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A direct comparison between Al-Cu and Al-Cu-Mg alloys shows that the mechanism, by which Mg affects the microstructure and the aging kinetics of the alloys, is related to the formation of vacancy-Cu-Mg complexes that act as embryos for the nucleation of coherent solute clusters. The presence of Mg stabilizes the vacancies in the Guinier-Preston ͑GP͒ zones. Experimental evidence of the association of the vacancies with the solute atoms was obtained by Doppler-broadening spectroscopy of the positron annihilation radiation using two Ge detectors in coincidence. 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 GP zones, after the solution heat treatment of the alloy.

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