Effects of Microstructure on Superplastic Behavior and Deformation Mechanisms in β‐Silicon Nitride Ceramics

Abstract

The influence of grain shape and size on superplastic behavior and deformation mechanisms was investigated in annealed β‐silicon nitride materials and compared with the results for hot‐pressed material. The microstructure of the annealed materials consisted of fine equiaxed β‐grains together with some elongated ones. Similar to the deformation behavior in the hot‐pressed material, strain hardening did not occur in these annealed materials. Moreover, in contrast to the deformation behavior under tension, grain alignment under compression resulting from the development of a mild texture did not give rise to strain hardening. An annealed material with small elongated 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 ∼1 and a grain‐size exponent of 3 were obtained for all materials. This suggests that the deformation mechanism in these annealed materials was the same as that of fine equiaxed β‐silicon nitride.