Revisiting the effect of Ag additions on Ω precipitation and heat resistance of Al–Cu–Mg–Si–Ag alloys

In heat-resistant Al–Cu–Mg–Ag alloys, the presence of AgMg segregation layer on Ω phase ensures its high thermal stability. It was reported Ω precipitation would be inhibited when Si was added such that the Mg/Si mass ratio was lower than 2. However, in this work, it has been proved the notable increase in Ag content from 0.5% to 0.8% could still promote precipitation of Ω besides θ′ in an Al–Cu–Mg–Ag–Si alloy with a Mg/Si mass ratio around 1.7. The key point is to obtain an Ag/Mgexcess atomic ratio greater than 1 under the premise of W(Mg)/W(Si) > 1.4 in the supersaturated solid solution, where Mgexcess denotes the remaining Mg solutes after the formation of C/L phase. Although the fine precipitation of Ω in the 0.8 Ag alloy increases the total volume fraction of precipitates and yield strength, their quick coarsening at high temperature leads to a more pronounced strength loss. Such a comparison reveals the fully enveloped Ω precipitate is less stable than the θ′ precipitate with multiple interface phases. These findings update our knowledge on the systematic composition design for tunning Ω/θ′ precipitates' ratio and properties of Al–Cu–Mg–Ag–Si alloys.