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In this work, the effect of random substitution modes of Fe and Ni on the stability and mechanical properties of Co3Ti-based intermetallics was investigated using first principles calculations through occupancy modes, electronic structures, energies, and elastic constants. The results show that FeCo type substitution favors ductility while NiCo type substitution favors strength. Compared to the above two types of substitution, FeTi type substitution is favorable for obtaining a balance between stability and ductility, but it is not easy to achieve energetically. Synergy of three types of substitutions enables stability and equilibrium mechanical properties of Co3Ti-based multi-component intermetallics. In addition, the anti-site substitution of neighboring Co and Ti atoms interchanging their positions may be a key step in the formation of FeTi type substitutions in terms of energy. This work provides useful data and insights into the design and utilization of Co3Ti-based multi-component intermetallics. [ABSTRACT FROM AUTHOR]

The thermomechanical behavior of materials is known to be sensitive to preexisting defects in their microstructure. In this paper, non-equilibrium molecular dynamics simulations have been used to investigate the effects of the microvoid size on the plastic deformation in single-crystal iron shock-compressed along the [ 001 ] crystallographic direction. The higher the microvoid radius, the faster the kinetics of dislocations. Thus, as the microvoid radius increases, the plastic activity evolves from a regime where the deformation is dominated by twin activities to a regime where both twin and dislocation activities play an essential role and then to a regime where the deformation is dominated by dislocation slip. Furthermore, in both defect-free and defective initial crystal states, the elastic precursor wave is observed to decay with propagation distance, resulting in a constitutive functional dependence of the yielding pressure, σ E , on the plastic deformation rate, ε ˙ p. In the regime where both deformation twinning and dislocation slip play important roles, the constitutive behavior is consistent with the original Swegle–Grady model and is in overall agreement with experimental data and thermomechanical simulations. [ABSTRACT FROM AUTHOR]

Meso-scale calculations of energy localization and initiation in energetic material microstructures must capture the deformation and collapse of pores and high-temperature shear bands, which lead to hotspots. Because chemical reaction rates depend sensitively on temperature, predictive continuum models need to get the pore-collapse dynamics and resulting hotspot temperatures right; this imposes stringent demands on the fidelity of thermophysical model forms and parameters and on the numerical methods employed to perform high-resolution meso-scale calculations. Here, continuum material models for β-HMX are examined in the context of shock-induced pore collapse, treating predictions from all-atom molecular dynamics (MD) simulations as ground truth. Using atomistics-consistent material properties, we show that the currently available strength models for HMX fail to correctly capture pore collapse and hotspot temperatures. Insights from MD are then employed to advance a Modified Johnson–Cook (M-JC) strength model, which is shown to capture key aspects of the physics of shock-induced localization in HMX. The study culminates in a MD-guided strength model for β-HMX that produces continuum pore-collapse results in better alignment on several aspects with those predicted by MD, including pore-collapse mechanism and rate, shear-band formation in the collapse zone, and temperature, strain, and stress fields in the hotspot zone and the surrounding material. The resulting MD-informed/MD-determined M-JC model should improve the fidelity of meso-scale simulations to predict the detonation initiation of HMX-based energetic materials in microstructure-aware multi-scale frameworks. [ABSTRACT FROM AUTHOR]

The cytoskeleton, a dynamic network of structural proteins within cells, is essential for cellular deformation and responds to external mechanical cues. Here, based on the structure of the cytoskeleton, combined with the biochemical reactions of the activator RhoA and the inhibitor F-actin, we develop a novel mechanochemical cytoskeleton model to investigate the mechanical behavior of cells. Interestingly, we find that active stress fibers exhibit diverse dynamical modes at specific inhibitor concentration thresholds. The existence of concentration differences and sustained mechanochemical feedback in activators and inhibitors trigger a global oscillation of isolated cells. In addition, under uniaxial and biaxial stretches, activators and inhibitors preferentially diffuse toward the more significantly deformed cytoskeletal elements, and their dynamic interactions regulate the cell to align with the main stretching direction. Our findings, consistent with many experimental results, provide fundamental insights into cytoskeletal remodeling and cellular mechanosensing mechanisms. [ABSTRACT FROM AUTHOR]

Flexible electronic devices with skin-like properties are hailed as revolutionary for the development of next-generation electronic devices, such as electric-skin and humanoid robotics. Graphene is intrinsically flexible due to its structural thinness in nature and are considered next-generation materials for wearable electronics. These devices usually experience a large mechanical deformation in use so as to achieve intimate conformal contact with human skin and to coordinate complex human motions, while heat dissipation has been a major limitation when the device is under a large mechanical strain. Unlike the small deformation (<1%) induced by intrinsic material factors such as lattice mismatch between material components in devices, a large mechanical deformation (>1%) by an external loading condition could lead to apparent changes to global geometric shapes and significantly impact thermal transport. In this study, we investigated the thermal conductivities of graphene under several large mechanical strains: 2.9%, 4.3%, and 6.1%. We used a refined opto-thermal Raman technique to characterize the thermal transport properties and discovered the thermal conductivities to be 2092 ± 502, 972 ± 87, 348 ± 52, and 97 ± 13 W/(m K) for the relaxed state, 2.9%, 4.3%, and 6.1% tensile strain, respectively. Our results showed a significant decreasing trend in thermal conductivities with an increasing mechanical strain. The findings in this study reveal new thermal transport mechanisms in 2D materials and shed light on building novel flexible nanoelectronic devices with enhanced thermal management. [ABSTRACT FROM AUTHOR]

Surface pattern formation of Smart Cut™ silicon-on-insulator (SOI) substrates is investigated using a new process derived from this technology. The local control of SOI surface roughness is achieved by using hetero-temperature surface activated bonding to introduce locally bespoke shear stresses into bonded silicon wafers. The finite element method is used to back up experimental measurements of these large deformations to determine the amount of shear stresses introduced into the structure, and to determine its impact on the fracture mechanism. [ABSTRACT FROM AUTHOR]

A gradient nano-grained (GNG) structure demonstrates satisfactory surface strength. However, the underlying mechanism responsible for its strengthening lacks sufficient research. To explain how gradient nano-grained structures improve surface strength in detail, large-scale parallel molecular dynamics simulations are utilized in this study to investigate the mechanical deformation behavior of BCC tungsten with varying grain sizes during spherical nanoindentation. The findings suggest that a well-designed gradient structure can promote rational plasticity and an appropriate distribution of internal atomic stress. The critical point of maximum stress and hardness is observed when the initial grain size is 4.5 nm, with an average grain size of 7.1 nm. The interaction between grain boundary slip and migration in small grains, along with the enhanced activity of grain boundary dislocations in large grains, collectively contributes to the enhancement of the strength and hardness of the GNG structure. Compared with a homogeneous nano-grained structure, the gradient nano-grained structure exhibits a more rational distribution of dislocations and stress relaxation effects to enhance strength. The present work utilizes the molecular dynamics nanoindentation method to study GNG materials, providing a methodology for investigating the surface strengthening effects of GNG structures at the atomic scale and effectively revealing potential mechanisms for resisting surface deformation in GNG structures. [ABSTRACT FROM AUTHOR]

