THE EFFECTS OF GRAIN-SIZE DISTRIBUTION ON CAVITY NUCLEATION AND CREEPDEFORMATION IN CERAMICS CONTAINING VISCOUS GRAIN-BOUNDARY PHASE

The grain size distribution in a polycrystalline ceramic material is n ot uniform. Such microstructural inhomogeneity may give rise to nonuni form local stress distributions. Here we investigate the effect of gra in size distribution on the generation of local stress concentration i n ceramic materials creeping by localized flow of a viscous grain boun dary phase. A simple bimodal grain size distribution is first consider ed. The critical stress for cavity nucleation calculated using classic al Becker-Doring nucleation theory, is compared with the local stress concentration. The results show that, because of the inhomogeneity, th e local stress in the grain boundary viscous phase at the locations of large grains can exceed the critical stress for cavity nucleation. Th e creep rate due to localized viscous flow of the grain boundary phase and cavity growth is evaluated. Although the creep behavior owing sol ely to viscous flow is linear with respect to applied stress, it can b e highly nonlinear when cavitation occurs. Moreover, as an example, th e model has been used to study creep behavior of a whisker-reinforced Si3N4 matrix composite in which long whiskers are surrounded by small equiaxed ceramic grains. (C) 1997 Acta Metallurgica Inc.