Grain-boundary migration in metals fatigued at high temperatures

Abstract

THE migration of grain boundaries in polycrystalline metals fatigued at high homologous temperatures has been reported for FCC, BCC and CPH metals^1–4. Migration results in a ‘diamond’ or orthogonal configuration of boundaries with segments aligned in the maximum shear stress directions. It has been suggested that this migration contributes to grainboundary failure by the absorption of point defects at migrating boundaries⁵ and/or by the development of the diamond configuration of boundaries, which puts these boundaries in a favourable arrangement for sliding^1–4. In contrast to the information on cavity growth during high temperature deformation, there seems to be little quantitative information on grain-boundary migration in these conditions. We report here the results of measurements of grain-boundary migration on Al, Cu, Zr and Zircaloy-2 specimens fatigued at temperatures where grain-boundary cavitation occurs. These results indicate that in the initial 10% of the fatigue life the rate of boundary migration decreases with time and the amount of migration is ∝time^2/3. The possible influence of boundary migration on cavitation is suggested to be limited to the initial part of the fatigue life.