Analysis of deformation mechanisms in superplastic yttria-stabilized tetragonal zirconia

There have been extensive experimental observations of changes in the appar ent rate controlling creep parameters in studies on superplastic materials. The three most common explanations associated with these changes in the st ress exponent, n, the activation energy Q and the inverse grain size expone nt, p involve the effect of concurrent grain growth, the operation of a thr eshold stress or transitions in creep mechanisms. Each of these factors may influence experimental creep data in a similar manner. Therefore, a carefu l analysis of the consequences of all three factors must involve the develo pment of a consistent set of experimental observations in order to adequate ly distinguish the effects of each. This paper discusses the role of concur rent grain growth, a threshold stress and transitions in creep mechanisms i n superplastic materials. Specific attention is given to the analysis of da ta on superplastic yttria-stabilized zirconia ceramics for which an increas e in n has been observed at low applied stresses. It is demonstrated that n either concurrent grain growth nor a threshold stress can account for all t he relevant experimental observations in this material. It is concluded tha t the changes in rate controlling creep parameters are associated with the operation of two distinct sequential mechanisms as part of a grain boundary sliding process. (C) 2001 Published by Elsevier Science Ltd. All rights re served.