The effect of time dependent void density on grain boundary creep fracture—i. continuous void coalescence

When voids in a creeping solid are non-uniformly distributed over a grain boundary facet void coalescence occurs simultaneously with void growth. The effect of such coalescence on subsequent grain boundary void growth can be analysed assuming a random (i.e. Poisson) initial void distribution. A simple relationship between the fraction of voids remaining on a grain boundary facet and their area fraction is found. This relationship is independent of the mechanism by which the voids grow. When voids grow by surface diffusion or by power-law creep very little effect of coalescence on the overall rate of damage accumulation is found. However, when void growth is controlled by grain boundary diffusion, the rate of void growth is decreased dramatically during coalescence, and effectively stops. Final fracture occurs by creep controlled void growth. The model predicts a non-linear stress dependence for this process, and an activation energy different from that for boundary diffusion alone. The model is compared with creep fracture data for pure metals implanted with water vapour bubbles. Good agreement with the model is found in two cases, while in a third case void growth appears to have been controlled by surface diffusion.