Effect of thermal treatment condition on the Ag precipitates in Al–Ag alloy studied by positron annihilation

Formation of Ag precipitates in an Al–1 wt%Ag alloy after aging at different temperatures was studied by positron annihilation spectroscopy. It is found that the aggregation of Ag atoms takes place during natural aging process after the Al–Ag alloy was homogenized at 550 °C and quenched to room temperature water. The Ag nanoclusters could trap positrons and thus positron annihilation measurements give information on the precipitation of Ag atoms. After artificial aging at 120 °C, the Ag signal is enhanced, which indicates further aggregation of Ag atoms. However, after artificial aging of the sample at 200 °C, no Ag nanoclusters are observed. Instead, the quenched-in vacancies show gradual recovery during this aging process. This is probably due to the dissolving of Ag clusters into Al matrix at 200 °C. Furthermore, after the sample was first heat treated at 200 °C and then aged at 120 °C, Ag nanoclusters appear again. This implies that the formation of Ag precipitates during natural aging process is assisted by the quenched-in vacancies. Temperature dependence of the positron annihilation measurements indicates that Ag nanoclusters are shallow positron traps, which makes it difficult to observe the real-time Ag precipitation formation by positrons during artificial aging of Al–Ag alloy.