3 - Diffusional Creep

Abstract

Nondislocation based diffusional creep at high temperatures and very low stresses in fine-grained materials was qualitatively suggested 50 years ago by Nabarro. This was rigorously (quantitatively) proposed and described by Herring. Mass transport of vacancies through the grains from one grain boundary to another was described. Excess vacancies are created at grain boundaries perpendicular to the tensile axis with a uniaxial tensile stress. These excess vacancies diffuse from the grain boundaries lying normal to the tensile direction toward those parallel to it. In 1963, it was proposed that a mechanism by which creep was instead controlled by grain-boundary diffusion. It was suggested that, at lower temperatures, the contribution of grain-boundary diffusion is larger than that of self-diffusion through the grains. Thus, diffusion of vacancies along grain boundaries controls creep. The existence of diffusional creep has been questioned over the last decade by some investigators and defended by others. One major point of disagreement is the relationship between denuded zones and diffusional creep. Many studies have been devoted to assess the separate contributions from diffusional creep and grain-boundary sliding to the total strain. Some claim that both diffusional creep and grain-boundary sliding contribute to the overall strain and that they can be distinctly separated; other claim that one of them is an accommodation process