Evolution of helium bubbles in aluminum during heavy-ion irradiation

The evolution of individual He bubbles in thin Al foils during 200-keV Xe irradiation at room-temperature has been followed with in situ transmission electron microscopy. He bubbles were produced by room-temperature implantation of 3-keV He ions into prethinned aluminum samples. During subsequent xenon irradiation, several distinct processes were observed to cause individual He bubbles to increase or decrease in size. Bubble growth was observed to take place by radiation-induced coalescence of bubbles without bubble motion. This coalescence was a result of the net displacement of Al atoms out of the volume between bubbles initially in close proximity. The resulting nonequilibrium-shaped bubble evolved towards a more energetically favorable spherical shape whose final size was determined by equilibrium bubble pressure. Bubbles were observed to disappear as the specimen surface was removed by sputtering. Bubbles unaffected by sputtering were observed to decrease in size at an average rate of 0.1 to 0.2 nm/(1015 Xe/cm2) or 0.024 to 0.048 nm/dpa (displacements per atom). This rate of bubble shrinkage can be understood on the basis of direct displacement of He out of the bubble while the bubble remains at equilibrium pressure. He resolution occurred at a rate of 0.005 to 0.01 (Heejected/He)/dpa. No examples were found that would indicate complete destruction of a bubble by a single Xe ion. Bubble centers remained fixed during bubble shrinkage indicating negligible bubble motion during room-temperature irradiation.