duplex stainless steels with transformation induced plasticity

Author(s): Huawei Zhang, Yulai Xu, Pengfei Hu, Wanjian Xu, Jun Li, Kunfang Li, Xueshan Xiao

The V and W modified Cr19 series duplex stainless steels with transformation induced plasticity (TRIP) have been newly developed and the effects of microstructures on room temperature mechanical properties have been investigated. The addition of ferrite forming elements V and W slightly increases the volume fraction of ferrite phase from about 55 to 57–60%, however, the cold rolling process hardly influence the fraction. When the duplex phases exhibit a coarse banding morphology before cold rolling, the ultimate tensile strength and elongation are improved significantly with the addition of V and W elements mainly due to the occurrence of TRIP effect. The ultimate tensile strength and yield strength of Cr19+V increase very slightly, but the elongation increases to about 61% after cold rolling. Both the ultimate tensile strength and elongation of Cr19+W significantly increase to about 800MPa and 67%, respectively. The improvement is attributed to TRIP effect and refinement of banded ferrite and austenite phases. After aging heat treatment, the precipitate of VN and Cr23C6 particles contributes to the further increase of ultimate tensile strength to about 1000 and 920MPa of Cr19+V and Cr19+W DSSs, respectively. Strain induced αʹ-martensite is transformed from austenite directly with an orientation relationship. The strengthening behaviors have been discussed based on the microstructural evolutions.

Science and Engineering A

The influence of minor boron on the precipitation behavior of LPSO phase and dynamic recrystallization in the Mg94Y2.5Zn2.5Mn1 alloys

Author(s): Kai Yang, Jinshan Zhang, Ximei Zong, Wei Liu, Jianqiang Hao, Chunxiang Xu

The influence of minor boron on the microstructure and mechanical properties of solution-treated and as-extruded Mg94Y2.5Zn2.5Mn1 alloys has been investigated in this paper. The mechanisms of long period stacking ordered (LPSO) phase transformation during solid-solution treatment and dynamic recrystallization (DRX) in the process of extrusion were analyzed. Experimental results demonstrate that the boron addition effectively promotes the precipitation of 14H LPSO phase and reduces the area fraction and average size of the W phase particles in the solution-treated Mg94Y2.5Zn2.5Mn1 alloy. Furthermore, boron delays the dynamic recrystallization process due to increased 14H LPSO phase during extrusion process. And two kinds of DRX were observed in as-extruded alloys. Finally, the boron addition improves the mechanical properties of both the solution-treated and as-extruded Mg94Y2.5Zn2.5Mn1 alloys. Especially, the as-extruded Mg94Y2.5Zn2.5Mn1 alloy with 0.003 wt% boron addition exhibited optimal mechanical performance with ultimate tensile strength (UTS) and elongation of 389.7 MPa and 24.9%, respectively.

Science and Engineering A

New beta-type Ti-Fe-Sn-Nb alloys with superior mechanical strength

Author(s): G.-H. Zhao, S.V. Ketov, J. Jiang, H. Mao, A. Borgenstam, D.V. Louzguine-Luzgin

Ti-Fe-Sn-Nb alloys comprised of single bcc phase were developed showing excellent mechanical strength and superior plasticity. The alloys exhibited increased yield strength as a function of Fe content, where the Ti80Fe14Sn3Nb3 alloy presented the highest yield stress (1.88GPa) comparable to that of Ti-based nanocrystalline alloys. Moreover, a significant strain-hardening (520MPa) was achieved along with the plastic deformation. The excellent mechanical strength was enhanced by the supersaturated β-Ti and the high density of lattice defects that restrict the dislocation motion and the high β-phase stability.

Science and Engineering A

Elevated Temperature Mechanical Behaviour of Nanoquasicrystalline Al93Fe3Cr2Ti2 Alloy and Composites

Author(s): S. Pedrazzini, M. Galano, F. Audebert, G.D.W. Smith

Rapidly solidified nano-quasicrystalline Al93Fe3Cr2Ti2 at.% alloy has previously shown outstanding tensile and compressive strength and microstructural stability up to elevated temperatures. Despite this, no study had previously assessed the effect of plastic deformation at elevated temperature to simulate thermal-mechanical forging processes for the production of engineering components. The present work analysed bars consisting of a nano-quasicrystalline Al93Fe3Cr2Ti2 at.% alloy matrix, with the addition of 10 and 20 vol.% pure Al ductilising fibres, produced through gas atomisation and warm extrusion. The microstructure was made primarily of nanometre-sized icosahedral particles in an α-Al matrix. Compression tests were performed across a range of temperatures and strain rates. The measured yield strength at 350°C was over 3x that of “high strength” 7075 T6 Al alloy, showing outstanding thermal stability and mechanical performance. However, the microstructure was shown by XRD to undergo a phase transformation which resulted in the decomposition of the icosahedral phase around ~500°C into more stable intermetallic phases. Serrated flow associated with dynamic strain ageing was observed and a semi-quantitative analysis matching elemental diffusion speeds with dislocation speed at specific strain rates was performed, which tentatively identified Ti as the solute species responsible within the selected range of temperatures and strain rates.

Science and Engineering A

Cu and Ag additions affecting the solidification microstructure and tensile properties of Sn-Bi lead-free solder alloys

Author(s): Bismarck L. Silva, Marcella G.C. Xavier, Amauri Garcia, José E. Spinelli

Over the past few years Sn-based solders containing third and fourth elements have become of great interest to try and improve the consistency of solders during application. In most reported cases this involved the addition of either Ni in Sn-Cu or Ag in Sn-Bi solder alloys. Still there is a lack of research showing how the combination of third element additions and varying cooling rates affect the mechanical properties of Sn-Bi-X solder alloys. As such the present investigation examines the effects of minor additions of Ag and Cu on a Sn-34wt.%Bi solder alloys produced by directional solidification. Directional solidification was used as the transient regime attained during directional solidification in a water-cooled mold may allow for similar cooling rates to those found in industrial reflow soldering operations. Microstructural analysis on the Sn-Bi-X alloys was conducted using eutectic spacing (λE), Bi precipitates spacing (λp) and the secondary dendritic spacing (λ2) measurements. These measurements represented the complex eutectic growth, the solid-state precipitation of Bi within the β-Sn phase and the length-scale of the Sn-rich dendritic array respectively. In conjunction with these measurements the evolution of tensile strength and ductility as a function of λ2 was examined. Considering the Sn-34wt.%Bi, Sn-34wt.%Bi-0.1wt.%Cu, Sn-34wt.%Bi-0.7wt.%Cu and Sn-33wt.%Bi-2wt.%Ag alloys, it was found that the modified alloys containing 0.7wt.%Cu and 2.0wt%Ag showed lower tensile properties and lower ductility. In contrast, the addition of 0.1wt.%Cu increased the ductility for λ2 <14μm while preserving the tensile strength, representing the best alternative of all alloys examined.

Science and Engineering A

Re-examining the relation between fracture strain and yield stress in Al-Mg-Si alloys

Author(s): M.R. Langille, B.J. Diak, S. Saimoto

A linear relationship between fracture strain, εf, and yield strength first observed by Lloyd (2003) in aluminum alloys is re-examined for age-hardenable AA6063. A breakdown in the relationship is observed if AA6063 is pre-strained prior to ageing. A linear relationship is recovered for εf versus dislocation initial inter-obstacle spacing, l.

Science and Engineering A

Sat Aug 19 2017

Precipitation of α′ in neutron irradiated commercial FeCrAl alloys

Author(s): Kevin G. Field, Kenneth C. Littrell, Samuel A. Briggs

Alkrothal 720 and Kanthal APMT™, two commercial FeCrAl alloys, were neutron irradiated up to damage doses of 7.0 displacements per atom (dpa) in the temperature range of 320 to 382°C to characterize the α′ precipitation in these alloys using small-angle neutron scattering. Both alloys exhibited α′ precipitation. Kanthal APMT™ exhibited higher number densities and volume fraction, a result attributed to its higher Cr content compared with Alkrothal 720. Trends observed as a function of damage dose (dpa) are consistent with literature trends for both FeCr and FeCrAl alloys.

Scripta Materialia

Thermally activated martensite formation in ferrous alloys

Author(s): Matteo Villa, Marcel A.J. Somers

Magnetometry was applied to investigate the formation of α/α ´ martensite in 13 ferrous alloys during immersion in boiling nitrogen and during re-heating to room temperature at controlled heating rates in the range 0.0083–0.83Ks1. Data shows that in 3 of the alloys, those that form {5 5 7}γ martensite, no martensite develops during cooling. For all investigated alloys, irrespective of the type of martensite forming, thermally activated martensite develops during heating. The activation energy for thermally activated martensite formation is in the range 8–27kJmol1 and increases with the fraction of interstitial solutes in the alloy.

Scripta Materialia

Cracking in a hot deformed SiCPA6061 composite investigated using synchrotron microtomography

Author(s): Chiradeep Gupta, Ashish K. Agarwal, Balwant Singh, Partha S. Sarkar, S.C. Gadkari, Amar Sinha, Dinesh Srivastava, Gautam K. Dey

We present the first investigation of cracking by hot compression of any metal matrix composite in zones deforming under tri-axial state of stress using synchrotron microtomography (SRμCT). The task required to reduce the unfavourable manifestations of phase contrast in the reconstructed slices is simplified by developing a Fourier domain based filter for processing of reconstructed slices. As a result for the first time an accurate characterisation of the state of 3-D damage developed by compression at 500°C and 1s−1 to strains of ~1 is achieved in the bulge region of a hot worked SiCP/A6061 composite. The composition of the damage is found to predominately consist of acicular shaped large to very large pores and “coalesced” micro-cracks. The quantification of the Fourier domain filtered dataset and the analysis of the spatial proximity of the different types of damage provided the mechanism of cracking in metal matrix composites near regions of tri-axial stress at high compressive strains. The propensity for cracking in bulge region of SiCP/A6061 composite is attributed to the development and growth of acicularly shaped pores facilitated by repeated particle fragmentation. It results in a damage architecture that promotes the coalescence of the closely spaced pores and micro-cracks. This mechanism rationalises the high efficiency of power dissipation observed in hot working of SiCP/A6061composite in the absence of microstructural restoration processes.

Science and Engineering A

Microstructural evolution and mechanical properties of solution annealed cast IN617B alloy during long-term thermal exposure

Author(s): Shuang Gao, Jieshan Hou, Yongan Guo, Lanzhang Zhou

Cast IN617B alloy, a nickel-based superalloy, is considered as a preferential candidate for castings or valve components in developing 700°C Advanced Ultra Supercritical (A-USC) power plant. Microstructural evolution and mechanical properties of solution annealed cast IN617B alloy during the long-term thermal exposure at 700℃ are systematically investigated. The results show that Ti (C, N), M6C and M23C6 are the primary precipitates in as-cast microstructure, and most of them completely dissolved after solution heat treatment. The major precipitates in the thermal exposed microstructure are M23C6, M6C and γʹ phases, and a trace amount of σ phases are also detected within grains. By comparison with previous studies on wrought IN617B alloy, it is identified that the processing method exerts a significant influence on the morphology of M23C6 carbides within grains. Moreover, the microstructure observation reveals that the growth of M23C6 facilitates the formation of γʹ by enriching γʹ-formation elements, and conversely, dense γʹ phase inhibites the coarsening and transformation of M23C6 by constraining the diffusion of M23C6-formation elements. The tensile strength of 700°C improves strikingly due to the precipitation strengthening of γʹ phases and carbides while the ductility decreases noticeably. The significant coarsening of carbides on grain boundaries (GBs) results in the fracture mechanism evolution during long-term thermal exposure.

Science and Engineering A

Microstructrual evolution and mechanical properties of the stir zone during friction stir processing a lean duplex stainless steel

Author(s): G.M. Xie, H.B. Cui, Z.A. Luo, R.D.K. Misra, G.D. Wang

A lean duplex stainless steel was friction stir processed at 300rpm-100mm/min using a W-Re stirring tool. The microstructural evolution on the advancing side, the center, and the retreating side of the stir zone were studied by a combination of electron microscopy and electron backscattered diffraction. The study indicated that the direction of fiber structure on the advancing side, center, retreating side were 0°, 90°, 45° relative to the processing direction because of the stirring effect of the tool. Given that the strain rate and temperature were different on the advancing side, center and retreating side, the phase fraction, misorientation, texture components varied in different parts of the stir zone. The deformation mostly occurred in the center and retreating side and the material on the advancing side almost did not deform during tensile test. As a result, strain induced martensitic transformation occurred in the center and retreating side. The voids initiated at the interface of ferrite and transformed martensite on the retreating side, and cracks also propagated along this interface during tensile test. Furthermore, because of the 45° direction of the fiber structure on the retreating side, the tensile strength of the FSP sample was slightly less than the base metal.

Science and Engineering A

Shear softening of Ta-containing metallic glass matrix composites upon dynamic loading

Author(s): Y.L. Tang, R.F. Wu, Z.M. Jiao, X.H. Shi, Z.H. Wang, J.W. Qiao

The deformation behaviors of in-situ Ti50Zr18Ni5Ta15Be12 bulk metallic glass matrix composites (MGMCs) were investigated upon quasi-static and dynamic loadings. The present MGMCs exhibit good quasi-static mechanical properties, combining high fracture strength (2460MPa) with remarkable plasticity (20%) at the strain rate of 5×10−4 s−1. When the strain rate is higher than 3080s−1, the strain rate effect of the yielding strength has an apparent negative strain rate sensitivity (SRS), which can be ascribed to the deteriorated ability of dendrites to impede the propagation of shear bands and the matrix-dominated fracture related to thermal softening at higher strain rates. Based on their deformation mechanisms, a constitutive relationship is obtained by cooperative shear model (CSM), which is employed to model the dynamic yielding stress behavior. The constitutive equations are established for describing the present MGMCs.

Science and Engineering A

Long-Term Annealing of High Purity Aluminum Single Crystals: New Insights into Harper-Dorn Creep

Author(s): K.K. Smith, M.E. Kassner, P. Kumar

Single crystals of 99.999 and 99.9999% pure aluminum were annealed at high elevated temperatures (0.98Tm) for relatively long times of up to one year, the longest in the literature. Remarkably, the dislocation density remains relatively constant at a value of about 109 m−2 over a period of one year. The stability suggests some sort of “frustration” limit. This has implications towards the so-called “Harper-Dorn creep” that generally occurs at fairly high temperatures (e.g. > 0.90Tm) and very low stresses. It is possible that ordinary five-power-law creep occurs within the tradition Harper-Dorn regime with very low initial dislocation densities in aluminum. Higher initial dislocation densities, such as with this annealing study, may lead to Harper-Dorn (Newtonian) creep.

Science and Engineering A

Evolution of nano-size precipitation and mechanical properties in a high strength-ductility low alloy steel through intercritical treatment

Author(s): G. Han, Z.J. Xie, L. Xiong, C.J. Shang, R.D.K. Misra

A two-step intercritical heat treatment was designed to obtain a multi-phase microstructure consisting of intercritical ferrite, tempered martensite/bainite and stable retained austenite in a low carbon and copper alloyed steel, characterized by high strength and high ductility combination. The evolution of copper precipitation during intercritical tempering was studied by transmission electron microscopy (TEM). Electron microscopy studies indicated that the precipitation of copper during tempering followed the sequence (as a function of time): twinned 9R-Cu (0.5h) → de-twinned 9R-Cu (1h) → ε-Cu (greater than 3h), which was accompanied by increase in the size of precipitates from ~11nm to ~30nm. Considering the cutting mechanism of precipitation strengthening, ε-Cu precipitation contributed to ~248MPa and ~207MPa toward yield strength for 3h and 5h tempering, respectively. The average size of niobium-containing carbides varied marginally from ~11–16nm and had a Baker–Nutting (B-N) orientation relationship with the ferrite matrix. The combination of transformation induced plasticity (TRIP) effect and nano-sized precipitation strengthening contributed to excellent mechanical properties (yield strength > 700MPa, tensile strength > 800MPa, the uniform elongation > 16% and the total elongation > 30%).

Science and Engineering A

Tensile Properties and Microstructure of a Cryomilled Nanograined Al-Mg Alloy Near the AA5083 Composition

Author(s): K.H. Kaisar, C. Hofmeister, A. Pedigo, A.K. Giri, Y. Sohn, K.C. Cho, M. van den Bergh, B.S. Majumdar

In an effort to develop high strength aluminum alloys, a near-AA5083 Al-Mg alloy powder was cryomilled in liquid nitrogen and consolidated by vacuum hot-pressing (VHP). The composition of the Al-Mg alloy was designed to minimize intermetallic particles that often lead to premature fracture especially when flow strength is high. The as-VHP material had poor ductility and therefore was extruded under different conditions to obtain a good combination of tensile strength and ductility. The microstructure of samples were characterized using X-ray diffraction, EBSD, FIB induced secondary electron orientation contrast imaging, and TEM techniques, which together provided unique assessment at multiple length scales. The samples exhibited a wide range of grain sizes that could be binned into three different groups: i) grain sizes of 50–100nm, (ii) sizes in the 100–300nm range, and (iii) elongated larger grains with widths in the range 0.5 to 2 μm. Room temperature tensile tests with cylindrical dog-bone geometry indicated a high ultimate strength of 730–770MPa and reproducible elongation to failure of about 3 − 4%. This combination of strength and ductility in the material are some of the best that have been reported for alloys close to the weldable and corrosion resistant AA5083 composition, and likely a result of the multi-scale microstructure resulting from the processing route.

