Microstructure Evolution and Synergistic Optimization of Mechanical Properties and Thermal Conductivity in Mg-xSn-0.2Ca-0.5Zr Alloys

To address the challenge of the trade-off between mechanical properties and thermal conductivity in magnesium alloys, Mg-xSn-0.2Ca-0.5Zr alloys (x = 3, 4, 5 wt.%) were investigated in this work. Permanent mold casting, semi-solid rheo-die casting, and heat treatment were employed to tailor the microstructure of the alloys. The inherent correlations between microstructure evolution, mechanical properties, and thermal conductivity were systematically explored. The results reveal that as Sn content increases in permanent mold cast alloys, grain refinement occurs and the volume fraction of secondary phases rises. Nevertheless, excessive Sn induces coarsening of grain-boundary Mg ₂ Sn, thereby degrading both ductility and thermal conductivity. Among the permanent mold cast alloys, Mg-4Sn-0.2Ca-0.5Zr exhibits optimal overall performance, with an ultimate tensile strength of 137 MPa, yield strength of 71 MPa, elongation of 7.6%, and thermal conductivity of 86.12 W/(m·K), and is accompanied by the lowest hot-tearing susceptibility. Near-spherical α-Mg grains are obtained in the semi-solid rheo-die cast Mg-4Sn-0.2Ca-0.5Zr alloy. After T6 heat treatment, fine Mg ₂ Sn precipitates are dispersed throughout the matrix. The ultimate tensile strength, yield strength, and elongation are improved to 202 MPa, 112 MPa, and 11.7%, respectively. Meanwhile, aging facilitates the precipitation of Sn from the α-Mg matrix and reduces electron scattering, contributing to an elevated thermal conductivity of 110.68 W/(m·K). Regulation of the solidification microstructure enables the simultaneous enhancement of strength, ductility, and thermal conductivity for Mg-Sn-Ca-Zr alloys.