A new methodology for the characterization of solute clusters leading to compositional fluctuations is presented and discussed. The methodology makes use of contrast variation arising from a combination of small‐angle scattering using neutrons and X‐rays, and adapts a model for solute correlation to extract the chemistry and length scale of clustered states after quench and after natural ageing. In three subsets of the Al–Cu system, Cu‐rich clusters are reported for all cases. The presence of Mg strongly enhances Cu clustering in the naturally aged state and results in more than double the number of clusters in the complex Al–Cu–Li–Mg system. The results are compared with those obtained using atom probe tomography.

The effect of Mg content on the natural ageing of Al-Cu-Li-(Mg) alloys has been investigated on a compositionally graded material made by linear friction welding, allowing to probe Mg contents between 0 and 0.4 at.%. High throughput time-and space-resolved characterization of natural ageing kinetics has been achieved using small-angle X-ray scattering, supplemented by differential scanning calorimetry. Natural ageing results mainly in the formation of Cu-rich clusters whose characteristics strongly depend on the presence of Mg. Larger precipitates of 2 nm size also form to a lesser extent above an Mg concentration of 0.1 at.%.Al-Cu-Li alloys have become a staple of the aerospace industry with their high strength to weight ratio, good toughness and corrosion resistance [1]. Research on these alloys has been focused on promoting precipitation of the T1 (Al2CuLi) phase through thermo-mechanical processing [2,3] and chemical alloying [4][5][6][7]. Dislocations were shown to dramatically promote the formation of high density T1 precipitates by acting as heterogeneous nucleation sites [8][9][10]. Mg additions above 0.2 wt. % strongly influence the precipitation pathway of the alloys through the formation of precursor phases rich in Cu and Mg during ageing, subsequently promoting formation of T1 phase [7,11]. Preceding the artificial ageing treatment where these precipitation processes occur, these alloys undergo clustering processes during natural ageing. Solute clustering and the associated strengthening has been well documented in most age hardening aluminum alloys [12][13][14] and plays a prominent role in determining the subsequent precipitation path. Starink and Wang [15] evidenced co-clustering between Cu and Mg in Al-Cu-Mg alloys and attribute it to a strong binding energy of a Cu-Mg pair. Decreus and co-workers, and Gumbmann and co-workers, report on the presence of Cu-rich clusters in Al-Cu-Li alloys with minor addition of Mg after ageing at room temperature [8,16]. In a former study, we have evidenced that adding a minor amount of Mg (0.4 at. %) to an Al-Cu-Li alloy changed dramatically the clustering behavior of Cu atoms during natural ageing [17], so that the presence of Mg seems to play a crucial role on the ability of Cu atoms to diffuse in the Al matrix and join in clusters. However, much is still unknown on this phenomenon, particularly what is the effect of the Mg concentration on the clustering kinetics of Cu, with or without a threshold content. Thus, the aim of the present contribution is to investigate the link between Mg concentration and cluster formation kinetics during natural ageing of an Al-Cu-Li alloy with a continuous variation of Mg addition.