Science and Engineering A

Synthesis and Characterization of the Mechanical Properties of Ti3SiC2Mg and Cr2AlCMg Alloy Composites

Author(s): Matt Nelson, Matthias T. Agne, Babak Anasori, Jian Yang, Michel W. Barsoum

Herein we report on the fabrication and characterization of Mg and Mg-alloy metal matrix composites (MMCs) reinforced with the MAX phases, Ti3SiC2 and Cr2AlC. Pure Mg and Al-containing Mg-alloys with varying Al content (AZ31, AZ61 & AZ91), were pressureless melt infiltrated into 55 ± 1 vol. % porous MAX preforms. The resulting microstructures and mechanical properties were characterized by X-ray diffraction, scanning electron microscopy, microhardness and uniaxial compression tests. Similar to the Ti2AlC/Mg composite system, increasing the Al content in the matrix enhanced the mechanical properties of the Mg/Ti3SiC2 composite system, but had little effect on the properties of the Mg/Cr2AlC composite system. The latter were inferior to those reinforced with the other MAX phases. The Ti3SiC2/AZ91 composite achieved the highest Vickers hardness (1.9 ± 0.1 GPa), yield strength (346 ± 4 MPa) and ultimate compressive strength (617 ± 10 MPa) obtained in this study. All composites exhibited fully and spontaneously reversible hysteresis loops, evidence of energy dissipation, during compression cycling. Having an elastic modulus of ≈160 GPa, the Ti3SiC2/AZ91 composite may be suited for high specific strength, high damping applications.

Science and Engineering A

Orientation dependence of twinning in single crystalline CoCrFeMnNi high-entropy alloy

Author(s): I.V. Kireeva, Yu.I. Chumlyakov, Z.V. Pobedennaya, I.V. Kuksgausen, I. Karaman

Tensile deformation behavior of the equiatomic CoCrFeMnNi high entropy alloy single crystals were studied along three different crystallographic orientations, i.e. [001], [ 1 ¯ 23 ] , [ 1 ¯ 11 ] , in order to reveal the orientation dependence of deformation twinning at room temperature. It was shown that initial yield behavior along these orientations is governed by dislocation slip mechanism and the critical resolved shear stresses for slip are independent of crystal orientation. Twinning starts at different strain levels after slip deformation, depending on the tensile testing orientation: in the [ 1 ¯ 11 ] -oriented crystals, twinning starts after 5% strain while in the [ 1 ¯ 23 ] -oriented crystals, it starts after about 25% strain. Deformation twinning was not detected in the [001]-oriented crystals under tensile loading. The critical resolved shear stresses for twinning are determined to be τ cr tw =110–140MPa. Onset of deformation twinning in the [ 1 ¯ 11 ] -oriented crystals at low strain levels, together with multiple active dislocation slip systems, leads to a significantly higher strain hardening coefficient and stress for fracture in comparison with the [001]-oriented crystals, where the plastic deformation occurs only by multiple slip systems, and the [ 1 ¯ 23 ] -oriented crystals, in which the deformation mainly occurs by single slip and then twinning in one system at later stages of the deformation.

Science and Engineering A

Characterization of microscopic deformation in Cu-Al-Mn superelastic alloy by in situ Laue diffraction study using white X-ray microbeam

Author(s): E.P. Kwon, S. Sato, S. Fujieda, K. Shinoda, K. Kajiwara, M. Sato, S. Suzuki

In this study, in situ white X-ray microbeam diffraction experiments were conducted to investigate the microscopic deformation behavior of a Cu-Al-Mn superelastic alloy during tensile testing. Local deformation both at a grain scale and an intragranular scale was revealed by analyzing Laue spot streaking that occurred within a grain and in some grains with different orientations. The streaking measured within a grain under tensile load showed inhomogeneous behavior because of an inhomogeneous martensitic transformation. Moreover, the effect was dependent on grain orientation. In a grain with a high Schmid factor, a large dispersion in the 2θ diffraction angle of Laue spots was observed due to significant Laue spot streaking, which can be attributed to that grain's favored martensitic transformation.

Science and Engineering A

Microstructures and mechanical behavior of aluminum-copper lap joints

Author(s): Xianglong Zhou, Gang Zhang, Yu Shi, Ming Zhu, Fuqian Yang

5052 aluminum alloy and pure copper (T2) are joined, using a low heat input pulsed double-electrode gas metal arc welding (DE-GMAW)-brazing method with AlSi12 filler metal. The effects of welding current (heat input) on the microstructure and mechanical behavior of the joints, which consist of Al-Al welding zone and Al-Cu brazing zone, are investigated. The Al-Cu welding zone mainly consists of α-Al solid solution and Al-Cu eutectic phase in coral-like shape. There exists a layer of Al2Cu intermetallic compound (IMC) in the Al-Cu brazing zone. Using the theory of thermal activation process, a quadratic relation between the thickness of the IMC layer and welding current intensity is derived. The experimental result supports this relationship. The shear strength of the Al-Cu joints first increases with the increase of the welding current (heat input), reaches a maximum of 17.66MPa, and then decreases with the increase of the welding current due to the dispersion of the Al2Cu IMCs of large sizes in the Al alloy. Fracture of the Al-Cu lap joints occurs at three different positions, and the corresponding failure mechanisms are discussed according to the morphologies of fracture surfaces.

Science and Engineering A

Deformation and fracture behavior of Mg alloy, WE43, after various aging heat treatments

Author(s): J.J. Bhattacharyya, F. Wang, P.J. McQuade, S.R. Agnew

Wrought Mg alloy, WE43, is normally artificially aged after hot working (T5 temper). A solutionizing heat treatment is revealed to weaken the initial basal texture of the hot-rolled plate, which resulted in a reduction of the yield strength anisotropy (by notably lowering the in-plane strength). Solutionizing followed by peak aging (T6 temper) only leads to a marginal increase in hardness. The precipitate size and volume fraction are slightly greater for T6 as compared to T5. Different strategies to increase the number density of the precipitates were explored. T8 treatments (where the solutionized material is cold-deformed prior to aging) were carried out, and although the aging kinetics improved, the peak hardness did not. Similarly, two-step aging (first at 140°C for 4h, followed by 210°C) treatment provided no measurable increase in hardness. The T6 samples showed large grain boundary precipitates and a larger precipitate free zone, as compared to T5, which resulted in extensive intergranular ductile fracture, and reduced ductility. Finally, the lower ductility along the plate normal direction is related to stringers of Y-rich cuboidal dispersoids. The results reveal the incumbent T5 temper to be an optimum in terms of strength, ductility, and energy consumption.

Science and Engineering A

Dislocation versus grain boundary strengthening in SPD processed metals: non-causal relation between grain size and strength of deformed polycrystals

Author(s): M.J. Starink

In metals that are heavily cold deformed, for instance by a severe plastic deformation process, significant strengthening is caused by the high density of defects such as grain boundaries and dislocations. In this work a model for volume-averaged dislocation and grain boundary (GB) creation is used to show that unless significant annihilation of defects post deformation occurs, the dislocation densities and GB densities in the deformed material are closely correlated. The dislocation strengthening effect thus shows a strong correlation with GB strengthening, and correlation of strength or hardness with d 1/2, where d is the grains size, as in a Hall-Petch type plot, can not be taken as an indication that GB strengthening dominates. Instead, in many SPD processed metals and alloys, dislocation strengthening is the dominant strengthening effect, even though a Hall-Petch type plot shows a good linear correlation.

Science and Engineering A

Influence of Tension-Compression Anomaly During Bending of Magnesium Alloy AZ31

Author(s): Sebastian Härtel, Marcel Graf, Thomas Lehmann, Madlen Ullmann

In this paper, the influence of the tension-compression anomaly of magnesium alloys on the numerical accuracy is quantified for the first time. For this, flow curves were evaluated for a twin-roll cast, rolled and annealed AZ31 strip using tensile and compression tests at different temperatures and constant strain rate. This is necessary to take into account the changing material behavior under tensional and compression load within the FEM.

Science and Engineering A

Localized creep characterization of 316LN stainless steel weld joint using Small Punch Creep test

Author(s): J. Ganesh Kumar, K. Laha

Type 316LN stainless steel (SS) weld joints are widely used in several high temperature applications. The variations of creep properties across the microstructurally heterogeneous regions of 316LN SS weld joint such as base metal, weld metal and heat affected zone (HAZ) have been evaluated using Small Punch Creep (SPC) technique at 923 K. Both the SPC rupture and deformation behaviors were investigated. The creep strength gradient across 316LN SS weld joint showed a smooth increasing trend from base metal to weld metal. However, the trend of the rupture life for weld metal shifted with change in load level. The SPC deformations of various regions of weld joint have been analyzed according to the equation, δ = δ 0 + δ T ( 1 e κ t ) + δ ̇ s t + δ 3 e [ ϕ ( t t r ) ] . The variations of transient, secondary and tertiary creep parameters were determined across the weld joint. The master curve for transient creep deflection was obtained for various regions of weld joint.

Science and Engineering A

Structural Metastability of “Cold” Repair Welds in 2.25Cr-1Mo (P22) Steel under Elevated Temperature and Stress Conditions

Author(s): Elizabeth Budzakoska-Testone, Druce Dunne, Huijun Li, Michael Drew, Tim Nicholls

This paper investigates the effects of accelerated creep testing of a temperbead (TB) repair weld for a P22 pressure vessel component that was produced without post-weld heat-treatment (PWHT). The mechanical properties of the “cold” (non-PWHT) weld metal were superior to those of aged parent metal and the original fabrication welds for all tests, with the notable exception of creep performance. The creep performance of the TB repair weld material was inferior to that of the aged parent metal, due to the metastability of the weld metal microstructure relative to restoration processes occurring at the “accelerated” temperatures used for testing. It is concluded that the test conditions induced structural changes that are unlikely to occur under the actual service conditions and resulted in a creep life assessment that was highly conservative. The reported results are relevant to the security and integrity of non-PWHT repair welding of high-energy piping components.

Science and Engineering A

Microstructure Evolution Characteristics of Inconel 625 Alloy from Selective Laser Melting to Heat Treatment

Author(s): C. Li, R. White, X.Y. Fang, M. Weaver, Y.B. Guo

Superalloy Inconel 625 has been widely used in selective laser melting (SLM). Since SLM-induced microstructure with columnar grains, strong texture, porosity, and undesired properties, heat treatment is often used to tune the microstructure and mechanical properties. However, the microstructure evolution of IN 625 from SLM to heat treatment is poorly understood. In this study, IN 625 samples were SLMed and then heat treated at elevated temperatures. Microstructure evolution characteristics of the processed IN 625 alloy have been characterized using optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction, and micro indentation. Fine dendrite microstructures with strong texture parallel to layer build-up direction was observed in the as-SLM samples due to rapid cooling and epitaxial growth. High dislocation density and high microhardness were found in the γ matrix which also contains high Z-contrast precipitates. After annealing at high temperatures, random grain growth accompanied by dislocation annihilation and twinning occurs. The decrease in lattice parameter and the prevalence of large grain boundary misorientation in the γ matrix suggests that SLM-induced residual stress be significantly reduced. In addition, the uncertainty of mechanical properties due to the process variations was quantified.

Science and Engineering A

Spatial valley separation in strained graphene pn junction

HongYu Tian and Jun Wang

Valleytronics in analogy to spintronics aims to use the electron valley degree of freedom to carry and manipulate information, and one of urgent tasks in this field is to generate valley-polarized electrons. In this work, we propose using the electron focusing effect in a strained graphene pn junction to separate valleys spatially through a beam of valley-unpolarized electrons, since the strain-induced pseudo-gauge potentials are opposite for K and {$K^\prime $} valleys and severely affect the trajectories of K and {$K^\prime $} electron propagation. We numerically simulate this valley-separated Veselago lens effect in a lattice model and demonstrate that pseudo-gauge potentials can efficiently control valley separation patterns.

Journal of Physics Condensed Matter

Pressure-induced superconductivity in CrAs and MnP

Jinguang Cheng and Jianlin Luo

Transition-metal monopnictides, CrAs and MnP, were studied over 50 years ago due to the presence of interesting magnetic properties: CrAs forms a double-helical magnetic structure below T N   ≈  270 K accompanied by a strong first-order structural transition, while MnP first undergoes a ferromagnetic transition at T C   ≈  290 K and then adopts a similar double-helical order below T s   ≈  50 K. Both compounds are correlated metals and exhibit distinct anomalies at these characteristic magnetic transitions. By using high pressure as a clean tuning knob, we recently observed superconductivity with a maximum superconducting transition temperature of T c   ≈  2 K and 1 K when their helimagnetic orders are suppressed under a critical pressure of P c   ≈  0.8 and 8 GPa for CrAs and MnP, respectively. Despite a relatively low T c , CrAs and MnP are respectively the first superconducto...

Journal of Physics Condensed Matter

Fri Aug 18 2017

Violation of Ohm’s law in a Weyl metal

Dongwoo Shin, Yongwoo Lee, M. Sasaki, Yoon Hee Jeong, Franziska Weickert, Jon B. Betts, Heon-Jung Kim, Ki-Seok Kim & Jeehoon Kim

The chiral anomaly in the Weyl metal phase is reported to lead to a breakdown of Ohm’s law.

Nature

The processing and heterostructuring of silk with light

Mehra S. Sidhu, Bhupesh Kumar & Kamal P. Singh

Femtosecond laser pulses can induce local bulging or plasma ablation of silk with limited structural damage, thus offering a technique for cutting, patterning, bending and welding of silk with various other materials.

Nature

Flexible shape-memory scaffold for minimally invasive delivery of functional tissues

Miles Montgomery, Samad Ahadian, Locke Davenport Huyer, Mauro Lo Rito, Robert A. Civitarese, Rachel D. Vanderlaan, Jun Wu, Lewis A. Reis, Abdul Momen, Saeed Akbari, Aric Pahnke, Ren-Ke Li, Christopher A. Caldarone & Milica Radisic

Cardiac repair usually requires highly invasive interventional procedures. Here, the authors develop an injectable shape-memory cardiac patch and demonstrated its applicability in a myocardial infarction model.

Nature

Bronze-mean hexagonal quasicrystal

Tomonari Dotera, Shinichi Bekku & Primož Ziherl

A two-dimensional quasicrystalline tiling based on the bronze-mean hexagonal pattern is proposed.

Nature

Thu Aug 17 2017

Engineering of magnetic softness and giant magnetoimpedance effect in Fe-rich microwires by stress-annealing

Author(s): V. Zhukova, J.M. Blanco, M. Ipatov, J. Gonzalez, M. Churyukanova, A. Zhukov

Magnetic softness and giant magnetoimpedance (GMI) effect of Fe-rich microwires has been considerably improved by stress-annealing: we obtained maximum GMI ratio about one order higher than in as-prepared Fe-rich microwire and of the same order as in as-prepared Co-rich microwire. High enough bias current allows drastic increasing of GMI ratio of Fe-rich microwires up to 120% and off-diagonal GMI effect but also produces irreversible changes of GMI effect related to Joule heating of the samples. For interpretation of observed dependences we considered internal stresses relaxation after annealing interplay of compressive “back-stresses” arising after stress annealing and axial internal stresses.

Scripta Materialia

Anisotropic strengthening of nanotwinned austenitic grains in a nanotwinned stainless steel

Author(s): F.K. Yan, Q. Li, N.R. Tao

Nanotwin lamellae orientation effect on tensile properties of nanotwinned grains strengthening stainless steels was investigated. It is found that the nanotwinned grains with twin lamellae oriented roughly parallel to loading direction exhibit the much higher strengthening effect (at least a GPa higher) than the nanotwinned grains containing twin lamellae inclined to loading direction, which is related to the anisotropic strengthening mechanisms of nanotwins.

Scripta Materialia

Large strain induced by the alignment of defect dipoles in (Bi3+,Fe3+) co-doped Pb(Zr,Ti)O3 ceramics

Author(s): Jiangtao Zeng, Kunyu Zhao, Xue Shi, Xuezheng Ruan, Liaoying Zheng, Guorong Li

(Bi3+,Fe3+) co-doped Pb(Zr,Ti)O3 (PZT-BF) and single Fe3+ dope Pb(Zr,Ti)O3 (PZT-F) ceramics were prepared by solid state reaction method and their ferroelectric properties were investigated. The results showed that ferroelectric property of (Bi3+,Fe3+) co-doped PZT ceramics were very different from that of single Fe3+ doped PZT ceramics due to the different mechanisms on the interaction of defect dipoles with spontaneous polarization or domain walls. Unpoled PZT-BF showed very low remnant polarization and low electric field induced strain. After poling, the remnant polarization and electric field induced strain increased markedly due to the alignment of defect dipoles.

Scripta Materialia

Effect of the interface energy on the pressure-induced superheating of metallic nanoparticles embedded in a matrix

Author(s): M. Zhao, X. Yao, Y.F. Zhu, Q. Jiang

The pressure-induced superheating of metallic nanoparticles embedded in a coherent or incoherent matrix was evaluated by considering the interface energy effect. As the size decreases, the superheating is weakened from the bulk value to zero for these two systems. Associated to the competing roles of different thermodynamic quantities, the weakening in the coherent system originates from the predominant lattice contraction and melting enthalpy reinforcement because of the negative interface energy, but that in the incoherent system results from the bulk modulus decline due to the positive interface energy. Our theoretical predictions are in agreement with available literature results.

Scripta Materialia

Revisit the role of deformation twins on the work-hardening behaviour of twinning-induced plasticity steels

Author(s): Z.C. Luo, M.X. Huang

The present study found that the work-hardening rate and dislocation density in a twinning-induced plasticity (TWIP) steel deformed at 373K and 473K are comparable to that deformed at 298K, but deformation twins are considerably prohibited at 373 and 473K. High dislocation density induced by dynamic strain aging (DSA) is the dominant mechanism responsible for the high work-hardening rate of TWIP steels at 373 and 473K. It indicates that TWIP steels can also achieve high working-hardening rate without the formation of deformation twins.

Scripta Materialia

Deformation mechanisms of spherical cell porous aluminum under quasi-static compression

Author(s): Zhiqiang Fan, Bingbing Zhang, Yubo Gao, Xuefeng Guan, Peng Xu

Quasi-static compression tests for a novel type of spherical cell porous aluminum prepared by the space-holder method were conducted. Formation and evolution mechanisms of deformation bands on the macro and micro scale were monitored and discussed by using a digital image correlation procedure. Effect of micro strain concentrations on the formation process and morphology of deformation bands at the macro level was analyzed in detail. In total three deformation modes of spherical cells and four failure modes of cell membranes including bending, shear, combined compression-shear, and eventually tearing were identified.

