Microstructure-dependent thermal conductivity and mechanical properties in cast Mg-4Sm-xAl alloys

The thermal conductivity of Mg-Sm binary alloys is relatively poor due to the severe lattice distortion introduced by the high solid solubility Sm atom in the Mg matrix, which restricts their widespread application. In this study, the viability of Al alloying (0-2.6 wt%) in permanent mold cast Mg-4Sm (wt.%) alloy was investigated to design cast Mg alloys with high thermal conductivity. It is found that the thermal conductivity significantly improved with the addition of Al content, and peaked at 136.8 W/(m & BULL;K) with an Al content of 1.5 wt%, which is attributed to the release of the Sm atoms from the Mg matrix. It is also confirmed that minimizing the lattice distortion effect is critical to achieving high thermal conductivity. The formation of Al2Sm and Al11Sm3 phases leads to a dramatic decline in solid solution strengthening due to its strong consumption of the dissolved Sm atoms, but in the meantime enhances secondary phase strengthening and promotes stronger interactions between different strengthening components. The optimum combination of mechanical properties and thermal conductivity is obtained in Mg-4Sm-2Al alloy, which possesses a yield strength of 102.6 MPa, elongation of 6.8% and thermal conductivity of 124.5 W/(m & BULL;K), compared with several commercial as-cast Mg alloys.