Strategic Ca addition for tailoring precipitates and enhancing mechanical properties in Mg‑Sm‑Zn alloys

Multi-alloying design of rare earth magnesium alloy is a new way to achieve strengthening, toughening and lightweight. This study investigated how Ca incorporation influences the microstructure and properties of extruded Mg-1.5Sm-1Zn (wt%) alloys. Ca addition refined grains and facilitated Mg₄₁Sm₅ phase dispersion. The refinement originated from finely dispersed Mg₂Ca particles acting as heterogeneous nucleation sites. Visco-plastic self-consistent simulations showed that increasing Ca shifted the deformation from multi-slip/twinning to predominantly basal <a> slip, with low Ca partially activating twinning and non-basal slip to relieve strain, while high Ca limited plasticity due to twinning exhaustion and non-basal slip suppression. Consequently, yield strength and ultimate tensile strength increased by 75% and 20%, respectively, with 0.6wt% Ca. The strengthening effect is resulted from refinement-induced and second-phase reinforcement. In contrast, excess Ca produced coarse, brittle Mg₂Ca particles with weak interfacial bonding, which acted as stress concentrators, accelerated crack initiation, and reduced ductility.