Scripta Materialia

Excellent photochromic properties of an oxygen-containing yttrium hydride coated with tungsten oxide (YHx:OWO3)

Author(s): Mao La, Ning Li, Ren Sha, Shanhu Bao, Ping Jin

Oxygen-containing yttrium hydride films (YHx:O) show interesting photochromic properties under ultraviolet irradiation. In this paper, oxygen-containing yttrium hydride films covered with tungsten oxide were prepared by a direct current magnetron sputtering method. This composite material showed reversible photochromic properties. Furthermore, its coloration speed was faster than that of a single layer of yttrium hydride, and its switching modulation was broader. The photochromic mechanism of the YHx:O/WO3 thin films was investigated by X-ray diffraction, X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy analysis. Oxygen and hydrogen played a key role in the photochromic process of the YHx:O/WO3 composite films.

Scripta Materialia

Structural properties and energy analysis of ZrxCu92−xAl8 ternary metallic glasses

Author(s): Jingfeng Zhao, C.T. Liu, A. Inoue, P.K. Liaw, Guang Chen, Cang Fan

In this work, classic molecular dynamics simulations were employed to study ZrxCu92−xAl8 (x=55, 60, 65, and 70) ternary alloys. When Cu atoms were gradually replaced by Zr atoms, the local atomic structural evolution and energy landscape were analyzed. Pair distribution functions were used to characterize the structural features. The results showed that Zr-Al, Zr-Cu and Cu-Al atom pairs showed a strong short range order (SRO) within the first shell and they exhibited a weak sensitivity to atomic concentrations while Zr-Zr and Cu-Cu atom pairs presented a strong sensitivity to atomic concentrations. The analyses of the coordination number and coordination polyhedra indicated that substituting Zr atoms for Cu atoms led to the reduction of high coordination polyhedra and precluded the formation of the icosahedral structure, resulting in a more loosely packed atomic structure of the metallic glass. Finally, the energy landscape revealed that, with the increase of Zr composition and the decrease of Cu composition, the energy distribution of the Zr-centered clusters shifted towards a higher energy significantly and this originated from the increase of the number of Zr-Zr atom pairs and the growth of the Zr-Zr atom pair energy. The simulation results revealed that, from both consideration of the structure and energy distributions, the high Zr content Zr-Cu-Al metallic glasses were composed of more weakly bonded regions and packed more loosely, in line with the experimental results that the high Zr content glassy alloys exhibited high plasticity and low glass-forming ability.

Computational Materials Science

Efficiency and fidelity of molecular simulations relevant to dislocation evolutions

Author(s): Fei Shuang, Pan Xiao, Fujiu Ke, Yilong Bai

The comparative simulations of dislocation evolution in nanoindentation are carried out in terms of both molecular dynamics (MD) and energy minimization (EM) methods, to explore what really govern the computational efficiency and fidelity in molecular simulations relevant to dislocation evolutions. It is found that although all simulations can present similar relationship between indentation force and depth, there still might be some significant differences in the simulated dislocation patterns and computational efficiency. Firstly, the EM simulations show more complicated dislocations. Secondly, the necessary computational effort of EM increases nonlinearly with indentation depth, compared to the linear dependence in MD simulations, namely EM shows higher efficiency than MD in shallow indentation, but vice versa in deeper ones. More importantly, it is revealed that the time consumption of the minimization iteration is strongly dependent on the moving of dislocation loops and increases greatly when dislocation loops move long distances. Whereas MD simulations of complicated dislocations patterns may need less time cost but present immature dislocation evolutions, since the relaxation steps in MD simulations are fixed beforehand, regardless of the dislocation loops moving to equilibrium state or not.

Computational Materials Science

Precipitation during cooling of 7XXX aluminum alloys

Author(s): Pikee Priya, David R. Johnson, Matthew J.M. Krane

Precipitation during post-homogenization cooling has been studied. A Particle Size Distribution (PSD) based numerical model has been developed to predict diffusion based nucleation, growth and coarsening of plate-like precipitates. The precipitation of S (Al2CuMg), η(MgZn2), T(Al2Mg3Zn3) and θ(Al2Cu) phases is predicted for an Al-6.2Zn-2.3Cu-2.35Mg-0.13Zr alloy. These phases have been experimentally observed in a lab-scale furnace cooled sample. The effect of cooling rates and compositions on the precipitation reactions during cooling are studied. A cooling rate >500°C/h which lead to precipitates <0.6μm in size, lower volume fractions of all phases and a composition of 6–8% Zn, 1–1.5% Cu and ∼2% Mg has been suggested for easily extruded microstructures with good mechanical properties. The Continuous Cooling Curves (CCC) and Time Temperature Transformation (TTT) curves have been predicted for AA7050.

Computational Materials Science

Phase-field simulations of thermomechanical behavior of MnNi shape memory alloys using finite element method

Author(s): Shushan Cui, Jianfeng Wan, Yonghua Rong, Jihua Zhang

The thermomechanical behavior and associated microstructure evolution of MnNi shape memory alloys (SMAs) are studied via two-dimensional phase-field simulations. Simulations with complex geometries and boundary conditions are realized by using finite element method. Shear-related surface relief, determined by the combination of Bain strain and crystal rotation, was observed. In the process of pseudoelastic bending, large bending degree was realized by formation of triangular martensitic domains, and the elastic strain energy did not apparently increase. The coupling strength of a SMA pipe coupling was greatly influenced by the proceeding of the reverse transformation upon heating. It was found that large shape change of SMAs was realized by the transition between different crystal structures, this transition was driven by the free energy difference between the structures, and the stress plateau during a pseudoelastic cycle was related to the thermodynamic equilibrium state of austenite–martensite interface.

Computational Materials Science

Fatigue of AlSi10Mg specimens fabricated by additive manufacturing selective laser melting (AM-SLM)

Author(s): Naor Elad Uzan, Roni Shneck, Ori Yeheskel, Nachum Frage

The fatigue resistance of AM-SLM AlSi10Mg samples built in the Z direction after various heat treatments was investigated. Specimens were tested in the as-built (AB) condition, after stress relief (SR) treatment and after SR and hot isostatic pressing (HIP) at either 250°C or 500°C. The AB machined and polished specimens displayed the highest fatigue limit (S f = 125MPa, N f = 107 cycles). SR and HIP cycles decrease the yield strength, hardness and fatigue limit. The SR and HIP treatment at 500°C resulted in the lowest fatigue resistance due to significant microstructural changes. A relation between the yield stress and fatigue resistance was established. Linear elastic fracture mechanics were employed for evaluating fracture surface morphology. Based on the results of fracture surface characterization, values of the critical stress intensity factor (K cr ) for AM-SLM AlSi10Mg specimens after various heat treatments were estimated.

Science and Engineering A

A bulk of uniform nanocrystalline copper with superior comprehensive mechanical properties electroformed in an ultra-low sulfate concentration bath without additives

Author(s): C.J. Shen, Z.W. Zhu, D. Zhu

Nanocrystalline copper has a high potential to meet high application requirement of a variety of new industries for its unique mechanical, physical and chemical properties. But its disappointingly low ductility limits its application. This paper provides a reliable and economical preparation method of nanocrystalline copper with superior comprehensive mechanical properties. In an ultra-low sulfate concentration bath (20g/L) without additives, by strong electrolyte flushing and regulating cathodic working current density from 1 to 6A/dm2, copper deposits with different nanostructure is electroformed because of high electrochemical polarization. The microstructure and mechanical properties of copper deposits are investigated. Except of the deposits of 6A/dm2, the copper deposits exhibit good comprehensive mechanical properties in tensile tests at room temperature. In particular, a bulk of nanocrystalline copper obtained at 4A/dm2, with uniform grains size (60–70nm) and random grain orientation, exhibits superior comprehensive mechanical properties, with a high tensile strength of 526MPa and a high elongation of 30%. The deformation mechanisms of copper deposits are discussed. Fracture observation indicates a feature of ductile fracture.

Science and Engineering A

Predicting failure modes in creep and creep-fatigue crack growth using a random graingrain boundary idealised microstructure meshing system

Author(s): Lei Zhao, Lianyong Xu, Kamran Nikbin

An idealised microstructure mesh model combined with a novel creep and creep-fatigue damage accumulation model was employed to simulate crack growth behaviour under creep/fatigue conditions for a modified 9Cr-1Mo steel. The influence of microstructures on the crack growth behaviour was studied using a random grains separated by idealised grain boundaries. For accurately representing the damage accumulation in creep-fatigue regime, a non-linear damage model was employed. In this case, creep damage was determined by multiaxial ductility exhaustion approach and fatigue damage was reliant on maximum stress and plastic range ahead of crack tip per loading cycle. When creep dominated, cracks mainly propagated along grain boundaries in steady crack growth stage. As an exception, the crack growth in the tertiary crack growth stage gradually changed from intergranular to mixed model and finally transgranular fracture. In contrast, in creep-fatigue regime, the crack growth behaviour was greatly reliant on the dwell time. For short duration period, the crack mainly propagated in a transgranular manner. But as the dwell time increased to greater than 600s, the creep intergranular fracture dominated once again. Furthermore, the grain size gradient had little influence on the crack growth model and only affected the fracture life in creep and creep-fatigue regimes.

Science and Engineering A

Achievement of high yield strength and strain hardening rate by forming fine ferrite and dislocation substructures in duplex lightweight steel

Author(s): Hyejin Song, Jisung Yoo, Seok Su Sohn, Minseo Koo, Sunghak Lee

Lightweight steels containing a considerable content of Al show high specific strength and ductility, but there are some drawbacks such as low yield strength and stringer-type bands formed along the rolling direction. Here we design new duplex lightweight steel in order to complement the drawbacks, and achieve ultra-high yield strength (865MPa), good ductility (41%). Submicron ferrite mainly affects high yield-to-tensile ratio, and high strain hardening is attributed to Lomer-Cottrell lock and planar slip, and cell structure by further deformation in austenite. These results are expected to provide a desirable possibility for applications to reinforcement components requiring high yield-to-tensile ratio.

Science and Engineering A

Tailoring in-situ TiB2 particulates in aluminum matrix composites

Author(s): X. Liu, Y. Liu, D. Huang, Q. Han, X. Wang

TiB2 particulates were produced in-situ in aluminum matrices through chemical reactions of K2TiF6 and KBF4 salts with aluminum melts. The formation and growth of the in-situ diboride particulates are studied in the presence of different alloying elements, Mg, Zr, Ti and Si respectively, modifying the reaction chemistry. Microstructural analyses revealed alloying elements playing a significant role on the formation and growth of the in-situ particulates. The addition of 5 w.t. pct. in-situ TiB2 particulates, for example, resulted in a 14% increase in ultimate tensile strength for an as-cast Al-Si alloy. The strengthening effect is attributed to both the direct strengthening of TiB2 particulates as a reinforcement and their effects on modifying eutectic Si in the castings. The morphology and dispersion of the in-situ reinforcing particulates are discussed in terms of reaction chemistry, the heterogeneous nucleation of alpha aluminum grains and the modification of Si in eutectic Al-Si phase.

Science and Engineering A

Influences of warm rolling and annealing processes on microstructure and mechanical properties of three parent structures containing Mg-Li alloys

Author(s): Yan Tang, Qichi Le, Weitao Jia, Xuan Liu, Jianzhong Cui

The microstructural evolution and mechanical properties of Mg-xLi-3Al-2Zn-0.2Y (x=5, 8 and 11) alloys under the different rolling temperatures and annealing routes were investigated. The results showed that the Li addition transformed gradually the Mg crystal structure from hcp to bcc. With ascending rolling temperature, the Mg-11Li-3Al-2Zn-0.2Y (LAZ1132-0.2Y) alloy achieved the dynamic recrystallization behavior in advance, as compared with Mg-5Li-3Al-2Zn-0.2Y (LAZ532-0.2Y) and Mg-8Li-3Al-2Zn-0.2Y (LAZ832-0.2Y). Moreover, {0002} basal texture in LAZ532-0.2Y was weakened gradually by ascending rolling temperature. In LAZ832-0.2Y, {11–20} texture component relatively kept stable when the {0002} basal texture was weakened. When rolling at 503K, a new {211} <201> texture was observed in LAZ1132-0.2Y. In the annealing process, a kind of abnormal grain growth took place in LAZ1132-0.2Y. In addition, a strengthening behavior could be accomplished by strengthening β matrix (bcc).

Science and Engineering A

Effects of powder ratio and annealing temperature on mechanical properties of CuAl composites with core-shell structure

Author(s): Youwei Wang, Zhihao Zhang, Fei Xiao

Al-based composites reinforced by Cu particles have been fabricated by powder extrusion method. Effects of Cu content and annealing temperature on the mechanical properties of the composites were studied. The results show that the mechanical properties of the composites are affected by the powder ratio and the diffusion degree between Cu particles and Al matrix, the strengthening mechanism is the synergistic strengthening, which results from the interaction of load transfer, dislocation strengthening and solid solution strengthening. Annealing significantly affects the mechanical strength of the samples with Cu content less than 5wt%. After annealing at 510°C for 0.5h, the tensile strength of 2wt%Cu and 5wt%Cu samples increases by 41% and 76%, respectively, compared with that of as-extruded sample. In addition, the annealed samples hold good ductility and the elongations are greater than 15%. However, for samples with Cu content more than 10 wt%, the effect of annealing on the tensile strength seems to be weak, while the plasticity decreases rapidly with an increase of annealing temperature. Suitable powder ratio and annealing temperature can improve mechanical properties of the samples effectively. The tensile strength of 5wt%Cu sample annealed at 510°C for 0.5h is up to 262MPa, and its elongation is 15.3%.

Science and Engineering A

Melting and High $P\text{−}T$ Transitions of Hydrogen up to 300 GPa

Author(s): Chang-sheng Zha, Hanyu Liu, John S. Tse, and Russell J. Hemley

High $P\text{−}T$ Raman spectra of hydrogen in the vibron and lattice mode regions were measured up to 300 GPa and 900 K using externally heated diamond anvil cell techniques. A new melting line determined from the disappearance of lattice mode excitations was measured directly for the first time ab...

Physical Review Letters

Fixed Points of Wegner-Wilson Flows and Many-Body Localization

Author(s): David Pekker, Bryan K. Clark, Vadim Oganesyan, and Gil Refael

Many-body localization (MBL) is a phase of matter that is characterized by the absence of thermalization. Dynamical generation of a large number of local quantum numbers has been identified as one key characteristic of this phase, quite possibly the microscopic mechanism of breakdown of thermalizati...

Physical Review Letters

Two Universality Classes for the Many-Body Localization Transition

Author(s): Vedika Khemani, D. N. Sheng, and David A. Huse

We provide a systematic comparison of the many-body localization (MBL) transition in spin chains with nonrandom quasiperiodic versus random fields. We find evidence suggesting that these belong to two separate universality classes: the first dominated by “intrinsic” intrasample randomness, and the s...

Physical Review Letters

Pressure-induced magnetic collapse and metallization of $\mathrm{TlF}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_{2}$

Author(s): P. G. Naumov, K. Filsinger, S. I. Shylin, O. I. Barkalov, V. Ksenofontov, Y. Qi, T. Palasyuk, W. Schnelle, S. A. Medvedev, M. Greenblatt, and C. Felser

The crystal structure, magnetic ordering, and electrical resistivity of $\mathrm{TlF}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_{2}$ were studied at high pressures. Below $∼7\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, $\mathrm{TlF}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_{2}$ is an antiferromagneticall...

Physical Review B

Spin currents during ultrafast demagnetization of ferromagnetic bilayers

A Eschenlohr, L Persichetti, T Kachel, M Gabureac, P Gambardella and C Stamm

Ultrafast spin currents induced by femtosecond laser excitation of ferromagnetic metals have been found to contribute to sub-picosecond demagnetization, and to cause a transient enhancement of the magnetization of the bottom Fe layer in a Ni/Ru/Fe layered structure. We analyze the ultrafast magnetization dynamics in such layered structures by element- and femtosecond time-resolved x-ray magnetic circular dichroism, for different Ni and Fe layer thicknesses, Ru and Ta interlayers, and by varying the pump laser fluence. While we do not observe the transient enhancement of the magnetization in Ni/Ru/Fe discovered previously, we do find a reduced demagnetization of the Fe layer compared to a Ni/Ta/Fe layered structure. In the latter, the spin-scattering Ta layer suppresses spin currents from the Ni layer into Fe, consistent with previous results. Any spin current arriving in the lower Fe layer will counteract other, local demagnetization mechanisms such as phonon-mediated spin-flip ...

Journal of Physics Condensed Matter

Topological magnon bands and unconventional thermal Hall effect on the frustrated honeycomb and

S A Owerre

In the conventional ferromagnetic systems, topological magnon bands and thermal Hall effect are due to the Dzyaloshinskii–Moriya interaction (DMI). In principle, however, the DMI is either negligible or it is not allowed by symmetry in some quantum magnets. Therefore, we expect that topological magnon features will not be present in those systems. In addition, quantum magnets on the triangular-lattice are not expected to possess topological features as the DMI or spin-chirality cancels out due to equal and opposite contributions from adjacent triangles. Here, however, we predict that the isomorphic frustrated honeycomb-lattice and bilayer triangular-lattice antiferromagnetic system will exhibit topological magnon bands and topological thermal Hall effect in the absence of an intrinsic DMI. These unconventional topological magnon features are present as a result of magnetic-field-induced non-coplanar spin configurations with nonzero scalar spin chirality. The relevance of the res...

