Of Grain Boundary Facets

Abatraet-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 tensik load am uniformly cavitated. This situation represents the opposite limiting case to when cavitated facets are we&sepamted and do not intemct with each other. The analysis is done for a 3-D periodic polycrystallii mode3 of grains in the shape of the Wigner-Seitz cells of a f.c.c. lattice. The grains have freely-sliding boundarks and deform in a nonlinear viscous manner in response to applied stress. Expressions for the avity growth mte 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 tempemtum over which cavity growth is constrained and rupture lifetime is increased. The time to rupture, which is taken here to mean avity coalcsccncc on the damaged facets, is seen to depend strongly on the proximity of avitated facets, at least when cavity growth is constrained. However, the strain to rupture is obaervcd to lack tbis strong dependence although for constrained conditions, the cavitation process contributes substantially to the total strain when cavitated facets are closely-spaced. When cavitated faoets am well-scpamted, the polycrystaf is seen to achieve a relatively constant strain rate. By comparison, the stmin rate is seen to vary substantially with time when cavitated facets are closely-spaced. The time and atrain to rupture as well as strain mte versus time curves are calculated as functions of applied load and tempemtum for nickel as a representative f.c.c. metal.