High-resolution electron microscopy observation of grain-boundary films insuperplastically deformed silicon nitride

The thickness distribution of grain-boundary films during the superplastic deformation of fine-grained beta-silicon nitride was investigated by high-r esolution electron microscopy. In particular, grain-boundary thickness was considered with respect to the stress axis in two orientations; namely, par allel and perpendicular to the direction of applied stress. The results sho wed that the thickness distribution in boundaries perpendicular to the dire ction of applied stress was unimodal, whereas in parallel boundaries it was bimodal, Moreover, it was found that the majority of film-free boundaries were parallel to the direction of applied stress in the extremely deformed sample. The variation in spacing reflects distribution of stresses within t he material due to irregular shape of the grains and the existence of perco lating load-bearing paths through the microstructure.