Effects of microstructure on superplastic behavior and deformation mechanisms in beta-silicon nitride ceramics

The influence of grain shape and size on superplastic behavior and deformat ion mechanisms was investigated in annealed beta -silicon nitride materials and compared with the results for hot-pressed material. The microstructure of the annealed materials consisted of fine equiaxed beta -grains together with some elongated ones. Similar to the deformation behavior in the hot-p ressed material, strain hardening did not occur in these annealed materials . Moreover, in contrast to the deformation behavior under tension, grain al ignment under compression resulting from the development of a mild texture did not give rise to strain hardening. An annealed material with small elon gated grains had a flow-stress dependency of n = 1, whereas other annealed materials with large elongated grains exhibited a flow-stress dependency of n = 1.6. In terms of texture development and the effect of grain shape on the creep rate when diffusion was the rate-controlling mechanism, a single curve with a stress exponent of similar to1 and a grain-size exponent of 3 were obtained for all materials. This suggests that the deformation mechani sm in these annealed materials was the same as that of fine equiaxed beta - silicon nitride.