Importance of stress in bubble ordering in the helium gas-bubble superlattice in copper

Abstract

Microstructural changes induced in metals by ion bombardment have important implications for technology (see, for example, refs 1–3). Helium irradiation can result in the formation of small (∼2nm diameter) helium bubbles in high concentration (∼1025 m–3), ordered on a superlattice. Current theories are all directed towards explaining the formation of a superlattice having the same alignment as the crystal lattice of the metal—the matrix or m orientation. In the face-centred-cubic metal copper, although there was evidence in previous work that some domains4 in the bubble array were in orientations other than m (ref. 5), it may have been assumed that the proportion of such domains was small. Here we report new results that show that a high proportion of the ordered bubble array is in domains that have orientations different from m. We propose that a new mechanism, based on the spatial characteristics of the stress field around an overpressurized bubble, must play a dominant role in the later stages of bubble ordering.