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Phase Diagram Evaluations: Section II

Fe-Si-U (Iron-Silicon-Uranium) V. Raghavan

The previous review of this system by [1992Rag] presented an isothermal section at 795
C near the U-corner. The update by [2003Rag] summarized the results of [1994Gon] on the ternary solid solutions that lie on the line of constant U content of 7.7 at.%. Recently, [2008Ber] determined an isothermal section at 900
C for the entire composition range, which depicts eleven ternary phases.

Table 1

Binary Systems The Fe-Si phase diagram [Massalski2] shows that the fcc solid solution based on Fe is restricted by a c loop. The intermediate phases are: a2 (B2, CsCl-type cubic), a1 (D03, BiF3-type cubic), Fe2Si (stable between 1212 and 1040
C; hexagonal), Fe5Si3 (D88, Mn5Si3-type hexagonal), FeSi

Fe-Si-U crystal structure and lattice parameter data [2008Ber]

Phase U2FeSi3 (s1 or A)

UFe2Si2 (s2 or B)

U3Fe2Si7 (s3 or C)

U2Fe3Si (s4 or D)

UFe12–xSix (x = 1-3) (s5 or E)

U2Fe17–xSix (x = 3.2-4) (s6 or F)

UFeSi (s7 or G)

U1.2Fe4Si9.7 (s8 or H)

U2Fe3Si5 (s9 or I)

UFe5Si3 (s10 or J)

U6Fe16Si7 (s11 or K)

Composition, at.% 16.7 Fe 50.0 Si 33.3 U 40 Fe 40 Si 20 U 16.7 Fe 58.3 Si 25 U 50 Fe 16.7 Si 33.3 U 84.6-69.2 Fe 7.7-23.1 Si 7.7 U 72.6-68.4 Fe 16.8-21.1 Si 10.5 U 33.3 Fe 33.3 Si 33.3 U 26.8 Fe 65.1 Si 8.1 U 30 Fe 50 Si 20 U 55.5 Fe 33.3 Si 11.1 U 55.2 Fe 24.1 Si 20.7 U

Pearson symbol

Space group

Prototype

Lattice parameter, nm

hP3

P6/mmm

AlB2

a = 0.4011 c = 0.3864

tI10

I4/mmm

ThCr2Si2

a = 0.3946 c = 0.9540

oC24

Cmmm

La3Co2Sn7

hP12

P63/mmc

MgZn2

a = 0.4013 b = 2.4324 c = 0.4023 a = 0.5154 c = 0.7686

tI26

I4/mmm

ThMn12

a = 0.8350 (a) c = 0.4705

hP38

P63/mmc

Th2Ni17

a = 0.8330 (b) c = 0.8201

oP12

Pnma

TiNiSi

hP16

P63/mmc

Er1.2Fe4Si9.7

a = 0.7001 b = 0.4065 c = 0.6857 a = 0.3960 c = 1.5075

…

C2/c

Lu2Co3Si5

…

P4/mmm

UFe5Si3

Fm3m

Mg6Cu16Si7

cF116

a = 1.0843 b = 1.1482 c = 0.5163 b = 119.38
a = 0.39296 c = 0.77235 a = 1.17817

(a) at x = 2; (b) at x = 3.2

Journal of Phase Equilibria and Diffusion Vol. 30 No. 1 2009

107

Section II: Phase Diagram Evaluations

Fig. 1 Fe-Si-U isothermal section at 900
C [2008Ber]. Narrow two-phase regions are omitted

(B20-type cubic), bFeSi2 (tetragonal), and aFeSi2 (orthorhombic). The Fe-U phase diagram [Massalski2] depicts two intermediate phases: Fe2U (C15, MgCu2-type cubic) and FeU6 (MnU6-type tetragonal). The Si-U phase diagram [Massalski2, 2008Ber] has a number of compounds: Si3U (L12, AuCu3-type cubic), Si2U (ThSi2-type tetragonal), Si2–xU (GdSi2-type orthorhombic at the Si-poor boundary and ThSi2-type tetragonal at the Si-rich boundary), Si5U3 (AlB2-type hexagonal and two orthorhombic distortions of AlB2-type), SiU (SiU-type tetragonal), Si4U5 (hexagonal), Si2U3 (D5a-type tetragonal), and SiU3 (AuCu3-type cubic at 900
C and other forms at lower temperatures).

15 days. The phase equilibria were studied with a scanning electron microscope equipped with an energy dispersive x-ray analyzer and with x-ray powder diffraction. The isothermal section constru