Suppression of Cavity Formation in Ceramics: Prospects for Superplasticity

Ceramics exhibit macroscopic stressistrain rate relations that should lead to superplastic extension. However, premature fracture is normally encountered, due to the formation and growth of grain-boundary cavities. Thus, cavity nucleation and growth were analyzed in an attempt to identify microstructures and/or strain-rate regimes that would suppress cavity evolution and hence allow superplasticity. Analysis of cavity nucleation indicates that fine-grained materials devoid of grain-boundary amorphous phases and inclusions should sustain substantial deformation rates without nucleating cavities, especially if solid-solution additions that encourage rapid grain-boundary diffusion (while not excessively decreasing surface energy) are identified. The analysis of void growth indicates that high relative surface diffusivities are also desirable, indicating that alloy additions that do not depress (and probably enhance) the relative surface diffusivities must be selected.