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High-strength Al-Cu-Mg-Ag-Sm alloy was fabricated and subjected to single-stage aging and pre-aging (two-stage aging).Effect of pre-aging on microstructure and mechanical properties of the alloy was investigated.It is found that the alloy is mainly composed of α-Al,Al2Cu,Al2CuMg and AlCu4Sm.The number of plate-like Ω Al2Cu precipitates is comparable to that of rod-like S Al2CuMg precipitates in the single-stage aged alloy,whereas,in the two-stage aged alloy,it is much higher than that of S precipitates.Ω precipitates have a smaller plate thickness and distribute more uniformly in the two-stage aged alloy than in the single-stage aged alloy.Ultimate tensile strength (UTS) and yield strength of the two-stage aged alloy are 12％ higher than those of the single-stage aged one,indicating a better aging hardening caused by the two-stage aging.The increased tensile properties mainly come from both stronger precipitation strengthening caused by more Ω precipitates in the twostage aged alloy and stronger solution strengthening from Mg atoms.The fracture surfaces consist of both dimple zones composed of microscale dimples and platform zones composed of nanoscale dimples.The total area of dimple zones for single-stage aged alloy is much higher than that for two-stage aged alloy,which can be attributed to different numbers of Ω precipitates in the two alloys.