Metallic glasses (MGs) are of high strength but limited plasticity at room temperature (RT) due to localized shear in the intrinsically heterogeneous structure. Here, we investigate the variation of structural heterogeneity and plasticity of a Zr-based MG after high-temperature (T) tension under different stresses (σ) at 579 K (0.9Tg, where Tg is the glass transition temperature). The correlation length (ξ) of the heterogeneous structure and the average Young's modulus ( E ¯ ) increase with σ when σ is below 160 MPa; when σ exceeds 160 MPa, both ξ and E ¯ decrease with σ, leading to the improvement of the plasticity. This research could be enlightening for improving the plasticity of MGs at RT through tuning their structural heterogeneity with high-T deformation. [ABSTRACT FROM AUTHOR]

This paper utilizes a coupled chemomechanical model that integrates chemical reactions, diffusion, and mechanical deformation to describe the high-temperature interfacial reactions and stress evolution within an enamel coating/Ni-based super-alloy system. Through finite element simulation, we conduct a comprehensive analysis of multiple reactions and processes at the interface. Experimental characterization of the concentration distribution of various elements is employed to verify the accuracy of the proposed chemomechanical model. Additionally, the simulation reveals that the interface region experiences compressive circumferential stress, which can potentially lead to interfacial failure due to buckling. On the other hand, the enamel coating undergoes tensile circumferential stress, resulting in the formation of vertical surface cracks. These findings hold practical implications for the development of advanced coatings with enhanced durability and performance. Future research can further optimize coating design and enhance the reliability of high-temperature applications based on these results. [ABSTRACT FROM AUTHOR]

In this paper, a Zr–5W alloy was fabricated via casting. In order to obtain the mechanical properties of the material, quasi-static compression tests at room temperature and split Hopkinson pressure bar tests at various temperatures were carried out. The x-ray diffraction result showed that the main components of the alloy were αZr and W2Zr, where αZr is the matrix and W2Zr is the reinforcement. The metallographic characterization results showed that there were two main forms of W2Zr in the material, namely, large particle boundary and small diffuse submicrometer precipitates. The reinforcements of both distributions have the effect of increasing the strength of the material, but the small submicrometer W2Zr precipitates would cause microcrack nucleation during the late plastic deformation stage, resulting in damage softening. In order to make theoretical calculations of the mechanical properties of materials, the Johnson–Cook (JC) constitutive model and Zerilli–Armstrong (ZAM) constitutive model of the material were obtained. It was found that the JC constitutive model had poor consistency in describing material properties. Although the consistency of the ZAM constitutive model was higher than that of the JC constitutive model, it still had obvious shortcomings. Combined with the deformation mechanism of the alloy, a modified constitutive relation was established by adding damage softening terms based on the hexagonal close-packed metal constitutive model inferred by the kinetics of heat-activated dislocations. The relative error results of all working conditions show that the correlation consistency of the improved constitutive model in this paper is significantly better than that of JC constitutive and ZAM constitutive. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW)-layered materials, such as graphite, exhibit unique mechanical properties owing to their structural and mechanical anisotropies. This study reports the development of a mechanical model that reproduces the characteristics of the nonlinear and reversible bending deformation of vdW-layered materials, while taking into account the microscopic mechanism of the discrete interlayer slips. The vdW-layered material was modeled as a stack of interacting discrete deformable layers (semi-discrete layer model), and the interlayer interaction was modeled using a cohesive zone model that reproduced the localized interlayer slip. Using the finite-element method, out-of-plane bending deformation analyses were performed on the cantilevers of the highly oriented pyrolytic graphite (HOPG) and MoTe2, and the validity of the model was verified by comparing it with the experimental results. The model accurately reproduced the loading and unloading behaviors in the experiments for the submicron HOPG cantilevers or the large nonlinear and reversible deformation with a hysteresis loop. Furthermore, the model reproduced well the characteristics of the bending experiments for the micro-MoTe2 cantilevers, or the intermittent decrease in stiffness during the loading process and deformation restoration during the unloading process. These results demonstrated that the designed semi-discrete layer model can be universally applied to reproduce the bending deformation characteristics of a variety of vdW-layered materials and can be employed to effectively elucidate the underlying deformation mechanisms. [ABSTRACT FROM AUTHOR]

Utilizing molecular dynamics simulation, this study aims to explore the phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) when subjected to cyclic deformation. The influences of porosity and cyclic deformation on the phase transformation of NC NiTi SMAs are examined and discussed. The simulation results show that the increase in the porosity and number of cycles leads to a decrease in both the critical phase transformation stress and peak stress whereas an increase in the residual martensite, phase boundary, and interstitial atoms; the related results can be supported by previous experiments. After cyclic deformation, the reduction in the potential energy for the entire system during the tensile phase occurs at an earlier stage, indicating that the martensitic transformation occurs earlier as the number of cycles increases. Notably, the dissipated energy demonstrates a decrease with an increasing number of cycles, and the potential energy during the austenite elastic unloading stage undergoes a transition from a decreasing to an increasing trend due to the presence of residual martensite increasing with the number of cycles. [ABSTRACT FROM AUTHOR]

The mechanical response of ferroelectric (piezoelectric) ceramics under high stress conditions has a crucial role from the perspective of engineering applications such as advanced actuator systems. In this study, we perform first-principles calculations to clarify the deformation behavior and (ideal) tensile strength for typical ferroelectric ceramics of PbTiO 3 (PTO) under high mechanical loading. We find the superelastic-like nonlinear deformation behavior in ferroelectric PTO. In addition, it has several inflection points and shows a large critical strain compared to the paraelectric phase. We conclude that the unique nonlinear deformation in PTO originates from attributable to the displacement of oxygen atoms due to the ferroelectric phase transition based on analyzing the interatomic distances of each atom and an integrated crystal orbital Hamiltonian population with respect to strain. Furthermore, we also calculate the piezoelectric coefficient for PTO and reveal that it shows the singular peak at inflection points of the stress–strain curve. Unveiling and engineering the hidden superelastic-like deformation in the ferroelectric phase may open promising paradigms for functional piezoelectric devices. [ABSTRACT FROM AUTHOR]

