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 (GP) 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.

The standard positron trapping model has often been applied, as a simple approximation, to the interpretation of positron lifetime spectra in situations of diffusion-controlled trapping. This paper shows that this approximation is not sufficiently accurate, and presents a model based on the correct solution of the diffusion equation, in the version appropriate for studying positron trapping at grain boundaries. The model is used for the analysis of new experimental data on positron lifetime spectra in a fine-grained Al-Ca-Zn alloy. Previous results on similar systems are also discussed and reinterpreted. The analysis yields effective diffusion coefficients not far from the values known for the base metals of the alloys.