Fig. 8-2 Brownian motion of helium bubbles in aluminum observed continuously by electron microscopy at 833K after the bubble formation by 17 keV helium ion irradiation with flux of 1 x 1013 ions/cm2/s at 523 K for 90 minutes Bubbles denoted by a and b show remarkable migration and coalesced in Photo. 4.

Fig. 8-3 The locus of the helium bubble denoted by c in Fig. 8-2 The positions measured at two second intervals are shown by successive numbers. Number 1 denotes the starting position.

Fig. 8-4 Relation between time, t, and the mean square distance of bubbles, , from the starting position. From the relation: t, the bubbles are concluded to migrate by Brownian motion.

Fusion reactor materials are subjected to irradiation by large quantities of neutrons and ions, such as helium ions. This results in the formation of many lattice defects and bubbles, which have a strong influence on the properties and durability of these materials. To clarify the dynamic behaviors and the mechanism of bubble formation, growth coalescence, and disappearence under various irradiation environments, systematic studies are now in progress using the JAERI developed "In-situ observation system of dynamic behavior of ion irradiation damage in an electron microscope." Recently a new phenomenon was observed in the migration of bubbles in metal. Figure 8-2 is a series of electron micrographs that show the migration of small bubbles produced in aluminum. The loci of bubbles are illustrated in Fig. 8-3. From such loci, the mean square distances of bubbles from the starting position, , can be measured and plotted against time t, as shown in Fig. 8-4. This figure indicates that is proportional to t. From this relation it was concluded that bubbles migrate in metal by Brownian motion, as small pieces of pollen on water show random motion. Further, it was found also that the diffusion coefficient of the bubbles is inversely proportional to the fourth power of the radii of the bubbles, which leads to the conclusion that the migration of bubbles is caused by diffusion of atoms on the surfaces of the bubbles.

Reference K. Ono et al., In Situ Observation of the Migration and Growth of Helium Bubbles in Aluminum, J. Nucl. Mater. 191-194, 1269 (1992).

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