First-principles density-functional theory calculations were performed to reveal the effect of surface polarization and structural deformation on the formation and stabilization of the polaron in two-dimensional Ruddlesden–Popper perovskites. Our results revealed that the orientational distribution of organic cations induces surface polarization. The surface dipole moment can be well featured by the c axis distances between N and the nearest I atoms. Structural deformation and surface dipole moments result in separate real-space distributions of hole and electron polarons. Our results also reveal that the structural deformation of the [PbI6] sublattices and surface polarization are closely related to the reorientation of organic cations and can be effectively modulated by it. This reorientation significantly impacts the stabilization of polarons. Our understandings provide insight into the nature of polarons in two-dimensional Ruddlesden–Popper perovskites and general guidance for the proper selection of organic cations in two-dimensional perovskites for suitable applications in photovoltaic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

The emerging class of multi-principal element alloy (MPEA) processes superior mechanical properties and has great potential for applications in extreme environments. In this work, the synergic effect of the Cr content and crystallographic orientation on the deformation behaviors of single-crystal CrCoFeNi MPEAs has been investigated by atomistic simulations. We have found distinct differences in dislocation activities, deformation microstructures, and mechanical behaviors in the model MPEAs, which depend on crystallographic orientations, Cr concentration, and the number of activated slip systems. When multiple slip systems are triggered along [100] and [111] orientations, Shockley partial activation and their interaction are predominant, leading to the formation of sessile dislocations and a dense dislocation network. When only two slip systems of Shockley partials are favored along the [110] direction, the influence of Cr concentration and planner defect energies emerges. At low Cr concentration, the double planar slip of Shockley partials results in deformation-induced nanotwins. At high Cr concentration, the partial dislocations of a single slip plane become dominant, attaining the highest volume fraction of deformation-induced phase transformation. The results provide a fundamental understanding of deformation mechanisms in MPEAs, elucidating the synergic effect of crystal orientation and composition on tunning the mechanical behaviors. [ABSTRACT FROM AUTHOR]

Metals often lose their ductility at cryogenic temperatures owing to the decreased mobility of dislocations. TRansformation-induced plasticity (TRIP), a toughening mechanism at room temperature, can increase damage susceptibility at low temperatures, as the resultant martensite phases can become more brittle than the parent phases. Herein, we develop a high-entropy alloy (HEA) with an improved low-temperature impact-damage tolerance through a sequential plasticity mechanism. We design a trip-assisted dual-phase HEA (TADP HEA) and investigate the effects of Al addition on its mechanical properties upon deformation at different temperatures, depending on stacking fault energy (SFE). Our analysis shows that a senary (Cr20Mn6Fe34Co34Ni6)98Al2 HEA exhibits superior mechanical properties, including a 641 MPa yield strength (σy), 964 MPa ultimate tensile strength (σUTS), and 40% uniform elongation (ɛUTS) at ambient temperature (25 °C), and a 1 GPa σy, 1.5 GPa σUTS, and 36% ɛUTS at −100 °C. Notably, despite the presence of hexagonal-close packed martensite, the HEA exhibits a higher Charpy impact energy (406 J) than Cantor HEA (344 J) at −100 °C. We attribute this improvement to the sequential deformation mechanism of mechanical twinning and martensitic transformation in the HEA at −100 °C, which results in sustainable steady strain-hardening during deformation. We suggest that optimizing the sequential deformation mechanism by manipulating SFE in multi-component alloys can be an effective route for improving the damage tolerance of metals at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

Nanostructures with high surface area to volume ratio, such as oxidized and coated Cu nanowires (NWs), exhibit unique mechanical properties due to their size and surface effects. Understanding the complex oxidation process of Cu NWs at nanoscale and quantifying its resulting effects on mechanical behavior and properties are significantly essential for effective usage of Cu NW devices in a wide range of applications in nanoelectronics. Here, we perform molecular dynamics simulations using ReaxFF (reactive force field) to investigate the oxidation process and mechanisms of [001]-oriented cylindrical Cu NWs and its contribution on the mechanical deformation behavior and material properties as a function of NW sizes. The relatively thin oxide CuxOy layer is formed on the surface of Cu NWs in an O2 environment, creating a core/shell (Cu/CuxOy) NW structure that played a key role in governing the overall tensile mechanical deformation behavior and properties of Cu NW. The formation of oxide layer effects, including the resulting interface and defects, leads to a reduction in the initial dislocation nucleation barrier, which facilitates the onset of plasticity and stress relaxation, ultimately resulting in a negative impact on the tensile strength, Young's modulus, yield stress and strain, and flow stress when compared to pristine counterparts. It is worth noting that the tensile mechanical response and properties of the Cu NWs are highly dependent on the pre-existing oxide shell layer associated with the size of NW, determining the overall mechanical performance and properties of Cu NWs. [ABSTRACT FROM AUTHOR]

The varied atomic arrangements in face-centered cubic (FCC) solid solutions introduce atomic-scale fluctuations to their energy landscapes that influence the operation of dislocation-mediated deformation mechanisms. These effects are particularly pronounced in concentrated systems, which are of considerable interest to the community. Here, we examine the effect of local fluctuations in planar fault energies on the evolution of deformation twinning microstructures in randomly arranged FCC solid solutions. Our approach leverages the kinetic Monte Carlo (kMC) method to provide kinetically weighted predictions for competition between two processes: deformation twin nucleation and deformation twin thickening. The kinetic barriers underpinning each process are drawn from the statistics of planar fault energies, which are locally sampled using molecular statics methods. kMC results show an increase in the fault number densities of solid solutions relative to a homogenized reference, which is found to be driven by the fluctuations in planar fault energies. Based on kMC relations, an effective barrier model is derived to predict the competition between deformation twinning nucleation and thickening processes under a fluctuating planar fault energy landscape. A key result from this model is a measurement of the length-scale over which the influence of local fluctuations in planar fault energies diminish and nucleation/thickening-dominated behaviors converge to bulk predictions. More broadly, the tools developed in this study enable examination of the influence of chemistry and length-scale on the evolution of deformation twinning mechanisms in FCC solid solutions. [ABSTRACT FROM AUTHOR]

