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Abstract: Simulations of a monatomic model amorphous matrix embedded with approximately 37% of a body-centered cubic phase demonstrate mechanisms by which nanocrystallites can alter the mechanical response of metallic glass. Three effects affect the resulting ductility: (i) the presence of weak amorphous–crystalline interfaces, (ii) the fraction of nanocrystallites oriented to prevent twinning relative to the loading stress, and (iii) the shear-induced growth and dissolution of the nanocrystallites when they are impinged by shear bands. While the first effect dominates in these simulations due to system size limitations, the third effect appears to be crucial for understanding the ductility of experimental samples. These simulations indicate that shear-induced growth of existing nanocrystallites, rather than nucleation of new crystalline regions, may account for the observed enhancement in ductility. [Copyright &y& Elsevier]

Abstract: Nanoindentation was simulated in a two dimensional model metallic glass thin film using molecular dynamics. Strain localization was observed in simulations where the system was sufficiently structurally relaxed prior to deformation. Indentation simulations utilized an atomic indenter that adhered to the surface or a frictionless indenter. Boundary conditions were varied to constrain the film or allow the film to relax in-plane to examine the effect on shear band formation. The most constrained system, i.e. that with the atomic indenter and the constrained boundaries exhibited the highest hardness. [Copyright &y& Elsevier]

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. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The yielding of metallic glasses (MGs) is generally considered to be dependent on pressure or normal stress, which is usually described by the Mohr–Coulomb yield criterion. Experimentally, the orientation of the shear band angle or fracture angles in MG is roughly 40° to 42° in uniaxial compression and 50° to 59° in uniaxial tension, both with respect to the loading direction. However, the amounts of deviation from the maximum shear stress direction (45°) under uniaxial tension and compression are quite different, which seems to be inconsistent with the Mohr–Coulomb yield criterion. Here we carried out molecular dynamics simulations to study the incipient plasticity of model Dzugutov glasses subjected to various uniaxial and biaxial stress states. The yield points are defined as the sharp rise of the population of atoms participating in non-affine deformations, in addition to the traditional offset method. The yield surfaces thus created agree well with the Mohr–Coulomb yield criterion. Furthermore, the orientations of the embryonic shear bands (instead of the fully-grown shear bands, or fracture planes) are measured, which also follow predictions based on the Mohr–Coulomb criterion. Our simulation results call for orientational measurement of incipient shear bands in experiments to critically examine the Mohr–Coulomb yield criterion in MG systems. [Copyright &y& Elsevier]

Simulated nanoindentation tests on a model metallic glass reveal that the crack initiates inside a shear band via cavitation. The load-displacement curve was shown to be insensitive to the crack initiation but sensitive to subsequent crack propagation. The critical conditions for crack initiation were identified at both the macroscopic and microscopic levels. At the macroscopic level, the indenter geometry affects the overall critical load for crack initiation. Interestingly, the indentation volume at crack initiation appears to be a constant for different indenter geometries, based on which an analytical formula of the critical load as a function of the indenter geometry was derived. At the microscopic level, cavitation occurs once the normal stress perpendicular to the shear band exceeds a temperature-dependent critical cavitation stress. This critical cavitation stress was shown to reduce significantly upon shear deformation. [ABSTRACT FROM AUTHOR]