Unification of Harper-Dorn and power law creep through consideration of internal stress

Abstract

A single relation has been developed to describe the steady state creep behavior of coarse grained polycrystalline metals over a wide range of stress. The range of stress includes low stress Harper-Dorn (H-D) creep, intermediate stress power-law creep, and high stress power-law-breakdown creep. The unified creep relation is based on a diffusion-controlled dislocation creep model incorporating an internal stress which is visualized both to assist and inhibit the motion of dislocations. The internal stress arises from the presence of dislocations within subgrains. An extension of the unified creep relation is made by introduction of a structure dependent term based on the influence of subgrain size on the creep rate. It is proposed that the creep rate in the H-D regime can be decreased by subgrain size refinement and by a decrease in random dislocation density. The unified creep relations are shown to predict accurately the steady state creep rate of aluminum for over four orders of magnitude of stress in the temperature range of 0.21–0.99 T m In addition, good correlations are obtained for analysis of creep data for tin and lead, including the previously anomalous micro-creep tin data of Chalmers.