He bubble coarsening by migration and coalescence in annealed Pu–Ga alloys

The α-decay of plutonium in Pu–Ga alloys continually generates inert He atoms within the lattice of the Pu–Ga matrix. In naturally aged Pu specimens, those He atoms form into bubbles, He-filled vacancy clusters, with a characteristic size distribution centered near 1.4 nm. Upon annealing, the He bubbles are subject to temperature induced changes in the thermodynamic parameters governing their size, which results in a coarsening of the bubble distribution yielding a lower bubble density but larger average bubble sizes. Herein, we investigate, by means of transmission electron microscopy, the coarsening of He bubbles with several annealing treatments. Micrographs subsequent to the annealing treatments as well as in situ observations implicate migration and coalescence as the mechanism for He bubble coarsening with annealing.