This paper demonstrates a novel Eulerian computational framework for modeling anisotropic elastoplastic deformations of organic crystalline energetic materials (EM) under shocks. While Eulerian formulations are advantageous for handling large deformations, constitutive laws in such formulations have been limited to isotropic elastoplastic models, which may not fully capture the shock response of crystalline EM. The present Eulerian framework for high-strain rates, large deformation material dynamics of EM incorporates anisotropic isochoric elasticity via a hypo-elastic constitutive law and visco-plastic single-crystal models. The calculations are validated against atomistic calculations and experimental data and benchmarked against Lagrangian (finite element) crystal plasticity computations for shock-propagation in a monoclinic organic crystal, octahydro-1,3,5,7-tetranitro-1,3,5,7 tetrazocine (β-HMX). The Cauchy stress components and the resolved shear stresses calculated using the present Eulerian approach are shown to be in good agreement with the Lagrangian computations for different crystal orientations. The Eulerian framework is then used for computations of shock-induced inert void collapse in β-HMX to study the effects of crystal orientations on hotspot formation under different loading intensities. The computations show that the hotspot temperature distributions and the collapse profiles are sensitive to the crystal orientations at lower impact velocities (viz., 500 m/s); when the impact velocity is increased to 1000 m/s, the collapse is predominantly hydrodynamic and the role of anisotropy is modest. The present methodology will be useful to simulate energy localization in shocked porous energetic material microstructures and other situations where large deformations of single and polycrystals govern the thermomechanical response. [ABSTRACT FROM AUTHOR]

A dual-heterogeneous structure with both heterogeneous grain structure and coherent L12 nano-precipitate was obtained in a (FeCoNi)86Al7Ti7 medium-entropy alloy (MEA) after cold rolling, critical annealing, and aging treatments. The volume fraction of the L12 phase is found to increase from 0% to 34.5% after aging treatment, resulting in severer heterogeneity. The unaged samples of (FeCoNi)86Al7Ti7 MEA are found to have a much better synergy of strength and ductility as compared to the FeCoNi MEA. Moreover, the tensile properties for the aged samples of (FeCoNi)86Al7Ti7 MEA with dual-heterogeneous structures are even better than those for the corresponding unaged samples. The hetero-deformation-induced hardening plays a more important role in the aged samples than in the unaged samples, producing a higher density of geometrically necessary dislocations. High density dislocations are discovered in grain interiors for the aged samples and the deformation is dominated by the planar slip in the unaged samples. Meanwhile, a high density of dislocations are observed inside L12 nano-precipitates; thus, L12 nano-precipitates can be sheared by dislocations, indicating that L12 nano-precipitates are strong but deformable. The strong interactions between dislocations and L12 nano-precipitates should provide additional strain hardening for better tensile properties in the aged samples. [ABSTRACT FROM AUTHOR]

Piezoelectric materials (PEMs) find a wide spectrum of applications that include, but are not limited to, sensors, actuators, semiconductors, memory devices, and energy harvesting systems due to their outstanding electromechanical and polarization characteristics. Notably, these PEMs can be employed across several length scales (both intrinsic and extrinsic) ranging from mesoscale (bulk ceramics) to nanoscale (thin films) during their applications. Over the years, progress in probing individual electrical and mechanical properties of PEM has been notable. However, proportional review articles providing the mechanical characterization of PEM are relatively few. The present article aims to give a tutorial on the mechanical testing of PEMs, ranging from the conventional bulk deformation experiments to the most recent small-scale testing techniques from a materials science perspective. The advent of nanotechnology has led materials scientists to develop in situ testing techniques to probe the real-time electromechanical behavior of PEMs. Therefore, this article presents a systematic outlook on ex situ and in situ deformation experiments in mechanical and electromechanical environments, related mechanical behavior, and ferroelectric/elastic distortion during deformation. The first part provides significant insights into the multifunctionality of PEM and various contributing microstructural length scales, followed by a motivation to characterize the mechanical properties from the application's point of view. In the midst, the mechanical behavior of PEM and related mechanical characterization techniques (from mesoscale to nanoscale) are highlighted. The last part summarizes current challenges, future perspectives, and important observations. [ABSTRACT FROM AUTHOR]

Solid-state material at high pressure is prevalent throughout the Universe, and an understanding of the structure of matter under such extreme conditions, gleaned from x-ray diffraction, has been pursued for the best part of a century. The highest pressures that can be reached to date (2 TPa) in combination with x-ray diffraction diagnosis have been achieved by dynamic compression via laser ablation [A. Lazicki et al., Nature 589, 532–535 (2021)]. The past decade has witnessed remarkable advances in x-ray technologies, with novel x-ray Free-Electron-Lasers (FELs) affording the capacity to produce high quality single-shot diffraction data on timescales below 100 fs. We provide a brief history of the field of dynamic compression, spanning from when the x-ray sources were almost always laser-plasma based, to the current state-of-the art diffraction capabilities provided by FELs. We give an overview of the physics of dynamic compression, diagnostic techniques, and the importance of understanding how the rate of compression influences the final temperatures reached. We provide illustrative examples of experiments performed on FEL facilities that are starting to give insight into how materials deform at ultrahigh strain rates, their phase diagrams, and the types of states that can be reached. We emphasize that there often appear to be differences in the crystalline phases observed between the use of static and dynamic compression techniques. We give our perspective on both the current state of this rapidly evolving field and some glimpses of how we see it developing in the near-to-medium term. [ABSTRACT FROM AUTHOR]

Nanotwinned metals have demonstrated the capacity for concomitant high strength and ductility. However, metals with high stacking fault energies, such as aluminum (Al), have a low propensity for twin formation. Here, we show the fabrication of supersaturated solid-solution Al–Zr alloys with a high density of growth twins. Incoherent twin boundaries (ITBs) are strong barriers to dislocation motion, while mobile partial dislocations promote plasticity. These deformable nanotwinned Al–Zr alloys reach a flow stress of ∼1 GPa, as demonstrated using in situ micropillar compression tests. Density functional theory calculations uncover the role Zr solute plays in the formation and deformation of the nanotwinned microstructure. This study features a strategy for incorporating ITBs and 9R phase into Al alloys for simultaneous benefits to strength and deformability. [ABSTRACT FROM AUTHOR]

Nano-kirigami-based deformable metasurfaces offer unique advantages in the dynamic modulation of optical fields and the realization of reconfigurable micro-/nano-optoelectronic devices. Here, we theoretically propose and numerically demonstrate a magnetically actuated deformation scheme for 2D nano-kirigami structures, which can be accurately and continuously transformed from 2D to 3D. Based on the traditional pinwheel structure, an Fe/Au bilayer anti-pinwheel nanostructure is designed and dynamically transformed by employing magnetic attraction in numerical simulations, in which the ratio between deformation height and lateral period could reach up to 0.563. More importantly, it is found that the anti-pinwheel structure has superior circular dichroism (CD), whose maximum CD response could reach 7 times that of the corresponding pinwheel structure. Further analysis of the scattering power of multipole moments reveals that such a superior CD response is found to be induced by the handedness-dependent excitation of a toroidal dipole moment. Such a straightforward magnetically actuated deformation and novel anti-pinwheel structure provides useful methodologies to explore and realize deformable metamaterials and their dynamic regulation capabilities, as well as applications in chiral spectroscopy, optical reconfiguration, optical sensing, etc. [ABSTRACT FROM AUTHOR]

