Available deformation data for superplastic yttriastabilized zirconia polycrystals with grain size <1 µm have been analyzed at temperatures between 1250°and 1450°C as a function of stress, grain size, and impurity content. The apparent stress exponent n for the higher-purity materials (residual impurity content <0.10 wt%) varies from 2 (region II) to ≥3 (region I), and then toward 1 when the stress is decreased. The stress for transition between region II and region I decreases when the temperature and/or grain size is increased. The activation energy Q for flow in region II is 460 kJ/mol, which is approximately that for cation lattice diffusion. The grain-size exponent p decreases continuously and Q increases continuously with decreasing stress in region I. The constitutive equation for superplastic flow in region II is identical to that for metallic systems when lattice diffusion is the rate-controlling mechanism. The experimental results have been correlated with a single deformation process that incorporates a threshold stress, below which grain-boundary sliding does not contribute to strain. The threshold stress may result from yttrium segregation at grain boundaries and its interaction with grainboundary dislocations. A single deformation regime with n = 2 exists for low-purity materials (impurity content >0.10 wt%) over the entire stress range. The strain-rate enhancement with respect to high-purity materials is related to the grain-boundary amorphous phase present in such materials.

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