Achieving superior synergy of strength and toughness in Al–Cu–Mg–Ag alloys via Mg content regulation

The mechanical properties and microstructural evolution of T8-tempered Al–5Cu-xMg-0.4Ag (x = 0.6, 0.8, 1.0, 1.5) alloys were systematically investigated to elucidate the strengthening mechanisms and optimize alloy performance. The Ω, S, and θ′ precipitates were characterized by transmission electron microscopy (TEM), and quantitative evaluation was performed on their individual contributions to precipitation strengthening. The strengthening contribution ratios of the Ω phase were determined to be 87.0%, 85.8%, 83.5%, and 68.3%, respectively, which validates that the Ω phase plays a dominant role in the overall precipitation strengthening of the alloys with varying Mg contents. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were employed to statistic and analyze the second-phase particles and grain structures. Results show that GNDs density is inversely proportional to grain size and positively correlated with the Schmid factor. Furthermore, heterogeneous grain structures exhibit a higher sensitivity to GNDs density compared to uniform fine-grain structures. TEM analysis of grain boundary precipitates revealed a clear correlation between grain structure, grain boundary precipitation, and fracture toughness. Tensile and Kahn tear tests demonstrated that the alloy with 0.8 wt% Mg achieved the highest yield strength (501 MPa) and ultimate initiation energy (265 N/mm), highlighting its superior strength-toughness balance.