Thin foils of an austenitic stainless steel with and without dispersions of aluminum oxide particles were irradiated in a high voltage electron microscope at 600°C; prior to irradiation, up to 1000 ppm of helium atoms are implanted in the thin foils. The void concentration and the swelling decreased with decreasing size of the grains irradiated, the effect of grain size being more marked in the dispersed specimens. The grain size effect persisted also in specimens containing high concentrations of helium. The influence of high concentrations of helium on void growth and swelling is investigated. The dose dependencies of void size and swelling for grains of different sizes and with different helium concentrations are described.

These results are analyzed in terms of grain size dependent vacancy supersaturation and are found to be consistent with the “defect depletion” model recently proposed by the author. It is concluded that void nucleation is critically dependent on vacancy supersaturation and that the grain size effect is predominantly nucleation controlled.