Work-hardening in sapphire (α-Al 2O 3)

Abstract

Quantitative measurements of the densities of the several components of the dislocation substructure in pure sapphire deformed by basal glide—glide dislocations, dipoles, and prismatic loops—indicate that the accumulation of loops, and the resultant loop-glide dislocation interactions, make the principal contribution to work-hardening. A work-hardening model based on the interaction of loops and glide dislocations is proposed and the work-hardening rate is calculated to be μ 225, in good agreement with experiment. In this model, the loops are generated from edge dislocation dipoles by self-climb. The effects of recovery through loop annihilation by climb is introduced into the expression for the work-hardening rate, and the plateau flow stress for a given strain-rate and temperature is found to be inversely proportional to the cube root of the bulk diffusion coefficient for the rate-controlling species (oxygen). Diffusion coefficients calculated from experimental plateau flow stress data are in good agreement with experimental values. The actual shape of the theoretical stress-strain curve obtained from the model is also in good agreement with experimental curves. The maximum work-hardening rate decreases with increasing temperature, a trend attributed to the fact that, at the higher temperatures, recovery begins during the yield-point region before work-hardening becomes fully established and also loop-dislocation interactions may be less effective.