Effect of additions of Si and Ag to ternary Al-Cu-Mg alloys in the α + S phase field

Atom probe analysis was performed on an Al-1.1Cu-1.7Mg-0.1Ag (at.%) alloy and an equivalent alloy containing 0.25 at.% Si. Analyses were made from samples in the as-quenched condition and following ageing for 5 min. at 150°C, with a view to characterizing the clustering and precipitation processes associated with the rapid hardening reaction noted in these alloys. Fine scale Mg-Ag co-clusters were detected in Al-1.1Cu-1.7Mg-0.1Ag, together with numerous Mg-rich clusters, some of which contained Cu. The addition of Si to this alloy accelerates the interaction of solute and fine scale precipitates, rich in Mg and Cu and containing traces of Ag and Si, were detected immediately following the rapid hardening reaction. Other precipitates, rich in Cu, were also observed. The formation of these various clusters and precipitates is linked to the rapid hardening reaction. The composition of the {111}α X′ phase was also analyzed by atom probe in the Si-free alloy and results in the range Cu:0.2-0.25, Mg:0.15-0.25 and Al:0.5-0.65 were found. Furthermore, Ag was detected in association with the X′ precipitate. Results suggest that Ag is enriched inside the X′ phase to levels as high as 5 at.%.