Journal of Physics Condensed Matter

The effect of disorder of small spheres on the photonic properties of the inverse binary NaCl-like

Harini Pattabhiraman and Marjolein Dijkstra

Inverse opal structures are experimentally realisable photonic band gap materials. They suffer from the drawback of possessing band gaps that are extremely susceptible to structural disorders. A binary colloidal NaCl lattice, which is also experimentally realisable, is a promising alternative to these opals. In this work, we systematically analyse the effect of structural disorder of the small spheres on the photonic properties of an inverse binary NaCl lattice with a size ratio of 0.30 between the small and large spheres. The types of structural disorders studied include the position of the small spheres in the octahedral void of the large spheres, polydispersity in size of the small spheres, and the fraction of small spheres in the crystal. We find a low susceptibility of the band gap of the inverse NaCl lattice to the disorder of the small spheres.

Journal of Physics Condensed Matter

Single crystal polarized neutron diffraction study of the magnetic structure of HoFeO 3

T Chatterji, A Stunault and P J Brown

Polarised neutron diffraction measurements have been made on HoFeO 3 single crystals magnetised in both the [0 0 1] and [1 0 0] directions ( Pbnm setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of {$H = 9$} T parallel to [0 0 1] at {$T = 70$} K and with the lower field {$H = 0.5$} T parallel to [1 0 0] at {$T = 5, 15, 25$} K. A Fourier projection of magnetization induced parallel to [0 0 1], made using the hk 0 reflections measured in 9 T, indicates that almost all of it is due to alignment of Ho mome...

Journal of Physics Condensed Matter

Quantum mechanical force fields for condensed phase molecular simulations

Timothy J Giese and Darrin M York

Molecular simulations are powerful tools for providing atomic-level details into complex chemical and physical processes that occur in the condensed phase. For strongly interacting systems where quantum many-body effects are known to play an important role, density-functional methods are often used to provide the model with the potential energy used to drive dynamics. These methods, however, suffer from two major drawbacks. First, they are often too computationally intensive to practically apply to large systems over long time scales, limiting their scope of application. Second, there remain challenges for these models to obtain the necessary level of accuracy for weak non-bonded interactions to obtain quantitative accuracy for a wide range of condensed phase properties. Quantum mechanical force fields (QMFFs) provide a potential solution to both of these limitations. In this review, we address recent advances in the development of QMFFs for condensed phase simulations. In parti...

Journal of Physics Condensed Matter

Inhomogeneous screening of gate electric field by interface states in graphene FETs

Anil Kumar Singh and Anjan Kumar Gupta

The electronic states at graphene-SiO 2 interface and their inhomogeneity is investigated using the back-gate-voltage dependence of local tunnel spectra acquired with a scanning tunneling microscope. The conductance spectra show two, or occasionally three, minima that evolve along the bias-voltage axis with the back gate voltage. This evolution is modeled using tip-gating and interface states. The energy dependent interface states’ density, {$D_{it}(E)$} , required to model the back-gate evolution of the minima, is found to have significant inhomogeneity in its energy-width. A broad {$D_{it}(E)$} leads to an effect similar to a reduction in the Fermi velocity while the narrow {$D_{it}(E)$}

Journal of Physics Condensed Matter

Wed Aug 16 2017

Surface sensitivity of four-probe STM resistivity measurements of bulk ZnO correlated to XPS

Alex M Lord, Jonathan E Evans, Chris J Barnett, Martin W Allen, Andrew R Barron and Steve P Wilks

Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventio...

Journal of Physics Condensed Matter

Interactions between low energy electrons and DNA: a perspective from first-principles simulations

Jorge Kohanoff, Maeve McAllister, Gareth A Tribello and Bin Gu

DNA damage caused by irradiation has been studied for many decades. Such studies allow us to better assess the dangers posed by radiation, and to increase the efficiency of the radiotherapies that are used to combat cancer. A full description of the irradiation process involves multiple size and time scales. It starts with the interaction of radiation—either photons or swift ions—and the biological medium, which causes electronic excitation and ionisation. The two main products of ionising radiation are thus electrons and radicals. Both of these species can cause damage to biological molecules, in particular DNA. In the long run, this molecular level damage can prevent cells from replicating and can hence lead to cell death. For a long time it was assumed that the main actors in the damage process were the radicals. However, experiments in a seminal paper by the group of Leon Sanche in 2000 showed that low-energy electrons (LEE), such as those generated when ionising biological ...

Journal of Physics Condensed Matter

Tue Aug 15 2017

Stress Transmission and Failure in Disordered Porous Media

Author(s): Hadrien Laubie, Farhang Radjai, Roland Pellenq, and Franz-Josef Ulm

Simulations of porous materials exhibit internal stress patterns like those in granular materials, despite the fact that these two systems are practically “negative images” of each other.

Physical Review Letters

Temperature-Independent Nuclear Quantum Effects on the Structure of Water

Author(s): Kyung Hwan Kim, Harshad Pathak, Alexander Späh, Fivos Perakis, Daniel Mariedahl, Jonas A. Sellberg, Tetsuo Katayama, Yoshihisa Harada, Hirohito Ogasawara, Lars G. M. Pettersson, and Anders Nilsson

Nuclear quantum effects (NQEs) have a significant influence on the hydrogen bonds in water and aqueous solutions and have thus been the topic of extensive studies. However, the microscopic origin and the corresponding temperature dependence of NQEs have been elusive and still remain the subject of o...

Physical Review Letters

<i>Ab initio</i>

Author(s): Dušan Plašienka, Roman Martoňák, and Marcus C. Newton

The temperature-induced structural and electronic transformation in ${\mathrm{VO}}_{2}$ between the monoclinic M1 and tetragonal rutile phases was studied by means of ab initio molecular dynamics, based on density functional theory with Hubbard correction (DFT+$U$). We compare the structure of both ...

Physical Review B

Phase coexistence and Landau expansion parameters for a $0.70\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{−}0.30\mathrm{PbTi}{\mathrm{O}}_{3}$ single crystal

Author(s): Hangbo Zhang, Xiaoyan Lu, Ruixue Wang, Chunying Wang, Limei Zheng, Zhen Liu, Chao Yang, Rui Zhang, Bin Yang, and Wenwu Cao

Multidomain relaxor-based ferroelectric single crystals $(1\text{−}x)\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{−}x\mathrm{PbTi}{\mathrm{O}}_{3}$ (PMN-PT) have extraordinarily large electromechanical properties, but the origin of their giant piezoelect...

Physical Review B

Interplay of phase sequence and electronic structure in the modulated martensites of ${\mathrm{Mn}}_{2}\mathrm{NiGa}$ from first-principles calculations

Author(s): Ashis Kundu, Markus E. Gruner, Mario Siewert, Alfred Hucht, Peter Entel, and Subhradip Ghosh

We investigate the relative stability, structural properties, and electronic structure of various modulated martensites of the magnetic shape memory alloy ${\mathrm{Mn}}_{2}\mathrm{NiGa}$ by means of density functional theory. We observe that the instability in the high-temperature cubic structure f...

Physical Review B

Energy landscape of ZnO clusters and low-density polymorphs

Author(s): Robabe Rasoulkhani, Hossein Tahmasbi, S. Alireza Ghasemi, Somayeh Faraji, Samare Rostami, and Maximilian Amsler

We report on an extensive study of ZnO materials with cagelike motifs in clusters and bulk phases through structural searches using the minima hopping method. A novel putative ground state was discovered for the ${(\mathrm{ZnO})}_{32}$ cluster with a tubelike structure, closely related to the previo...

Physical Review B

Sun Aug 13 2017

High tensile-strength and ductile titanium matrix composites strengthened by TiB nanowires

Author(s): Liqing Huang, Lihua Wang, Ma Qian, Jin Zou

A three-dimensional network-woven architecture made of TiB nanowires has been designed and realized in the matrix of a Ti6Al4V alloy. The architecturally nanostructured design was achieved by dispersing nanoparticles of B4C or B onto the surfaces of spherical Ti6Al4V powder particles via mechanical mixing and subsequent consolidation by spark plasma sintering. The as-sintered nanostructured Ti6Al4V-TiB composites demonstrated excellent tensile strengths and ductility that are required for critical applications. The in situ formed TiB nanowires with aspect ratios up to 300 contributed to the high tensile strengths while the architectural design of the TiB nanowires ensured the good tensile ductility.

Scripta Materialia

High-strain-rate superplasticity of fine-grained Mg–6Zn–0.5Zr alloy subjected to low-temperature indirect extrusion

Author(s): Beomcheol Kim, Jong Chan Kim, Sooseok Lee, Ki-Suk Lee, Jung Gu Lee, Sung Soo Park

A commercial Mg–6Zn–0.5Zr alloy was subjected to low-temperature indirect extrusion for grain refinement and the tensile properties of the extruded alloy at 250°C were investigated. After extrusion, the alloy showed finely recrystallized grains with an average size of 1.6μm. High-strain-rate superplasticity was observed in the fine-grained alloy, which exhibited a tensile elongation of up to 800% at a strain rate of 0.01s−1. Experimental results demonstrating the occurrence of grain boundary sliding during the high-strain-rate superplastic deformation are presented.

Scripta Materialia

Ab initio investigation on the slip preference of 〈a〉-dislocations in hexagonal metals and alloys

Author(s): G. Zhou, Y.J. Li, H. Wang, L.H. Ye, D.S. Xu, C.G. Meng, R. Yang

Slip preference is investigated via ab initio calculations in hexagonal metals, Be, Mg, Ti, Zn and Zr, as well as alloyed Mg and Ti. In general, the unstable stacking fault energy, γus , which is relevant with the Peierls barrier, plays the key role in determining slip preference. In the materials studied, slip is preferred on the plane that possesses a lower γus . Detailed studies indicate that γus , determined by the directionality of electron bonding, is primarily relevant to the c/a ratio. In alloyed Mg and Ti, alloying varies the c/a ratio, but due to different dependences of γus on c/a, certain alloying elements significantly affect 〈a〉-slip preference in Mg, while alloying exhibits negligible influence on 〈a〉-slip preference in hexagonal Ti.

Computational Materials Science

Effects of morphology, tension and vibration on wettability of graphene: A molecular dynamics study

Author(s): Chengpeng Huang, Fei Xu, Yu Sun

In the present study, we investigated the effects of morphology, tension and vibration on the wettability of graphene by performing classical molecular dynamics simulations. The contact angle of water droplet on ideal graphene is calculated to be 88.27° using the density profile method, which is in good agreement with the experimental data. The wrinkled morphology slightly decreases the wettability of graphene in most cases, and the droplet contact line always prefers to stay at the crest of wrinkles. The wrinkled morphology also brings extra barrier energy which leads to the pinning effect and discretized wetting effect with the droplet contact angle fluctuating up and down when the droplet volume increases. Tension and vibration strongly influence the wettability of graphene. The droplet contact angle linearly increases with increasing of biaxial tensile strain when the stain is lower than 10%, then remains at about 110°. The graphene becomes more and more hydrophobic with increasing of vibrational amplitude and decreasing of vibrational period. The contact angle of droplet on vibrational graphene and corresponding vibrational energy of graphene can be described by a logarithmic function.

Computational Materials Science

Development and application of Ni-Ti and Ni-Al-Ti 2NN-MEAM interatomic potentials for Ni-base superalloys

Author(s): Young-Kwang Kim, Hong-Kyu Kim, Woo-Sang Jung, Byeong-Joo Lee

Interatomic potentials for the Ni-Ti and Ni-Al-Ti systems have been developed based on the second nearest-neighbor modified embedded-atom method (2NN-MEAM) formalism. The Ni-Ti binary potential reproduces fundamental materials properties (structural, elastic, thermodynamic, and thermal stability) of alloy systems in reasonable agreement with experiments, first-principles calculations and thermodynamic calculations. Atomistic simulations using the Ni-Al-Ti ternary potential validate that the potential can be applied successfully to atomic-scale investigations to clarify the effects of titanium on important materials phenomena (site preference in γ′, γ-γ′ phase transition, and segregation on grain boundaries) in Ni-Al-Ti ternary superalloys.

Computational Materials Science

Elastic constants and mechanical properties of PEDOT from first principles calculations

Author(s): R.O. Agbaoye, P.O. Adebambo, J.O. Akinlami, T.A. Afolabi, Smagul Zh. Karazhanov, Davide Ceresoli, G.A. Adebayo

In this work, we report about the electronic and elastic properties of crystalline poly(3,4-ethylenedioxythiophene), known as PEDOT, in the crystalline form, studied in the framework of semilocal DFT, using the PBE and PBEsol exchange-correlation functional and PAW pseudopotentials. Contrary to previous molecular dynamics simulations, our calculations revealed that the most stable state structure of pristine PEDOT is monoclinic. We calculated the 13 independent elastic constants and the elastic compliance which enables us to establish other elastic properties of pristine PEDOT; the Pugh’s ratio and the Vicker’s hardness computed with PBE and PBEsol are in good agreement with each other. Finally, we compute the directional elastic modulii and found remarkable differences between different DFT functionals.

Computational Materials Science

First principles determination of static, dynamic and electronic properties of liquid Ti near melting

Author(s): B.G. del Rio, O. Rodriguez, L.E. González, D.J. González

Several static and dynamic properties of bulk liquid titanium at a thermodynamic state near its melting point have been evaluated by ab initio molecular dynamics simulations. The calculated static structure shows very good agreement with the available experimental data, including a marked shoulder on the second peak in the structure factor, which underlines a notable local icosahedral short-range order in the liquid. The dynamical structure reveals propagating density fluctuations whose dispersion relation and damping closely follow the inelastic X-ray scattering measurements. The dynamical processes behind the S ( q , ω ) have been analyzed in terms of a model with two decay channels (a fast and a slow) associated to the relaxations of the collective excitations. Finally, results are also reported for several transport coefficients.

Computational Materials Science

Enhanced plasticity in a Ti-Ni-Nb-Zr shape memory bulk metallic glass composite with high Nb addition

Author(s): Shuangshuang Chen, Jiajia Wu, Jianxin Tu, Xiaohua Li, Jizhao Zou, Qiang Hu, Xierong Zeng

In this work, we reported a new Ti-Ni-Nb-Zr bulk metallic glass composite with high Nb addition, which is composed of amorphous, β-Nb and shape memory B2-Ti(Ni, Nb) phases. The volume fraction of the amorphous phase in the rod specimen with a diameter of 1.5mm was determined to be ~15vol%. The room-temperature mechanical test suggested that the composite exhibited a significant engineering plastic strain of 22.1% and a maximum compressive strength of ~2400MPa, together with a dominating strain-hardening behavior during the whole deformation process. The large plasticity of the material was contributed the pile-ups of dislocation and severe lattice distortion, as well as generation of abundant shear bands during deformation. This result provides a promising approach to improve the mechanical properties of metallic glasses through compositional and structural design.

Science and Engineering A

Hydrogen embrittlement behavior of high strength rail steels: A comparison between pearlitic and bainitic microstructures

Author(s): Weijun Hui, Zhibao Xu, Yongjian Zhang, Xiaoli Zhao, Chengwei Shao, Yuqing Weng

The present study was attempted to evaluate the hydrogen embrittlement (HE) behavior of two high strength rail steels with pearlitic and bainitic microstructures by using slow strain rate test (SSRT) with notched round bar specimens. The results show that the bainitic rail steel with a mixed microstructure of bainite, tempered martensite and M/A constituents exhibits superior mechanical properties in comparison with the pearlitic rail steel. The results of SSRT revealed that the bainitic rail steel is more susceptible to HE than the pearlitic rail steel, which is ascribed mainly to the higher strength level and the microstructural characteristics of the bainitic rail steel. The attempt of re-tempering treatment of the as-received bainitic rail steel exhibits no notable improvement in the resistance to HE at no expense of strength. It is thus suggested that further efforts concerning obtaining low susceptibility to HE besides obtaining excellent combination of strength and toughness should be conducted to guarantee the safety service of the bainitic steel rails.

Science and Engineering A

Isotropy in large-size Al2O3Y3Al5O12 eutectic ceramic grown by Horizontal Directional Solidification method

Author(s): Ying Nie, Jiecai Han, Yang Liu, Mingfu Zhang, Jin Zhang

The eutectic Al2O3/Y3Al5O12 with the dimension of 150mm×90mm×20mm was grown in vacuum by Horizontal Directional Solidification (HDS) method. The thermal and mechanical properties were investigated from room temperature to 1273K, on specimens whose longitudinal directions were parallel (L-specimen) and perpendicular (T-specimen) to the solidification direction. Even though new preferred orientations of (1 1 2 ¯ 3) Al2O3 and (5 3 2) Y3Al5O12 emerged in L-specimen, neither in T-specimen, isotropic properties of residual stresses in Al2O3 phase, thermal expansion coefficients, flexural and compressive strengths appeared. For HDS-Al2O3/Y3Al5O12, the reasons of improved Vickers hardness were discussed.

Science and Engineering A

An evaluation of the hexagonal close-packed to face-centered cubic phase transformation in a Ti-6Al-4V alloy during high-pressure torsion

Author(s): Hamed Shahmir, Terence G. Langdon

A Ti-6Al-4V alloy was used to examine the effect of martensitic (αʹ) or lamellar (α + β) microstructures on grain refinement and the hcp to fcc phase transformation during HPT processing. There was significant grain refinement with grain sizes of ~ 30 and ~ 40nm in the αʹ and α + β microstructures, respectively, and with the occurrence of an allotropic hcp to fcc phase transformation in the αʹ sample after HPT. A high volume fraction of boundaries, together with a substructure containing initial defects of the martensitic sample, promotes the formation of the fcc phase during the HPT processing.

