Capturing Shear Bands and Explaining Plasticity of Metallic Glasses with Continuum Mechanics

Capturing a shear band in a metallic glass during its propagation experimentally is very challenging. Shear bands are very narrow but extend rapidly over macroscopic distances, therefore, characterization of large areas at high magnification and high speed is required. Here we show how to control the shear bands in a pre-structured thin film metallic glass in order to directly measure local strains during initiation, propagation, or arrest events. In-situ scanning electron microscopy with digital image correlation was utilized to measure local strain fields within, and in the vicinity of propagating shear bands in PdSi thin film metallic glasses. Dynamic stages of shear band propagation as well as multiple shear band arrest events are documented and quantified in terms of local von Mises strain fields. Quantification of local conditions for shear band propagation and arrest allowed to formulate a consistent model of shear banding purely within the framework of continuum mechanics. We claim that, at the nanoscale, metallic glasses always exhibit an elastic limit of about 5% which must be exceeded either at a stress concentrator to initiate a shear band, or at the tip of a shear band to support its propagation. At the same time, the “universal” elastic limit of about 2%, reported for various metallic glasses, reflects the violation of the shear band arrest condition within a large enough sample volume so that generated shear bands can escape from the sample and form surface steps. The presented continuum mechanics model of shear banding does not imply the existence of atomic-scale phenomena that are specific to metallic glasses, such as structural rejuvenation or collective activation of shear transformation zones. The model can successfully connect micro- and macroscopic plasticity of metallic glasses and suggests an alternative interpretation of controversial experimental observations.