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 tra nsmission electron microscopy. He bubbles were produced by room-temper ature implantation of 3-keV He ions into prethinned aluminum samples. During subsequent xenon irradiation, several distinct processes were o bserved to cause individual He bubbles to increase or decrease in size . Bubble growth was observed to take place by radiation-induced coales cence 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 bubb le evolved towards a more energetically favorable spherical shape whos e final size was determined by equilibrium bubble pressure. Bubbles we re observed to disappear as the specimen surface was removed by sputte ring. Bubbles unaffected by sputtering were observed to decrease in si ze at an average rate of 0.1 to 0.2 nm/(10(15) Xe/cm2) or 0.024 to 0.0 48 nm/dpa (displacements per atom). This rate of bubble shrinkage can be understood on the basis of direct displacement of He out of the bub ble while the bubble remains at equilibrium pressure. He resolution oc curred at a rate of 0.005 to 0.01 (He(ejected)/He)/dpa. No examples we re found that would indicate complete destruction of a bubble by a sin gle Xe ion. Bubble centers remained fixed during bubble shrinkage indi cating negligible bubble motion during room-temperature irradiation.