Abstract
In order to elucidate some of the differences between Al-Mg-Si and Al-Mg-Ge alloys and the role of Cu, a series of Al-Mg-Ge, Al-Mg-Si and Al-Mg-Ge-Si alloys, some of them containing Cu, are investigated by positron annihilation lifetime spectroscopy during natural ageing. Al-Mg-Ge alloys show qualitatively the same evolution of positron lifetime τ1C with time as Al-Mg-Si alloys, namely an initial decrease, followed by a re-increase, after which τ1C drops to an equilibrium value. However, for alloys with equal Mg contents, Ge gives rise to a notably slower ageing kinetics than Si, pointing at effects of atomic size or solute-vacancy binding energies. Adding Cu to both Al-Mg-Ge and Al-Mg-Si alloys slows down the initial formation of clusters but promotes their further growth.