Positron-annihilation study of the aging kinetics of AlCu-based alloys. II. Ag microalloying

Positron-annihilation spectroscopy has been applied for studying the effects of microalloying additions of Ag to Al-4%Cu and Al-4%Cu-0.3%Mg (wt %) alloys on the aging kinetics and on the association of vacancies to solute elements. The results show that: (i) in Al-Cu, the addition of Ag increases (from 0.32 to 0.61 eV) the effective activation energy that controls the formation of solute aggregates; (ii) on the contrary, in the Al-Cu-Mg alloy, the activation energy is decreased (from 0.65 to 0.22 eV); (iii) in Al-Cu and in Al-4%Cu-0.3%Ag (wt %) alloys the solute aggregates (clusters or GP zones) formed after long aging at temperatures below 70 °C do not contain vacancies; (iv) in Al-Cu-Mg, the addition of Ag enhances the formation of co-clusters (or GPB zones) containing Mg and vacancies; (v) the formation of Mg-rich aggregates is enhanced by Ag also at a high aging temperature (180 °C), and this leads to a better stabilization of the structure produced by artificial aging. The above effects are interpreted as due to the tendency of Ag atoms to bind vacancies in the Al-Cu and vacancy-Mg complexes in Al-Cu-Mg.