Recently the extruded Mg–Zn–Y alloy is known as attractive for automotive components with increased strength at elevated temperature. In order to achieve these attractive properties in cast component, we focused on grain refinement of Mg
96Zn
2Y
2 (at%) cast alloy with long-period stacking ordered (LPSO) structures and investigated the effects of zirconium addition and cooling rate on microstructures and mechanical properties. Mg
96Zn
2Y
2 cast alloy with the addition of zirconium consisted of α-Mg phase, Mg
12ZnY phase and Mg
3Zn
3Y
2 phase as well as the alloy without zirconium. However, the coarse dendritic structure was found to transform into fine equiaxed grains by an addition of 0.2 at%Zr. Furthermore, the increased cooling rate led to the crystallization of the fine Mg
3Zn
3Y
2 and Mg
12ZnY phases forming a net-like structure. Consequently, this alloy exhibited a tensile strength of 223 MPa at 523 K, which is about twice that of A4032-T6 alloy and excellent fatigue property at high temperature of 473 K. Additionally, high elongation of 10.5% at room temperature was achieved even in the as-cast condition. This alloy also had thermal properties equivalent or higher than those of commercial magnesium alloys.
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