B-Fe-Si (Boron-Iron-Silicon)

PDF / 304,491 Bytes
2 Pages / 593.972 x 792 pts Page_size
56 Downloads / 160 Views

Section II: Phase Diagram Evaluations

B-Fe-Si (Boron-Iron-Silicon) V. Raghavan

The previous review of this system by [1992Rag] presented a liquidus projection for B-lean alloys, a reaction sequence for liquid-solid reactions, an isothermal section for B-lean alloys at 1050 C from [1989Des], and a full isothermal section at 900 C from [1970Cha]. Recently, [2004Tok] obtained new experimental results, optimized the thermodynamic parameters, and computed isothermal sections at 1000 and 900 C. Three vertical sections were also computed at 65 at.% Fe, 10 at.% B, and 10 at.% Si, respectively.

rhombohedral. The Fe-Si phase diagram [Massalski2] depicts a c loop, restricting the Fe-based face-centered cubic (fcc) solid solution to 3.2 at.% Si. The body-centered cubic (bcc) solid solution a exists in the disordered A2 form, as well as in the ordered forms of B2 and D03. The intermediate phases are: Fe2Si (hexagonal), Fe5Si3 (D88, Mn5Si3-type hexagonal), FeSi (B20-type cubic), aFeSi2 (orthorhombic), and bFeSi2 (tetragonal).

Ternary Phase Equilibria Binary Systems The Fe-B phase diagram [1993Lia] depicts two intermediate phases: Fe2B (C16, CuAl2-type tetragonal) and FeB (B27-type orthorhombic). Fe3B is a metastable phase with several crystal modiﬁcations. The B-Si phase diagram [1992Rag] depict three intermediate phases. SiB3 (D1g, B4C-type rhombohedral) is stable below ~1270 C. SiB6 is orthorhombic and SiBn (93.8-98 at.% B) is b B-type

The structural details of the three conﬁrmed ternary compounds in this system, Fe5SiB2 ðs1 Þ; Fe4.7Si2B ðs2 Þ; and Fe3(Si,B) ðs3 Þ were listed by [1992Rag]. It is possible that Fe3(Si,B) is a Si-stabilized phase originating from one of the metastable forms of Fe3B. A fourth compound Fe2Si0.4B0.6 ðs4 Þ reported by [1960Aro] was not conﬁrmed by [1970Cha]. With starting metals of 99% B, 99.998% Fe, and 99.9% Si [2004Tok] arc-melted 13 Fe-rich ternary alloys. Differ-

Fig. 1 B-Fe-Si computed isothermal section at 1000 C [2004Tok]. Narrow two-phase regions are omitted

380

Journal of Phase Equilibria and Diffusion Vol. 28 No. 4 2007

Phase Diagram Evaluations: Section II

Fig. 2 B-Fe-Si computed vertical section at 65 at.% Fe [2004Tok]

ential scanning calorimetry (DSC) was performed at a cooling/heating rate of 5 C per min. Peak DSC temperatures during heating were used to identify the phase changes. In the thermodynamic modeling by [2004Tok], the Gibbs energies of the liquid and fcc phases were described by the regular solution model. The third component solubility in all the binary phases was neglected. The three ternary compounds Fe5SiB2 ðs1 Þ; Fe4.7Si2B ðs2 Þ; and Fe2 Si0:4 B0:6 ðs4 Þ were treated as stoichiometric, neglecting the small homogeneity ranges. The ternary compound Fe3(Si,B) ðs3 Þ was not considered by [2004Tok]. The bcc phase was described by a two-sublattice model, so that the B2 ordering is accounted for. Two isothermal sections at 1000 and 900 C were computed. The computed section at 1000 C is shown in Fig. 1. In place of the three-phase equilibria of ðs1 + FeB + FeS

Data Loading...