Science and Engineering A

Change in microstructure of selectively laser melted AlSi10Mg alloy with heat treatments

Author(s): Naoki Takata, Hirohisa Kodaira, Keito Sekizawa, Asuka Suzuki, Makoto Kobashi

In the present study, we examined changes in the microstructure and mechanical properties of AlSi10Mg alloy, initially fabricated using selective laser melting (SLM) combined with a powder-bed system, by applying heat treatments at temperatures of either 300 or 530°C. The as-fabricated samples exhibited a characteristic microstructural morphology and {001} texture. Melt pools corresponding to the locally melted and rapidly solidified regions were found to be composed of several columnar α-Al grains surrounded by fine eutectic Si particles. A fine dislocation substructure consisting of low-angle boundaries is present within the columnar α-Al grains. At elevated temperatures, fine Si phase precipitates within the columnar α-Al phase and coarsening of the eutectic Si particles occurs. These fine Si particles inhibit grain growth in the α-Al matrix, resulting in the microstructural morphology and [001] texture observed in the heat-treated samples. The dislocation substructure disappears in the columnar α-Al grains. Furthermore, the formation of a stable intermetallic phase occurs, reaching microstructural equilibrium after long-term exposure. The as-fabricated specimen exhibits a high tensile strength of approximately 480MPa. The strength is independent of the tensile direction, that is, normal and parallel to the building direction. In contrast, the tensile ductility is found to be direction-dependent, and is therefore responsible for a fracture preferentially occurring at a melt pool boundary. The direction-dependence of the tensile ductility was not found in the specimen that had been heat-treated at 530°C. The present results provide new insights into the control of the direction-dependence of the tensile properties of AlSi10Mg alloys fabricated by SLM.

Science and Engineering A

Sat Aug 12 2017

Molecular dynamics simulation of the effects of affinity of functional groups and particle-size on the behavior of a graphene sheet in nanofluid

Author(s): JinHyeok Cha, Woomin Kyoung

The practical application of thermally enhanced nanofluids in automotive industries for higher energy efficiency requires that nanoparticles be buoyant, leading to Brownian motion in the base fluid without aggregation. Numerous studies have reported the long-term stability of dispersions resulting from steric hindrance and an affinity for the surrounding base fluid via modification of nanoparticles with functional groups. In this study, we performed molecular dynamics simulations of nanofluids containing a single graphene sheet with various functional groups to investigate the influence of affinity, as well as particle-size, on the behavior of the graphene sheet. Using the concept of the speed of nanoparticle, we quantitatively evaluated the dependence of behavior on affinity to investigate whether having more functional groups with more attractive interactions has an impact on the stable dispersion in nanofluid. In addition, the simulation results for the particle-size effect revealed that the larger nanoparticles produced more stable dispersion in nanofluid. We concluded that the behavior of the graphene sheet depends on a combination of two factors: different charge assigned to atoms due to a nitrogen atom, and the difference in mass, which influences the speed of the nanoparticles.

Computational Materials Science

Microstructure development of ultra fine grained Mg-22wt%Gd alloy prepared by high pressure torsion

Author(s): J. Čížek, P. Hruška, T. Vlasák, M. Vlček, M. Janeček, P. Minárik, T. Krajňák, M. Šlapáková, M. Dopita, R. Kužel, T. Kmječ, J.G. Kim, H.-S. Kim

A hardenable lightweight Mg-22wt%Gd alloy with ultra fine grained (UFG) structure was prepared by high pressure torsion (HPT) at ambient temperature. The development of microstructure during HPT processing was investigated. A homogeneous UFG structure with grain size of 300nm was achieved after 15 HPT revolutions. The UFG alloy exhibits enhanced strength due to work strengthening by tangled dislocations forming a dense forest throughout grains. Dislocation density in the sample was determined by positron annihilation spectroscopy (PAS) and X-ray line profile analysis (XLPA). It was found that there is an additional source of X-ray profile broadening in addition to small crystallites and micro-strains caused by dislocations. The additional micro-strain component was attributed to lattice modulation by Gd-rich nano-wires formed by agglomeration of Gd solutes and to strains arising from boundaries of crystallite domains and inter-domain interactions. Analysis of the influence of the crystallite size on the strength of UFG Mg-22wt%Gd alloy revealed a breakdown in the Hall-Petch relationship when the crystallite size decreased below a critical value of ≈30nm.

Science and Engineering A

Determination of the Density and Temperature Dependence of the Shear Viscosity of a Unitary Fermi Gas Based on Hydrodynamic Flow

Author(s): Marcus Bluhm, Jiaxun Hou, and Thomas Schäfer

We determine the shear viscosity of the ultracold Fermi gas at unitarity in the normal phase using hydrodynamic expansion data. The analysis is based on a generalized fluid dynamic framework which ensures a smooth transition between the fluid dynamic core of the cloud and the ballistic corona. We us...

Physical Review Letters

High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures

Author(s): Xiao-Di Liu, Ross T. Howie, Hui-Chao Zhang, Xiao-Jia Chen, and Eugene Gregoryanz

In situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium demonstrate the presence of a novel phase, phase ${\mathrm{II}}^{′}$, unique to deuterium and distinct from the known phase II. Phase ${\mathrm{II}}^{′}$ of ${\mathrm{D}}_{2}$ is not observed in hydrogen, making...

Physical Review Letters

Successive Dimensional Transition in ${(\mathrm{TMTTF})}_{2}{\mathrm{PF}}_{6}$ Revealed by Synchrotron X-ray Diffraction

Author(s): Shunsuke Kitou, Tatsuya Fujii, Tadashi Kawamoto, Naoyuki Katayama, Sachiko Maki, Eiji Nishibori, Kunihisa Sugimoto, Masaki Takata, Toshikazu Nakamura, and Hiroshi Sawa

A quasi-one-dimensional organic charge-transfer salt ${(\mathrm{TMTTF})}_{2}{\mathrm{PF}}_{6}$ undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a “structureless transition,” and these detailed structures were unknown for many years. With synchro...

Physical Review Letters

Stability of Electrodeposition at Solid-Solid Interfaces and Implications for Metal Anodes

Author(s): Zeeshan Ahmad and Venkatasubramanian Viswanathan

We generalize the conditions for stable electrodeposition at isotropic solid-solid interfaces using a kinetic model which incorporates the effects of stresses and surface tension at the interface. We develop a stability diagram that shows two regimes of stability: a previously known pressure-driven ...

Physical Review Letters

Phase transition of tetragonal copper sulfide ${\mathrm{Cu}}_{2}\mathrm{S}$ at low temperatures

Author(s): D. Zimmer, J. Ruiz-Fuertes, L. Bayarjargal, E. Haussühl, B. Winkler, J. Zhang, C. Q. Jin, V. Milman, E. Alig, and L. Fink

The low-temperature behavior of tetragonal copper sulfide, ${\mathrm{Cu}}_{2}\mathrm{S}$, was investigated by powder and single-crystal x-ray diffraction, calorimetry, electrical resistance measurements, and ambient temperature optical absorption spectroscopy. The experiments were complemented by de...

Physical Review B

Model for topological phononics and phonon diode

Author(s): Yizhou Liu, Yong Xu, Shou-Cheng Zhang, and Wenhui Duan

The quantum anomalous Hall effect, an exotic topological state first theoretically predicted by Haldane and recently experimentally observed, has attracted enormous interest for low-power-consumption electronics. In this work, we derived a Schrödinger-like equation of phonons, where topology-related...

Physical Review B

Thermally assisted ordering in Mott insulators

Author(s): Hunter Sims, Eva Pavarini, and Erik Koch

Landau theory describes phase transitions as the competition between energy and entropy: The ordered phase has lower energy, while the disordered phase has larger entropy. When heating the system, ordering is reduced entropically until it vanishes at the critical temperature. This picture implicitly...

Physical Review B

Effect of Pt substitution on the magnetocrystalline anisotropy of ${\mathrm{Ni}}_{2}\text{MnGa}$: A competition between chemistry and elasticity

Author(s): L. Caron, B. Dutta, P. Devi, M. Ghorbani Zavareh, T. Hickel, R. Cabassi, F. Bolzoni, S. Fabbrici, F. Albertini, C. Felser, and Sanjay Singh

The magnetocrystalline anisotropy (MAE) of ${\mathrm{Ni}}_{2−x}{\mathrm{Pt}}_{x}\mathrm{MnGa}(0≤x≤0.25)$ alloys are investigated using the singular point detection technique and density functional theory. A slight reduction in MAE as compared to that of ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ is observed d...

Physical Review B

Temperature of emergence of a transformational process zone at a crack tip

Author(s): A. Boulbitch and A. L. Korzhenevskii

We analyze the emergence of a process zone of improper ferroelastic class. We derive expressions for the zone emergence temperature and the size of the incipient zone in terms of experimentally accessible physical parameters. Our numerical estimates suggest that the region of existence of the proces...

Physical Review B

Thermoelectric and structural correlations in $(\mathrm{S}{\mathrm{r}}_{1−x−y}\mathrm{C}{\mathrm{a}}_{x}\mathrm{N}{\mathrm{d}}_{y})\mathrm{Ti}{\mathrm{O}}_{3}$ perovskites

Author(s): H. Somaily, S. Kolesnik, B. Dabrowski, and O. Chmaissem

Structural and thermoelectric properties are reported for a specially designed class of $A$-site substituted perovskite titanates, $(\mathrm{S}{\mathrm{r}}_{1−x−y}\mathrm{C}{\mathrm{a}}_{x}\mathrm{N}{\mathrm{d}}_{y})\mathrm{Ti}{\mathrm{O}}_{3}$. Two series synthesized with various $A$-site Sr-rich o...

Physical Review B

Near-field limitations of Fresnel-regime coherent diffraction imaging

Author(s): Benjamin A. Pound, John L. Barber, Kimberly Nguyen, Matthew C. Tyson, and Richard L. Sandberg

Coherent diffraction imaging (CDI) is a rapidly developing form of imaging that offers the potential of wavelength-limited resolution without image-forming lenses. In CDI, the intensity of the diffraction pattern is measured directly by the detector, and various iterative phase retrieval algorithms ...

Physical Review B

Robustness of topological states with respect to lattice instability in the nonsymmorphic topological insulator KHgSb

Author(s): D. Chen, T.-T. Zhang, C.-J. Yi, Z.-D. Song, W.-L. Zhang, T. Zhang, Y.-G. Shi, H.-M. Weng, Z. Fang, P. Richard, and H. Ding

We report a polarized Raman scattering study of nonsymmorphic topological insulator KHgSb with hourglasslike electronic dispersion. Supported by theoretical calculations, we show that the lattice of the previously assigned space group $P{6}_{3}/mmc$ (No. 194) is unstable in KHgSb. While we observe o...

Physical Review B

Lattice dynamics of Dirac node-line semimetal ZrSiS

Author(s): Wei Zhou, Heng Gao, Junran Zhang, Ruiyang Fang, Hao Song, Tao Hu, Alessandro Stroppa, Ling Li, Xuefeng Wang, Shuangchen Ruan, and Wei Ren

We report a comprehensive study of lattice dynamics of the Dirac node-line semimetal ZrSiS single crystal by Raman spectroscopy and first-principles calculations. The weak covalent bonding between ZrSiS layers is confirmed by the absence of low-frequency shear or breathing Raman modes down to $15\ph...

Physical Review B

Structural characterization of water-metal interfaces

Author(s): Kevin Ryczko and Isaac Tamblyn

We analyze and compare the structural, dynamical, and electronic properties of liquid water next to prototypical metals including Pt, graphite, and graphene. Our results are built on Born-Oppenheimer molecular dynamics (BOMD) generated using density functional theory (DFT) which explicitly include v...

Physical Review B

Semirelativity in semiconductors: a review

Wlodek Zawadzki

An analogy between behavior of electrons in narrow-gap semiconductors (NGS) and relativistic electrons in vacuum is reviewed. Energy band structures {$\varepsilon \left(\mathbf{k}\right)$} are considered for various NGS materials and their correspondence to the energy-momentum relation in special relativity is emphasized. It is indicated that special relativity for vacuum is analogous to a two-band {$\mathbf{k}\centerdot \mathbf{p}$} description for NGS. The maximum electron velocity in NGS is {$u\simeq 1\times {{10}^{8}}~\text{cm}~{{\text{s}}^{-1}}$} , which corresponds to the light velocity in vacuum. An effective mass of charge carriers in semiconductors is introduced, relating t...

Journal of Physics Condensed Matter

Nano- and microparticles at fluid and biological interfaces

S Dasgupta, T Auth and G Gompper

Systems with interfaces are abundant in both technological applications and biology. While a fluid interface separates two fluids, membranes separate the inside of vesicles from the outside, the interior of biological cells from the environment, and compartmentalize cells into organelles. The physical properties of interfaces are characterized by interface tension, those of membranes are characterized by bending and stretching elasticity. Amphiphilic molecules like surfactants that are added to a system with two immiscible fluids decrease the interface tension and induce a bending rigidity. Lipid bilayer membranes of vesicles can be stretched or compressed by osmotic pressure; in biological cells, also the presence of a cytoskeleton can induce membrane tension. If the thickness of the interface or the membrane is small compared with its lateral extension, both can be described using two-dimensional mathematical surfaces embedded in three-dimensional space. We review recent work ...

Journal of Physics Condensed Matter

Fri Aug 11 2017

Crystallographic study of plasticity and grain boundary separation in FeCo alloy using small single- and bi-crystalline specimens

Author(s): Daichi Kishi, Tsuyoshi Mayama, Yoji Mine, Kazuki Takashima

To study the relationship between the plasticity and intergranular fracture of FeCo intermetallic alloys, small, single- and bi-crystalline specimens were subjected to micro-tensile testing and crystal plasticity finite element analyses. The single-crystalline specimens exhibited a high strength and a large strain-to-failure compared with the bi-crystalline and polycrystalline specimens. The critical resolved shear stress for the {1 1 ¯ 0} 〈111〉 slip was ~100MPa. For the bi-crystalline specimen, the intergranular fracture was determined by stress concentration due to the difference in the strain between the deformed and undeformed crystals.

Scripta Materialia

Fabrication and magnetization reversal of L10 FeMnPt dots surrounded by paramagnetic A1 phase formed by ion irradiation

Author(s): Takashi Hasegawa, Kaori Sasaki, Craig W. Barton, Thomas Thomson

A nanopatterning method involving a ferromagnetic (FM)–paramagnetic (PM) phase transformation caused by ion irradiation has been developed. A (Fe0.56Mn0.44)50Pt50 film with the ordered L10 structure and uniaxial magnetocrystalline anisotropy constant (K u) of 2.1×106 Jm3 was fabricated. Mn ion irradiation produced a smooth FM dot array with dot diameters of 100nm surrounded by the PM phase, which was obtained from the induced FM–PM phase transition caused by the structural transformation from the L10 structure to the disordered A1 structure and a slight increase in the Mn content. The magnetization reversal process was studied by investigating the angle dependence of the dot reversal field.

Scripta Materialia

Remarkable strength of CoCrFeNi high-entropy alloy wires at cryogenic and elevated temperatures

Author(s): Wenyi Huo, Feng Fang, Hui Zhou, Zonghan Xie, Jianku Shang, Jianqing Jiang

CoCrFeNi high-entropy alloy wires containing nano-sized twins were produced by heavy cold-drawing process. The alloy wires exhibit high tensile yield strength of 1.2GPa, as well as a considerably high percentage elongation (i.e., ductility) of 13.6% at 223K. Such cryogenic and room temperature properties were found to result from the blockage of dislocation glide by primary and secondary nano-scale twin boundaries and, to less extent, by other dislocations. The high level of strength remained with the increase of temperature up to 923K. The deformation mechanism of the cold-drawn wires was observed to be governed by dislocation slip and dynamic recovery at elevated temperatures.

Scripta Materialia

Martensite formation in Fe-C alloys at cryogenic temperatures

Author(s): M. Villa, M.F. Hansen, M.A.J. Somers

Magnetometry was applied to quantify the fraction of austenite retained in Fe-C alloys subjected to various treatments. These treatments consisted of: (i) water quenching; (ii) water quenching followed by immersion in boiling nitrogen and again in water; (iii) as for (ii) but re-heating from 77K at a rate of 0.0083Ks1; (iv) as for (iii) but (re-)heating at 0.167Ks1 interrupted by an isothermal step. Data was coupled with hardness measurements and demonstrates that the re-heating conditions from 77K significantly influence the fraction of austenite retained at the end of the thermal cycle.

Scripta Materialia

From atomistic structure to thermodynamics and mechanical properties of epoxyclay nanocomposites: Investigation by molecular dynamics simulations

Author(s): Van Son Vo, Vu-Hieu Nguyen, Samia Mahouche-Chergui, Benjamin Carbonnier, Devis Di Tommaso, Salah Naili

Polymer/clay nanocomposites (PCNs) are multi-functional materials that have superior mechanical and thermal properties than polymer-based materials while maintaining their characteristic properties of lightweight and optical clarity. These materials are obtained by adding small amounts of clay nanofillers to a polymer matrix. In this work, we proposed a molecular model to investigate the morphology and thermomechanical properties of thermosetting clay nanocomposites. The epoxy matrix was composed of several representative cross-linked epoxy units, and different structures of PCNs, which correspond to intercalated or exfoliated status, were considered by varying the amount of polymer phase in the interlayer space. Molecular dynamics simulations of different nanocomposite structures were used to provide atomistic insights into the arrangement of the constitutive components of PCNs, the molecular interactions occurring in the interphase zone, and the influence of silicate layer on the thermodynamic and elastic properties of these multi-functional materials.

Computational Materials Science

Compositional and microstructural optimization and mechanical-property enhancement of cast Ti alloys based on Ti-6Al-4V alloy

Author(s): J.H. Luan, Z.B. Jiao, W.H. Liu, Z.P. Lu, W.X. Zhao, C.T. Liu

The effects of alloying additions on the microstructure and mechanical properties of cast Ti alloys based on Ti-6Al-4V were systematically investigated through a combination of thermodynamic calculations and experimental verifications. New cast titanium alloys with significantly improved properties were developed through combining thermodynamic calculations and physical metallurgy principles, which achieve an optimum combination of high strength and high ductility at both room and elevated temperatures. Our analysis indicates that the improved strength is due mainly to the α colony and lath size refinement induced by both macro- and micro-alloying of Cr, Fe, B and C elements, whereas the mechanisms for such property optimization include: (1) an increased volume fraction of ductile β phase in the cast state, (2) a refined α colony and lath size by the Cr, Fe, B and C alloying additions, and (3) an enhanced grain-boundary cohesion and refined grain size of prior-β grains induced by proper amount of B additions. The newly developed cast Ti alloys with substantially enhanced mechanical properties for engineering applications.

