A study was made of the growth of He gas bubbles on the Cu side of Cu/Ni diffusion couples. It was found that a bimodal distribution of the gas bubbles existed, due to the effect of the surface in the near-surface region and to diffusion-generated excess vacancies in the diffusion zone. An analysis of the results showed that the presence of a vacancy gradient, generated by the flux of diffusing atoms, was insufficient to cause bubble migration. A temperature or stress gradient was required in order to induce migration when the intrinsic atomic mobility was sufficiently high.

G.P.Tiwari: Journal of Nuclear Materials, 1996, 232[2-3], 119-24