Breakaway bubble growth during the annealing of helium bubbles in metals

Recent experimental studies on the annealing of helium implanted copper showed unusual surface pinholes which could not be explained using conventional bubble migration and coalescence theory. However, computer simulation results indicated that at some threshold, breakaway bubble swelling was taking place, thus giving a plausible picture of the experimental results. In the present study the computer simulations have been repeated for an initially uniform helium level in bulk material, allowing the threshold effects to be examined without any surface effects and any loss of helium. Assuming equilibrium bubble conditions, the results showed that when bubble coarsening reached an average of 20% swelling, breakaway swelling occurred. This phenomenon has been examined as a function of helium content and results extrapolated (with some assumptions) to other temperatures and metals. The implications for fusion reactor materials is briefly discussed.