Development of New High-Strength and Heat-Resistant Mg-Zn-Y-X (X=Zr and Ag) Casting Alloys

In order to develop a high strength and heat-resistant magnesium alloy, we focused on controlling microstructure of Mg96Zn2Y2 (at %) casting alloy by the addition of a 4th element. Initially, we investigated the effects of zirconium addition and cooling rate for grain refinement on microstructure and mechanical properties. Consequently, Mg95.8Zn2Y2Zr0.2 casting alloy contains fine equiaxed grains (approx. 0.01 mm), and it exhibits tensile and fatigue properties equivalent to or higher than those of commercial aluminum alloys at high temperature above 473 K. At 523 K, this alloy exhibited a tensile strength of 223 MPa nearly twice that of A4032-T6 alloy used in typical automotive pistons. The Mg95.8Zn2Y2Zr0.2 casting alloy also reveals sufficient ductility and good castability, characteristics not common in current heat-resistant magnesium alloys. Next, we focused on controlling microstructure of Mg96Zn2Y2 casting alloy by the addition of Ag. Mg96Zn2Y2 cast alloy is composed of alpha-Mg phase, long-period stacking ordered phase and Mg3Zn3Y2 phase; on the other hand, Mg-Zn-Y-Ag cast alloy had 4th phase by an addition of Ag. A substantial increase in yield strength at room temperature, without grain refining, was the result.