A direct comparison of Al-Cu and Al-Cu-Mg alloys showed that the mechanism, by which Mg affected the microstructure and the aging kinetics of the alloys, was related to the formation of vacancy-Cu-Mg complexes that acted as embryos for the nucleation of coherent solute clusters. The presence of Mg stabilized the vacancies in the Guinier-Preston zones. Experimental evidence of the association of vacancies with solute atoms was obtained by Doppler-broadening spectroscopy of the positron annihilation radiation, using two Ge detectors. The evolution of the average content of Cu in the proximity of

the vacancies was monitored at different stages of aging at temperatures below the solvus of Guinier-Preston zones, following solution heat-treatment of the alloy.

Stability of Vacancies during Solute Clustering in Al-Cu-Based Alloys. A.Somoza, M.P.Petkov, K.G.Lynn, A.Dupasquier: Physical Review B, 2002, 65[9], 094107 (6pp)