In previous studies, the influence of flaws on macro-scale and nano-scale metallic glasses (MGs) has always been a topic of great interest, with important applications in engineering. Moreover, through experiments and simulations, many researchers draw contradictory conclusions of the notch sensitivity, including the notch strengthening, weakening, and insensitivity in different MGs. In order to better understand its atomic mechanism, classical molecular dynamics simulations have been performed on the nano-scale notched MGs in the present work to investigate the deformation modes under the uniaxial tension. According to the results, after setting notches, the MG samples exhibit a significant change from the shear to necking failure, which indicates a notch strengthening phenomenon. At the same time, the evolution of atomic structures suggests that the introduction of notches will increase the degree of shear localization and further disturb the structural order of MGs. The atomic displacement, which is different from shear strain, can also be used as an index to visually describe the deformation behavior of MGs and shows the high notch sensitivity. [ABSTRACT FROM AUTHOR]

We numerically investigate non-uniformly strained Si-based systems to demonstrate that when a well focused laser beam locally excites the sample, the lattice distortion, impacting the band edge profile, causes a spatially dependent photovoltaic effect. It follows that, scanning the sample surface with the pump spot, a photovoltage signal can be acquired and used to quantitatively map the non-uniform strain field. To provide numerical evidence in this direction, we combine mechanical simulations with deformation potential theory to estimate the band edge energy landscape of a Si lattice strained by an array of SiN stripes fabricated on the top surface. These data are then used to simulate the voltage signal obtained scanning the sample surface with a normal incident pump beam. Our analysis suggests that strain deformations as small as 0.1% can trigger at room temperature robust photovoltaic signals. These results allow us to envision the development of a fast, cost-effective, and non-destructive setup, which leverages on the bulk-photovoltaic effect to image the lattice deformation in semiconductor crystals. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs) break the design concept of traditional alloys and exhibit excellent mechanical properties. However, as a new member of the alloy family in recent years, the dependence of the deformation behavior of the HEAs on alloy composition and twin boundary (TB) is still unclear, and many phenomena urgently need to be revealed. Here, the effects of TB spacing and Ni concentration on the mechanical properties and deformation behavior of the nano-twinned (CoCrFeCu)1−XNiX HEA (nt-HEA) under tensile loading are investigated by molecular dynamics simulation. The results show that with the decrease in TB spacing, the average flow stress of the nt-HEA changes from Hall–Petch strengthening to inverse Hall–Petch softening. When the TB spacing is greater than a critical value, the plastic deformation mechanism is dominated by the slip of partial dislocations. However, when the TB spacing is less than the critical value, the plastic deformation mechanism is transformed into the formation of voids induced by the amorphous phase, which becomes the key factor for the softening of the nt-HEA. It is also found that the mechanical properties of the nt-HEA can also change from strengthening to softening by adjusting Ni concentration, which is closely related to the change of stacking fault energy of the nt-HEA. In addition, the plastic deformation mechanism and voids formation mechanism of the nt-HEA are also discussed in detail. [ABSTRACT FROM AUTHOR]

We study the ion-irradiation-induced deformation of free-standing ultrathin Si wafers with a thickness of 8 μ m. The time-response spectrum of the deformation was measured using a laser displacement meter with a time resolution of 1 ms. The results showed that the deformation appeared during irradiation and disappeared after irradiation. The deformation was composed of a fast deformation with a millisecond time constant and a slow deformation with a second time constant. We performed a model calculation to identify the deformation mechanisms. We found that the fast deformation originated from expansion or shrinkage of crystal lattice caused by beam heating and deduced that the slow deformation resulted from the topological defect formation in Si crystals. The relaxation time of the slow deformation is related to the coordination number of disappeared topological defects. In this experiment, we conclude that the deformation of Si crystals maintains reversible behavior in the formation of topological defects up to the coordination number 5. [ABSTRACT FROM AUTHOR]

While microalloying of metallic glasses (MGs) using hydrogen emerged a few years ago, the underlying mechanism of how hydrogen microalloying influences mechanical behaviors is currently still under debate. Herein, we perform atomistic simulations to investigate the effect of hydrogen microalloying on the mechanical profiles of MGs. The mitigation of strain localization accompanied with plenty of shear transformation zones (STZs) is exhibited in H-alloyed MGs and becomes more significant with the increase of H content. The addition of hydrogen induces a loose atomic structure with fertile liquid-like regions and abundant free volume, which triggers more STZ events and the softening behavior. Furthermore, hydrogen doping lowers the STZ activation energy barrier, which is verified by a theoretical model based on the concept of energetic criterion. Our work provides some new insight on how to tune the local packing of MGs and further alter the deformation behavior. [ABSTRACT FROM AUTHOR]

Solid particles can be fragmented by a fast-moving fluid if their velocity difference is great enough, such as during the atmospheric entry of meteoroids or the shock compression of engineered particulate composites. The extent of particle deformation and breakup in such systems is poorly understood because the necessary extreme conditions make observation difficult and data scarce. To meet this need, experiments combining ultrafast synchrotron-based radiography with plate impact loading were performed at the dynamic compression sector at the advanced photon source. Metal microspheres of several densities and strengths (Au, Ta, and W) were placed inside a polymer matrix. A planar shock wave was then produced in the polymer by the impact of a gun-launched flyer plate. X-ray images of the resulting flow were collected at ∼150 ns intervals. These images document the progression of particle deformation across a range of flow conditions and particle materials. They show that the extent of deformation is sensitive to the ratio of drag stress to particle strength. The deforming particle's shape is determined by the initial shock–particle interaction, fluid stagnation pressure, and vorticity, each acting on its own timescale. A set of scaling relationships is presented to capture these observations and enable comparison with prior hydrodynamic data. The result is a framework for predicting the conditions under which strong particles are severely deformed by a shock-driven flow. [ABSTRACT FROM AUTHOR]

