The Helium Gas Bubble Superlattice in Copper

Abstract

Transmission electron microscopy is used to examine the structures formed by helium gas bubbles in copper for various doses of helium ions and at a range of depths below the specimen surface. Changes in the mean bubble size and density are studied and the bubble array is examined for signs of ordering into a superlattice structure. The development of this structure as dose or depth is increased is noted. Direct electron micrographs and selected area diffraction patterns of the bubble array in {110} copper grains are analysed in detail for a dose and depth at which the bubble array appears ordered. The various structures that make up this array are identified. It is found that only specimens at some doses and depths contain a bubble array that is ordered enough to give rise to diffraction. There appears to be a close link between features of the bubble array such as the bubble size and the superlattice spacing, and the type of ordering in the bubble array. The main changes in the bubble size and the superlattice spacing occur between doses and depths for which the bubble array either gains or loses widespread ordering. Analysis of the selected area diffraction patterns and of the direct electron micrographs has shown that the ordered bubble array can not be modelled as a perfect fcc lattice in the copper matrix orientation alone. The results of the analysis are consistent with the presence of particular variant structures in the bubble array. In a {110} grain seven variant structures, at least, are thought to be present. The observed composition and development of the ordered bubble array suggests that stress-stress interactions play an important part in the ordering of overpressurized bubbles.