NUCLEATION AND GROWTH-CHARACTERISTICS OF CAVITIES DURING THE EARLY STAGES OF TENSILE CREEP DEFORMATION IN A SUPERPLASTIC ZIRCONIA-20 WT-PERCENT ALUMINA COMPOSITE

Constant-stress tensile creep experiments on a superplastic 3-mol%-ytt ria-stabilized tetragonal zirconia composite with 20 wt% alumina revea led that cavities nucleate relatively early during tensile deformation . The number of cavities nucleated increases with increasing imposed s tress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attai n dimensions on the order of the grain facet size. It is suggested tha t coarser-grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet-cracks and their i nterlinkage to form a macroscopic crack of critical dimensions.