Science and Engineering A

Twinning induced plasticity in austenitic stainless steel 316L made by additive manufacturing

Author(s): M.S. Pham, B. Dovgyy, P.A. Hooper

Additively manufactured (AM) 316L steel exhibits extraordinary high yield strength, and surprisingly good ductility despite the high level of porosity in the material. This detailed study sheds light on the origins of the observed high yield strength and good ductility. The extremely fine cells which are formed because of rapid cooling and dense dislocations are responsible for the macroscopically high yield strength of the AM 316L (almost double of that seen in annealed 316L steel). Most interestingly, twinning is dominant in deformed samples of the AM316. It is believed that twinning-induced plasticity (TWIP) behaviour to be responsible for the excellent ductility of the steel despite the high level of porosity. The dominant twinning activity is attributed to Nitrogen gas used in 3D printing. Nitrogen can lower the stacking fault energy of the steel, leading to the disassociation of dislocations, promoting the deformation twinning. Twinning induces large plasticity during deformation that can compensate the negative effect of porosity in AM steel. However, twinning does not induce significant hardening because (1) the porosity causes a negative effect on hardening and (2) twinning spacing is still larger than extremely fine solidification cells.

Science and Engineering A

Strengthening of A2024 alloy by high-pressure torsion and subsequent aging

Author(s): Intan Fadhlina Mohamed, Takahiro Masuda, Seungwon Lee, Kaveh Edalati, Zenji Horita, Shoichi Hirosawa, Kenji Matsuda, Daisuke Terada, Mohd Zaidi Omar

An age-hardenable A2024 alloy is processed by high-pressure torsion (HPT) for grain refinement and further aged for fine precipitation. The HPT is conducted under an applied pressure of 6GPa for 0.75, 1 and 5 turns with a rotation speed of 1rpm at room temperature and this results in a significant grain size reduction to a grain size of ~ 240 ± 80nm. The hardness sharply increases with imposing strain at an early stage but level off after 5 turns. Further aging at temperatures of 373K and 423K leads to extra hardening above the elevated hardness of the HPT-processed condition. Components contributing to the strengthening were evaluated in terms of grain refinement and fine precipitation including the contributions from dislocation accumulation and solid solution. In this study, a conclusion is reached such that simultaneous strengthening due to grain refinement and fine precipitation is achieved by application of HPT processing and subsequent aging.

Science and Engineering A

Effect of La inoculation on composition, content, granularity and mechanical properties of in-situ Al-30wt%Mg2Si Composite

Author(s): Yuyan Ren, Tongyu Liu, Yingmin Li, Hao Hu

In this paper, the phase composition and content of Al-30wt%Mg2Si composite prepared by in-situ process have been investigated; and the effect of different addition of La inoculation on microstructure, granularity and mechanical properties of Al-30%Mg2Si composite have been studied. Scanning electron microscope (SEM) and X-ray diffraction (XRD) images show that the phase composition and content of Al-30wt%Mg2Si composite agree with pseudo-binary Al-Mg2Si phase diagram. The effect of La inoculation on microstructure indicates that La addition can change the morphology of primary Mg2Si particles and refine the particle size of primary Mg2Si. When the La addition reaches to 0.8wt%, the refinement effect of Al-30%Mg2Si composite is the best. Similarly, the results reveal that La addition can effectively improve the ultimate tensile stress (UTS), breaking elongation (BE) and Vickers microhardness (VM) values. When La addition is over 0.8wt%, primary Mg2Si particles become coarsening and mechanical properties decline. The crystal size was characterized by XRD and deduced by Cauchy and Gaussian approximation. The relationship between crystal size and UTS or VM can be described by Hall-Petch relation. The linear fitting degree of UTS is better.

Science and Engineering A

Microstructure evolution of a Cu-15Ni-8Sn-0.8Nb alloy during prior deformation and aging treatment

Author(s): Yi Ouyang, Xueping Gan, Zhou Li, Kechao Zhou, Shizhong Zhang, Yexin Jiang, Xianwei Zhang

The microstructure evolution of Cu-15Ni-8Sn-0.8Nb alloy during prior cold deformation by rotary swaging (RS) and subsequent aging treatment was investigated. Electron back scattered diffraction (EBSD) results indicated that the deformed microstructure mainly consisted of refined and elongated grains. Discontinuous precipitates appeared at previous grain boundaries (PGBs) and shear bands after deformation of 45 pct reduction. Transmission electron microscope (TEM) studies revealed that the transformation kinetics of both DO22 ordered phases and discontinuous precipitates were significantly accelerated by prior deformation, and the transformation from DO22 to L12 ordering was suppressed to a certain degree. The maximum yield strength of the specimens with 45 pct reduction reached to about 1230MPa after aging treatment, but the corresponding ductility was poor due to the rapid growth of discontinuous precipitates.

Science and Engineering A

Stress softening and hardening during compression and tensile consecutive cyclic loading of Mn18Cr18N austenitic stainless steel

Author(s): Fei Li, Huayu Zhang, Wenwu He, Xiaodong Zhao, Huiqin Chen

The compression and tensile consecutive deformation behavior and microstructure evolution of Mn18Cr18N steel were investigated by cyclic loading tests at room temperature and strain amplitude from 0.005 to 0.15. The results indicated that the cyclic loading stress-strain curves of the steel show plastic deformation characterized by almost the same working hardening rates without yield plateaus at various strain amplitudes. At the lower strain amplitudes from 0.005 to 0.01, the stress amplitudes and subsequent yield surface radius decreased with cycles, which suggest that stress softening occurs during cyclic loading. At the strain amplitudes from 0.02 to 0.15, the stress amplitudes and subsequent yield surface radius increased with cycles, which imply that stress hardening occurs during cyclic loading. With the increasing of strain amplitudes, the cyclic hardening coefficients monotonically increased, while the cyclic softening coefficients rapidly decreased. The maximum flow stress increased up to 1549.6MPa after three cycles at the strain amplitude of 0.15, which increased by 395.4MPa compared to the maximum uniaxial tensile flow stress. It was also demonstrated that the cyclic loading stress-strain behaviors (cyclic softening and hardening) depend on internal stress component rather than effective stress component. TEM observation shows that planar slip with parallel substructures along one direction at high number cycles evolved from various dislocation configurations at low number cycles occurs at the lower strain amplitude of 0.005; double planar slip with grid substructures along two directions at high number cycles evolved from parallel substructures along one direction at low number cycles occurs at the higher strain amplitude of 0.04. Internal stress could be released by dislocation rearrangement and activation of cross slips during cyclic loading process, which induce stress softening at lower strain amplitudes; while slipping and twining alone two intersectional directions caused stress hardening at higher strain amplitudes.

Science and Engineering A

Effect of the cooling rate on the mechanical properties of Ti-Ni-Cu-Zr-based crystalglassy alloys

Author(s): J. Jiang, S. Ketov, H. Kato, D.V. Louzguine-Luzgin

Ti-Ni-Cu-Zr-based crystal/glassy dual-phase alloy samples with different sizes and good mechanical properties were manufactured. The structure of these samples was examined via X-ray Diffraction, Scanning Electron Microscopy, and Transmission Electron Microscopy. The developed dual-phase structure alloys combine the high strength of glassy alloys and plasticity of crystalline alloys. Plastic deformation was enhanced by this dual-phase structure. Mechanical characterization of the alloys revealed three deformation stages, namely a martensitic transformation, dislocation slip in a crystalline phase, and shear deformation of the glassy matrix. Two types of effects involving the martensitic transformation were observed: superelastic behavior and the transformation induced plasticity (TRIP) effect, which provide an additional deformation mechanism and lead to a significant increase in the plasticity of these dual-phase samples. The results indicate that the fraction of glassy phase occurring in large rods formed at lower cooling rates is lower than that occurring in smaller rods; therefore, the deformation-induced martensitic transformation of the crystalline cP2 phase in the large samples occurs at lower stresses.

Science and Engineering A

Spark plasma sintering and spark plasma upsetting of an Al-Zn-Mg-Cu alloy

Author(s): Mehmet Masum Tünçay, José Alberto Muñiz-Lerma, Donald Paul Bishop, Mathieu Brochu

Al-Zn-Mg-Cu alloy powder Alumix 431D was sintered at 400°C by spark plasma sintering (SPS) and upset forging was applied to the sintered sample through SPS. Densities of 99.1 ± 0.3% and 99.8 ± 0.1% of theoretical were obtained for the sintered and forged samples, respectively. T6 temper was carried out on the samples and microstructure analysis and mechanical properties before and after heat treatment were evaluated. Microhardness of 173 ± 3 and 172 ± 3 HV were attained in the T6 temper of as-sintered and forged samples, respectively. The flexural strength and strain values were significantly improved after the forging process, which can be mainly attributed to the better particle bonding in addition to the occurrence of some recrystallization. Significant loss in the ductility was observed after the T6 temper.

Science and Engineering A

Dynamic strain aging in DP steels at forming relevant strain rates and temperatures

Author(s): Berkay Bayramin, Caner Şimşir, Mert Efe

Mechanical testing of dual phase (DP) steels at low strain rates (10−3 s−1) have shown that they are susceptible to dynamic strain aging (DSA) between 100°C–400°C. During industrial forming processes at intermediate strain rates (1–102 s−1), the local temperatures may rise to the DSA range due to deformation heating which may disturb the exceptional formability of these steels. In this study, two grades of DP steel (DP590 and DP800) are tested at thermomechanical conditions relevant to forming and the effects of DSA on the formability are established. Test results show that the DSA controls the deformation between 200°C–300°C through serrations in the stress-strain curves of both grades. With increasing strain rates (up to 1s−1) and temperatures, DSA intensifies and results in severe drops in uniform and total ductility with negative strain rate sensitivity, indicating poor formability at these conditions. A detailed analysis of the serrations coupled with dislocation density measurements by x-ray analysis suggests that the serrations can be linked to a periodic microstructural feature.

Science and Engineering A

On the rule of mixtures for bimetal composites

Author(s): Bo Feng, Yunchang Xin, Zheng Sun, Huihui Yu, Juan Wang, Qing Liu

In the present study, mechanical behavior of bimetal composite rods was systematically studied by tensile and compressive tests on three types of bimetal rods, Al 6082/Al 7050, Mg AZ31/Mg ZK60 and Mg AZ31/Al 7050 and compared with the rule of mixtures (ROM) predictions. The influence of the interface characteristic (i.e. the diffusion layer) and the strain hardening behavior of two components on the ROM was addressed. Our results show that when the two components have a similar work hardening behavior, the ROM for flow curves is applicable irrespective of the interface characteristic. When there is a large mismatch in strain hardening, the efficiency of ROM is highly dependent on the thickness of diffusion layer. With a diffusion layer about 10µm thick, the ROM calculation is in good agreement with the experimental one. In contrast, the experimental flow curve would largely deviate from the ROM calculation with a diffusion layer about 300µm thick. The corresponding causes are related to cracks on the interface and load transfer. It is also found that the ROM for yield strength can work well in all bimetal composite rods.

Science and Engineering A

Justifying quasiparticle self-consistent schemes via gradient optimization in Baym–Kadanoff theory

Sohrab Ismail-Beigi

The question of which non-interacting Green’s function ‘best’ describes an interacting many-body electronic system is both of fundamental interest as well as of practical importance in describing electronic properties of materials in a realistic manner. Here, we study this question within the framework of Baym–Kadanoff theory, an approach where one locates the stationary point of a total energy functional of the one-particle Green’s function in order to find the total ground-state energy as well as all one-particle properties such as the density matrix, chemical potential, or the quasiparticle energy spectrum and quasiparticle wave functions. For the case of the Klein functional, our basic finding is that minimizing the length of the gradient of the total energy functional over non-interacting Green’s functions yields a set of self-consistent equations for quasiparticles that is identical to those of the quasiparticle self-consistent GW (QS GW ) (van Schilfgaarde ...

Journal of Physics Condensed Matter

Excitonic Aharonov–Bohm effect in QD-on-ring nanostructures

Yuanzhao Yao, Martin Elborg, Takashi Kuroda and Kazuaki Sakoda

We show by the first-order perturbation theory and the configuration interaction method that the Coulomb interaction in quantum rings mixes electron–hole pair states with the same total angular momentum, which makes it difficult to observe a clear excitonic Aharonov–Bohm (A–B) effect. To avoid this situation, we propose the use of a combined structure of a quantum dot on the top of a quantum ring with an applied static electric field. Under moderate experimental conditions with respect to the applied electric and magnetic fields, we show that we can observe the excitonic A–B effect due to the reduction of the Coulomb interaction and an increase in the difference between the average radii of the electron and hole trajectories.

Journal of Physics Condensed Matter

Antiferromagnetism, charge density wave, and d -wave superconductivity in the extended t - J - U

M Abram, M Zegrodnik and J Spałek

In the first part of the paper, we study the stability of antiferromagnetic (AF), charge density wave (CDW), and superconducting (SC) states within the t - J - U - V model of strongly correlated electrons by using the statistically consistent Gutzwiller approximation (SGA). We concentrate on the role of the intersite Coulomb interaction term V in stabilizing the CDW phase. In particular, we show that the charge ordering appears only above a critical value of V in a limited hole-doping range δ . The effect of the V term on SC and AF phases is that a strong interaction suppresses SC, whereas the AF order is not significantly influenced by its presence. In the second part, separate calculations for the case of a pure SC phase have been carried out within an extended approach (the diagrammatic expansion for the Gutzwiller wave function , DE-GWF) in order to analyze the influence of the intersite Coulomb repulsion on the S...

Journal of Physics Condensed Matter

Modeling elastic anisotropy in strained heteroepitaxy

Gopal Krishna Dixit and Madhav Ranganathan

Using a continuum evolution equation, we model the growth and evolution of quantum dots in the heteroepitaxial Ge on Si(0 0 1) system in a molecular beam epitaxy unit. We formulate our model in terms of evolution due to deposition, and due to surface diffusion which is governed by a free energy. This free energy has contributions from surface energy, curvature, wetting effects and elastic energy due to lattice mismatch between the film and the substrate. In addition to anisotropy due to surface energy which favors facet formation, we also incorporate elastic anisotropy due to an underlying crystal lattice. The complicated elastic problem of the film-substrate system subjected to boundary conditions at the free surface, interface and the bulk substrate is solved by perturbation analysis using a small slope approximation. This permits an analysis of effects at different orders in the slope and sheds new light on the observed behavior. Linear stability analysis shows the early evol...

Journal of Physics Condensed Matter

Thu Aug 10 2017

Raman spectroscopy: Enhanced by organic surfaces

John R. Lombardi

Nanostructured films of organic semiconductors are now shown to enhance the Raman signal of probe molecules, paving the way to the realization of substrates for Raman spectroscopy with molecular selectivity.

Nature

Phase coexistence and electric-field control of toroidal order in oxide superlattices

A. R. Damodaran, J. D. Clarkson, Z. Hong, H. Liu, A. K. Yadav, C. T. Nelson, S.-L. Hsu, M. R. McCarter, K.-D. Park, V. Kravtsov, A. Farhan, Y. Dong, Z. Cai, H. Zhou, P. Aguado-Puente, P. García-Fernández, J. Íñiguez, J. Junquera, A. Scholl, M. B. Raschke, L.-Q. Chen, D. D. Fong, R. Ramesh & L. W. Martin

Metal-oxide superlattices were found to possess coexisting phases; a ferroelectric phase and a vortex phase with electric toroidal order. Electric fields interconverted from one phase to another, potentially enabling new functionality.

Nature

High-pressure synthesis of tetragonal iron aluminide FeAl2

Author(s): Kazuki Tobita, Naoki Sato, Yukari Katsura, Koichi Kitahara, Daisuke Nishio-Hamane, Hirotada Gotou, Kaoru Kimura

Tetragonal FeAl2 has been predicted a narrow-gap semiconductor stable at ambient pressure. However, only a complex, triclinic FeAl2 has been experimentally observed. We calculated the stability of tetragonal FeAl2 under various pressures and revealed that tetragonal FeAl2 is more stabilized under high pressure. We successfully synthesized tetragonal FeAl2 at 10GPa and 1873K with diamond-anvil cell techniques. The X-ray diffraction pattern of the sample was unchanged after releasing the pressure; that is, the pressure-induced transformation is irreversible. We also calculated the transport properties of tetragonal FeAl2. The results indicate that tetragonal FeAl2 is a promising candidate for thermoelectric materials.

Scripta Materialia

Fabricating interstitial-free steel with simultaneous high strength and good ductility with homogeneous layer and lamella structure

Author(s): Ling Zhang, Zhen Chen, Yuhui Wang, Guoqiang Ma, Tianlin Huang, Guilin Wu, Dorte Juul Jensen

Annealed interstitial-free steel (IF steel) and deformed IF steel sheets were stacked alternatively into multi-layers to produce laminated IF steel through thermal-mechanical processing. After proper processing, a yield strength of 500MPa, an ultimate tensile strength of 600MPa (comparable to cold rolled one) and a uniform elongation around 17% can be realized. Microstructural observation by electron back-scatter diffraction revealed a characteristic hierarchical layer+heterogeneous lamella structure, namely L2 structure. The reasons for the good mechanical properties were discussed.

Scripta Materialia

Reinforcement of nanoglasses by interface strengthening

Author(s): Constanze Kalcher, Omar Adjaoud, Jochen Rohrer, Alexander Stukowski, Karsten Albe

Nanoglasses consist of glassy grains connected by an amorphous interface. While internal interfaces in nanoglasses help to prevent brittle failure, they are usually not beneficial to the glasses overall strength. In this molecular dynamics study, we manipulate the glass–glass interfaces of a Cu–Zr nanoglass, such that they are replaced by stronger crystalline interphases. Analogous to grain boundary strengthening in crystalline materials, we show that it is possible to reinforce the nanoglass without compromising its ductility.

