Helium bubble nucleation at grain boundaries

Abstract

Helium bubble nucleation has been studied at grain boundaries in a ternary austenitic steel after helium implantation in the temperature range 450–600°C. Transmission electron microscopy shows that all interfaces except the coherent twin are preferred nucleation sites for bubbles, and that the density of bubbles is greater on interfaces containing resolvable grain boundary dislocations (GBDs), A model has been developed which accounts for the bubble density as a function of GBD spacing. This model implies that there is a positive binding energy between He and GBDs until the dislocation spacing becomes very small. The diffusion coefficient of helium along GBDa is much lower than that arising from interstitial diffusion in the lattice. It appears that helium migrates in grain boundaries and along GBDs by a slow vacancy-type mechanism.