Deformation of structural superplastics, nanostructured materials and metallic glasses: a unified approach

Abstract

Structural superplasticity up to the point of inflection in the log stress ~ log strain rate curve and the deformation of nanostructured materials were recently traced co grain/interphase boundary sliding controlled flow (1-9], Random close packed atomic ensembles around a free volume site at a boundary constitutes the basic unit of sliding, Sequential shearing of a basic unit till the edge of a boundary causes a measurable sliding displacement Mesoscopic (cooperative) sliding results from boundary migration and plane interface formation involving two or more contiguous boundaries. By interconnection the plane interfaces cause extensive flow. High temperature deformation of metallic glasses also can be understood in terms of this mechanism with the lower density regions of intense shear corresponding to grain/interphase boundaries and the higher density islands of atomic aggregates that experience little deformation being analogous to the quasi-rigid grains of the superplastics. Decrease in the rate sensitivity of flow in superplastics at high strain rates is due to the emergence of multiple dislocation slip as an independent additional mechanism but in metallic glasses to the slope change inherent in a hyperbolic sine dependence of the log strain rate - log stress reelationship. It is not clear if Argon's dilatation retaining deformation process [10] can lead to significant flow in a metallic glass. In this paper, the deformation behaviour of structural superplastics, nanostructured materials and metallic glasses at high temperatures - all materials of highly disordered struetures [11] - is described in a unified manner.