The description of elevated temperature deformation in terms of threshold stresses and back stresses: A review

Abstract

Description of elevated temperature creep are often based on the idea that threshold stresses or back stresses which oppose deformation play a key role in the deformation process. In this review the use of these concepts to characterize fluid flow, diffusional creep, superplastic deformation, power law creep in relatively pure metals and dislocation creep in dispersion-strengthened materials is examined. Both the experimental data and the theoretical justifications for threshold stresses and back stresses are considered. The operational methods for determining these parameters are also described. It is noted that the enthusiasm with which these concepts are accepted and used is often not justified by the available experimental evidence. Particular attention is devoted to the terminology which is used to described these ideas, since it is undoubtedly a source of confusion in furthering the understanding of elevated temperature deformation mechanisms. In this discussion the term “threshold stress” is restricted to cases where the deformation is opposed by a fixed resistance which is independent of the imposed stress or strain rate. Back stresses are classified as arising from the prior deformation history or from the deformation process itself. Finally, it is suggested that a certain amount of care is needed in using a net or effective stress to correlate data, in that fundamental details of the deformation mechanism may be obscured by this procedure.