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Oxide glass, one of the most transformative materials in the modern world, breaks easily under load due to its brittleness. Using classical molecular dynamics simulations, we prepared amorphous alumina by consolidating glass nanoparticles at room temperature. We showed that consolidated amorphous alumina exhibits work hardening ability, hence deforms homogeneously and fractures via necking under tension, while amorphous alumina obtained from the traditional melt‐quench process fractures catastrophically due to severe shear banding. This finding suggests that if processed properly, amorphous oxides could deform and fracture like ductile metals, which will significantly expand the applications of oxide glasses into new areas where load bearing or mechanical reliability is necessary.

 

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.