High-strain-rate superplasticity in oxide ceramics:a trial of microstructural design based on creep-cavitation mechanisms

From existing knowledge about high-temperature cavitation mechanisms,necessary conditions were discussed for the suppression of cavitation failure during superplastic deformation in ceramic materials.The discussion,where special attention was placed on the relaxation of stress concentrations during grain-boundary sliding and cavity nucleation and growth,leaded to a conclusion that cavitation failure could be retarded by the simultaneous controlling of the initial grain size,the number of residual defects,diffusivity,dynamic grain growth and the homogeneity of microstructure.On the basis of this conclusion,high-strain-rate superplasticity (defined as superplasticity at a strain rate higher than 0.01 s-1) could be intentionally attained in some oxide ceramic materials.This was shown in tetragonal zirconia and composites consisting of zirconia,α-alumina and a spinel phase.