Microstructures and fatigue fracture behavior of an Al–Cu–Mg–Ag alloy with a low Cu/Mg ratio

The influence of Ag addition on the microstructure and fatigue fracture behavior of a low Cu/Mg ratio Al–Cu–Mg alloy was investigated. Results showed that the highest hardening response was revealed by initial slight artificial aging because pre-aging at 170 °C for 0.5 h produced high density Mg–Ag co-clusters for the subsequent precipitation of Ω plates. Also, pre-stretching facilitates the formation of S(S′) needles but has a negative effect on the precipitation of Ω phase. A close correlation between the fatigue behavior and the microstructures of the studied Al–Cu–Mg–Ag alloy was revealed. For both underaged conditions, slight artificial aging was found to improve the fatigue crack propagation (FCP) resistance of the studied Al–Cu–Mg–Ag alloy as compared to T351 treatment. Although long term aging at 150 °C following initial aging treatments was found to significantly degrade the FCP resistance, the relative high FCP resistance was still revealed in the Ω-dominated microstructure rather than the S(S′)-dominated one.