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ection II: Phase Diagram Evaluations

Al-Fe-Nb (Aluminum-Iron-Niobium) V. Raghavan

The early review of this system by [1987Rag] presented an isothermal section at 800 °C from the studies of [1966Ram] and [1970Bur]. The fcc-bcc equilibrium in Fe-rich alloys of this system was calculated by [1989Har] and isothermal sections were presented at 1250, 1150, 1050 and 950 °C. An unpublished study of this system by [1993Bej] (quoted by [2009Pal]) presented a partial liquidus projection. Recently, [2009Pal] determined three isothermal sections at 1150, 1000 and 800 °C for Fe-rich alloys, to clarify the bcc-Fe2Nb and B2-Fe2Nb equilibria.

Binary Systems In the Fe-Al phase diagram [1993Kat], the solid solution c based on face-centered cubic (fcc) Fe is restricted by a loop. The solid solution based on the body-centered cubic (bcc) Fe (a) exists in both the disordered (A2) and ordered (B2 and D03) forms. Apart from the high temperature phase e (D82, Cu5Zn8-type cubic), there are three intermediate phases in the system with restricted ranges of homogeneity: FeAl2 (triclinic), Fe2Al5 (orthorhombic) and FeAl3 or Fe4Al13 (monoclinic). The Al-Nb phase diagram [Massalski2] depicts the following intermediate phases: Nb3Al (A15, Cr3Si-type cubic), Nb2Al (D8b , rCrFe-type tetragonal) and NbAl3 (D022 , Al3Ti-type tetragonal). The Fe-Nb phase diagram [2000Tof] has the following intermediate phases: Fe2Nb (C14, MgZn2-type hexagonal) and Fe7Nb6 (D85, Fe7W6-type rhombohedral).

Ternary Isothermal Sections

alloys containing up to 10 at.% Nb and 40 at.% Al. The alloys were given a final anneal at 1150, 1000 and 800 °C for 100, 200 and 1000 h respectively and quenched in iced brine. The phase equilibria were studied with scanning electron microscopy, x-ray powder diffraction and electron probe microanalysis. The isothermal sections for Fe-rich alloys constructed by [2009Pal] at 1150, 1000 and 800 °C are shown in Fig. 1-3. At all the three temperatures, the C14 Laves phase (Fe2Nb ) is in equilibrium with bcc (or B2) phase. The solubility of Nb in bcc increases with increasing Al and even more so in B2. At 1150 °C (Fig. 1), a three-phase equilibrium was found, but the nature of the third phase is not known. [2009Pal] ruled out of the possibility of its being a Heusler-type phase. Lattice parameter measurements for the above phases were also reported by [2009Pal].

Fig. 2 Al-Fe-Nb partial isothermal section at 1000 °C for Fe-rich alloys [2009Pal]

With starting metals of 99.9999% Al, 99.9% Fe and 99.99% Nb, [2009Pal] levitation/induction melted five

Fig. 1 Al-Fe-Nb partial isothermal section at 1150 °C for Fe-rich alloys [2009Pal]

166

Fig. 3 Al-Fe-Nb partial isothermal section at 800 °C for Fe-rich alloys [2009Pal]

Journal of Phase Equilibria and Diffusion Vol. 31 No. 2 2010

Phase Diagram Evaluations: Section II References 1966Ram: A. Raman, X-ray Investigations of Some T-T5-Al Systems, Z. Metallkd., 1966, 57(7), p 535-540, in German 1970Bur: V.V. Burnashova, V.R. Ryabov, and V.Ya. Markiv, Investigation of the Nb-Fe-Al and Nb-Co-Al Systems, Do