It was noted that the predominance of phenomenological power laws, in describing the creep of crystalline materials, indicated that the dislocation mechanics of inelastic deformation of crystalline materials was not yet fully understood. In general, dislocation-mediated plasticity led to the generation of dislocations in the crystal interior. Creep (plasticity at constant stress) continued only if the dislocations were able to disappear again (recovery). A simple model for the creep of subgrain-free materials reproduced the characteristic features of steady-state creep, but exhibited significant quantitative deficiencies; thus indicating that sub-grain formation could not be neglected. It was proposed that the migration of low-angle sub-grain boundaries was the process which controlled creep in most single- and multi-phase materials having conventional grain sizes. High-angle boundaries began to play a significant role when their spacing, d, approached the steady-state sub-grain size, w∞, which developed in coarse-grained materials. Depending upon the w∞/d ratio and the deformation conditions, high-angle boundaries could harden or soften the material during steady-state deformation.

Dislocation Mechanics of Creep. W.Blum, P.Eisenlohr: Materials Science and Engineering A, 2009, 510-511, 7-13