MICROSTRUCTURAL ASPECTS OF SUPERPLASTIC TENSILE DEFORMATION AND CAVITATION FAILURE IN A FINE-GRAINED YTTRIA STABILIZED TETRAGONAL ZIRCONIA

A fine-grained yttria stabilized tetragonal zirconia exhibits an optimum superplastic elongation to failure of approximately 700% at 1823 K and a strain rate of 8.3 x 10(-5) s-1. A detailed microstructural investigation of the superplastically deformed specimens reveals the occurrence of extensive concurrent grain growth and internal cavitation. An expression is developed to characterize the extent of deformation enhanced concurrent grain growth, as influenced by experimental factors such as true strain, strain rate and temperature. The variation in the level of concurrent cavitation with strain rate conforms closely to the variation in elongation to failure with strain rate. It is demonstrated that the tendency towards cavity interlinkage in a direction perpendicular to the tensile axis is an important factor influencing the total elongation to failure obtained in superplastic materials.