SUPERPLASTIC BEHAVIOR OF A FINE-GRAINED, YTTRIA-STABILIZED, TETRAGONAL ZIRCONIA POLYCRYSTAL (Y-TZP)

The microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated. Both hot indentation and tensile tests were carried out at temperatures between 1273 and 1923K over the strain rate range from 2.7 × 10-5 to 2 × 10-3 s-1. It was found that the material exhibited extensive plasticity at temperatures higher than 1473K; a maximum tensile elongation of over 800% was recorded. Microstructural examination did not indicate the presence of a glassy phase at grain boundaries. Yttrium, however, was found to segregate to the grain boundaries. The microstructure of the Y-TZP was thermally unstable and appreciable grain growth was observed at emperattures higher than 1723 K; the grain growth was enhanced by external stresses, i.e. dynamic grain growth was observed. Grain growth at elevated temperatures resulted in apparent strain rate sensitivity exponents of approximately 0.33 at 1723K. This value decreased with increasing temperature. The grain size-compensated strain rate, however, was found to depend approximately on the square of the flow stress, i.e. to exhibit a true strain sensitivity value of 0.5, which suggests a grain boundary sliding mechanism. Microstructures from samples that were deformed superelastically indicated that grains remained equiaxed; this observation is consistent with a grain boundary sliding mechanism. The activation energy for superplasticity, under the conditions of constant structure, in Y-TZP was calculated to be 720 kJ/mol. © 1990.