The previous review of this ternary system by [1987Rag] presented a liquidus surface for Fe-rich alloys from the studies of [1938Vog] and an isothermal section at 800 °C from [1966Mar]. Recently, [2008Wei] reinvestigated the solidification characteristics over the entire composition range and presented a liquidus projection, a corresponding reaction sequence and a tentative isothermal section at 900 °C. Earlier, [2002Lof] investigated the Si-lean region and presented three partial isothermal sections at 1150, 1000, and 800 °C. This unpublished thesis result was quoted by [2005Ste], who presented the isothermal sections at 1150 and 800 °C. The isothermal section at 800 °C of [2002Lof] was also reproduced by [2004Lof], who studied the mechanical properties of Si-lean alloys.

Binary Systems

The Fe-Si phase diagram [Massalski2] is characterized by a gamma loop enclosing (γFe) (fcc). The bcc α phase is present in the disordered α (A2) form and the ordered α1 (D03, BiF3-type cubic) and α2 (B2, CsCl-type cubic) forms. The intermediate phases are: Fe2Si (hexagonal), Fe5Si3 (D88, Mn5Si3-type hexagonal), FeSi (B20-type cubic), αFeSi2 (orthorhombic), and βFeSi2 (tetragonal). The Fe-Ti phase diagram [1998Dum] depicts the following intermediate phases: Fe2Ti (C14, MgZn2-type hexagonal) and FeTi (B2, CsCl-type cubic). The Si-Ti phase diagram [Massalski2, 2008Wei] depicts the following intermediate phases: Ti3Si (Ti3P-type tetragonal), Ti5Si3 (D88, Mn5Si3-type hexagonal), Ti5Si4(o) (Sm5Ge4-type orthorhombic; low-temperature modification), Ti5Si4(t) (Zr5Si4-type tetragonal; high-temperature modification), TiSi (B27, FeB-type orthorhombic), and TiSi2 (C54-type orthorhombic).

Ternary Phases

[1987Rag] summarized the then-available structural details of the ternary phases of this system: FeSi2Ti (τ1) (MnSi2Ti-type orthorhombic), FeSiTi (τ2) (FeSiTi-type orthorhombic), Fe4Si3Ti (τ3) (Pd40Sn31Y13-type hexagonal), Fe10Si44Ti46 (X′) (unknown structure), and Fe15Si40Ti45 (X′′) (unknown structure). In the ordered phase Fe3Si (α1), Ti dissolves up to x = 0.7 in Fe3-x SiTi x . The L21-type Fe2SiTi is a metastable superstructure based on Fe3Si. [2008Wei] reported a number of additional ternary phases τ4, τ5, τ6, τ7, τ8, and τ9. The compositions of these phases were determined, but the crystal structures were not. The compositions of the ternary phases X′ and X′′ reported by [1966Mar] were found by [2008Wei] to be two-phase mixtures of τ9 + Ti5Si4(o) and τ7 + τ8, respectively. In addition to the above, [2002Lof] reported a ternary phase at the composition Fe7Si2Ti. The composition region within which this phase occurs was not investigated by [2008Wei]. The above phases are listed in Table 1 with updated structural data.

Table 1 Fe-Si-Ti crystal structure and lattice parameter data [2008Wei]
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Liquidus Projection

With starting metals of 99.98% Fe, 99.99% Si, and 99.98% Ti, [2008Wei] arc-melted in Ar atm more than 80 ternary alloys. The alloys were annealed at 1000, 950, and 900 °C for 2 weeks (or more) and quenched in water. Differential thermal analysis was carried out at a heating/cooling rate of 5 °C per min. The arrests were read from the heating curves. X-ray powder diffraction and energy dispersive x-ray analysis on a scanning electron microscope were employed for structure identification and for determination of local composition.

To identify the invariant reactions, [2008Wei] assigned the first strong peak during the heating cycle in thermal analysis with “the formation of liquid phase from three solid phases.” The three co-existing solid phases were identified from the annealing experiments in the solid state. All three solid phases melt to form the liquid in a ternary eutectic reaction. One out of the three solid phases melts during a ternary peritectic reaction. The transition reactions, on the other hand, are characterized by the interchange of the tie-lines with no fresh melting at the invariant temperature. Using the reaction sequence derived from the invariant temperatures, the liquidus projection was constructed by [2008Wei], as shown in Fig. 1. The fields of primary crystallization are marked. For clarity, the U-type transition reactions are not numbered. The ternary eutectic reactions E1, E2, E3, and E4 occur at 1254, 1175, 1151, and 1034 °C, respectively. The ternary phases τ3, τ4, τ5, τ6, τ8, and τ9 form through ternary peritectic reactions P7 (1241 °C), P5 (1414 °C), P6 (1263 °C), P4 (1450 < T < 1480 °C), P1 (1640 °C), and P2 (1597 °C), respectively. The ternary phases τ1 and τ2 form at temperature maxima of C3 (1328 °C) and C1 (>1662 °C), respectively. The binary phases Fe2Ti and Fe5Si3 nucleate in the ternary region through peritectic reactions P3 and P8 at 1591 and 1201 °C, respectively. The ternary phase τ7 does not take part in the liquid-solid equilibria. Among the ternary phases, τ1 and τ2 have large areas of primary crystallization.

Fig. 1
figure 1

Fe-Si-Ti liquidus projection [2008Wei]

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The tentative isothermal section constructed by [2008Wei] at 900 °C is shown in Fig. 2. The ternary phases τ1, τ2, τ3, τ5, τ7, τ8, and τ9 are present. The phases τ4 and τ6 decompose above 1000 °C [2008Wei]. The phase τ7 forms in the solid state and is found to be stable at 900 °C. The isothermal sections at 1150 and 800 °C in the Si-lean region determined by [2002Lof] are redrawn in Fig. 3 and 4 from [2005Ste]. At 1150 °C (Fig. 3), the ternary phases FeSiTi (τ2), Fe4Si3Ti (τ3) and Fe7Si2Ti are present. At 800 °C (Fig. 4), only FeSiTi (τ2) is present. The binary phase Fe5Si3 is stable between 1050 and 825 °C. The presence of Ti increases the temperature range of stability of this phase. It is seen within the ternary region at 1150 and 800 °C in Fig. 3 and 4. At all three temperatures (Fig. 2-4), Fe2Ti dissolves large amounts of Si.

Fig. 2
figure 2

Fe-Si-Ti tentative isothermal section at 900 °C [2008Wei]. Narrow two-phase regions are omitted

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Fig. 3
figure 3

Fe-Si-Ti isothermal section at 1150 °C [2002Lof, 2005Ste]

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Fig. 4
figure 4

Fe-Si-Ti isothermal section at 800 °C [2002Lof, 2005Ste]

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The work of [2008Wei] marks a step forward in characterizing the phase equilibria in this complex system. More detailed studies, especially on the crystal structures of ternary compounds, are needed to establish a complete picture.