Strength–Ductility Synergy in a Peak-Aged Low-Cu Al-Zn-Mg-Cu-Zr Thin-Walled Extruded Profile: Coupling of η′ Precipitation and Heterogeneous Grain Structure

To elucidate the strength–ductility synergy in a peak-aged low-Cu Al-Zn-Mg-Cu-Zr thin-walled extruded profile, the mechanical properties, precipitation behavior, and grain structure of the alloy were systematically investigated after solution treatment at 470 °C for 2 h followed by single-step aging at 120 °C for 8–48 h. The results showed that the strength, hardness, and elongation initially increased and then decreased with increasing aging time. The optimal combination of mechanical properties was achieved after aging at 120 °C for 24 h, with an ultimate tensile strength of 644 ± 6 MPa, a yield strength of 592 ± 4 MPa, an elongation to failure of 15.6 ± 0.4%, and a Vickers hardness of 189.3 ± 1.5 HV. Precipitation strengthening induced by η′ precipitates was identified as the primary source of the high strength, while grain-boundary strengthening and dislocation/substructure strengthening provided additional contributions. Compared with the Rod-T6 condition, the Profile-24 h condition exhibited a comparable yield strength, whereas its elongation to failure increased from 9.16% to 15.6%. The fine-grained regions contributed to grain-boundary strengthening, while the coarse-grained regions provided enhanced plastic strain accommodation. Meanwhile, the retained low-angle grain boundaries and deformation substructures promoted dislocation storage and work hardening. These microstructural features collectively improved deformation compatibility and delayed local strain concentration and fracture, thereby enabling a favorable combination of high strength and high ductility.