Abstract: Microstructure and its thermal stability are critical in the development of high-performance Al-Mg alloys. Here, we attempt to tailor Al₃(Sc,Zr) precipitates and thus microstructure characteristics to manipulate mechanical properties and microstructural stability of Al-7Mg alloys fabricated by hot extrusion combined with two-pass hard-plate rolling via changing Sc/Zr ratio. Increasing Sc/Zr ratio leads to improved strength without any loss of ductility. A strength-ductility synergy, i.e. yield strength of ~548 MPa and ultimate tensile strength of ~605 MPa with an impressive ductility of ~10% elongation was achieved in the Al-7Mg-0.3Sc-0.1Zr alloy. The good strength-ductility synergy is ascribed to the multi-scale heterogeneous microstructure promoted by the high Sc/Zr ratio, i.e. a bimodal grain structure, profuse low angle grain boundaries, dispersed nano-sized Al₃(Sc,Zr) precipitates coexisting with intragranular Mg-Zr co-clusters segregated at dislocations. Upon thermal exposure, the Al-7Mg-0.3Sc-0.1Zr alloy maintained higher hardness at below 250 °C, whereas Al-7Mg-0.2Sc-0.2Zr and Al-7Mg-0.1Sc-0.3Zr alloys exhibited higher hardness in moderate-and high-temperature range of 250-350 °C and≥400 °C, respectively. Atom-probe tomography analysis illustrates that slow-diffusing Zr atoms enhance Al₃(Sc,Zr) coarsening resistance through forming a higher-content Zr-enriched protective shell around a Sc-enriched core in Al-7Mg-0.1Sc-0.3Zr. Meanwhile, the high Zr content promotes concurrent Al₃(Sc,Zr) precipitation during thermal exposure at high temperatures. The improved microstructural thermal stability in Al-7Mg-0.1Sc-0.3Zr alloy is further discussed in terms of the recrystallization resistance and grain growth behavior. The present study reveals the feasibility for designing high-strength and thermally stable hetero-structured Al-Mg-Sc-Zr alloys via tailoring Sc/Zr ratios for different application temperature ranges.

Key words: Al-Mg alloys, Al₃(Sc,Zr), Strength, Thermal stability, Bimodal grain structure