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Shimizu, Futoshi; Ogata, Shigenobu*; Li, J.*
no journal, ,
The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for bulk metallic glass, based on small-scale molecular dynamics simulations (up to 20,000 atoms) and thermomechanical analysis. The model predicts the existence of a critical lengthscale 
100 nm and timescale 100 ps, above which melting transition occurs in shear-alienated glass. Large-scale molecular dynamics simulations with up to 20 million atoms have directly verified this prediction. When the applied stress exceeds the glue traction (computed separately before), we indeed observe maturation of the shear band embryo into bona fide mode-II or III shear crack, accompanied by melting. In contrast, when the applied stress is below the glue traction, the shear band embryo does not propagate, becomes diffuse, and eventually dies. Thus this all-important quantity, the glue traction (a property of shear-alienated glass), controls the macroscopicyield point of well-aged glass.
Li, J.*; Shimizu, Futoshi; Ogata, Shigenobu*
no journal, ,
Shear bands form in most bulk metallic glasses (BMGs) within a narrow range of uniaxial strain 2%. We propose this critical condition corresponds to embryonic shear band (ESB) propagation, not its nucleation. To propagate an embryonic shear band, the far-field shear stress must exceed the quasi steady-state glue traction of shear-alienated glass until the glass-transition temperature is approached internally due to frictional heating, at which point ESB matures as a runaway shear crack. The incubation lengthscale necessary for this maturation is estimated to be 10
nm for Zr-based BMGs, below which sample size-scale shear localization does not happen. We model 4 metallic glasses: a binary Lennard-Jones system, two binary embedded-atom (EAM) potential systems, and a quinternary EAM system. Despite vast differences in the structure and interatomic interactions, the four MD calculations give yield strain predictions of 2.4%, 2.1%, 2.6% and 2.9%, respectively.