Diffusional Creep and Cavitational Strains in High-Purity Alumina under Tension and Subsequent Hydrostatic Compression

Diffusional creep and cavitation in pure alumina prepared with three different fabrication processes are compared under tension and subsequent hydrostatic compression. The deformation rates are separated into a volume-conserving creep rate and cavitational rate by measuring the longitudinal and transverse strains intermittently during deformation. Concurrent grain growth causes the volume-conserving strain rate to decrease in a manner consistent with Nabarro-Herring creep. The creep stress index of n = 1.3 and the average activation energy of Q = 480 kJ/mol are also consistent with Nabarro-Herring creep controlled by aluminum lattice diffusion. Anelastic loading and unloading transients are also identified and separated from the creep strains. High-voltage electron microscopy indicates that cavities nucleate at grain edges early and continuously in the creep process. The surfaces of these cavities tend after some growth to exhibit negligible curvature and various dihedral angles. The activation energy of Q = 450 kJ/mol and stress dependence of the cavitation rate of n = 1.3 are consistent with a grain boundary diffusional growth mechanism. The loading mode is found to have no significant effect on the cavitation rate during tensile creep and the subsequent decavitation rate during hydrostatic compression. The cavitation and decavitation rates are in good agreement with the model proposed by Speight and Beere when the effects of grain growth on cavity accumulation on grain boundaries are included. Exaggerated grain growth in high-density specimens can lead to early cavity coalescence and failure.