CAVITY GROWTH IN POLYCRYSTALLINE MATERIALS UNDER GRAIN-BOUNDARY DIFFUSION CREEP AND TRANSITION FROM CAVITY TO CRACK

The stress distribution along grain boundaries in polycrystalline mate rials under the grain boundary diffusion creep (Coble creep) condition is numerically analyzed in order to elucidate the effect of the grain boundary network on cavity growth. The stress gradient at the cavity tip, which is proportional to the changing rate of the cavity volume, points out that the triple junction of grain boundaries interferes wit h cavity growth, though the cavity grows faster after the tip goes bey ond it. The deceleration effect due to the triple junction diminishes as the cavity becomes larger. The transition from ''crack-like cavity' ' to ''crack'' is also discussed on the basis of the stress field near the tip. The ''crack'' and the ''cavity'' are distinguished by the fa ct that the former possesses the stress singularity in the vicinity of the tip while the latter does not. The calculation reveals that the t ransition under the Coble creep condition does not take place before t he cavity length exceeds 15 grain boundaries. On the other hand, the t ransgranular deformation due to dislocation creep drastically accelera tes the transition.