Softening-induced plastic flow instability and indentation size effect in metallic glass

Despite the absence of microstructural features, metallic glasses (MGs) could display size-dependent hardness at the submicron scale. While most early studies attributed this size effect to Weibull statistics, here we propose a mechanism related to shear softening induced flow instability that can give rise to a deterministic indentation size effect in MGs. In line with this mechanism, an explicit relation is derived linking the size dependency of hardness to a critical length scale that governs the transition from a stable to unstable plastic flow in MGs. Through a series of carefully designed spherical indentation tests, this mechanism is experimentally justified, from which we are able to extract the critical transition length for a Zr-based MG at different indentation strain rates. On the basis of the combined theoretical/experimental efforts, our current work provides a quantitative insight into the indentation size effect in MGs.