Tracing the evolution of bubbles in helium-injected aluminum by means of positron annihilation

Pure aluminum was injected with 0.6 appm of helium. Positron lifetime measurements were performed following isochronal annealing between 25 and 6000 C. Transmission electron micrographs were taken after several of these anneals. The results are consistent with a model in which the injected helium quickly becomes substitutional. This substitutional helium is a trapping site for positrons. By monitoring the concentration and size of this trapping site through the measurement of the positron lifetimes it is concluded that migration of the helium begins at approximately 1000 C and that helium continually agglomerates up to approximately 2500 C. Above this temperature the increasing size of the trapping site indicates bubble growth. Bubble growth proceeds up to 6000 C, but whether by continued arrival of helium at nucleation sites or by bubble migration and coalescence (or both) being indeterminable. The major stage of bubble growth is between 300 and 6000 C. The electron microscopy verified the presence of bubbles both in grain boundaries and matrix after annealing to 6250 C, but only cavities in grain boundaries were observed in annealing to 5000 C. Positron-lifetime measurements are a useful complement to electron microscopy in the study of bubbles in metals, especially in the early stages of the nucleation and growth of small cavities. (Auth.)