Effect of Ca and Y on the Oxidation of Mg Alloys

The AZ91D magnesium alloys containing 1% Ca and 1% Y were cast and oxidized between 350 and 500^oC in atmospheric air to study the effect of Ca and Y on the oxidation of Mg alloys. The Mg+1% Ca alloy consisted of α-Mg, Al₁₂Mg₁₇, and Al₂Ca. The AZ91D+1% Y alloy consisted of α-Mg, Al₁₂Mg₁₇, and Al₂Y. Al₂Ca and Al₂Y were present in the form of platelets or blocky particles along the boundaries of primary α–Mg grains.

The oxidation rates observed for the three alloys in the study decreased in the following order: AZ91D, AZ91D+1% Y, and AZ91D+1% Ca. As the oxidation progressed, the compact MgO oxide scale became porous, non-adherent, thick, and fragile. Ca, in the AZ91D+1% Ca alloy, was the most active element, and it appeared to diffuse outward toward the oxide layer rather easily. The presence of CaO in the oxide layer retarded the oxidation of the Mg alloy. Both the oxide scale and the matrix of the AZ91D+1% Y alloy contained oxygen. The dissolution of oxygen beyond the oxide scale indicates that Y could not effectively deter the ingress of oxygen, although the 1% Y content did increase the oxidation resistance of the alloy.

Acknowledgement. This work is the outcome of "Environment-friendly and Energy-efficient Manufacturing Technology of Eco-Mg" program (No. 10035292-2010-01) funded by the Korea government Ministry of Trade, Industry and Energy.