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 precipi tation 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 f rom the solution treatment temperature; (ii) after aging at temperatures be low 68 degrees C, the vacancies are associated with Cu and with Cu-Mg aggre gates [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 degrees C; (iv) immediately after aging at 180 degree s 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 sampl e is cooled and aged at room temperature. The kinetics of formation of solu te aggregates can be described, after scaling, by a temperature-independent function. From the temperature dependence of the scaling factors, an effec tive activation energy of 0.65 eV was obtained.