Competition between shear band nucleation and propagation across rate-dependent flow transitions in a model metallic glass

Abstract

Shear transformation zone (STZ) dynamics is used to examine the transition between different regimes of flow serration in the strain rate dependent deformation of metallic glass. To capture the strain rate independent yield strength of Vitreloy 1 at low to moderate strain rates, the model is adapted to include STZ volume and activation energy that decrease with increasing strain rate. The different stages of shear banding are examined in a statistical fashion over six different strain rates ranging from 10−5 to 100 s−1, with twelve replicates at each strain rate. Examination of flow serration, shear band nucleation rates, propagation rates, and sliding rates in each simulation find support for the hypothesis that the flow transition is caused by high shear band propagation and sliding rates at low strain rates, and high shear band nucleation rates at high strain rates. The underlying cause for the flow transition is hypothesized to be a strain rate dependent critical shear band nucleus size that increases with increasing strain rate. This critical shear band nucleus size results from the strain rate dependent STZ volume and activation energy, in which very small variations can cause a large change in shear banding behavior.