Nucleation and growth of precipitates and helium bubbles in a high-purity Al-Mg-Si alloy irradiated with 600 MeV protons

Abstract

Irradiation-induced changes in the precipitation behaviour of age-hardening alloys are not yet well established. Very little is known about the problem of precipitation and precipitate stability particularly under fusion irradiation conditions leading to concurrent production of displacement damage and gaseous impurities at high rates. The main objective of the present work has been to study the effect of displacement damage, in the range from 0.01 to 1.63 dpa, with simultaneous production of helium atoms at a high rate ( ~ 214 appm/dpa) in a high purity Al-0.75% Mg-0.42% Si alloy; the composition is similar to that of the commercial type 6061 Al-Mg-Si alloy. In order to study the effect of irradiation on the nucleation and growth of precipitates and the aggregation of concurrently produced helium atoms, the alloy was irradiated in the solution annealed condition (535°C, 35 min). The irradiation temperature varied between 120 and 260°C. After irradiation, the nucleation, growth and dissolution of Mg2Si-type precipitates as a function of irradiation dose and temperature were studied by transmission electron microscopy (TEM). The results show that the precipitation of Mg2Si-type particles occurs on a finer scale during irradiation than during thermal aging of the solution treated Al-Mg-Si alloy (at the temperature of irradiation). The needle-shaped Mg2Si-type precipitates in the thermally aged and the irradiated specimens are found to lie along the 〈100〉 matrix directions. Information regarding nucleation and growth of helium bubbles in the matrix, at precipitate-matrix interfaces and at grain boundaries has also been obtained by TEM. Long rows of bubbles are found to be associated with the Mg2Si-type precipitates. The bubbles at the precipitate-matrix interfaces and at the grain boundaries are found to grow faster than those in the matrix. The implications of these results will be briefly discussed.