Mechanism of negative strain rate sensitivity in metallic glass film

The observation of negative strain rate sensitivity during nanoindentation is a common, yet poorly understood phenomenon in metallic glasses. In this study, a combination of experimental investigation and atomistic simulation has been employed to understand the mechanism behind this perplexing effect. The results suggest that the negative strain rate sensitivity arises out of the influence of a slow time-dependent relaxation process during the driven deformation at a controlled rate. By complementing the experimental outcome by simulation results, it is possible to show that this unification of the rate-driven and relaxation mechanisms is the consequence of their overlapping timescales. In addition, we demonstrate that by using the numerical results of simulated nanoindentation, it is possible to calculate the size and activation volume of the shear transformation zone, which is otherwise difficult using only experimental data in the regime of negative strain rate sensitivity. (C) 2019 Elsevier B.V. All rights reserved.