Observations and theory of thermally-induced helium bubble shrinkage in gold

Abstract

In an annealing study of small helium bubbles in gold, bubble migration previously found above 500 K was unexpectedly not observed. Instead, near-neighbour bubbles were seen to coalesce via Lidiard-Nelson interactions over the range 800 to 1200 K. Of particular interest was the observation at 1200 K that isolated bubbles reduced their radius, slowly at first, but with a rapid increase of bubble shrinkage at small radii. This novel behaviour demonstrates the dissociation of helium from a cavity with an associated loss of vacancies to maintain bubble equilibrium. The shrinkage observations have been modelled theoretically by considering the thermal (quasi-) equilibrium between helium in the bubble and in HeV defects at the bubble periphery. The helium release is then described by the subsequent flux away from the bubble via thermal vacancy assisted diffusion of the HeV's.