Sc/Zr ratio-dependent mechanisms of strength evolution and microstructural thermal stability of multi-scale hetero-structured Al–Mg–Sc–Zr alloys

Abstract

• A tensile strength of ∼605 MPa and ∼10% elongation were obtained in Al–7Mg–0.3Sc–0.1Zr. • The strength-ductility synergy owes to a multi-scale heterogeneous microstructure. • While Al–7Mg–0.1Sc–0.3Zr had higher thermal stability at ≥ 400 °C than high-Sc/Zr ratio alloys. • High-content Zr atoms are helpful in enhancing precipitation and coarsening resistance of Al 3 (Sc,Zr). Microstructure and its thermal stability are critical in the development of high-performance Al–Mg alloys. Here, we attempt to tailor Al 3 (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 Al 3 (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 Al 3 (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 Al 3 (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.