Cavitation behaviour in fine grain 3Y-TZP during tensile and compressive superplastic flow

Studies of cavitation in Y-TZP during superplastic flow have been made for both tensile and compressive deformation conditions. It was observed that t he morphologies of cavities near the fracture faces of tensile specimens va ried markedly with testing conditions and in most cases differed from those near the gauge heads. Two quite different forms of cavitation behaviour we re observed leading to high and low strains to failure, respectively. For o ptimum conditions of superplastic flow, of high temperature/low strain rate (low stress), when large elongations were observed, cavities were either s pherical or elongated parallel to the tensile axis. Those near the fracture face interlinked in a plastic (necking) mode to give transverse cavities a nd subsequent failure. At high strain rate/low temperature (high stress), transverse intergranular cracking played a dominant role in failure at low elongations. For intermediate conditions of temperature/strain rate, elonga ted cavities developed parallel to the tensile axis, but near the fracture face these usually interlinked by transverse cracking. These conditions wer e associated with intermediate elongations to failure. For the assessment o f cavity growth mechanisms, artificial pores were introduced into fine grai n Y-TZP specimens and changes in their shape and size during tensile or com pressive deformation were investigated. Results show that the change of por e volume, in the superplastic regime, is controlled by plastic deformation of the matrix and can be described by the relationship of dR/d epsilon = et a R, where epsilon is the true strain, eta the cavity growth rate parameter and R is the radius of the pore. (C) 1999 Kluwer Academic Publishers.