Despite the absence of microstructural features, metallic glasses (MGs) could display sizedependent 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.