Scripta Materialia

Improved multi-level data storage properties of germanium-antimony-tellurium films by nitrogen doping

Author(s): Xiao Yu, Yue Zhao, Chao Li, Chaoquan Hu, Liang Ma, Shihao Fan, Yi Zhao, Nan Min, Shuaipeng Tao, Yulong Wang

Although multi-level storage is an effective approach to increase data-storage capacity, it is currently still a big challenge to prepare chalcogenide phase-change films with excellent storage property. Here, we find that the storage property of germanium-antimony-tellurium films is significantly improved by nitrogen doping, and the underlying mechanism is well elucidated by our experiments and theoretical calculations. It is shown that nitrogen doping induces a wide quasi-platform of intermediate resistance state and simultaneously increases insulator-metal-transition temperature due to changes in electric structures. This study shows the important role of doping and provides a new strategy for realizing multi-level data storage.

Scripta Materialia

Design of the P-surfaced shellular, an ultra-low density material with micro-architecture

Author(s): Ban Dang Nguyen, Seung Chul Han, Yoon Chang Jung, Kiju Kang

The P-surface is an intersection-free smooth surface that has constant mean curvature everywhere on the surface, and periodicity in three directions in 3D space. A cellular material composed of thin shells in this configuration, named Shellular, reveals stretching-dominated deformation under external loading. The thin continuous shell of a Shellular could play the role of a transfer interface between the two sub-volumes, as well as a mechanical load support. This paper presents a preliminary study to design an optimal shape of the P-surfaced Shellular. Analysis of the geometry and finite element analyses are performed to investigate the effects of the geometric parameters and boundary conditions on their mechanical properties. It is shown that the geometry and mechanical properties of a P-surfaced Shellular can be expressed in empirical equations with only two independent dimensionless parameters, i.e., the volume fraction and the ratio of the wall thickness to cell size; and that the effect of the volume fraction is substantial. A low volume fraction is very beneficial to achieve high strength, and suppresses elastic buckling to delay the transition of the failure mode from plastic yielding to elastic buckling as the wall thickness is decreased.

Computational Materials Science

Tunable electronic properties and optical properties of novel staneneZnO heterostructure: First-principles calculation

Author(s): Hanxing Cao, Zhaobo Zhou, Xiaolong Zhou, Jianchun Cao

In this work, the structural stability, electronic and optical properties of novel stanene/ZnO heterostructure were investigated by using first-principles calculation. The results show that the lattice constants of Sn/ZnO heterostructure are in good agreement with the previous studies, and the stability of Sn/ZnO with interlayer distance of 3.0Å (h3.0-Sn/ZnO) is better than that of other constructed Sn/ZnO. The band gap of Sn/ZnO can be effectively tuned by interlayer distances, external electric field and strain. Notably, h3.2-Sn/ZnO HTS occurs an indirect-to-direct band gap transition after −1% strain. The absorption coefficient of Sn/ZnO HTS exhibit strongest peak in ultraviolet zone, illustrating that they possess an excellent absorption capability. In general, the calculated results provides a new perspective for the potential application of novel Sn/ZnO in further nanoelectronics.

Computational Materials Science

Friction welding of selective laser melted Ti6Al4V parts

Author(s): K.G. Prashanth, R. Damodaram, T. Maity, P. Wang, J. Eckert

Ti6Al4V alloy samples fabricated by selective laser melting (SLM) were subjected to solid-state welding friction welding (FW). The welded alloy exhibits a α´-martensitic microstructure in the form of platelets with dimensions in the submicron regime. The base alloy has a relatively coarser microstructure consisting of both α´ and β-phases, as compared to the as-prepared SLM microstructure (single-phase α´martensite). Hardness measurements revealed an increase of hardness in the weld zone due to the refined α´platelets. A marginal drop in hardness along the base alloy is observed that may be attributed to the imposed thermal cycle during the FW process. Tensile tests reveal an improved ductility for the FW samples at the expense of a marginal drop in strength, compared to the as-prepared SLM samples. The present work illustrates the ability of solid-state welding processes for successfully joining SLM parts and in improving, the ductility of the SLM parts and offers the opportunity to work with the additive manufacturing processes without size limits.

Science and Engineering A

Effect of Sn and Zr content on superelastic properties of Ti-Mo-Sn-Zr biomedical alloys

Author(s): Kazuki Endoh, Masaki Tahara, Tomonari Inamura, Hideki Hosoda

The effects of Sn and Zr content on the shape memory and superelastic properties of Ti-3Mo-(1−8)Sn-(0−10)Zr alloys were investigated over a wide alloy composition range. Ti-3Mo-5.5Sn-9Zr, Ti-3Mo-6Sn-7Zr, Ti-3Mo-6.5Sn-5Zr, and Ti-3Mo-7Sn-3Zr alloys exhibited superelasticity, and large shape recovery strains of over 4% were obtained. The critical stress for slip of the Ti-3Mo-6Sn-5Zr alloy was evaluated as 652MPa. Moreover, the stress hysteresis between the stress for inducing the martensitic transformation and for the reverse martensitic transformation decreased with increasing Sn content or decreasing Zr content.

Science and Engineering A

Dendrite size dependence of mechanical properties of in-situ Ti-based bulk metallic glass matrix composites

Author(s): Jun Wang, Jie Bai, Liyuan Li, Hongchao Kou, Jinshan Li

The relations between dendrite size and mechanical properties in an in-situ Ti-based bulk metallic glass composite (BMGC) have been investigated by using copper mold casting and semi-solid processing methods. Three Ti-based BMG composites with different dendrite sizes without changing glass matrix simultaneously were successfully obtained, which gives a new way to quantitively understand the relationship between dendrite size and mechanical properties. Results show that the yield strength decreases as the dendrite size increases and there exists an optimum dendrite size for both the plasticity and work hardening capacity.

Science and Engineering A

Structure and mechanical properties of B2 ordered refractory AlNbTiVZrx (x = 0–1.5) high-entropy alloys

Author(s): N.Yu. Yurchenko, N.D. Stepanov, S.V. Zherebtsov, M.A. Tikhonovsky, G.A. Salishchev

Structure and mechanical properties of the AlNbTiVZrx (x = 0; 0.1; 0.25; 0.5; 1; 1.5) refractory high-entropy alloys were investigated after arc melting and annealing at 1200°C for 24h. The AlNbTiV alloy had a B2 ordered single phase structure. Alloying with Zr resulted in (i) change of the degree of order of the B2 phase; and (ii) precipitation of the Zr5Al3 and C14 Laves ZrAlV phases. The density of the AlNbTiVZrx alloys varied from 5590kgm−3 for the AlNbTiV alloy to 5870kgm−3 for the AlNbTiVZr1.5 alloy. The compression yield strength at 22°C increased with an increase in the Zr content from 1000MPa for the AlNbTiV alloy to 1535MPa for the AlNbTiVZr1.5 alloy. The plasticity raised from 6% for the AlNbTiV alloy to >50% for the AlNbTiVZr0.5 alloy and then dropped to 0.4% for the AlNbTiVZr1.5 alloy. At 600°C, the strongest alloy was also the AlNbTiVZr1.5, whereas, at 800°C, the AlNbTiVZr0.1 alloy demonstrated the maximum strength. The plasticity of the AlNbTiV alloy at 600°C increased up to 14.3%, while the Zr-containing alloys had lower plasticity. At 800°C, all the AlNbTiVZrx alloys could be plastically deformed up to 50% of strain without fracture. Ordering in the alloys and the reasons of a complicated dependence of mechanical properties of the AlNbTiVZrx alloys on the Zr content and temperature were discussed.

Science and Engineering A

Nanoporous MoS 2 monolayer as a promising membrane for purifying hydrogen and enriching methane

Yadong Zhang, Zhaoshun Meng, Qi Shi, Haiqi Gao, Yuzhen Liu, Yunhui Wang, Dewei Rao, Kaiming Deng and Ruifeng Lu

We present a theoretical prediction of a highly efficient membrane for hydrogen purification and natural gas upgrading, i.e. laminar MoS 2 material with triangular sulfur-edged nanopores. We calculated from first principles the diffusion barriers of H 2 and CO 2 across monolayer MoS 2 to be, respectively, 0.07 eV and 0.17 eV, which are low enough to warrant their great permeability. The permeance values for H 2 and CO 2 far exceed the industrially accepted standard. Meanwhile, such a porous MoS 2 membrane shows excellent selectivity in terms of H 2 /CO, H 2 /N 2 , H 2 /CH 4 , and CO 2 /CH 4 separation (>10 3 , >  10 3 , >  10 6 , and  >  10 4 , respectively) at room temperature. We expect that the findings in this work will expedite theoretical or experimental exploration on gas separation membranes ...

Journal of Physics Condensed Matter

Self-interaction free local exchange potentials applied to metallic systems

S J Clark, T W Hollins, K Refson and N I Gidopoulos

We extend the formalism of local exchange methods to calculate and investigate the electronic structure of metals. It is well-known that the Hartree–Fock method when applied to metals shows unphysical behaviour, however the accurate treatment of exchange via DFT’s exact exchange method and using our local Fock exchange method can be used to describe metallic band structures accurately.

Journal of Physics Condensed Matter

Exploring the effect of hole localization on the charge–phonon dynamics of hole doped delafossite

Nilesh Mazumder, Prasanta Mandal, Rajarshi Roy, Uttam Kumar Ghorai, Subhajit Saha and Kalyan Kumar Chattopadhyay

For weak or moderate doping, electrical measurement is not suitable for detecting changes in the charge localization inside a semiconductor. Here, to investigate the nature of charge–phonon coupling in the presence of gradually delocalized holes within a weak doping regime (~10 16 cm −3 ), we examine the temperature dependent Raman spectra (303–817 K) of prototype hole doped delafossite {${\rm CuC}{{{\rm r}}_{1-x}}{\rm M}{{{\rm g}}_{x}}{{{\rm O}}_{2-y}}{{{\rm S}}_{y}}$} ( x   =  0/0.03, y   =  0/0.01). For both {${{E}_{{\rm g}}}$} and {${{A}_{1{\rm g}}}$} phonons, negative lineshape asymmetry and relative thermal hardening are distinctly observed upon ...

Journal of Physics Condensed Matter

Mobility dependent recombination models for organic solar cells

Alexander Wagenpfahl

Modern solar cell technologies are driven by the effort to enhance power conversion efficiencies. A main mechanism limiting power conversion efficiencies is charge carrier recombination which is a direct function of the encounter probability of both recombination partners. In inorganic solar cells with rather high charge carrier mobilities, charge carrier recombination is often dominated by energetic states which subsequently trap both recombination partners for recombination. Free charge carriers move fast enough for Coulomb attraction to be irrelevant for the encounter probability. Thus, charge carrier recombination is independent of charge carrier mobilities. In organic semiconductors charge carrier mobilities are much lower. Therefore, electrons and holes have more time react to mutual Coulomb-forces. This results in the strong charge carrier mobility dependencies of the observed charge carrier recombination rates. In 1903 Paul Langevin published a fundamental model to descr...

Journal of Physics Condensed Matter

Light-induced photoisomerization of a diarylethene molecular switch on solid surfaces

Fabian Nickel, Matthias Bernien, Martin Herder, Sandro Wrzalek, Pantelis Chittas, Kai Kraffert, Lucas M Arruda, Lalminthang Kipgen, Dennis Krüger, Stefan Hecht and Wolfgang Kuch

Diarylethenes are molecular switches, the state of which can efficiently be controlled by illumination with ultraviolet or visible light. To use the change in the molecular properties when switching between the two states for a specific function, direct contact with solid surfaces is advantageous as it provides immobilization. Here we present a study of a diarylethene derivate (T-DAE, 1,2-bis(5-methyl-2-phenylthiazol-4-yl)cyclopent-1-ene) in direct contact with highly ordered graphite as well as with semimetallic Bi(1 1 1) surfaces by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy and simulated spectra based on density functional theory. On both surfaces, the molecule can be switched from its open to its closed form by 325–475 nm broadband or ultraviolet illumination. On the other hand, back isomerization to the ring-open T-DAE was not possible.

Journal of Physics Condensed Matter

Nonlinear dielectric effects in liquids: a guided tour

Ranko Richert

Dielectric relaxation measurements probe how the polarization of a material responds to the application of an external electric field, providing information on structure and dynamics of the sample. In the limit of small fields and thus linear response, such experiments reveal the properties of the material in the same thermodynamic state it would have in the absence of the external field. At sufficiently high fields, reversible changes in enthalpy and entropy of the system occur even at constant temperature, and these will in turn alter the polarization responses. The resulting nonlinear dielectric effects feature field induced suppressions (saturation) and enhancements (chemical effect) of the amplitudes, as well as time constant shifts towards faster (energy absorption) and slower (entropy reduction) dynamics. This review focuses on the effects of high electric fields that are reversible and observed at constant temperature for single component glass-forming liquids. The exper...

Journal of Physics Condensed Matter

Wed Aug 9 2017

A third-generation charge optimized many body (COMB3) potential for nitrogen-containing organic molecules

Author(s): Jackelyn Martinez, Tao Liang, Susan B. Sinnott, Simon R. Phillpot

This work presents a new empirical, variable charge potential for NCOH systems in the third-generation charge-optimized many-body (COMB3) potential framework. The potential parameters were determined by fitting each binary system separately to the experimental enthalpy of formation and geometries for experimentally important molecules. The resulting parameter set reproduces the energetics and geometries of many nitrogen-based organic molecules with a similar average error as other reactive potentials for the same systems.

Computational Materials Science

Effect of ordered domains on the fracture toughness of high Co-Ni secondary hardening steel

Author(s): Huiping Duan, Xiao Liu, Xianzhe Ran, Jia Li, Dong Liu

The mechanism on the sharp decrease in fracture toughness of the overaged high Co-Ni secondary hardening steel has been investigated. Experimental results showed that ordering process occurred and ordered domains were formed in the steel after overaging. The ordered domains will impede the movement of the dislocations and act as local brittle centers during deformation, which leads to the sharp decrease in the fracture toughness of the steel. Based on the effect of the reverted austenite films and ordered domains, a model was proposed to explain the behavior of the fracture toughness of the steel.

Science and Engineering A

Effect of carbon nanotubes and high temperature extrusion on the microstructure evolution of Al-Cu alloy

Author(s): Chunhong Li, Risheng Qiu, Baifeng Luan, Zhiqiang Li

CNTs/Al composites were prepared using flake powder metallurgy. The effects of CNTs and high temperature extrusion on mechanical properties and density of the material were investigated through analyzing crystal size, distribution and the change (size, chemistry and crystal structure) of precipitates and CNTs. Results show that hardness and density of the composites can be increased via extrusion and CNTs addition. Moreover, extrusion leads to refined precipitates with uniform distribution and partial dissolution compared to the as-sintered. The grains become smaller after addition of CNTs. Four types of phases distributed homogeneously in CNTs/Al composites: CNTs, Al2Cu, Al2O3 and nano-sized Al4C3. The intermetallic compound Al4C3 as interlayer is beneficial to strengthen adhesion between CNTs reinforcement and the Al matrix. The uniformly distributed dispersions of CNTs, Al2Cu, Al2O3 and Al4C3 can refine the crystalline structure, thus improving mechanical performance. Combined strengthening mechanisms are identified in the composites, containing grain boundary strengthening, dispersion strengthening and better load transfer from the Al-Cu matrix to CNTs though Al4C3.

Science and Engineering A

Crystal growth and morphology evolution of D88 (Mn, Fe)5Si3 phase and its influence on the mechanical and wear properties of brasses

Author(s): Hang Li, Jinchuan Jie, Shichao Liu, Yubo Zhang, Tingju Li

The crystal growth and morphology evolution of D88 transition metal silicide and its influence on the mechanical properties of alloys have yet to be determined. In this paper, the effect of in situ-formed D88 (Mn, Fe)5Si3 phase on the mechanical and wear performances of brasses is presented. By means of deep etching and phase extraction technologies, the morphology of (Mn, Fe)5Si3 phase is found to exhibit long hexagonal prism, due to the higher growth velocity in the <0001> direction than that of <11 2 ̅ 0> according to the Mn–Si bond chain arrangement. Two kinds of hollows, i.e. internal hollow and prism facet hollow, are formed on the coarse primary (Mn, Fe)5Si3 prisms, which can be ascribed to the restrained solute diffusion within the prism and attachment limited kinetics on the prism facet, respectively. With increasing the (Mn, Fe)5Si3 content, the hardness and yield strength increase effectively, which mainly results from the load transferring effect and grain refinement, supported by the good agreement between calculated and experimental results. The fracture mode transforms from ductile failure to quasi-cleavage and cleavage failure. Severe cracking tendency of high aspect ratio primary (Mn, Fe)5Si3 particles containing large-sized hollows leads to slight increment in ultimate tensile strength and great ductility decrease at high (Mn, Fe)5Si3 content. In addition, the fine eutectic (Mn, Fe)5Si3 particles are beneficial to improve the ductility by acting as effective barriers to crack cleavage and void growth. The proper combination of thinner primary and eutectic (Mn, Fe)5Si3 particles is found to efficiently restrain the crack creation and result in a good wear resistance. This study may be helpful to design high performance alloys reinforced with transition metal silicides by tailoring their morphology.

Science and Engineering A

Effects of Al and Zn contents on the microstructure and mechanical properties of Mg-Al-Zn-Ca magnesium alloys

Author(s): Fang Wang, Tong Hu, Yitan Zhang, Wenlong Xiao, Chaoli Ma

The microstructure and tensile properties of Mg-xAl-yZn-2Ca (x + y = 8wt%) alloys with various Al and Zn contents have been investigated. The results showed that all the alloys exhibited dendritic microstructure with α-Mg matrix and eutectic compounds distributed at dendritic boundaries. The category and amount of eutectic compounds were influenced obviously by Al and Zn contents. Two Laves phases, i.e. C36 and C15, Q phase and/or Ca2Mg6Zn3 isomorphs phases were observed in as-cast alloys. The majority of second phase was C36 in high Al-containing alloys. By decreasing Al content, the C36 was transformed to C15 Laves phase and Ca2Mg6Zn3 isomorphs phase. Both C36 and C15 Laves phases contained a certain amount of Zn. The tensile strength and ductility of Mg-Al-Zn-Ca alloys were greatly dependent on the category and the morphology of second phases. The C36 Laves phase exhibited better strengthening effect, while the Q phase easily led to the formation of cavities at high temperature, thus decreasing heat resistance of the alloy. The alloy containing the majority of networked C36 phase shows optimal mechanical properties at both room temperature (RT) and elevated temperature.

