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Phase Diagram Evaluations: Section II

Fe-Ga-Tb (Iron-Gallium-Terbium) V. Raghavan

Recently, [2009Li] determined an isothermal section at 500
C for this ternary system, which depicts six ternary compounds.

Binary Systems The Fe-Ga phase diagram [2004Oka, Massalski2] has the following intermediate phases: a¢ (B2, CsCl-type cubic), a¢¢ (D03, BiF3-type cubic), aFe3Ga (L12, AuCu3-type cubic), bFe3Ga (D019, Ni3Sn-type hexagonal), aFe6Ga5 (Fe6Ge5-type monoclinic), bFe6Ga5 (D810, Al8Cr5-type rhombohedral), Fe3Ga4 (monoclinic) and FeGa3 (CoGa3type tetragonal). The Fe-Tb phase diagram [Massalski2] depicts the following compounds: aFe17Tb2 (Th2Zn17-type rhombohedral), bFe17Tb2 (Th2Ni17-type hexagonal), Fe23Tb6 (D8a, Mn23Th6-type cubic), Fe3Tb (Be3Nb-type rhombohedral) and Fe2Tb (C15, MgCu2-type cubic). The Ga-Tb phase diagram [Massalski2] depicts the following compounds: Ga6Tb (tetragonal), aGa3Tb (D019, Ni3Sn-type hexagonal), bGa3Tb (L12, AuCu3-type cubic), Ga2Tb (C32, AlB2-type hexagonal), GaTb (Bf, CrB-type orthorhombic) and Ga3Tb5 (D8l, Cr5B3-type tetragonal).

Table 1

Ternary Compounds Six ternary compounds are known in this system: TbFe0.32Ga1.68 (labeled s1 here and as a by [2009Li]), TbFe2.4Ga0.6 (s2 or b), Tb4FeGa12 (s3 or c), Tb2FeGa8 (s4 or g), TbFe6Ga6 (s5 or k), and TbFe5Ga7 (s6 or h). The structural characteristics of these compounds are listed in Table 1. The compounds Tb2FeGa8 (s4 or g) and TbFe6Ga6 (s5 or k) were known earlier and confirmed by [2009Li]. The other compounds were reported for the first time by [2009Li] in alloys annealed at 500
C.

Isothermal Section With starting metals of 99.9+% purity, [2009Li] arcmelted about 220 samples under Ar atm. The alloys were given a final anneal at 500
C for 3 days and quenched in liquid nitrogen. The phase equilibria were studied by x-ray powder diffraction. The isothermal section constructed by [2009Li] at 500
C is shown in Fig. 1. All six ternary compounds are present. A number of them show a small homogeneity range around the nominal composition listed

Fe-Ga-Tb crystal structure and lattice parameter data [2009Li]

Phase TbFe0.32Ga1.68 (s1or a)

TbFe2.4Ga0.6 (s2or b)

Tb4FeGa12 (s3or c)

Tb2FeGa8 (s4or g)

TbFe6Ga6 (s5or k)

TbFe5Ga7 (s6or h)

Composition, at.%

Pearson symbol

Space group

Prototype

10.7 Fe 56 Ga 33.3 Tb 60 Fe 15 Ga 25 Tb 5.9 Fe 70.6 Ga 23.5 Tb 9.1 Fe 72.7 Ga 18.2 Tb 46.15 Fe 46.15 Ga 7.7 Tb 38.5 Fe 53.8 Ga 7.7 Tb

hP6

P63/mmc

Ni2In

a = 0.43727 c = 0.72930

hP24

P63/mmc

CeNi3

a = 0.51877 c = 1.6621

cI34

Im3m

U4Re7Si6

a = 0.85576

tP11

P4/mmm

Ho2CoGa8

a = 0.42678 c = 1.1131

oI26

Immm

ScFe6Ga6

tI26

I4/mmm

ThMn12

a = 0.85551 b = 0.86826 c = 0.50717 a = 0.86779 c = 0.50923

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

Lattice parameter, nm

175

Section II: Phase Diagram Evaluations

Fig. 1 Fe-Ga-Tb isothermal section at 500
C [2009Li]. Narrow two-phase regions are omitted

in Table 1. The Fe-Tb binary compounds Fe2Tb, Fe3Tb, and Fe17Tb2 dissolve up to 19.6, 8, and 37.5 at.% Ga, respectively. The third