On the applicability of diffusional flow to the creep of polycrystalline materials at low stresses and intermediate temperatures

Abstract

It is shown that no evidence exists for diffusional (Coble) creep in single-phase metal-base polycrystals at intermediate temperatures and low stresses. At stresses above about 10−5E (where E is Young's modulus), grain boundary sliding is more facile than diffusional flow and, at stresses below 10−5E, plastic flow mechanisms not associated with diffusional flow dominate deformation. The recent statements of Sritharan and Jones regarding the inapplicability of a phenomenological relation based on grain boundary sliding in describing low stress creep are shown to be incorrect. Specifically, their arguments are invalid regarding (1) the representation of creep data as dotϵ ∫ σ2 (dotϵ is the creep rate and σ is the stress), (2) the use of an “empirical” equation for describing the grain boundary diffusivity and (3) the position of the boundary between diffusional creep and grain boundary sliding in a deformation mechanism map. Phenomenological creep relations for describing grain boundary sliding and diffusion-controlled dislocation creep correctly predict the influence of grain size on the range of stress and strain rate where dotϵ ∫ σ2.