Diffusional growth of intergranular cavities in uniform stress field and ahead of crack-like stress concentrator

Abstract

The diffusional growth of an isolated intergranular cavity is studied in a uniformly loaded body and also ahead of a crack. The corresponding rates of cavity growth as functions of the applied stress and stress intensity factor, respectively, are found. In the former case, a cavity which possesses an equilibrium shape with constant curvature can, for physically reasonable applied stresses, grow independently of other cavities only if the average separation of the cavities is in excess of 10³ times the cavity size. The reason is that the transition towards the Hull and Rimmer type steady state takes place for smaller cavity spacing due to the large extent of the region of material deposition onto the boundary. On the other hand, crack-like cavities may grow independently even if the average cavity separation is small, provided their width is about two orders of magnitude smaller than their length which may be the case when the surface diffusion is appreciably slower than the grain boundary diffusion. However, ahead of a crack both the equilibrium and crack-like cavities can grow independently since the extent of the region of the material deposition is comparable with the size of the cavities for physically reasonable values of the crack stress intensity factor. Since there are factors which may prevent the development of the Hull and Rimmer type diffusional mechanism in regions which are large compared with the cavity size, it is suggested that cavity growth controlled purely by grain boundary diffusion is more likely to be observed in creep crack growth experiments than in creep experiments on uncracked uniformly loaded specimens.