Revisiting the effect of Ag additions on 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 omega phase ensures its high thermal stability. It was reported omega 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 omega besides theta ' 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 omega 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 omega precipitate is less stable than the theta ' precipitate with multiple interface phases. These findings update our knowledge on the systematic composition design for tunning omega/theta ' precipitates' ratio and properties of Al-Cu-Mg-Ag-Si alloys.