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Fe-Gd-Nb (Iron-Gadolinium-Niobium) V. Raghavan

An isothermal section for this system was determined by [1995Zhu] at 500 ⬚C. This section depicts one ternary compound GdFe11Nb.

Binary Compounds An updated version of the Fe-Gd phase diagram is given by [1998Zha]. There are four line compounds in this system: Fe17Gd2, Fe23Gd6, Fe3Gd, and Fe2Gd. [1993Bej] reinvestigated the Fe-Nb phase diagram. The homogeneity ranges of the compounds of this system, Fe2Nb and Fe7Nb6, used in this review are taken from their work. There are no intermediate phases in the Gd-Nb system and the mutual solubility between Gd and Nb is negligible.

Ternary Isothermal Section With starting metals of purity 99.95% Fe, 99.9% Gd, and 99.9% Nb, [1995Zhu] melted 133 alloy samples in an induction furnace under Ar atm. The alloys were homogenized at 800 ⬚C for 40 days and annealed further at 500 ⬚C for 10 days; they were then quenched in an ice-water mixture. The phase equilibria were studied mainly by x-ray powder diffraction. Their isothermal section at 500 ⬚C is redrawn in Fig. 1 to agree with the accepted binary data. One ternary compound GdFe11Nb (denoted herein 1:12) is present at this temperature. This compound has the ThMn12-type tetragonal structure. The maximum solubility of Gd in Fe2Nb and

Fe7Nb6 and that of Nb in Fe17Gd2 are 1.8, 1.2, and 2.1 at.%, respectively [1995Zhu]. It is interesting to compare Fig. 1 with the isothermal section at ⬃25 ⬚C determined by [1998Hua] (reviewed by [2000Rag]). At ⬃25 ⬚C, two ternary compounds, GdFe11.4Nb0.6 (1:12) and Gd3(Fe,Nb)29, (3:29) are present. Also, in samples annealed at 700 ⬚C, [1998Hua] found both compounds. On the other hand, the isothermal section at 500 ⬚C (Fig. 1) depicts only the 1:12 phase. [1998Hua] noted that the 3:29 compound in other similar systems such as FeNd-Ti is stable only at elevated temperatures in contrast to its stability at room temperature found by them in the FeGd-Nb system. Clearly, the results of [1995Zhu] and [1998Zha] are contradictory and further experimental evidence is required to conclude whether the 3:29 phase is stable at low temperatures. References 1993Bej: J.M.Z. Bejarano, S. Gama, C.A. Ribeiro, and G. Effenberg: Z. Metallkd., 1993, vol. 84 (3), pp. 160-64. 1995Zhu: Y. Zhuang, J. Huang, and H. Zhou: Z. Metallkd., 1995, vol. 86 (4), pp. 253-55. 1998Hua: F. Huang, J.K. Liang, Q.L. Liu, X.L. Chen, and G.Y. Huo: J. Phys.: Condens. Matter, 1998, vol. 10, pp. 9183-88. 1998Zha: W. Zhang, C. Li, X. Su, and K. Han: J. Phase Equilibria, 1998, vol. 19 (1), pp. 56-63. 2000Rag: V. Raghavan: J. Phase Equilibria, 2000, vol. 21 (5), pp. 454-55.

Fig. 1 Fe-Gd-Nb isothermal section at 500 ⬚C [1995Zhu]. The thin two-phase fields around tie-triangles are omitted

Journal of Phase Equilibria Vol. 22 No. 5 2001

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