Co-strengthening of the multi-phase precipitation in high-strength and toughness cast Al–Cu–Zn–Mg alloy via changing Zn/Mg ratios

The effects of the Zn/Mg ratio on the as-cast microstructure, aging-precipitation behavior, and mechanical properties of cast Al–Cu–Zn–Mg alloys, an interesting precipitation-hardened Al alloy different from the 2XXX and 7XXX, were investigated. The primary phase of the as-cast alloy changed from lamellar θ and S phases to a block S phase with an increased Zn/Mg ratio. The number density of the solely S phases in the peak-aged alloy increased with the Zn/Mg ratio from 0.4 to 1.5, because the Zn promoted the formation of GPB zones. At a Zn/Mg ratio of 2.5, the major precipitates are the S, η, and T phases, due to the preferential interaction of Mg atoms with Zn, which promotes the formation of Mg–Zn clusters and GP zones in the initial stages of aging. The yield strength, ultimate tensile strength, and elongation reached maximum values of 372 MPa, 502 MPa, and 12.3%, respectively. This illustrates the cooperative strengthening of the S, T, and η phases, which pin the moving dislocations and inhibit crack initiation and propagation. A large amount of the T phase in the high Mg content alloy suppresses the formation of the S phase. This multi-phase precipitation addressed the “strength ductility trade-off” in cast Al alloys.