Gd. Zhan et al., Effects of microstructure on superplastic behavior and deformation mechanisms in beta-silicon nitride ceramics, J AM CERAM, 83(12), 2000, pp. 3179-3184
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.