Neutralization of Impurity Elements of Cu and Ni in Mg-Zn Alloy by Dissolution into MgZn2 Phase

Cu and Ni impurities in Mg alloys are deleterious contaminants that reduce the corrosion resistance of the alloy. Mg 2 Cu and Mg 2 Ni precipitates can cause significant anodic dissolution of the Mg matrix, owing to their potential difference. Suppression of these phases can prevent the deterioration of corrosion resistance. The neutralization of these impurities through the formation of MgZn 2 intermetallic phases has been studied, because the atomic radii of Cu and Ni are similar to that of Zn. As a result, the MgZn 2 phase may precipitate during the rapid cooling that occurs during the solidification of the Mg-6 mass% Zn alloy, and introduce substitutional impurity atoms in the crystal lattice. Mg (Zn, Cu, Ni) 2 phase can be formed instead of Mg 2 Cu and Mg 2 Ni, in the presence of both of Zn and these impurities. In this study, the solubility of Cu and Ni into MgZn 2 phase when they contain simultaneously was investigated by preparing the intermetallic compounds and the crystal structure evaluation. The lattice parameter of the MgZn 2 phase containing both Cu and Ni impurity decreased to about 5.14 Å with increasing impurity concentration. Cu and Ni impurities are concentrated in the MgZn 2 phase while maintaining the crystal structure, when the impurity concentrations are within the composition range of Cu + 2Ni ≦ 20 at.%. When the concentrations of Cu and Ni exceeded this range, the crystal structure of the MgZn 2 phase transformed from hexagonal (hP12) to cubic (cF24).