The effect of time-dependent void density on grain boundary creep fracture—II. Continuous nucleation

The density of voids on a grain boundary facet changes continuously due to the nucleation of fresh voids with increasing strain and due to the coalescence of voids as they grow and touch each other. In this paper a previous treatment which considered only void coalescence effects is generalized to include continuous nucleation. An equation for the evolution of void density is derived and is found to depend on the rate of void growth. Thus void growth and void density change must be treated as coupled processes. When this is done a set of equations for creep life are found which differ significantly from those derived on the basis of a constant void density. In particular stress exponents and activation energies between those expected for diffusion processes and for creep processes acting alone are predicted. An analysis of transverse creep rupture data in an oxide-dispersion strengthened (ODS) superalloy is in agreement with the predictions of such a model.