A study of the composition dependence of the rapid hardening phenomenon in Al–Cu–Mg alloys using diffusion couples

A diffusion couple approach is demonstrated for the combinatorial study of the compositional dependence of the rapid hardening phenomena in Al–Cu–Mg alloys. A series of Al–Cu/Al–Cu–Mg and Al–Mg/Al–Cu–Mg diffusion couples have been successfully fabricated to contain a gradient in Mg or Cu concentration, respectively. The effects of both total solute content and Cu:Mg ratio of the rapid hardening phenomena are considered. The rapid hardening response has been monitored by measuring the hardness profile across the diffusion gradients before and after artificial ageing. The composition profiles were quantitatively verified by electron probe microanalysis (EPMA) and the microstructure of the ternary end members has been characterised using atom probe tomography (APT). It is demonstrated that a critical Cu content exists, above which the rapid hardening phenomena diminishes. A change in the chemistry of the Cu–Mg clusters that form, and which are thought to determine the rapid hardening increment, also occurs with changes in the bulk alloy chemistry. The results suggest that Cu–Mg clusters rich in Mg have greater strengthening potency and this is consistent with recent suggestions by Marceau et al. The need for the development of combinatorial approaches to characterisation (microstructural and mechanical) for the full utilisation of such approaches in physical metallurgy is emphasised.