Positron-annihilation study of the aging kinetics of AlCu-based alloys. I. Al-Cu-Mg

Positron-annihilation spectroscopy has been applied for studying the aging kinetics and the association of vacancies to solute elements during precipitation in an Al-Cu-Mg alloy with high Cu/Mg ratio. The results show that: (i) isolated vacancies--Mg complexes exist in the alloy following quenching from the solution treatment temperature; (ii) after aging at temperatures below 68 ^∘{}C, the vacancies are associated with Cu and with Cu-Mg aggregates [solute clusters, Guinier-Preston GP(Cu) and GP(Cu, Mg) zones]; (iii) the contribution of Cu-Mg aggregates decreases as the aging temperature is raised from 20 to 68 ^∘{}C; (iv) immediately after aging at 180 ^∘{}C for times from 15 s to 10 h, the vacancies are present only in pure Cu regions, but vacancies in Cu-Mg aggregates become observable when the sample is cooled and aged at room temperature. The kinetics of formation of solute aggregates can be described, after scaling, by a temperature-independent function. From the temperature dependence of the scaling factors, an effective activation energy of 0.65 eV was obtained.