Mg-1.88Zn-0.75Y Cast Alloys with High Thermal Conductivity of 141 Wm<sup>−1</sup>K<sup>−1</sup>

High thermal conductivity ternary Mg alloys composed of Zn and Y pair with a negatively large mixing enthalpy was developed by optimization of alloy composition and heat-treatment conditions. Optimal alloy composition was Mg-1.88Zn-0.75Y (at%) alloy, in which the ratio of Zn content to Y content was 2.5 and the Y content was 0.75 at%. The alloy was composed of α-Mg+W phase (Mg3Zn3Y2)+I phase (icosahedral Mg3Zn6Y). Heat treatment under the optimal heat treatment conditions, where temperature, time and cooling rate were 633 K, 15 h and air cooling, respectively, improved the thermal conductivity from 114 to 141 Wm−1K−1 that corresponds to 90% of the pure Mg thermal conductivity. Fine W phase precipitation in α-Mg matrix by the heat-treatment caused a reduction of solute Y element in α-Mg matrix, resulting in improvement of the thermal conductivity.