Defects Associated with Nanostructures in AlZnMg and AlCu(Mg) Alloys

The present work reports on a positron annihilation study addressing the structural and chemical characterization of solute aggregates containing open volumes (vacancies and/or misfit regions at precipitate–matrix interfaces) in AlZnMg, AlCu, and AlCuMg alloys. High resolution transmission electron microscopy results for selected ageing conditions are also presented. Two main points are discussed: (i) differentiation of the origin of hardening during artificial ageing between the AlZnMg and AlCu(Mg) alloy systems; (ii) structural origin of secondary ageing at low temperature, after a few minutes of high temperature exposure. It is shown that in AlZnMg alloys hardening at a high temperature is concomitant with the loss of coherency; on the contrary, in AlCu(Mg) alloys hardening is associated with coherent structures. Positron lifetime and coincidence Doppler broadening data taken during secondary ageing indicate that the residual solute supersaturation leads to the formation of coherent Zn–Mg–v (or Cu–Mg–v) aggregates.