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Phase Diagram Evaluations: Section II

Al-Mg-Zn (Aluminum-Magnesium-Zinc) V. Raghavan

A review of the experimental and computed data on this ternary system was reported recently by [2007Rag] and [2006Pet]. The (Mg)-solvus in Mg-rich alloys was recently determined by [2009Ren] at 320 C.

Binary Systems The Al-Mg phase diagram has the following intermediate 3m), R or phases: Mg2Al3 (denoted b; cubic, space group Fd  e (rhombohedral) and Mg17Al12 (denoted c; a-Mn type cubic). The Al-Zn phase diagram [Massalski2] contains no intermediate phases. A miscibility gap occurs in the Al-based face centered cubic (fcc) solid solution below 351 C, where the fcc phase splits into (Al)¢ and (Al)¢¢. The monotectoid reaction (Al)¢¢ M (Al)¢ + (Zn) follows at 277 C. The Mg-Zn phase diagram [Massalski2] depicts the following intermediate phases: Mg7Zn3 (high-temperature phase), MgZn, Mg2Zn3 (monoclinic), MgZn2 (C14-type hexagonal) and Mg2Zn11 (D8c-type cubic).

Fig. 1 Al-Mg-Zn isothermal section at 320 C [2009Ren]

Ternary Phases Two ternary phases are known in this system. (Al,Zn)49Mg32 (denoted s) is cubic, space group Im3. Al2Mg5Zn2 (denoted /) is orthorhombic and has a range of homogeneity.

Isothermal Section at 320 C With starting metals of 99.99% Al, 99.99% Mg and 99.999% Zn, [2009Ren] induction-melted under Ar atm 14 Mg-rich ternary alloys containing up to 33 at.% Al and 40 at.% Zn. The alloys were annealed at 320 C for 720 h and quenched in water. The phase equilibria were studied with x-ray powder diffraction and scanning electron microscope equipped with energy dispersive x-ray analyzer. The identified phases and their compositions were listed. The partial isothermal section constructed by [2009Ren] at 320 C is shown in Fig. 1. Figure 2 shows the details at the Mg corner. (Mg) is in equilibrium with both s and / phases. The homogeneity range of / was found to be 14.7-27 at.% Al and 30.5-17.7 at.% Zn, agreeing with the result of [1997Don] at 335 C. Aluminum preferentially substitutes for zinc, with the variation in Mg content of / being small (53.7-57 at.%). The maximum solubility of Zn in c is about 8.3 at.% and that of Al in MgZn is also about 8.3 at.%.

Fig. 2 Al-Mg-Zn isothermal section at 320 C near the Mg corner [2009Ren]

References 1997Don: P. Donnadieu, A. Quivy, T. Tarfa, P. Ochin, A. Dezellus, M.G. Harmelin, P. Liang, H.L. Lukas, H.J. Seifert, F. Aldinger, and G. Effenberg, On the Crystal Structure and

Journal of Phase Equilibria and Diffusion Vol. 31 No. 3 2010

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Section II: Phase Diagram Evaluations Solubility Range of the Ternary / Phase in the Mg-Al-Zn System, Z. Metallkd., 1997, 88(12), p 911-916 2006Pet: D. Petrov, A. Watson, J. Grobner, P. Rogl, J.C. Tedenac, M. Bulanova, and V. Turkevich, Aluminum-Magnesium-Zinc, Ternary Alloy Systems, Vol 11A3, G. Effenberg and S. Ilyenko, Ed., Springer, Germany, 2006, p 191-209

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2007Rag: V. Raghavan, Al-Mg-Zn (Aluminum-MagnesiumZinc), J. Phase Equilib. Diffus., 2007, 28(2), p 203-208 2009Ren: Y.P. Ren, G.W. Qin, W.L. Pei, Y. Guo, H.D. Zhao, H.X. Li, M