In-situ observation of Brownian motion of helium bubbles along grain boundaries in aluminium

The diffusion profile of small helium bubbles along grain boundaries in aluminium has been examined by in-situ electron microscopy observation. It was found that bubbles moved two-dimensionally in the general grain-boundary interface with a misorientation about the axis [122]. The occurrence probability P*(R x 2 , t) of the square of the bubble migration distance along the boundary can be well fitted to a relation of the random-walk theory. The mean square (R x 2 ) of the migration distance is proportional to the time which is required to move the distance. These facts provide clear quantitative evidence of Brownian-type motion of the bubble along the grain boundary and yield the bubble diffusivity; for example, D = 5.6 x 10- 18 m 2 s -1 at 833K for a bubble of diameter 2.3 nm. The diffusivity obtained is much higher than that in the lattice matrix.