Dual-phase nanoscale lamellar microstructures containing alternating regions of soft phase and hard phase are known to produce alloys with an exceptional combination of strength and plasticity. Here, the effect of layer thickness on the mechanical properties and deformation mechanism of the fcc/bcc dual-phase CoNiFeAlxCu1−x high-entropy alloys multilayers are investigated by the molecular dynamics simulation method. The results show that the deformation behavior of the multilayers is strongly related to the layer thickness. At the yield point, the deformation behavior of the small thickness multilayer is caused by the dislocation slip in the fcc phase, while the plastic deformation of the large thickness multilayer is initiated by the bcc → hcp phase transformation in the bcc phase. During the subsequent plastic deformation, the phase transformation of bcc → fcc also occurred in the bcc phase of the multilayer, which depended on the relative size of the bcc phase in the multilayer. Especially for the multilayer with the layer thickness of 2 nm, the bcc → fcc phase transformation promotes the formation of twins, and the fcc/bcc phase interface transforms into a perfect twin boundary. The twin formation mechanism and phase transformation mechanism are also discussed in detail. [ABSTRACT FROM AUTHOR]

Constitutive relations for slip and deformation twinning behaviors of a number of metals, including α-titanium, copper, α-iron, and tantalum materials, are described over a very large range in imposed strain rates. The focus is on application to higher-rate Taylor (solid cylinder) impact tests, split-Hopkinson pressure bar measurements, and shock as compared with ramp-type isentropic compression tests and including simulations of the corresponding dislocation-based deformation behaviors. [ABSTRACT FROM AUTHOR]

We first revisit the mathematical modeling of the flexoelectric effect in the context of continuum mechanics at infinitesimal deformations. We establish and clarify the relation between the different formulations, point out theoretical and numerical issues related to the resulting boundary value problems, and present the natural extension to finite deformations. We then present a simple B-spline based computational technique to numerically solve the associated boundary value problems, which can be extended to handle unfitted meshes, hence allowing for arbitrarily-shaped geometries. Several numerical examples illustrate the flexoelectric effect in simple benchmark setups, as well as in new flexoelectric devices and metamaterials engineered for sensing or actuation. [ABSTRACT FROM AUTHOR]

While metallic glasses exhibit exceptionally high strength, their relatively low ductility, accompanied by catastrophic failure caused by the formation of shear bands, is the major obstacle to using these materials in practical applications. Despite discovering some methodologies for improving the near-zero ductility of metallic glasses, overcoming this deficiency is still the most active field of research in designing and fabricating bulk metallic glasses. This work utilizes computational studies at the atomistic scale to demonstrate that adding graphene to metallic glasses is a superior method to improve their ductility. Our results show that the graphene layers in metallic glass-graphene composites will enhance the ductility by activation of three deformation mechanisms, including (i) confining the space for shear band formation, (ii) retarding the propagation of embryonic shear bands, and (iii) increasing the resistance of the metallic glass matrix against shearing during the nucleation and propagation of shear bands. [ABSTRACT FROM AUTHOR]

In situ x-ray synchrotron diffraction experiments were carried out on nickel-based high-strength superalloys under pressure to understand their deformation mechanism using a diamond anvil cell (DAC). Radial x-ray diffraction determines the room-temperature equations of state and yield strengths of binary Ni3Al alloy and 500 ppm boron-doped Ni3Al to pressures of 20 and 46 GPa, respectively. Crystallographic preferred orientations observed in these superalloys due to anisotropic stress field in DAC indicate the onset of plastic deformation. Inverse pole figure analysis reveals that the underlying deformation mechanisms change from an octahedral slip to a simultaneous activation of octahedral and cube slips upon doping with boron. The yield-strength values were found to increase with pressure and are comparable to those determined from axial diffraction experiments. The results indicate that the yield strength of Ni3Al:B is about 0.5 GPa higher (at pressures below 20 GPa) due to grain boundary strengthening by boron. It is shown that due to high elastic anisotropy of Ni3Al alloy, the yield-strength estimations from diffraction experiments strongly depend on the micromechanical model used to convert the measured elastic strains into stresses. [ABSTRACT FROM AUTHOR]

Large compressive strains may introduce giant tetragonality and morphotropic phase boundaries in perovskite BiFeO3 films, where the coexisting tetragonal and rhombohedral phases (T like and R like phases) are identified to exhibit large piezoelectric response. Here, we have further achieved mechanical bending deformations in R like BiFeO3 through its neighboring T like BiFeO3 phases, where a strain gradient of ∼106/m was identified. Aberration-corrected scanning transmission electron microscopy revealed not only the strain distributions but also the atomic scale Fe polar displacement in the gradient R like BiFeO3. In spite of the giant strain gradient, the polarization direction in each R like BiFeO3 unit cell was found mainly along its diagonal direction, suggesting that potential flexoelectric coupling induced polarization in BiFeO3 is smaller than its spontaneous polarizations, while a large built-in electric field can be obtained via the large strain gradient. Our results indicate that the common phase coexistences in oxide materials could be further manipulated to introduce elastic strain gradients and tune the properties for oxide films. [ABSTRACT FROM AUTHOR]

Single-phase hexagonal close-packed structure of the ScYLaGdTbDyHoErLu high-entropy alloy was studied in detail. The applicability of the rule of mixture was analyzed with respect to the lattice constant, mechanical parameters, elastic properties, melting point, and hardness of the alloy. Significant tension-compression asymmetry has been found and explained by the strength differential effect during the uniaxial tests. Numerous deformation twins and high densities of stacking faults can be observed from the morphological characterization by a transmission electron microscope, which governs the main deformation mechanism during the plastic deformation in the current high-entropy alloy. [ABSTRACT FROM AUTHOR]

Engineering structures or materials are often subjected to multiple shock loadings. Mechanical response and its physical mechanism under such loadings are extremely complex and need to be studied in depth. To reveal double shock-induced deformation and microstructural evolution in the key structural material Ta, large-scale non-equilibrium molecular dynamics simulations of monocrystalline and polycrystalline Ta under double shock loading were performed. The results show that the activation and re-evolution of twins and dislocations introduced by the first shock dominate the plastic deformation during the second one. Some crystallographic orientation dependent mechanisms of plastic deformation under the second shock are revealed. Twin-dislocation conversion is dominant in the ⟨100⟩-orientated monocrystalline and polycrystalline Ta, while dislocation slipping is dominant in the ⟨110⟩- and ⟨111⟩-orientated Ta. The dependence of flow strength on the loading-paths of single and double shocks was also investigated. Shock-induced amorphization and recrystallization are observed in the single shock-loaded Ta models, leading to lower flow strengths than those of the double shock-loaded ones. These results help understand the complex relationship between the dynamic strength and intrinsic deformation mechanism of Ta under multiple shock loadings. [ABSTRACT FROM AUTHOR]

