EFFECT OF THERMAL CONDITIONING AND STRAIN ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ALUMINA.

Yielding and fracture in dense, pure, fine-grained aluminum oxide have been studied using stress-strain curves obtained from four-point bend tests by means of a bent beam plastic stress analysis. Transitions from brittleness to ductility in bending were shown to occur at temperatures from 1200C to 1400C. Yielding was found to be a thermally activated process, having an apparent activation energy of 59,000 calsmole, similar to that for oxygen ion diffusion in alumina. The mechanism of plastic deformation was attributed primarily to stress enhanced diffusion in which the small grain size of the experimental material promoted ductility. Observations of grain boundary sliding and separation were also made. The effect of strain on the brittle fracture stress was investigated by prestraining specimens above the transition temperature and unloading, cooling, and testing to fracture below the transition temperature. It was found that for strains up to about 0.4 there was no significant effect on the fracture stress, but for higher prestrains, up to 1.5, reduction of the brittle fracture stress, averaging about 30, was observed. This weakening was believed to be caused by intergranular separation. The brittle fracture strength also appeared to decrease with increased porosity. Author