The key role of Zn in enhancing precipitation kinetics and refinement of Mg₁₇Al₁₂ and Mg₂Sn phases through trace Na additions

This study investigates zinc's (Zn) key role in enhancing the precipitation kinetics and refinement of Mg₁₇Al₁₂ and Mg₂Sn phases in magnesium alloys through trace sodium (Na) additions. Magnesium alloys with varying compositions of aluminum (Al), tin (Sn), Zn, and Na were prepared and aged at 453 K. Microstructural analyses were conducted using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and atom probe tomography (APT). Trace additions of Na significantly enhanced the precipitation responses of both Mg₁₇Al₁₂ and Mg₂Sn phases. When Zn was co-added with Na, as in the ATZ641N3 alloy (Mg–6Al–4Sn–1Zn–0.3Na), there was a pronounced refinement in precipitate morphology and acceleration of precipitation kinetics. The ATZ641N3 alloy achieved a peak hardness of 103 Hv at 36 hours, compared to 91 Hv at 72 hours for the ATZ641 alloy without Na. The simultaneous addition of Zn and Na led to the formation of Sn–Na–Zn particles that acted as effective nucleation sites for Mg₂Sn, promoting aluminum partitioning and accelerating the precipitation of Mg₁₇Al₁₂ through Al-rich regions. Additionally, Zn and Na co-segregated within the Mg₁₇Al₁₂ phase, reducing misfit strain caused by Zn substitution and improving precipitate stability and refinement. These findings highlight Zn's critical role, alongside trace Na additions, in refining and accelerating the precipitation of Mg₁₇Al₁₂ and Mg₂Sn phases, thereby enhancing the age-hardening response of magnesium alloys.