Constrained creep cavitation of grain boundary facets

Constraints on diffusive creep cavity growth along grain boundary facets are studied for the limiting case when all facets oriented approximately normal to an applied tensile load are uniformly cavitated. This situation represents the opposite limiting case to when cavitated facets are well-separated and do not interact with each other. The analysis is done for a 3-D periodic polycrystalline model of grains in the shape of the Wigner-Seitz cells of a f.c.c. lattice. The grains have freely-sliding boundaries and deform in a nonlinear viscous manner in response to applied stress. Expressions for the cavity growth rate and the strain and time to rupture are compared with results of prior work in which cavitated facets are well-separated, and this gives a good understanding of the ranges of stress and temperature over which cavity growth is constrained and rupture lifetime is increased. The time to rupture, which is taken here to mean cavity coalescence on the damaged facets, is seen to depend strongly on the proximity of cavitated facets, at least when cavity growth is constrained. However, the strain to rupture is observed to lack this strong dependence although for constrained conditions, the cavitation process contributes substantially to the total strain when cavitated facets are closely-spaced. When cavitated facets are well-separated, the polycrystal is seen to achieve a relatively constant strain rate. By comparison, the strain rate is seen to vary substantially with time when cavitated facets are closely-spaced. The time and strain to rupture as well as strain rate versus time curves are calculated as functions of applied load and temperature for nickel as a representative f.c.c. metal.