Designing High‐Strength and High‐Ductility Cast Mg–Zn–Al–Mn–Sn Alloys via τ‐Mg ₃₂ (Al,Zn) ₄₉ Phase Tuning and Twin Boundary Toughening Optimisation

ABSTRACT Achieving a simultaneous enhancement of strength and ductility remains a critical bottleneck for the large‐scale and cost‐effective engineering application of magnesium alloys. In this work, guided by first‐principles calculations, we tailored the τ‐Mg 32 (Al,Zn) 49 phase and optimised twin boundary, thereby designing and successfully synthesising a series of low‐cost high‐performance Mg–10Zn–4Al–0.4Mn– x Sn ( x = 0.0, 0.2, 0.4, 0.6; labelled as ZAM– x T) alloys. First‐principles calculations demonstrated that the τ‐Mg 32 (Al,Zn) 49 phase reached optimal elastic properties at a Zn/Al ratio of 2:1, and that Sn was the most effective alloying element for Mg twin boundary strengthening. Moreover, experimental results showed that the main phases in the ZAM– x T alloys consisted of α‐Mg, τ‐Mg 32 (Al,Zn) 49 , AlMn and Al 8 Mn 5 , with the Zn/Al atomic ratio in the τ‐Mg 32 (Al,Zn) 49 phase maintaining a 2:1 proportion. The aged ZAM–0.6T alloy exhibited an ultimate tensile strength of 334 MPa, a yield strength of 191 MPa and an elongation of 11.8%, whereas the aged ZAM–0.4T alloy had the highest Young's modulus, reaching 48 GPa. In addition, Orowan and grain boundary strengthening were identified as the primary strengthening mechanisms, contributing 108 and 40 MPa, respectively, to the yield strength of the ZAM–0.6T alloy. This study demonstrated the use of first‐principles calculations to design phase stability and interfacial strength, guiding alloying element selection to accelerate the development of high‐strength ductile magnesium alloys, minimise experimental trial‐and‐error and advance lightweight cast alloy applications.