A cylindrical rubber fiber subject to a twist will also elongate: a manifestation of Poynting's effect in large strain elasticity. Here, we construct an analogous treatment for an active rubber fiber actuated via an axisymmetric pattern of spontaneous distortion. We start by constructing an exact large-deformation solution to the equations of elasticity for such fiber subject to imposed twist and stretch, which reveals spontaneous warping and twisting of the fiber cross section absent in passive rubbers. We then compute the corresponding non-linear elastic energy, which encompasses the Poynting effect but is minimized by a finite spontaneous twist and stretch. In the second half of the paper, we apply these results to understand the twist-contraction actuation of nematic elastomer fibers fabricated with director fields that encode helical patterns of contraction on heating. We first consider patterns making a constant angle with respect to the local cylindrical coordinate system (conical spiral director curves) and verify the predicted spontaneous twist, contraction, and cross-section deformation via finite elements. Second, we consider realistic director distributions for the experimentally reported fibers fabricated by cross-linking while simultaneously applying stretch and twist. Counterintuitively, we find that the maximum actuation twist is produced by applying a finite optimal twist during fabrication. Finally, we illustrate that spontaneously twisting fibers will coil into spring-like shapes on actuation if the ends are prevented from twisting relative to each other. Such a twist–torsion coupling would allow us to make a tendril-like "soft-spring" actuator with low force and high linear stroke compared to the intrinsic contraction of the elastomer itself. [ABSTRACT FROM AUTHOR]

Nematic elastomers are programmable soft materials that display large, reversible, and predictable deformation under an external stimulus such as a change in temperature or light. While much of the work in the field has focused on actuation from flat sheets, recent advances in 3D printing and other methods of directed synthesis have motivated the study of actuation of curved shells. Snap-through buckling has been a topic of particular interest. In this work, we present theoretical calculations to motivate another mode of actuation that combines programmable soft materials as well as instabilities associated with large deformation. Specifically, we analyze the deformation of a cylindrical shell of a patterned nematic elastomer under pressure, show that it can undergo an enormous change of volume with changing temperature and suggest its application as a pump with extremely high ejection fraction. [ABSTRACT FROM AUTHOR]

The effects of the carbon nanotube (CNT) length and material structure on the mechanical properties of free-standing thin CNT films with continuous networks of bundles of nanotubes and covalent cross-links are studied in large-scale simulations. The simulations are performed based on a dynamic mesoscopic model that accounts for stretching and bending of CNTs, van der Waals interaction between nanotubes, and inter-tube cross-links. It is found that the tensile modulus and strength of the CNT films strongly increase with increasing CNT length, but the effect of the nanotube length is altered by the cross-link density. The mutual effect of the nanotube length and cross-link density on the modulus and strength is primarily determined by a single parameter that is equal to the average number of cross-links per nanotube. The modulus and strength, as functions of this parameter, follow the power-type scaling laws with strongly different exponents. The film elongation at the maximum stress is dominated by the value of the cross-link density. The dispersion of nanotubes without formation of thick bundles results in a few-fold increase in the modulus and strength. The variation of the film properties is explained by the effects of the CNT length, cross-link density, and network morphology on the network connectivity. The in-plane compression results in the collective bending of nanotubes and folding of the whole film with only minor irreversible changes in the film structure. Depending on the CNT length, the reliefs of the folded films vary from a complex two-dimensional landscape to a quasi-one-dimensional wavy surface. [ABSTRACT FROM AUTHOR]

Progress in the synthesis of semiconductors of nanoscale structures has revived an interest in using semiconductors of different structures in the sensing of mechanical deformation through the change of photoluminescent characteristics under external stress/strain. In the heart of the deformation-induced change of photoluminescent characteristics is the deformation-induced change of the bandgap of semiconductors. In this work, we investigate the bandgap changes of bulk semiconductors of the wurtzite structure due to elastic indentation by a rigid, axisymmetric indenter. Using deformation potentials and the analytical solutions for the contact problems, we obtain closed-form solutions of the contact-induced change of the bandgaps for the elastic indentation by three different indenters of flat-ended, spherical, and conical shapes. For the elastic indentation by a flat-ended indenter, the bandgap changes are proportional to the indenter displacement/indentation load. For the elastic indentation by a spherical indenter, the bandgap changes are proportional to the cubic root of the indention load. For the elastic indentation by a conical indenter, the bandgap changes exhibit the characteristic of shape-similarity, independent of the indentation depth/load. [ABSTRACT FROM AUTHOR]

The mechanical deformation induced by reactive ion etching (RIE) of rectangular ridge waveguides in GaAs and InP has been investigated by photoluminescence and cathodoluminescence techniques. Several trends were identified and are discussed. First, it is concluded that the RIE process itself is the source of the mechanical deformation. A compressive volume change occurs mainly within the ridge (with a maximum close to the vertical etched sidewalls), extending outside the ridges, up to several micrometers below the bottom etched surface. An anisotropic deformation also appears, again mainly close to the etched sidewalls and below the bottom etched surface. A narrow area under tensile stress was also identified, localized outside the ridges and in a shallow region below the bottom etched surface. Cumulative, overlapping effects are seen inside the ridges where the compressive stress fields originating at the vertical etched sidewalls contribute to an overall compression inside the ridge which increases as the ridge width decreases. In addition, a tensile stress is also observed outside the ridge, strongly enhanced by the presence of neighboring ridges. These conclusions are significant for the design of photonic structures. Because of the photoelastic effect, which is important in GaAs and InP, the properties of devices such as waveguides might be affected by the mechanical stress described herein. [ABSTRACT FROM AUTHOR]

The introduction of a glass–glass interface is an effective way to improve the plasticity of metallic glass. However, the strength–plasticity trade-off has not still been effectively overcome. Here, the effect of the composition on the mechanical properties and deformation behavior of the CuZr nanoglass (NG) is investigated under tensile loading by a molecular dynamics simulation. The results indicate that high-performance NGs can be obtained by adjusting the percentage of Cu atoms. There is a critical Cu content (i.e., 75%), which makes the NGs have both high strength and high plasticity. The results show that with the increase in the Cu content, the deformation mechanism of the NGs changes from necking to uniform plastic deformation and then to the nucleation and the growth of the main shear band. Our results underscore the importance of the composition in the design and preparation of high-performance metallic glass. [ABSTRACT FROM AUTHOR]

