Abstract
High temperature creep strength at 650 K and microstructures of Mg-Y-Zn-based alloys with fourth elements (nickel and calcium) have been investigated. The microalloying of 0.5 mass% calcium markedly increases the stacking fault density in the Mg-3 mass%Y-0.5 mass%Zn alloy. Furthermore, the addition of calcium can decrease the size of stacking faults. On the other hand, in the nickel-added alloy, many precipitates were formed and the estimated stacking faults density decreased significantly. The creep strength of the Mg-Y-Zn-Ca alloy is higher than that of the base alloy (Mg-3 mass%Y-0.5 mass%Zn, WZ305). Transmission electron microscopic (TEM) observations revealed that many a-dislocations on the basal planes in the magnesium matrix are extended during creep deformation in Mg-Y-Zn-based alloys. There was significant consumption of solute atoms in the matrix and deterioration of creep strength in the Mg-Y-Zn-Ni alloy, because the formation of a large number of precipitates began before creep. On the other hand, both the low mobile dislocation density and the large separation width of extended a-dislocations were observed in the Mg-Y-Zn-Ca alloy, which exhibits excellent creep strength at 650 K under 20 MPa.
