Precipitation processes in an Al-2.5Cu-1.5Mg (wt. %) alloy microalloyed with Ag and Si

We have produced precipitation on and along both the <100>α and the {111}α crystal systems by microalloying an Al-2.5Cu-1.5Mg (wt. %) base alloy with Ag and Si. Using electron microscopy and atom probe field ion microscopy, we have demonstrated that Ag+Si additions produce multi-component clustering reactions. When compared to comparable quaternary compositions, the quinary alloy exhibits a higher hardness. The first precipitates observed were GPB zones, rich in Cu and Mg and containing traces of both Si and Ag. Following ageing at 200 deg. C, the peak hardness microstructure consisted of rod-shaped GPB zones along <100>α together with X' and Ω precipitation, both as plates on the {111}α planes. The Ω phase was less stable than the X' phase and dissolved soon after ageing to peak hardness. This was associated with a rapid drop in hardness. The overaged microstructure was dominated by S phase