Chromium – Iron – Vanadium

Abstract

The constitution of many ferritic steels includes chromium and vanadium as basic alloying elements. Knowledge of the phase diagram and thermodynamic properties of the Cr-Fe-V system is essential to understand the behavior of such iron-based alloys. The first experimental work on the phase equilibria of the Cr-Fe-V system was undertaken by [1952Mar] who investigated the phase boundaries between the bcc and σ phases at 700°C, and found that the σ phase extends across the ternary diagram from the Fe-V to the Cr-Fe binary side. These results were confirmed experimentally by [1954Kor, 1957Kor1, 1957Kor2] as well as the calculations of [1977Bra1, 1977Bra2], who used a cluster model. The isothermal section presented by [1957Kor1] was almost the same as that of [1952Mar]. The effect of vanadium on the miscibility gap was investigated by [1967Mim, 1968Mim, 1970Mim, 1971Yam]. They reported that the miscibility gap in the Cr-Fe system extends into the Cr-FeV ternary system and takes the form of a mountain with a peak. The thermodynamic properties of some solid ternary alloys were measured by [1973Mal] and [1987Che]. The enthalpies of formation of the αδ solid solution as well as enthalpies of transformation from the σ phase to the bcc structure were determined. The activities of Fe and V in liquid Cr-Fe-V alloys were reported by [1975Fur]. The composition dependence of the lattice parameters of the αδ and σ phases were presented by [1988Vas1, 2002Ham] and [1952Mar]. Thermodynamic computations of the phase equilibria were carried out [1973Spe, 1988Kum, 1992Lee]. [1973Spe] calculated isothermal sections illustrating the αδ/σ phase relationships at 427, 700 and 900°C, and compared them with the experimental results of [1952Mar]. The results of [1973Spe] had certain discrepancies with respect to the results of [1952Mar], in particular with regard to the relative positions of the tie lines. [1973Spe] pointed out that errors could arise from the methods used to fit the experimental data and in the summation of the three binary expressions for calculation of the ternary Gibbs energy values, mostly for the σ phase region. Later, [2001Spe] showed that calculations of the Cr-Fe-V system using different interaction models gave different results in the σ phase domain. [1988Kum] computed the (γ/γ+αδ) and (γ+αδ/γ) phase boundaries near the Fe corner at 1350, 1250, 1150, 950, and 900°C. A single-lattice magnetic solution model was used. Isothermal sections of the Cr-Fe-V system at 1727, 1552, 1527, 1027, 700, and 337°C were calculated by [1989Kau]. [1992Lee] made a detailed thermodynamic analysis of this system. The binary description for the Cr-Fe was taken from [1987And] and that for Fe-V from [1991Hua]. The Cr-V system was computed by [1992Lee]. In the last case, the depression in the Cr-V fusion diagram was ignored. The ternary interaction parameters were obtained by optimization using experimental data. A three-sublattice model was used to express the Gibbs energy of the σ phase. The computed results were presented as isothermal sections at 700 and 480°C, as a metastable miscibility gap at 480°C, a vertical section at 50 at.% Fe, and solidus and liquidus isotherms at 1900, 1800, 1700, 1600 and 1500°C. Metastable vertical sections at 76 and 60 mass% Fe were also presented by suspending the σ phase in the calculation. They show that the addition of V to Cr-Fe binary alloys increases the critical temperature of the miscibility gap. These results are in good qualitative agreement with experimental data [1967Mim, 1968Mim, 1970Mim, 1971Yam]. The results of the calculations of [1992Lee] are in better agreement with the experimental phase equilibrium data than the work of [1973Spe] and [1989Kau], and for this reason they are preferred. Reviews of the Cr-Fe-V system have been presented by [1986Ban], [1988Ray] and [1994Rag]. Investigations of phase relations, structures and thermodynamics are listed in Table 1. Physical and mechanical properties of some Cr-Fe-V alloys were studied theoretically and experimentally by [1974Rus, 1979Kat, 1981Kon, 1986Gal, 1988Vas1, 1988Vas2, 1995Nic, 1996Cer, 1997Gal, 1998Gal1, 1998Gal2, 1998Gal3, 2000Som]. The experimental techniques as well as measured properties are listed in Table 4. Cr–Fe–V 1