Mg–CaおよびMg–Ca–Zr合金の組織と機械的性質

Abstract

Cast Mg–Ca and Mg–Ca–Zr alloys were prepared from elemental metals, Mg–5Ca, and Mg–33Zr master alloys. The microstructure was examined and tensile tests performed. The microstructures of the cast Mg–Ca alloys consist of primary α-Mg dendrites and a degenerated lamellar eutectic. An addition of zirconium transformed coarse primary α-Mg dendrites to fine globular grains of approximately 20∼35 μm in diameter. Hardness increased for both Mg–Ca and Mg–Ca–Zr alloys with calcium contents, and there was a slope change in hardness increase at about 0.3%Ca, at which eutectic containing Mg2Ca started to appear at grain boundaries. The value of 0.2% proof stress also increased for both alloys, showing higher values due to a grain refining effect. The values of tensile strength for Mg–Ca–Zr alloys were also higher than those of the Mg–Ca alloys, while the strength did not improve beyond 0.3%Ca. Fracture occurred owing to the decohesion within eutectic regions and the cracks propagated though the regions. Thus, an increase in the eutectic regions leads to a reduction in ductility. In particular, the Mg–Ca–Zr alloys, with calcium content as little as 0.3%, showed a significant reduction in ductility, negating the effect of grain refinement.