Quantitative study of Brownian motion of helium bubbles in fcc metals

Dynamical process of helium bubbles in face‐centred cubic (fcc) metals has been studied by in situ irradiation and electron microscopy with a video recording system. Brownian type motion of helium bubbles was demonstrated quantitatively by establishing a proportional relation between the mean square of the bubble migration distance and time. This relation yielded the diffusion coefficient of the bubble. Examples of Brownian motion in the matrix and along general grain boundaries in pure aluminium are shown comparatively. Similar Brownian motion was also observed in an Fe‐16Cr‐17Ni austenitic alloy. It was revealed from scanning transmission electron microscopy–electron energy‐loss spectroscopy that Cr was depleted around the bubble surface, but Ni was enriched. Response of the bubble motion to irradiation with high‐energy self‐ions was examined in pure aluminium and both effects of enhancement and retardation of the bubble mobility were found.