Fe-Gd-Mo (Iron-Gadolinium-Molybdenum)

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Section II: Phase Diagram Evaluations

Fe-Gd-Mo (Iron-Gadolinium-Molybdenum) V. Raghavan

This ternary system was reviewed recently by [2001Rag]. Based mainly on the results of Zinkevich et al. [1999Zin1, 1999Zin2, 1999Zin3, 2000Zin1], the review presented a liquidus projection, three isothermal sections at 1200, 800, and 600 °C, and a reaction scheme from the solidification range to room temperature. Recently, [2002Zin] carried out a thermodynamic assessment of the system.

Binary Systems In the Fe-Gd system, the four line compounds, Fe17Gd2, Fe23Gd6, Fe3Gd, and Fe2Gd, form through successive peritectic reactions. The Fe-Mo system shows a closed ␥ loop and a significant solubility of Mo in body-centered-cubic (bcc) Fe and of Fe in Mo. The intermediate phases R and ␴ form through peritectic reactions and decompose at or above 1200 °C. Two other intermediate phases ␮ and ␭ form through peritectoid reactions and are stable at lower temperatures. In the Gd-Mo system, there are no intermediate phases. A liquid miscibility gap is present with a monotectic reaction at 2472 °C. The final solidification is through a eutectic reaction at 1299 °C at the Gd end. In their ternary assessment, [2002Zin] used the binary descriptions of [2000Zin2] for Fe-Gd, [1988And] for Fe-Mo, and [2001Zin] for Gd-Mo.

Thermodynamic Assessment of the Ternary System [2002Zin] modeled the Gibbs energy of the individual phases on the basis of a generalized sublattice formalism. The liquid phase was described using a regular solution model. It was found necessary to introduce a ternary interaction parameter to account for the presence of the liquid phase in the ternary region down to 650 °C. The terminal solid solutions face-centered cubic (fcc), bcc, and close packed hexagonal (cph) were modeled on the basis of a two-sublattice model. The excess free energy due to magnetic ordering was taken into account. The ␴, R, and ␮ phases were treated by a three-sublattice model. The first and second sublattices are filled with Fe and Mo atoms, respectively, the third one being a solution sublattice con-

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taining both Fe and Mo atoms. The ␭ phase was treated as a stoichiometric phase. The ternary compound Gd(Fe,Mo)12 (ThMn12-type, tetragonal, denoted 1:12 by [2001Rag], and ␶ by [2002Zin]) was described by a four-sublattice model. This allowed modeling the homogeneity range of the compound represented by Gd1+nFe12−x−yMox䊐y, where 䊐 is a vacancy, 0 < n < 0.2, 1.3 < x < 3.7, and 0.6 < y < 0.9. The other ternary compound Gd3(Fe1−xMox)29 {Nd3(Fe,Ti)29type monoclinic, denoted 3:29 by [2001Rag] and ␨ by [2002Zin]} was not modeled. It is stable only over a narrow temperature range of ∼1200-1000 °C. [2002Zin] computed a liquidus projection and three isothermal sections at 1200, 800, and 600 °C. These are in satisfactory agreement with the experimental diagrams reviewed in [2001Rag]. In addition, [2002Zin] computed three vertical sections (not shown here) at 8 and 9 at.% Gd and at 20 at.% Mo. There are no experimental results to compare with these computed sections

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