Abstract
The thickness of intergranular films in polycrystalline beta-Si3N4 ceramics, both before and after superplastic deformation, was systematically studied using high-resolution TEM. In characterising the film thickness, care was taken to correlate the grain boundary orientation with the direction of the compressive stress applied during the hot-pressing and the superplastic deformation. The film thickness shows a dependence on the intersection angle between the grain boundary and the applied force direction, typically ranging from around a characteristic value for most of the boundaries to zero for a boundary which has an overall short length and is perpendicular to the applied force direction. The film thicknesses in the deformed material, as compared with those before deformation, are marked by a wider distribution and an increased fraction of boundaries free of films, unequivocally demonstrating that during the superplastic deformation the liquid phase is redistributed within short ranges, a process governed by the local stress level as well as kinetic factors. Possible consequences of the liquid-phase redistribution on the deformation behaviour are also discussed. 25 refs.