Like-metal superplasticity of fine-grained Y2O3-stabilized zirconia ceramics

The superplastic deformation of fine-grained yttria-stabilized zirconia pol ycrystals (YSZPs) has been revised through a detailed analysis. The results indicate that high purity YSZPs (residual impurity content below 0.1 wt%) display a transition in the apparent stress exponent n(ap) from 2 (region I I) to higher values (region I) with decreasing stress; a further change tow ards 1 (region 0) is found at very low stresses. A continuous variation in the apparent activation energy and grain size exponent with stress, grain s ize and temperature is found in region I. By contrast, low-purity YSZPs dis play only region II over the entire stress range. The threshold stress appr oach used to explain the conventional and high-strain-rate superplasticity of metallic alloys accounts for the mechanical characteristics of high-puri ty YSZP ceramics. The constitutive equation for superplastic flow is indeed identical with that found for metals when lattice diffusion is rate contro lling. Yttrium segregation at grain boundaries is compatible with a thresho ld stress for grain-boundary sliding.