Evolution of solute clustering in Al–Cu–Mg alloys during secondary ageing

The evolution of an atomistic-level nanostructure during the early stages of secondary ageing of a rapid hardening Al–1.1Cu–1.7Mg (at.%) alloy has been characterized by a combination of atom probe tomography (APT) and transmission electron microscopy. Quantitative APT analysis reveals key changes in the evolution of solute clusters during secondary ageing (T6I4 condition) that correlate with secondary hardness increments. The microstructures that cause peak hardness differ between the T6 and T6I4 tempers – the former is a result of solute clustering as well as the precipitation of GPB zones and S phase, whereas in the latter, secondary ageing promotes only the formation of solute clusters. Cu–Mg clusters with high Mg:Cu ratio have the most strengthening potency during secondary ageing in T6I4 heat treatment.