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

Positron-annihilation spectroscopy has been applied for studying the effect s of microalloying additions of Ag to Al-4%Cu and Al-4%Cu-0.3%Mg (wt %) all oys on the aging kinetics and an the association of vacancies to solute ele ments. The results show that: (i) in;U-Cu, the addition of Ag increases (fr om 0.32 to 0.61 eV) the effective activation energy that controls the forma tion 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 AI-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 degrees C do not contain vacancies; (iv) in Al-Cu-Mg, the addition of Ag enhances the formation of eo-clusters (or GPB zones) containing Mg and vacancies; (v) the formation o f Mg-rich aggregates is enhanced by Ag also at a high aging temperature (18 0 degrees C), and this leads to a better stabilization of the structure pro duced by artificial aging. The above effects an interpreted as due to the t endency of AE atoms to bind vacancies in the AI-Cu and vacancy-Mg complexes in Al-Cu-Mg.