The transition from localized to homogeneous plasticity during nanoindentation of an amorphous metal

Using nanoindentation, we examine the fundamental nature of plasticity in a bulk amorphous metal. We find that the mechanics of plasticity depend strongly on the indentation loading rate, with low rates promoting discretization of plasticity into rapid bursts. For sufficiently slow indentations, we find that plastic deformation becomes completely discretized in a series of isolated yielding events. As the loading rate is increased, a transition from discrete to continuous yielding is observed. These results are fundamentally different from the classical expectations for metallic glasses, in which the transition from discrete to continuous yielding occurs upon a decrease in deformation rate. The present experimental results are analysed with reference to the theoretical ideal-plastic strain field beneath an indenter and rationalized on the basis of mechanistic models of glass plasticity.