Influence of Pb impurities on He-bubble formation and growth in Al studied by positron annihilation and transmission electron microscopy.

Abstract

Positron-lifetime and transmission-electron-microscopy measurements are reported for He (4.5 at. ppm), in pure Al, and Pb-contaminated ({similar to}1 at. ppm) Al:0.5 wt. % Ni alloy. The observed He-bubble growth in pure Al was found to follow at {ital t}{sup 1/5} dependence, i.e., was in good agreement with previously published data. The Pb-contaminated alloy exhibited accelerated He agglomerate and bubble growth. Bubbles observed by transmission electron microscopy in the Pb-contaminated alloy were found to have precipitates attached to them. No precipitates were observed that were not associated with bubbles and no bubbles observed that did not have a precipitate attached. Agglomerate and bubble radii were calculated using the positron-lifetime data. Again the results for pure Al were found to be in good agreement with previously published data. In the Pb-contaminated alloy agglomeration and coalescence occurred at temperatures as low as {similar to}80 {degree}C, while accrual of free volume started at {similar to}180 {degree}C. An increase in bubble growth rate, which occurred between {similar to}350{degree} and {similar to}400 {degree}C, was tentatively associated with the melting of Pb precipitates.