Abstract:
Metallic glasses, due to the long-range disorder of atoms, expand locally or globally in response to shear, bearing a resemblance to Reynolds' dilatation of granular media. It has been widely accepted that shear flow is allowed in metallic glasses via a cascade of local dilatation events, which was mainly modeled by Spaepen's free volume theory or Argon's shear transformation theory. The shear-dilatation coupling leads to the pressure (or normal stress) sensitivity of macroscopic failure (flow and fracture) of metallic glasses, which essentially differs from that of their crystalline counterparts. Recently, it has been revealed that the dilatation itself can dominate brittle fracture of metallic glasses, where the crack tip propagates via cavitating events that involve a series of nanovoids nucleation and coalescence with very limited plastic growth. Therefore, the dilatation holds the key to understand the flow and fracture of metallic glasses. In this talk, we attempt to provide an up-to-date review on this aspect, but from a viewpoint of the dilatation. We first review the representative flow modes for metallic glasses, where the shear-dilatation correlation is highlighted. Theoretical as well as atomistic modeling and experimental work on the dilatation degree during shear banding at different stages are all discussed. Typical yielding criteria accounting for the dilatation effect are then described. We further present the atomic-level mechanism for local shear and dilatation events, respectively. Based on the inherent shear-dilatation competition, a united failure criterion is constructed, in which the governing factors to determine the ductile or brittle failure of metallic glasses are revealed.
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