Grain Boundary Diffusion in Severely Deformed Metals: State of the Art and Unresolved Issues

Abstract

An overview of current understanding of diffusion properties of grain boundariesin severely deformed materials is presented. The results are evaluated with respect to thetypes of severe plastic deformation, defects introduced by such processing, and processing pa-rameters. An extended hierarchy of diffusion paths is shown to exist in ultra- ne grainedmaterials, with relative diffusion rates being different by orders of magnitude. Properties ofdeformation-modi ed grain boundaries, such as width, diffusivity, diffusion mechanism and freevolume excess, are examined. A model of deformation-modi ed state of general high-angle grainboundaries is discussed with respect to the underlying concept of free volume accumulation.The relaxation stages of the deformation-modi ed state of grain boundaries are analyzed.