Al-Ho-Mg (Aluminum-Holmium-Magnesium)
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hase Diagram Evaluations: Section II
Al-Ho-Mg (Aluminum-Holmium-Magnesium) V. Raghavan
Recently, this ternary system was investigated experimentally by [2003Den] and assessed thermodynamically by [2003Cac]. To minimize the number of experiments required, an iterative procedure was adopted, where additional critical experiments were designed on the basis of the preliminary calculations. An Al-rich ternary compound s with the MgNi2type hexagonal structure was found at 400 °C.
Binary Systems The Al-Ho phase diagram [2003Den, 2003Cac] shows the following intermediate phases: HoAl3 (HoAl3-type (hR20) rhombohedral), HoAl2 (C15, MgCu2-type cubic), HoAl (ErAl-type orthorhombic), Ho3Al2 (Zr3Al2-type tetragonal) and Ho2Al (C23, Co2Si-type orthorhombic). The Al-Mg phase diagram [1998Lia] has the following intermediate phases: Mg2Al3 (cubic, labeled b), R or e (rhombohedral) and Mg17Al12 (A12, aMn-type cubic, denoted c). The Ho-Mg phase diagram [Massalski2, 2003Cac] has the following intermediate phases: Ho5Mg24 (A12, aMn-type cubic), HoMg2 (C14, MgZn2-type hexagonal), and HoMg (B2,
CsCl-type cubic). A body centered cubic (bcc) phase is stable above 680 °C with a homogeneity range extending from 18 to 42 at.% Mg.
Ternary Isothermal Section With starting metals of 99.999% Al, 99.9% Ho, and 99.99% Mg, [2003Den] induction-melted seven ternary alloys. Differential thermal analysis (DTA) was carried out at a heating/cooling rate of 10 °C/min. The samples were annealed at 400 °C for 850 h and quenched in water. The phase equilibria were studied with optical and electron microscopy, x-ray powder diffraction and electron probe microanalysis. These experimental results were used in the thermodynamic optimization by [2003Cac]. The computed isothermal section at 400 °C shown in Fig. 1 agrees well with the experimental results. An Al-rich ternary phase s is present at 400 °C. It has a composition near Al2Ho0.39Mg0.61 and the C36, MgNi2-type hexagonal structure, with a = 0.5471 nm and c = 1.7671 nm [2003Den]. The DTA data show that s forms peritectically at about 450 °C.
Fig. 1 Al-Ho-Mg computed isothermal section at 400 °C [2003Den, 2003Cac]. Narrow two-phase regions are omitted
Journal of Phase Equilibria and Diffusion Vol. 28 No. 5 2007
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Section II: Phase Diagram Evaluations
Fig. 2 Al-Ho-Mg computed isothermal section at 800 °C [2003Cac]
At 400 °C, the binary phases HoMg (denoted B2) and HoAl2 (denoted C15) dissolve up to 35 at.% Al and 10 at.% Mg respectively. An isothermal section computed at 800 °C by [2003Cac] is shown in Fig. 2. The s phase is absent at this temperature. The solubility of Mg in HoAl2 is larger here, as compared to that in Fig. 1. A liquidus projection was also computed by [2003Cac]. The extent of the primary solidification ranges of the phases in the computed projection agrees well with the metallographic observations of [2003Den]. The C15 phase shows a large field of primary crystallization. However, the agreement of the computed surface with DTA data was found to be not satisfactory.
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References 1
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