Direct observation of grain rearrangement during superplastic creep of a fine-grained zirconia

A 2 mol% yttria partially stabilised zirconia specimen has been deformed at 1300 °C under a constant stress of 100 MPa by strain increments of 14% up to an ultimate true strain of −0.56. At each stage of creep test, grain arrangement inside two zones of a side face of specimen was investigated by scanning electron microscopy. Grain rearrangement proceeded essentially by two elementary transformations: the neighbour switching event and the grain disappearance corresponding to grain movement perpendicularly to the analysed surface. These observations highlighted unambiguously that grain boundary sliding was not simply an accommodation mechanism but acted as the deformation mechanism, excluding that a pure diffusion creep may account for deformation. Influence of cooperative grain boundary sliding was very limited. Grain redistribution analysis showed that, on average, each grain was implied in four neighbour exchange events during test and that topological stability was nearly reached after a strain of 28%.