Yielding and shear banding of metallic glasses

In this study the yielding and subsequent shear banding evolution process of Zr-, Cu-, Fe- and Mg-based metallic glasses (MGs) were investigated using carefully designed cyclic microcompression tests. It was found that yielding starts from a stable plastic flow with a viscosity of the order of 1012 Pa s, resembling that of the glass transition. This provides critical evidence that yielding is caused by a stress-induced glass transition with internal randomly distributed liquid-like cores get connected. Up to a critical point the liquidized layer penetrates the entire sample with an initiation viscosity of 108 Pa s, comparable with that of the liquid-like cores. Along the liquid-like layer shear band propagation involves shear band sliding and is succeeded by shear band arrest. Dynamic softening leads to an increase in the velocity of shear band sliding, with resultant shear offset, which can be successfully captured by a linear softening model. Once the elastic energy in the shear band is dissipated its internal structure begins to recover, with the solid-like matrix being reconstructed, resulting in shear band arrest. A simple diagram elucidating the yielding and shear banding dynamics is constructed, which sheds light on the fundamental nature of the deformation mechanisms of MGs.