Atom probe tomography study of Mg-dependent precipitation of Ω phase in initial aged Al-Cu-Mg-Ag alloys

The association between Mg variations and the precipitation of Ω phase in Al-Cu-Mg-Ag alloys were investigated by transmission electron microscopy and quantitative atom probe tomography analysis. After aging at 165 °C for 2 h, the highest number density of Ω phase was revealed in 0.81Mg alloy, leading to the highest strength properties. The lowest strength properties of 0.39Mg alloy was related to the lowest precipitation kinetics of Ω phase. The parabolic change in the plate number density with increasing Mg highlighted the existence of a critical Mg content that contributed to the strongest precipitation kinetics of Ω phase. The number density of Mg-Ag co-clusters was not the sole factor in controlling the Ω precipitation. It was found that the precipitation of Ω phase was not only determined by initial Mg-Ag co-clustering but also related to the effective competition for solutes. In addition, the cluster-dominated microstructure facilitated the dense precipitation of Ω phase.