Structure and mechanical properties of Mg-Ca and Mg-Ca-Zr alloys

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 Mg 2Ca 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. © 2013 The Japan Institute of Light Metals.