Constraints on the diffusive cavitation of isolated grain boundary facets in creeping polycrystals

Following concepts introduced by B. F. Dyson (Metal Sci. 349 1976), the diffusive cavitation of grain facets is considered in circumstances for which the cavitated facets are well separated from one another. In this case the requirement of geometric compatibility between the opening grain facets and the creep-deforming polycrystalline surroundings reduces the stress transmitted to the cavitated facets, and hence increases the rupture lifetime. An evaluation of the rupture time, t r , based on diflusional cavity growth to coalescence shows that t r , is given by the sum of two terms, one proportional to 1 Dσ∞ (where D is the grain boundary diffusion parameter and σ ∞ the stress which would act on a non-cavitated facet) and another proportional to 1 E∞ (here E ∞ is the creep rate of a similarly loaded polycrystal with uncavitated boundaries). The latter term is found to be much larger than the first at sufficiently low stress and temperature, as long as the cavitated facets are indeed well separated. This circumstance leads to results in which the cavity growth process and strain to rupture are consistent with the diffusional mechanism, but in which the rupture time t r , follows a Monkman-Grant correlation with t r , proportional to 1 E∞ .