Positron spectroscopy analysis of vacancy-solute nanoaggregates in Al-Cu

This work addresses the process of ageing in Al-4wt.% Cu at two temperatures (150°C, 180°C) and in conditions of secondary ageing at temperatures up to 60°C, after artificial ageing treatment at 150°C. Positron lifetime and coincidence Doppler broadening measurements are reported. The analysis shows an initial association of positron trapping sites with Cu (about 9 at. % Cu at the annihilation site) that decreases to 7+6 % in the initial stages of ageing, but then increases sharply (up to 25 at. % Cu after 300 hours at 180°C). The positron lifetime also displays an initial decrease followed by a final increase. The results confirm the hypothesis that positrons are not trapped in GP[1] zones, but show that trapping occurs in GP[2] zones formed at 150°C. During secondary ageing, the vacancy-Cu association as well as the positron lifetime decrease with the ageing time. This is attributed to the formation of GP[1] zones due to residual supersaturation.