A nanoindentation study of serrated flow in bulk metallic glasses

Plastic deformation of two Pd- and two Zr-based bulk metallic glasses (BMGs) is investigated through the use of nanoindentation, which probes mechanical properties at the length scale of shear bands, the carriers of plasticity in such alloys. These materials exhibit serrated flow during nanoindentation, manifested as a stepped load-displacement curve punctuated by discrete bursts of plasticity. These discrete "pop-in" events correspond to the activation of individual shear bands, and the character of serrations is strongly dependent on the indentation loading rate; slower indentation rates promote more conspicuous serrations, and rapid indentations suppress serrated flow. Analysis of the experimental data reveals a critical applied strain rate, above which serrated flow is completely suppressed. Furthermore, careful separation of the plastic and elastic contributions to deformation reveals that, at sufficiently low indentation rates, plastic deformation occurs entirely in discrete events of isolated shear banding, while at the highest rates, deformation is continuous, without any evidence of discrete events at any size scale. All of the present results are consistent with a kinetic limitation for shear bands, where at high rates, a single shear band cannot accommodate the imposed strain rapidly enough, and consequently multiple shear bands must operate simultaneously. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.