Using density functional theory calculations, we investigate and report the electronic structure basis of symmetry-breaking in atomic structures and variations in extreme mechanical properties of a number of widely used polymers, namely, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, and three structural variants of PVDF. All of these polymers consist of C–C bonds in the backbone and a combination of C–H and C–F bonds attached to the backbone. Our results show that the relative proportion and arrangement of the C–H and C–F bonds, hereafter called the chemical heterogeneity, lead to two distinct sets of structural symmetries and mechanical properties. The set with the lower chemical heterogeneity possesses higher structural symmetry and exhibits higher strength and toughness. Electronic population analysis shows that chemical heterogeneity breaks the structural symmetry and alters deformation induced "electron redistribution patterns" in the C–C backbone. Strong variations in electron redistribution serve as the basis for distinctive mechanical behavior among the polymers: the higher the structural variation and electron redistribution, the lower the bond rupture force and toughness of the polymer. The bond rupture process is initiated by a decrease in the electronic population in the backbone, while the electronic interactions at the C–H and C–F bonds dictate where, how, and which bonds rupture. Furthermore, in the linear regime of mechanical deformation, the force–strain behavior is insensitive to the chemical heterogeneity and controlled by the strong covalent interactions of the C–C bonds. In the nonlinear regime, the chemical heterogeneity dramatically reduces the strength and toughness of the polymer chain. [ABSTRACT FROM AUTHOR]

Nanocages are structurally complex hollow low-dimensional materials that offer unique properties and functions that are inaccessible in bulk materials. They have tremendous potential in revolutionizing a number of fields including drug delivery and nanotechnology, but their applications remain limited primarily due to inadequate understanding of their extreme mechanical properties. Here, using reactive force field-based classical molecular dynamics simulations, we explore the bulk modulus, strength, and toughness modulus for a number of dodecahedral silica nanocages. The results show that, both under hydrostatic tension and compression, the mechanical properties vary nonlinearly with the structural parameters. Also, unlike bulk silica—which shows softening under tension in the nonlinear regime of mechanical deformation—silica nanocage exhibits stiffening at higher deformation that originates from the structural resistance of the nanocage. We show that the surface-area to volume ratio accurately describes the softening and stiffening behavior of the cage. Likewise, under compression, the nanocage shows three distinctive regimes: (i) linear decrease in stress with increasing strain, (ii) constant stress states with increasing strain representing shrinkage of the empty space, and (iii) exponential decrease in stress with increasing strain dominated by strong repulsion. These findings highlight the possibility of attaining a diverse set of mechanical properties from a nanocage by tailoring its structural parameters. [ABSTRACT FROM AUTHOR]

Applying a combination of atomistic and continuum scale simulations, we show that stress-localization forms the fundamental basis for toughening in "carbon nanotube reinforced amorphous silica" (CNT − aSiO 2). Depending on the cohesive strength of the interface, a propagating crack renders three distinct types of failure conditions: (i) with stronger cohesive interactions both silica and nanotube undergo catastrophic failure, (ii) with moderate cohesive interactions the nanotube debonds from the matrix and undergoes severe mechanical deformation but fracture remains in the matrix, and (iii) with lower cohesive strengths the nanotube debonds from the matrix easily and allows quicker failure of the matrix, compared to the previous two failure conditions. For either of the cases, continued propagation of the crack requires renucleation at the opposite side of the nanotube. However, the renucleation criteria are mostly unaffected by the strength of interfacial interactions. Also, the effective toughness of the nanocomposite increases nonlinearly with increasing interfacial strength and the maximum possible toughness enhancement is strictly controlled by the strength of elastic interactions between the nanotube and the matrix. The overall toughening behavior of the nanocomposite is governed primarily by stress-localization at the nanotube–silica intersections along the projected crack path direction in the composite. The observations highlight the dramatic role of site-selective interatomic interactions that can affect the macroscopic mechanical behavior of the nanocomposite substantially. [ABSTRACT FROM AUTHOR]

Here, we use molecular dynamics simulations as a tool to investigate vacancy clustering in pure aluminum single crystals. A 1% superconcentration of single vacancies are randomly introduced into an otherwise perfect lattice, and the system is allowed to evolve for 500 ns at an elevated temperature of 728 K. Under these conditions, the individual vacancies rapidly agglomerate into larger clusters to reduce their overall energy. The systems are then subject to mechanical deformation to failure. The results of a total of 35 molecular dynamics simulations are reported. The mechanical behavior of these systems is found to be highly sensitive to the vacancy cluster microstructure, with the largest cluster size being most closely correlated with the cavitation strength. Since the largest cluster size evolves, an interesting time–structure–property coupling governs the behavior of these supersaturated metals. Despite the idealizations of the microstructure and loading conditions, we find a remarkably favorable agreement with laser-driven spall experiments. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed to study the mechanical behaviors and microstructural evolution in nanocrystalline Cu films created by the phase field model under different strain rates and temperatures. The results indicate that grain boundaries' (GBs) migration caused by shear stress difference of GBs is found in the initial deformation stage. The migration on the site with a small curvature radius of curved GBs is large due to the high stress difference. The migration process of curved GBs in the initial stage is that atoms migrate from FCC structures to GBs along the (111) surface, which is different from the mechanism of atomic shuffling for the flat GBs. Meanwhile, the initial GBs migration can make curved GBs become flat. In addition to temperature and stress difference, the hexagonal-close-packed (HCP) structures including stacking faults and twin boundaries can accelerate GBs' migration. The influence of initial GB migration on mechanical properties is achieved by changing the fraction and distribution of HCP structures. Larger initial GB migration at a higher temperature significantly reduces stress concentration on GBs, which leads to the distribution of HCP changing from the grains with large initial GB migration to other grains. Therefore, the sites of crack nucleation at conditions of low and high temperatures are different due to different magnitudes of initial GB migration. [ABSTRACT FROM AUTHOR]

Achieving tunable magnetism in low-dimensions is an essential step to realize novel spintronic applications. In this manner, two-dimensional transition metal carbides/nitrides (MXenes) with intrinsic magnetism have attracted significant interest. In this study, we extensively examine the structural and magnetic properties of 1T- and 2H-Ti 2 C monolayers by using first-principles techniques. We reveal the dynamical stability of both phases by using phonon spectra analysis and a b i n i t i o molecular dynamics simulations. The magnetic ground state is determined by considering all possible spin configurations and taking into account spin–orbit coupling effects, strong onsite Coulomb interaction, and corrected self-interaction terms. Our results indicate that while 1T-Ti 2 C is anti-ferromagnetic, 2H-Ti 2 C exhibits ferromagnetism, which is stable at/above room temperature. The electronic structure analysis demonstrates that 1T-Ti 2 C is an indirect bandgap semiconductor and 2H-Ti 2 C is a half-metal with 100% spin-polarization. Additionally, it is shown that the magnetic state is robust against low mechanical deformations and fundamental bandgap (also half-metallic bandgap) can be tuned by compressive/tensile strain. Phase-dependent and tunable electronic and magnetic properties of Ti 2 C monolayers offer new opportunities in the field of low-dimensional magnetism. [ABSTRACT FROM AUTHOR]