Ca microalloying-enabled β″/β′ precipitation and its effect on strength and electrical conductivity in Al–5Mg-xSi alloys

This study investigates the influence of Si content (1–4 wt. %) and T5/T6 heat treatments on precipitation behavior, electrical conductivity, and mechanical properties of a Ca-modified high-Mg Al–5Mg–xSi (Almag5–xSi) alloy system. Thermodynamic and kinetic analyses indicate that increasing Si accelerates Mg 2 Si precipitation by reducing nucleation barriers, while trace Ca (nominally 0.02 wt.%) promotes heterogeneous nucleation via Al 4 Ca/Ca–Si clusters, refining eutectic Mg 2 Si from lamellar to fibrous morphology and enhancing intragranular β″ precipitation. TEM–FFT–IFFT analysis confirms that T6 treatment produces dense, coherent β″ (Mg 5 Si 6 ) precipitates, with partial β′ (Mg 9 Si 5 ) formation in Si-rich alloys. Ca modification induces multi-oriented β″ nucleation and suppresses coarsening. The electrical conductivity increases with Si content and is higher after T6, reaching 48.3±0.5% IACS for Almag5–3Si owing to the near-stoichiometric Mg/Si ratio and solute depletion. The yield strength increases with Si, peaking at 361.7±1.5 MPa in Almag5–4Si, while the ultimate tensile strength reaches 368.3±1.3 MPa. The maximum ductility (13.3%) is observed in Almag5–2Si owing to the stable β″ precipitates. Notably, the Ca-modified Almag5–2Si alloy outperformed the Ca-free Al–5Mg–2Si counterpart, showing a superior strength–conductivity–ductility balance due to refined dense β″ precipitation and early β′ formation. Overall, Si and Ca synergistically optimize strength, conductivity, and ductility.