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Subcritical crack growth can occur under a constant applied load below the threshold value for catastrophic failure, also known as static fatigue. Here, we report how a crack grows under a combination of stress-intensity factor (K) and temperature in a model brittle glass using molecular dynamics simulations. The model glass is under dry conditions, thus avoiding the complexity of corrosion chemistry. The crack growth rate is shown to be inconsistent with the commonly used subcritical crack growth model rooted in the transition state theory (TST), in which the applied stress-intensity factor reduces the transition barrier. A new subcritical crack growth model is proposed with a constant barrier and a K-dependent prefactor in TST, representing the size of the region for potential bond breaking. The thermomechanical condition for subcritical crack growth is also mapped in the K-T domain, in between elastic deformation and catastrophic fracture regimes. Finally, we show substantial crack self-healing once the applied load is removed, under the thermodynamic driving force of surface energy reduction. Our findings provide new insights into the mechanochemical coupling during static fatigue and call for experimental investigation of whether the activation energy is K-dependent. 

Abstract The discovery of ferroelectricity in AlN‐based thin films, including Al_1‐xSc_xN and Al_1‐xB_xN, over the past few years has spurred great research interests worldwide. In this review, we carefully examined the latest developments for these ferroelectric films with respect to alloy composition, temperature, film thickness, deposition condition, and fatigue endurance by electric field cycling. Looking ahead, there is an urgent need to resolve the challenge of large current leakage faced by these films, which necessitates a combined efforts from both simulations and experiments to identify the root cause and eventually come up with engineering strategies to suppress such leakage. In addition, overcoming the thickness scaling challenge to push ferroelectric thin film down to a few nanometers for better device miniaturization will also be of great interest. Considering the somewhat unexpected discovery of AlN‐based thin films with potential ferroelectric application, we believe that it will be also rewarding to further explore other III‐V‐based semiconductor materials. 

Understanding how nanoparticles deform under compression not only is of scientific importance but also has practical significance in various applications such as tribology, nanoparticle-based probes, and the dry grinding of raw materials. In this study, we conducted compression tests on model brittle glassy nanoparticles using molecular dynamics simulations. We found that during the early stages of plastic deformation, shear bands formed in a similar pattern regardless of the nanoparticle size. However, as the deformation continued, dominant cracks emerged in large nanoparticles while being suppressed in smaller ones. This size-dependent brittle-to-ductile transition can be explained by a simple model based on Griffith's theory. We also investigated the effect of the surface stress state on fracture using thermally tempered nanoparticles. We observed that the presence of compressive surface stress strengthened the nanoparticle by suppressing crack formation, even when a pre-notch was present. On the other hand, tensile surface stress had the opposite effect. Interestingly, nanoparticles with both tensile and compressive surface stress promoted shear deformation, which could potentially compromise the mechanical performance of tempered glass despite delayed crack formation. 

Understanding how nanoparticles deform under compression is not only of scientific importance, but also has practical significance in various applications such as tribology, nanoparticle-based probes, and dry grinding of raw materials. In this study, we conducted compression tests on model brittle glassy nanoparticles using molecular dynamics simulations. We found that during the early stages of plastic deformation, shear bands formed in a similar pattern regardless of nanoparticle size. However, as the deformation continued, dominant cracks emerged in large nanoparticles while being suppressed in smaller ones. This size-dependent brittle to ductile transition can be explained by a simple model based on Griffith's theory. We also investigated the effect of surface stress state on fracture using thermally tempered nanoparticles. We observed that the presence of compressive surface stress strengthened the nanoparticle by suppressing crack formation, even when a pre-notch was present. On the other hand, tensile surface stress had the opposite effect. Interestingly, nanoparticles with both tensile and compressive surface stress promoted shear deformation, which could potentially compromise the mechanical performance of tempered glass despite delayed crack formation. 

Understanding the dynamics of shear band propagation in metallic glasses remains elusive due to the limited temporal and spatial scales accessible in experiments. In micron-scale molecular dynamics simulations on two model metallic glasses, we studied the propagation of a dominant shear band under uniaxial tension with a macroscopic strain of 3-5%. For both materials, the shear band can be intersonic with a propagation speed exceeding their respective shear wave speeds. The propagation exhibits intrinsic instability that manifests itself as microbranching and considerable fluctuations in velocity. The shear strain singularity ahead of propagating shear band tip scales as 1/r (r is the distance away from the tip), independent of the macroscopic tensile strain. In addition, we studied the intersection of two shear bands under uniaxial tension, during which path deflection, speed slowing-down, and temperature rise at the junction region were observed. The dynamics of propagating shear band shown here indicate that shear band in metallic glasses can be viewed as shear crack under the framework of weakly nonlinear fracture mechanics theory. 

Abstract The recent realization of ferroelectricity in scandium‐ and boron‐substituted AlN thin films has spurred tremendous research interests. Here we established a molecular dynamics simulation framework to model the ferroelectricity of AlN thin films. Through reparameterization of Vashishta potential for AlN, the coercive field strength and the AlN polarization were found to be close to experimental values. Furthermore, we examined the effects of film thickness, temperature, in‐plane strain on polarization‐electric field hysteresis loop, and the thickness‐dependent Curie temperature. Lastly, we incorporated electrodes towards atomic‐level modeling of ferroelectric device, by considering the induced charge at the interface between electrodes and ferroelectric film. We found that low dielectric contrast significantly lowers the coercive field for switching AlN.