Clustering kinetics during natural ageing of Al-Cu based alloys with (Mg, Li) additions

TL;DR

This study investigates the natural ageing clustering process in Al-Cu alloys with Mg and Li additions, revealing Mg's crucial role in accelerating clustering and diffusion at room temperature using advanced scattering techniques.

Contribution

It introduces a correlative SANS and SAXS technique to analyze chemically resolved clustering kinetics in Al-Cu alloys with Mg and Li, highlighting Mg's pivotal role.

Findings

Mg triggers diffusion and clustering in Al-Cu alloys.

Strong natural ageing observed in Al-Cu-Mg, minimal in Al-Cu-Li.

Adding Mg rapidly forms Cu-rich clusters, with Mg joining later.

Abstract

Room temperature solute clustering in aluminium alloys, or natural ageing, despite its industrial relevance, is still subject to debate, mostly due to its experimentally challenging nature. To better understand the complex multi-constituents' interactions at play, we have studied ternary and quaternary subsystems based on the Al-Cu alloys, namely Al-Cu-Mg, Al-Cu-Li and Al-Cu-Li-Mg. We used a recently introduced correlative technique using small-angle neutrons and X-ray scattering (SANS and SAXS) to extract the chemically resolved kinetics of room temperature clustering in these alloys, which we completed with differential scanning calorimetry (DSC) and micro-hardness measurements. The comparison of the clustering behaviours of each subsystem allowed us to highlight the paramount role of Mg as a trigger for diffusion and clustering. Indeed, while a strong natural ageing was observed in the Al-Cu-Mg alloy, virtually none was shown for Al-Cu-Li. A very slight addition of Mg (0.4%) to this system, however, drastically changed the situation to a rapid formation of essentially Cu-rich hardening clusters, Mg only joining them later in the reaction. This diffusion enabling effect of Mg is discussed in terms of diffusion mechanism and complex interactions with the quenched-in vacancies.