Solute clustering and solute nanostructures in an Al–3.5Cu–0.4Mg–0.2Ge alloy

An Al–3.5Cu–0.4Mg alloy with 0.2 wt.% Ge added exhibited an enhanced age–hardening response as indicated by 50% reduction in peak ageing time and 25% increase in the peak hardness during ageing at 200 °C. The clustering and partitioning behaviours of solutes in the alloy during ageing were investigated in detail using atom probe tomography. For the first time, two types of solute clusters, i.e. MgGe-rich clusters and Cu-rich clusters, are found to form in the alloy microalloyed with Ge, and they have different formation kinetics. In the as-quenched state, these small MgGe-rich clusters, rapidly formed in a high number density, did not produce an additional strengthening effect above the strength of an as-quenched Al–3.5Cu–0.4Mg base alloy. They assisted the subsequent formation of fine MgGe-rich needles during ageing. These fine needles serve as a major strengthening component and are responsible for an accelerated age-hardening response of the alloy. By contrast, the later formed Cu-rich clusters assisted the formation of strengthening θ′ precipitates (θ′ platelets and elongated θII′ precipitates) in the alloy. The effective stimulation of forming fine MgGe-rich needles by Ge addition makes the alloy possess a high strength after a prolonged ageing.