Observations on the fracture and deformation behaviour during annealing of residually stressed polycrystalline aluminium oxides

Abstract

Highly residually stressed polycrystalline aluminium oxides were found to exhibit residual stress relaxation, as evidenced by changes in load-bearing ability, at temperatures as low as about 850° C. This temperature is much too low for such relaxation to occur by dislocation, Nabarro—Herring or Coble creep. Irreversible changes in specimen dimension coupled with SEM-fractography revealed that the stress relaxation resulted from creep by intergranular cavitation and crack propagation. In one aluminium oxide, such cavitation and crack propagation appeared to take place in a stable mode along a viscous glassy grain boundary phase. In high-purity fine-grained aluminium oxide, crack propagation occurred in a frequently totally catastrophic and highly unstable manner. This latter material was also observed to exhibit spontaneous fatigue during isothermal anneal. Implications of the findings of this study for the use of thermal anneals to promote residual stress relaxation in structural ceramic materials are discussed.