The influence of various Ag additions on the nucleation and thermal stability of Ω phase in Al-Cu-Mg alloys

The effect of the bulk Ag content ranging from 0.14 to 0.57 wt% on the aging hardening response and microstructures of Al-Cu-Mg alloys was investigated in the present study. The strong dependence of the hardening response on the bulk Ag content was obtained as the hardening response at 165 °C was accelerated by increasing Ag content. The highest hardening response was observed in the alloy containing 0.57 wt% Ag, whereas the least one was obtained in the alloy containing 0.14 wt% Ag. The tensile testing at various temperatures also revealed a similar trend. Based on the TEM study on peak aging conditions at 165 °C, it was found that the competitive precipitation kinetics between Ω and θ′ phases was sensitive to the variation of Ag content. The dense precipitation of the Ω plates promoted by increasing Ag content was responsible for the increasing hardening response. Meanwhile, the formation of θ′ phase was greatly suppressed. Subsequent long term aging at 200 °C for 500 h following the peak aging treatment further suggested that the thermal stability of the Ω plates was enhanced with increasing Ag as the presence of the Ω plates could be clearly confirmed in 0.57 wt% Ag alloy but was hard to distinguish in the alloy containing 0.14 wt% Ag.