Effect of Cu and Ge on solute clustering in Al-Mg-Si alloys

The clustering kinetics after quenching and during natural ageing in pure or Cu/Ge-containing Al-Mg-Si alloys was studied by positron annihilation lifetime spectroscopy and hardness measurements. The evolution of positron lifetime follows qualitatively the same pattern for all the alloys investigated, but also differs in some details: adding Cu to both Al-Mg-Si and Al-Mg-Ge alloys slows down the initial formation of clusters in all alloys. However, in Al-Mg-Si alloys, it promotes their growth in later stages. For Ge-containing Al-Mg-Si alloys with equal Mg contents, Ge notably retards the ageing kinetics compared to Si. Possible reasons of these effects are related to the interactions between vacancies and solutes or solute clusters and to the diffusion kinetics of various solute-vacancy complexes.