High temperature deformation behavior of a fine-grained tetragonal zirconia

The stress exponent, n, defined in the following creep equation has often been regarded as a primary parameter to characterize superplastic deformation in fine-grained tetragonal zirconia containing 2.5 approximately 4 mol% yttria (Y-TZP): varepsilon = Aσn/dp where varepsilon is the strain rate, σ is the stress, d is the grain size, n is the stress exponent, p is the grain size exponent and A is a material constant. Recent studies have noted that the stress exponents of high-purity Y-TZP can be divided into two categories: n approximately 3 at low stresses and n approximately 2 at high stresses, where the stress dividing the deformation regions depends on both temperature and grain size. To argue the origins of such regions and relating mechanisms, however, some additional examination seems to be necessary for confirming that the regions characterized with n approximately 2 and approximately 3 are the genuine ones. This is because experimental limitations have tended to prevent the examination of deformation behavior by Eq. (1) in a strict sense. For example, the n-values have been derived from the overall strain rates that may indispensably include the effects of deformation around the grips of tensile specimens or those of constrained deformation near both sides of compression specimens. Furthermore, the data were obtained under an assumption that the effects of grain growth on the strain rate is negligible in Y-TZP. There seems to be rather little assurance, however, that these situations did not affect the evaluation of the stress exponent. From this point of view, the present study was conducted (1) to examine the effects of grain growth and some other experimental factors on creep behavior and (2) to evaluate the stress exponent from creep strain-rate curves corrected for both instantaneous stress and strain in a high purity Y-TZP