We investigated the effect of the cooling rate and molten metal temperature on the microstructure and mechanical properties of a subrapidly solidified magnesium alloy, Mg10Al0.2Mn1Ca, prepared by antigravity suction casting using a water-cooled steel mold. The microstructure of the antigravity-suction-cast material without water cooling consisted of coarse grains (grain size: 780 µm), with networks of an AlCa compound at the grain boundaries. The higher cooling rate of the water-cooled steel mold promoted the formation of the AlCa compound and voids in accordance with increases in the internal and external temperature gap and differences in the solidification rate of the mold. The formation of voids and the shrinkage were suppressed, however, by adjusting the cooling rate and decreasing the molten metal temperature. The particle size of the Mg phase was refined to 135 µm and the grain-boundary compounds were finely dispersed in the Mg phase. The as-cast alloy showed an ultimate tensile strength of 166 MPa and an elongation of 8%. The microstructure and mechanical properties of the as-cast alloy were dependent on the cooling rate and molten metal temperature, but they were not dependent on the casting speed.

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The effects of Al concentration and Zn addition on the corrosion resistance of MgAl(Zn)Ca-based magnesium alloy rolled sheets were investigated. AXM (MgAlCa) alloys containing 1 mass% Ca and AZX (MgAlZnCa) alloys containing 1 mass% Ca plus 1 mass% Zn with the Al concentration adjusted to 6, 7, 8, 9, and 11 mass% were prepared. The corrosion behaviors of the alloys in a 5 mass% NaCl solution were evaluated by weight loss and the penetration depth. Electrochemical methods based on corrosion potential measurements and impedance spectroscopy was applied to monitor the corrosion behavior. The hydroxide film formed on the alloy surface was characterized by GDOES and SEM/EDS. The AXM alloy with an Al content of approximately 8 mass% exhibited minimum corrosion rate, whereas the AZX alloy showed the opposite result in the same concentration range of Al. This phenomenon was analyzed in terms of the Al concentration in the matrix (¡-phase), which was reduced by the precipitation of Mg 17 Al 12 (¢-phase) and the presence of Zn. The tendency of localized corrosion in high Al-containing AZX alloys was pointed out.

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