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hase Diagram Evaluations: Section II

Fe-Ni-Ta (Iron-Nickel-Tantalum) V. Raghavan

The review of [1990Gup] presented a tentative, partial isothermal section of this ternary system (presumably at 1340  C) in the Ta-lean region. The subsequent studies of [1991Usk] presented an isothermal section at 1000  C. Recently, [2010Tan] determined an isothermal section at 1100  C covering the entire composition range.

Binary Systems The Fe-Ni phase diagram is characterized by a very narrow solidification range with a peritectic reaction at 1514  C, that yields (cFe). A continuous fcc solid solution c between cFe and Ni is stable over a wide range of temperature. At 517  C, an ordered phase FeNi3 (L12 ; AuCu3 type cubic) forms congruently from c. The Fe-Ta system [Massalski2] has two intermediate phases: Fe2 Ta (C14; MgZn2 -type hexagonal) and FeTa (D85 ; Fe7 W6 -type rhombohedral; denoted l). The Ni-Ta system [Massalski2] has five intermediate phases: Ni8 Ta (Ni8 Nb-type tetragonal),

Fig. 1

Ni3 Ta (Pt3 Ta-type monoclinic), Ni2 Ta (C11b ; MoSi2 -type tetragonal), NiTa (D85 ; Fe7 W6 -type rhombohedral), and NiTa2 (C16; CuAl2 -type tetragonal).

Ternary Isothermal Section With starting metals of 99.9% Fe, 99.9% Ni and 99.99% Ta, [2010Tan] arc-melted under Ar atm six Fe-Ni binary alloys and ten ternary alloys with Ta content ranging from 11 to 65 at.%. Fe-Ni/Ta diffusion couples were annealed at 1100  C for 600 h and quenched in water. The ternary alloys were annealed at 1100  C for 300 h and quenched in water. The phase equilibria were studied with optical and scanning electron microscopy and x-ray powder diffraction. The compositions of the coexisting phases were determined with energy dispersive x-ray spectroscopy and electron probe microanalysis and listed. The isothermal section at 1100  C constructed by [2010Tan] is shown in Fig. 1. The isostructural compounds FeTa an NiTa form a continuous solid solution denoted l.

Fe-Ni-Ta isothermal section at 1100  C [2010Tan]

Journal of Phase Equilibria and Diffusion Vol. 32 No. 2 2011

153

Section II: Phase Diagram Evaluations The Laves phase Fe2 Ta dissolves up to 40 at.% Ni [2010Tan]. This substitution occurs mainly by an exchange between Fe and Ni atoms, in contrast to the results of [1991Usk], who indicated a decreasing Ta content with increasing substitution in Fe2 Ta. Ni2 Ta and Ni3 Ta dissolve up to 18 and 7.8 at.% Fe respectively. The solubility of Ta in c increases with increasing Ni content, which is in agreement with the results of [1990Gup] and [1991Usk]. No ternary compounds were found [1991Usk, 2010Tan].

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References 1990Gup: K.P. Gupta, The Fe-Ni-Ta (Iron-Nickel-Tantalum) System, Phase Diagrams of Ternary Nickel Alloys, Part 1, Ind. Inst. Metals, Calcutta, 1990, p 316-320 1991Usk: E.N. Uskova and L.L. Meshkov, Phase Diagram of Fe-Ni-Ta System at 1000  C, Moscow Univ. Chem. Bull., 1991, 46(4), p 81-84 2010Tan: C. Tang, M. Tong, H. Xu, Y. Du, J. Lee, Q. Yao, Y. He, and H. Zhou, Isothermal Section at 1100  C of the Fe-Ni-Ta System, J. Alloys Compd., 2010,