Deformation kinetics of Zr-based bulk metallic glasses Temperature and strain rate influences on shear banding

Abstract

At low homologous temperatures metallic glasses exhibit inhomogeneous flow behaviour, which is associated with narrow shear banding. Based on the width of the shear bands and the time-dependent heat conduction, we show here that this process is not fully adiabatic at low strain rates, even though temperatures are sufficiently high to create a drop in viscosity within either a new or a pre-existing shear band. Evaluation of the deformation kinetics at cryogenic temperatures suggests an increase in viscosity within the shear band, although temperatures are still sufficiently high to cause localised melting at fracture. In addition, a change from serrated to non-serrated flow can be observed if the temperature is lowered below a critical value. This macroscopic change in the flow behaviour is directly related to a change in the strain rate sensitivity from negative to positive values, suggesting a clear change in deformation behaviour. We propose a refined shear dilatation model to explain the experimental findings.