DEVELOPMENT OF SUPERPLASTIC STRUCTURAL CERAMICS

Superplastic structural ceramics (Y‐TZP, Al2O3, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or α phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain‐growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second‐phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain‐boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain‐boundary cracking during large strain deformation. Existing evidence suggests that a lower grain‐boundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid‐phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by grain‐boundary diffusion. However, deformation characteristics are frequently altered by interface reactions, secondphase hardening/softening, and dynamic grain‐growth‐induced strain hardening. Ductility and formability, on the other hand, are controlled by the flow stress and flaw distribution, not by deformation instability as in superplastic metals. Analytical models and empirical correlations are presented to describe various constitutive relations pertaining to superplastic ceramics. Copyright © 1990, Wiley Blackwell. All rights reserved