The Role of Diffusion Creep in the Superplastic Deformation of 3 mol% Yttria Stabilized Tetragonal Zirconia

The ability of some polycrystalline materials to exhibit very large elongations to failure, termed superplasticity, has been studied extensively for metallic alloys (1). Following Wakai's (2) initial report of an elongation of >100% in a 3 mol% yttria stabilized tetragonal zirconia (3YTZ), there have been numerous reports of superplasticity in ceramics and ceramic composites (3-8). The mechanical properties of 3YTZ have been characterized in terms of the standard equation for high temperature deformation: $\varepsilon = \frac {ADGb}{kT}(\frac {b}{d})^p(\frac {\sigma}{G})^n$ where A is a dimensionless constant, D is the appropriate diffusion coefficient, G is the shear modulus, b is the magnitude f the Burger's vector, k is Boltzmann’s constant, T is the absolute temperature, d is the grain size, \sigma is the stress, and p and n are constants termed the inverse grain size and stress exponent, respectively. Superplasticity is associated with stress exponents of n< 3