Science and Engineering A

Tue Aug 8 2017

Ground-state phase diagram, fermionic entanglement and kinetically-induced frustration in a hybrid

R C P Carvalho, M S S Pereira, I N de Oliveira, J Strečka and M L Lyra

We introduce an exactly solvable hybrid spin-ladder model containing localized nodal Ising spins and interstitial mobile electrons, which are allowed to perform a quantum-mechanical hopping between the ladder’s legs. The quantum-mechanical hopping process induces an antiferromagnetic coupling between the ladder’s legs that competes with a direct exchange coupling of the nodal spins. The model is exactly mapped onto the Ising spin ladder with temperature-dependent two- and four-spin interactions, which is subsequently solved using the transfer-matrix technique. We report the ground-state phase diagram and compute the fermionic concurrence to characterize the quantum entanglement between the pair of interstitial mobile electrons. We further provide a detailed analysis of the local spin ordering including the pair and four-spin correlation functions around an elementary plaquette, as well as, the local ordering diagrams. It is shown that a complex sequence of distinct local orderin...

Journal of Physics Condensed Matter

Influence of TiO 2 and Si on the exciton–phonon interaction in PbI 2 and CdS semiconductors

A Nila, I Baltog, D Dragoman, M Baibarac and I Mercioniu

The exciton–phonon interaction, considered as a stimulated Raman scattering process, is studied in different semiconductor mixtures: PbI 2 /TiO 2 , PbI 2 /Si and CdS/Si. Raman spectra recorded at excitation wavelengths of 514.5 and 488 nm for PbI 2 and CdS, respectively, reveal a strong enhancement of the Raman lines peaked at 97 and 305 cm −1 , evaluated by the ratio I TK / I 300 K between the relative intensities of the spectra recorded in the temperature range of 88–300 K. It is found that PbI 2 and CdS exhibit a decrease in the Raman intensity modes with decreasing temperature, while in TiO 2 and Si an increase in the Raman lines intensities peaked at 138 and 520 cm −1 is observed. This behavior can be explained by an energy transfer process from PbI 2 or CdS towards TiO 2 and Si. This explanation is supported by the schematic potential energy lev...

Journal of Physics Condensed Matter

Generation mechanism of negative permittivity and Kramers–Kronig relations in BaTiO 3 /Y 3 Fe 5 O 12

Zhongyang Wang, Kai Sun, Peitao Xie, Yao Liu and Runhua Fan

Recently, negative parameters such as negative permittivity and negative permeability have been attracting extensive attention for their unique electromagnetic properties. Usually, negative permittivity is well achieved by plasma oscillation of free electrons in conductor–insulator composites or metamaterials, while some attention has been paid to attaining negative permittivity in ceramics to reduce dielectric loss. In this paper, negative permittivity in barium titanate and yttrium iron garnet composites are reported which was well fitted by the Lorentz model. Further, negative permittivity behavior was verified via Kramers–Kronig relations, and it revealed that the causal principle still valid for negative permittivity resulted from dielectric resonance. The interrelationships among negative permittivity, capacitive–inductive transition and ac conductivity are discussed.

Journal of Physics Condensed Matter

A full understanding of oxygen reduction reaction mechanism on Au(1 1 1) surface

Yang Yang, Changqing Dai, Adrian Fisher, Yanchun Shen and Daojian Cheng

Oxygen reduction and hydrogen peroxide reduction are technologically important reactions in energy-conversion devices. In this work, a full understanding of oxygen reduction reaction (ORR) mechanism on Au(1 1 1) surface is investigated by density functional theory (DFT) calculations, including the reaction mechanisms of O 2 dissociation, OOH dissociation, and H 2 O 2 dissociation. Among these ORR mechanisms on Au(1 1 1), the activation energy of {$\text{O}_{2}^{*}$} hydrogenation reaction is much lower than that of {$\text{O}_{2}^{*}$} dissociation, indicating that {$\text{O}_{2}^{*}$} hydrogenation reaction is more appropriate at the first step than

Journal of Physics Condensed Matter

Crystallographic structure and energetics of the Rh(1 0 0)-(3 × 1)-2O phase

T Kißlinger, P Ferstl, M A Schneider and L Hammer

In this study we investigate the crystallographic structure of the Rh(1 0 0)-( {$3\times1$} )-2O phase by quantitative low energy electron diffraction (LEED) and scanning tunnelling microscopy as well as the energetics of the system applying density functional theory calculations (DFT). The ( {$3\times1$} ) structure forms upon exposing the clean Rh(1 0 0) surface to 1200 L of oxygen at 520 K. A full-dynamical LEED intensity analysis (Pendry R-factor {$R= 0.095$} ) reveals an oxygen-induced shifted row-reconstruction of the rhodium top layer where every third Rh-row is displaced by half a surface lattice parameter along the [0 1 1]-direction. There are two oxygen atoms within the unit c...

Journal of Physics Condensed Matter

Theory and computer simulation of hard-core Yukawa mixtures: thermodynamical, structural and phase

Anele Mkanya, Giuseppe Pellicane, Davide Pini and Carlo Caccamo

We report extensive calculations, based on the modified hypernetted chain (MHNC) theory, on the hierarchical reference theory (HRT), and on Monte Carlo simulations, of thermodynamical, structural and phase coexistence properties of symmetric binary hard-core Yukawa mixtures (HCYM) with attractive interactions at equal species concentration. The obtained results are throughout compared with those available in the literature for the same systems. It turns out that the MHNC predictions for thermodynamic and structural quantities are quite accurate in comparison with the MC data. The HRT is equally accurate for thermodynamics, and slightly less accurate for structure. Liquid-vapor (LV) and liquid–liquid (LL) consolute coexistence conditions as emerging from simulations, are also highly satisfactorily reproduced by both the MHNC and HRT for relatively long ranged potentials. When the potential range reduces, the MHNC faces problems in determining the LV binodal line; however, the LL ...

Journal of Physics Condensed Matter

Strain-tuning of edge magnetism in zigzag graphene nanoribbons

Guang Yang, Baoyue Li, Wei Zhang, Miao Ye and Tianxing Ma

Using the determinant quantum Monte-Carlo method, we elucidate the strain tuning of edge magnetism in zigzag graphene nanoribbons. Our intensive numerical results show that a relatively weak Coulomb interaction may induce a ferromagnetic-like behaviour with a proper strain, and the edge magnetism can be enhanced greatly as the strain along the zigzag edge increases, which provides another way to control graphene magnetism even at room temperature.

Journal of Physics Condensed Matter

Crystal structures, magnetic properties, and DFT calculation of B-site defected 12L-perovskites Ba 2

Yoshihiro Doi, Makoto Wakeshima, Keitaro Tezuka, Yue Jin Shan, Kenji Ohoyama, Sanghyun Lee, Shuki Torii, Takashi Kamiyama and Yukio Hinatsu

The synthesis, crystal structures and magnetic properties of Ba 2 La 2 MW 2 O 12 (M  =  Mn, Co, Ni, Zn) were investigated. They crystallize in the 12-layer polytype of the perovskite structure with a regular cation defect in the B-site. The results of neutron diffraction measurements reveal that they adopt a rhombohedral structure with a space group R   −  3 and have a cation ordering between Ba and La ions in the A-site. In these compounds, the magnetic M ions form the 2D triangular lattice. From the results of magnetic measurements, the ferromagnetic ordering of M 2+ ions for M  =  Co ( T C   =  1.3 K) and Ni (6.2 K) and the paramagnetic behavior ( T   >  1.8 K) with an antiferromagnetic interaction for M  =  Mn are observed. From the DFT calculation, their band structures and magnetic interactions are discussed.

Journal of Physics Condensed Matter

Quantum tunneling recombination in a system of randomly distributed trapped electrons and positive

Vasilis Pagonis, Christopher Kulp, Charity-Grace Chaney and M Tachiya

During the past 10 years, quantum tunneling has been established as one of the dominant mechanisms for recombination in random distributions of electrons and positive ions, and in many dosimetric materials. Specifically quantum tunneling has been shown to be closely associated with two important effects in luminescence materials, namely long term afterglow luminescence and anomalous fading. Two of the common assumptions of quantum tunneling models based on random distributions of electrons and positive ions are: (a) An electron tunnels from a donor to the nearest acceptor, and (b) the concentration of electrons is much lower than that of positive ions at all times during the tunneling process. This paper presents theoretical studies for arbitrary relative concentrations of electrons and positive ions in the solid. Two new differential equations are derived which describe the loss of charge in the solid by tunneling, and they are solved analytically. The analytical solution compa...

Journal of Physics Condensed Matter

Sun Aug 6 2017

Molecular simulation of reverse osmosis for heavy metal ions using functionalized nanoporous graphenes

Author(s): Yaping Li, Zhijun Xu, Shuyan Liu, Jinwen Zhang, Xiaoning Yang

Currently, elimination of heavy metal ions from contaminated water resource is an important issue in environmental protection. In this study, we simulated the separation performance of heavy metal ions using nanoporous graphene surfaces as reverse osmosis membranes with functionalized groups (boron, nitrogen and hydroxyl groups). We show these nanoporous graphenes could realize high water permeation and ion rejection for various conditions. The simulated water permeability is 2–5 orders of magnitude greater than that of currently commercial membranes. The interfacial water structures and flow velocity of water molecules within the nanopores were characterized. The calculations of the potential of mean force reveal water molecules generally face lower free energy barrier than ions when passing through graphene pores. The free energy barriers for ions can be explained as the combining contributions from the ion dehydration effect and the surface electrostatic interaction. Overall, the functionalized nanoporous graphene membranes exhibit potential application in the removal of heavy metal ions, and meanwhile our simulation results provide new insights into the ion rejection mechanism.

Computational Materials Science

Destabilisation of nanoporous membranes through GB grooving and grain growth

Author(s): Chaitanya Joshi, T.A. Abinandanan, Rajdip Mukherjee, Abhik Choudhury

We have used a phase field model to study destabilization of cylindrical pores in a polycrystalline membrane; a key feature in the model is that it incorporates surface diffusion as the mechanism for mass transport. Using a model system in which a cylindrical pore runs through a material in which all the grain boundaries (GBs) are perpendicular to the pore axis, we identify two elementary mechanisms for pore failure. The first one is based just on grain boundary (GB) grooving, which causes a circular trench at the groove, and a constriction of the pore on either side of the GB; as the groove deepens, the constriction narrows, and eventually closes the pore. Pore closure through this mechanism is possible only when the grain size exceeds a critical size (below which the open pore surface acquires an inverse-bamboo morphology), and therefore, it is controlled by grain growth kinetics. In the second mechanism, the groove profiles of unequal sized grains is such that the curvature differences trigger a coarsening-like process in which atoms from the surface of smaller grains are transported to that of larger grains, causing an ever-narrowing constriction there. A simplified model that incorporates these two mechanisms acting in parallel is used to rationalize our observations of pore failure in polycrystalline systems.

Computational Materials Science

Molecular dynamics study of deformation and fracture in SiC with angular dependent potential model

Author(s): Atsushi Kubo, Shijo Nagao, Yoshitaka Umeno

An interatomic potential for SiC based on the angular-dependent potential (ADP) model is developed based on reference data obtained by first-principles calculations. The reference data include not only the energy, stress and interatomic forces of equilibrium and strained crystals but also the stress–strain relationship obtained in ideal strength analyses, with the aim to make the potential suitable for simulations of fracture. The constructed potential successfully reproduces the critical stress of the 3C and 4H structures under various loading conditions. Another distinct feature of the developed potential is the inclusion of point charges in the pairwise term, which enables the description of an environment with excess electric charge. Molecular dynamics simulations using the potential demonstrate fracture of thin films, where ductile and brittle fracture behaviors are found depending on crystal orientations and local stress conditions, and dislocation motion under shear stress, where excess electric charge is seen to influence the mobility of C-core and Si-core partial dislocations.

Computational Materials Science

Topological design of phononic band gap crystals with sixfold symmetric hexagonal lattice

Author(s): Zhaoxuan Zhang, Yang Fan Li, Fei Meng, Xiaodong Huang

Phononic band gap crystals offer great flexibility for manipulating elastic waves and can be used for many applications. The occurrence of band gaps highly depends on the spatial distribution of material phases in phononic crystals. This paper investigates topology optimization of two-dimensional (2D) solid/solid hexagonal-latticed phononic crystals with sixfold symmetry for maximizing specified band gaps. The optimization algorithm based on the bi-directional evolutionary structural optimization (BESO) method is established and verified by numerical examples. Various novel patterns with large band gaps for out-of-plane and in-plane waves are obtained and optimized solutions are discussed and compared with those of square-latticed ones. Based on the optimized solutions for out-of-plane waves and in-plane waves, the proposed method is extended to the design of the complete band gaps. The transmission analysis of the finite phononic structure formed by optimized phononic crystals shows that out-of-plane waves and in-plane waves can be transmitted or prohibited, which agrees well with the obtained band gaps in optimization. The further improvement of the proposed BESO method is also recommended.

Computational Materials Science

A key role of tensile strain and surface termination in formation and properties of La0.7Sr0.3MnO3 composites with carbon nanotubes

Author(s): Evgenia A. Kovaleva, Alexander A. Kuzubov, Pavel V. Avramov, Anastasia S. Kholtobina, Artem V. Kuklin, Felix N. Tomilin, Pavel B. Sorokin

Atomic and electronic structure of LSMO-based composites with carbon nanotubes were studied by means of density functional theory with respect to the termination of LSMO surface. The deformation of the tubes caused by the lattice mismatch with the substrate leads to a major change in their electronic structure. The surface terminated with Mn-O layer provides much stronger interaction with carbon nanotubes than Sr-O terminated one does. The interaction with transition metal atoms is essential for spin polarization of the nanotube while no spin injection was observed for Sr-O-supported tubes.

Computational Materials Science

Theoretical simulations on the glass transition temperatures and mechanical properties of modified glycidyl azide polymer

Author(s): Ying-ying Lu, Yuan-jie Shu, Ning Liu, Yao Shu, Ke Wang, Zong-kai Wu, Xiao-chuan Wang, Xiao-yong Ding

Molecular dynamics (MD) simulations were carried out to study the glass transition temperature and mechanical properties of glycidyl azide polymer (GAP) most widely used in propellants and explosives as an energetic binder. Other three polymers derived from GAP with different side groups were designed and simulated under the COMPASS force field, with the ensembles of constant particle number, volume, temperature (NVT) and constant particle number, pressure, temperature (NPT). It was found that the densities and free volumes of polymers changed regularly along with the decrease of temperature and the transition occurred at the turning point. Thus the glass transition temperatures (Tg ) were predicted in two methods based on density and free volume theory. The simulation results showed that the introduction of –NO2 made important contribution on reducing the T g of polymers, and the probable mechanism was discussed. Moreover, several mechanical properties of four pure polymer systems such as Young’s modulus, Shear modulus, Bulk modulus and Poisson’s ratio, etc, were calculated. The calculated results revealed that the single introduction of –NO2 increased the mechanical properties parameters more obviously than other two polymers. Overall, a new method was used for GAP modification in order to improve its mechanical properties at low temperatures, and provides some guidance to the GAP modification via changing substituent groups of side chain.

Computational Materials Science

Atomate: A high-level interface to generate, execute, and analyze computational materials science workflows

Author(s): Kiran Mathew, Joseph H. Montoya, Alireza Faghaninia, Shyam Dwarakanath, Muratahan Aykol, Hanmei Tang, Iek-heng Chu, Tess Smidt, Brandon Bocklund, Matthew Horton, John Dagdelen, Brandon Wood, Zi-Kui Liu, Jeffrey Neaton, Shyue Ping Ong, Kristin Persson, Anubhav Jain

We introduce atomate, an open-source Python framework for computational materials science simulation, analysis, and design with an emphasis on automation and extensibility. Built on top of open source Python packages already in use by the materials community such as pymatgen, FireWorks, and custodian, atomate provides well-tested workflow templates to compute various materials properties such as electronic bandstructure, elastic properties, and piezoelectric, dielectric, and ferroelectric properties. Atomate also enables the computational characterization of materials by providing workflows that calculate X-ray absorption (XAS), Electron energy loss (EELS) and Raman spectra. One of the major features of atomate is that it provides both fully functional workflows as well as reusable components that enable one to compose complex materials science workflows that use a diverse set of computational tools. Additionally, atomate creates output databases that organize the results from individual calculations and contains a builder framework that creates summary reports for each computed material based on multiple simulations.

Computational Materials Science

Sat Aug 5 2017

Tue Aug 22 2017

Journal of Physics Condensed Matter

Mon Aug 21 2017

Scripta Materialia
Science and Engineering A

Sun Aug 20 2017

Scripta Materialia
Computational Materials Science
Science and Engineering A

Sat Aug 19 2017

Scripta Materialia
Science and Engineering A
Journal of Physics Condensed Matter

Fri Aug 18 2017

Nature

Thu Aug 17 2017

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

Wed Aug 16 2017

Journal of Physics Condensed Matter

Tue Aug 15 2017

Physical Review Letters
Physical Review B

Sun Aug 13 2017

Scripta Materialia
Computational Materials Science
Science and Engineering A

Sat Aug 12 2017

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

Fri Aug 11 2017

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

Thu Aug 10 2017

Nature
Scripta Materialia
Computational Materials Science
Science and Engineering A
Journal of Physics Condensed Matter

Wed Aug 9 2017

Computational Materials Science
Science and Engineering A

Tue Aug 8 2017

Journal of Physics Condensed Matter

Sun Aug 6 2017

Computational Materials Science

Sat Aug 5 2017