Selective Oxidation and the Third-Element Effect on the Oxidation of Mg-Y Alloys at High Temperatures
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COMMONLY, magnesium alloys are melted under the protection of the fluxes or gases (CO2, SO2, and SF6) to prevent the melts from serious oxidation and even burning.[1–4] However, both methods have inherent disadvantages such as environmental pollution, required complicated equipment, and increased cost. Since the 1950s, much interest has been focused on the investigation and development of ignition-proof magnesium alloys.[5–12] Beryllium and calcium were proved to be effective elements for improving the oxidation resistance of magnesium alloys. Foerster[5] found that beryllium additions of 3 to 8 ppm could greatly improve the oxidation resistance of magnesium. Huddle et al.[13] thought that Ca addition could strengthen the oxidation resistance of magnesium alloys at their melting point temperatures. Fan et al.[14] found that the addition of 0.3 wt pct Ca increases the ignition point of pure magnesium by 120 K. However, until now, ignitionproof magnesium alloys with sufficient beryllium and calcium additions have not been extensively applied in industry due to the poor mechanical properties of the alloys and the toxicity of beryllium. For these reasons, it is necessary to find new ignition-proof methods. Recent research indicates that yttrium can improve not only the oxidation resistance of magnesium alloys but also their high-temperature mechanical properties.[15,16] However, The Y content for forming external oxide film must be J.F. FAN, Associate Professor, and Y.H. WEI and B.S. XU, Professors, are with the College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China. Contact e-mail: xubs.tyut.edu.cn G.C. YANG and Y.H. ZHOU, Professors, are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. Manuscript submitted October 14, 2008. Article published online July 9, 2009 2184—VOLUME 40A, SEPTEMBER 2009
larger than 8 wt pct, and the higher Y content will increase cost and degrade the mechanical properties of magnesium alloys. Moreover, the mechanism of oxidation resistance of magnesium alloys bearing Y has not been studied extensively, and the structure of the oxide film on the molten alloy and its thermodynamic and kinetic feature are far from being understood. Wagner[17] declared that in Cu-Zn-Al alloys, the addition of Zn decreased the critical concentration of Al for the exclusive formation of Al2O3. Pickering[17] found that in Ni-Al alloys, the stable protective Al2O3 film formed only when Al concentration was higher than 18 wt pct. However, when Cr was added to Ni-Al alloys, for example, Ni-20Cr-3Al alloy, the critical Al concentration of forming protective Al2O3 film was decreased from 18 to 3 wt pct. This phenomenon was called the third-element effect. In other words, in an A-B alloy, the addition of the third-element C, whose chemical activity is between that of A and B, can remarkably decrease the critical composition of B for the external formation of B oxide. Afte
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