Regulating the GP Zone to T′ Phase Evolution and Achieving Strength–Ductility Synergy in an Al-Mg-Zn-Cu Alloy via a Two-Step Aging (T4P-BH) Process

To address the technical challenge of balancing formability and strength in automotive aluminum alloys, this study examined an Al-4.35Mg-3.6Zn-0.2Cu alloy subjected to a combined heat-treatment schedule consisting of a two-step solution treatment (470 °C for 24 h followed by 460 °C for 30 min) and a subsequent two-step aging process (T4P: 80 °C for 12 h, followed by BH: 180 °C for 30 min). Microstructural evolution was characterized using transmission electron microscopy, and uniaxial tensile tests were performed in accordance with the GB/T 228.1-2021 standard at a strain rate of 0.2 mm/min. In the T4P condition, the matrix contained both GPI zones (~0.9 nm) and GPII zones (~1.2 nm), with no detectable T-phase precipitation. The presence of GPII zones enhanced ductility by promoting dynamic recovery after dislocation shearing, resulting in a yield strength (YS) of 178 MPa, an ultimate tensile strength (UTS) of 310 MPa, and an elongation (El) of 9%. After BH treatment, the GPII zones transformed into semi-coherent T′-Mg32(AlZnCu)49 precipitates (~2.4 nm), which strengthened the alloy through their semi-coherent interfaces. The retained GPII zones mitigated the loss of ductility, and the final mechanical properties reached a YS of 275 MPa, a UTS of 340 MPa, and an El of 8.5%, corresponding to a BH response of 97 MPa. Strengthening-mechanism calculations indicated that GP zones contributed approximately 120 MPa to the yield strength in the T4P state, whereas T′ precipitates contributed about 169.64 MPa after BH treatment. The calculated values agreed well with the experimental results, with a deviation of less than 3%. This study clarifies the precipitation sequence in the alloy—supersaturated solid solution → GPI zones → GPII zones → T′ phase—and establishes the relationship between microstructure and strength–ductility behavior. The findings provide theoretical guidance for the design and optimization of high-strength, high-formability aluminum alloys for automotive outer-panel applications.