Subgrain-boundary migration during creep of lif III. Stress reduction experiments

Abstract

The migration of subgrain boundaries and the development of the dislocation structure have been studied with the etch pit technique in the transient range of creep of pure -oriented LiF single crystals after large stress reductions, which were performed in the steady state of compressive creep at 773 and 923 K (i.e. homologous temperatures of 0·68 and 0·81). It was observed that in the initial part of the transients the migration distances per unit strain interval are eight to ten times larger than under steady-state conditions, which means that subgrain-boundary migration makes the dominant contribution to strain shortly after a large stress reduction. It is concluded that subgrain-boundary migration is an independent deformation process which at the same time is associated with recovery of the dislocation density. The dominance of subgrain-boundary migration after large stress reductions explains the experimental observation that the transient creep characteristics (variation in the creep rate with strain, and stress sensitivity of the constant-structure creep rate) are qualitatively different for small and large stress reductions. It is suggested that the rate of creep is determined by cooperative action of glide in the subgrains and migration of subgrain boundaries.