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 2 Ca 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 MgCa-Zr alloys, with calcium content as little as 0.3%, showed a significant reduction in ductility, negating the effect of grain refinement.

In a previous investigation concerning the ductility of Mg-La-Zr alloys, it was revealed that the tensile-tested MgLa-Zr alloys developed ultrafine grains within the deformed areas of the specimens. On the basis of those observations, in the present study, cast Mg-La-1.5Zr alloys with lanthanum content varying between 0 and 5.4 wt.% were hot-rolled and the microstructures examined by optical microscopy, X-ray diffraction, EBSD and KAM analyses to establish the characteristics of the hot-rolled and annealed specimens. It was revealed that the microstructures within the primary and eutectic αMg of hot-rolled Mg-5.4La-1.5Zr materials consisted of fine grains of 3.6 and 0.3 μm in average size, respectively. The grain growth of these fine grains within the primary αMg did not expand into the eutectic regions during annealing, while the fine grains within the eutectic αMg remained as fine as those observed in the hot-rolled materials even after annealing at 400 ºC. This is most likely due to the proximity of a eutectic Mg12La phase that may act as an obstacle to grain growth, suggesting the possibility of developing materials with bimodal microstructures. It was also found that the level of the basal texture in the hot-rolled and annealed materials of Mg-La-1.5Zr alloys was significantly lower than that of Mg-Zr alloy, demonstrating the beneficial effects of lanthanum. Key words:Mg-La-Zr alloys, hot-rolled materials, ultrafine grains, eutectic, bimodal microstructures. 緒 言

In the present study, tensile and creep properties of cast Mg-La-Zr and Mg-Ce-Zr alloys were examined and the microstructures of the alloys were observed by optical microscopy, TEM, and Cs-STEM to evaluate creep and work hardening characteristics. The room temperature tensile properties, plotted with lanthanum or cerium contents, did not reveal significant differences between the alloys except for slightly lower elongation for Mg-Ce-Zr alloys. However, at 150˚C, the values of tensile strength for Mg-Ce-Zr alloys were higher than those for Mg-La-Zr alloys, while the values of elongation for Mg-Ce-Zr alloys became significantly lower than those for Mg-La-Zr alloys. It was also found that Mg-Ce-Zr alloys tested at 150 ˚C exhibited a higher work hardening rate than Mg-La-Zr alloys. In creep curves, Mg-La-Zr alloys exhibited an initial strain after which the direction of curves abruptly changed to almost horizontal during creep testing, whereas Mg-Ce-Zr alloys did not exhibit the initial strain. It was found that Mg-Ce-Zr alloys contained numerous stacking faults on 0001 of the primary α-Mg grains, while Mg-La-Zr alloys did not, and these might have contributed to the differences in creep and work hardening characteristics.