Gas-induced swelling near grain boundaries in helium-loaded nickel during post-implantation annealing

Abstract

Transmission electron microscopy and metallographic analysis of cross sections have been used to investigate (1) the parameters of coarse gas cavities near grain boundaries and at the outer surface, as well as (2) the widths of the corresponding boundary and surface zones formed in helium-loaded Ni foils as a result of post-implantation annealing at temperatures between 973 and 1373 K and for times ranging from 0.17 to 9.0 h. It was shown that some grain boundaries acted as sources of thermal vacancies as effective as the surface. During annealing, helium enrichment of zones near such grain boundaries cannot be excluded, but it is certain that this enrichment does not appear locally in the grain boundary itself. An adequate kinetic model for the development of boundary and surface zones has been proposed. The model is based on treating the grain boundaries and the surface as vacancy sources, near which the primary gas porosity (population of tiny overpressurized helium bubbles arising under conditions of a thermal vacancy deficit) is transformed into the secondary gas porosity (large near-equilibrium bubbles).