Phase equilibria in the Fe–Co binary system

doi:10.1016/S1359-6454(01)00337-8

Acta Materialia

Volume 50, Issue 2, 22 January 2002, Pages 379-393

https://doi.org/10.1016/S1359-6454(01)00337-8 Get rights and content

Abstract

α (A2)/γ (A1) phase equilibria of the Fe–Co system between 400 and 800°C were determined by means of lattice parameter measurement using thin film specimens. Bulk specimens were also analyzed to compare the extent of attainment to the equilibrium. The thin film technique was found to be greatly advantageous for obtaining the phase equilibria at lower temperatures where solid-state reactions are too slow to reach the equilibrium state in the conventional methods using bulk specimens. It was confirmed that the α+γ two-phase region extends below the temperature at which the α (A2)/α′ (B2) transus meets the α/α+γ boundary. Thermodynamic analysis was also conducted by taking the magnetic and chemical ordering contributions of the B2 structure into account, findings of which confirmed the extension of the α+γ two-phase region below the α/α′ ordering temperature.

Introduction

The phase diagram of the Fe–Co system has been extensively investigated because of its practical importance in structural and functional alloys, especially magnetic materials, and particular interest has been generated because of the apparent contributions of magnetic and chemical ordering to the phase equilibria. Fig. 1 shows a compiled phase diagram of the Fe–Co system [1]. It has been suggested that the peculiar shape of the α (A2)/γ (A1) phase boundary in the Fe–Co system, i.e., Co raises both the A3 and A4 temperatures, is mainly due to the magnetic contribution. Kaufman and Nesor [2] have reported pioneering work on the phase diagram of the Fe–Co system using an empirical equation to describe the thermodynamics parameters. In their assessment, the magnetic contribution to the Gibbs energy was not taken into account, which resulted in a complex temperature dependence of the lattice stability and interaction parameters. Inden [3] and Hillert and Jarl [4] proposed a thermodynamic description of the magnetic contribution and Fernandez Guillermet applied it for the thermodynamic assessment of the Fe–Co system [5]. In his assessment, however, the α (A2)/α′ (B2) order–disorder transition was not taken into account. The α/α′ boundary was precisely examined by Ellis and Greiner [6], Asano et al. [7], Oyedale and Collins [8] and Mills et al. [9], and thermodynamic calculations using the cluster variation method (CVM) [10] and coupled description [11] of CVM and CALPHAD were performed by Colinet et al., although they did not present the effect of the ordering on the low-temperature α (A2)/γ (A1) equilibrium. Their calculation shows that the α (A2)/α′ (B2) transus under a fictitious non-magnetic condition deviate to a lower temperature than that of the magnetic system [10], which indicates the presence of an interaction between the magnetic and chemical ordering suggested in our previous paper [12].

Experimental determination of the phase equilibria at low temperatures is very difficult because of the low diffusivity where the conventional method using bulk specimens cannot be applied. The thin-film method, however, might overcome such difficulty due to the enhanced role of surface and grain boundary diffusion. The thin-film method for examining the ternary phase diagram was first reported by Kennedy et al. [13]. They demonstrated that the equilibrium state could be reached rapidly by annealing thin-film specimens and succeeded in determining isothermal sections of the Fe–Ni–Cr system. It is expected, therefore, that this technique enables attainment of low-temperature equilibria between the α′ (B2) and γ (A1) phases. The purpose of the present study was to examine the effect of the A2/B2 ordering reaction on the b.c.c. (α and α′)/f.c.c. (γ) equilibria by experiment and thermodynamic calculations.

Section snippets

Experimental procedure

Thin films with a thickness of about 0.4–1.0 μm were prepared on a rock-salt substrate by a conventional DC magnetron sputtering method, the composition of the films being controlled at Co–22 mass% Fe so as to have the α+γ two-phase microstructure by equilibration between 400 and 800°C. After the evacuation to a pressure of $1.1×10^{−4} Pa$ , Ar gas was conducted into the chamber to 4.0–5.3 Pa and the deposition was carried out with an applied voltage of 610 V and a discharge current of 0.15 A. The

Lattice parameters of b.c.c. and f.c.c. phases

Fig. 2 shows the lattice parameters of the f.c.c.-γ (A1) phase, a_α, in the bulk specimens annealed at 1000°C. The lattice parameters can be seen to increase linearly with increasing Fe content, which is in accordance with the previous work [6]. The relationship between the lattice parameter and the composition was determined to be $a_{γ} =0.35438+0.00010233× [at.% Fe]/nm .$

The lattice parameters of the b.c.c. phase, on the other hand, were reported to differ depending on the heat treatment conditions,

Thermodynamic assessment

The Gibbs energies of the stable and metastable structures for the pure elements are taken from the SGTE database compiled by Dinsdale [14]. In the Fe–Co system, four disordered solution phases, the liquid, α (A2), γ (A1) and ε (A3), and an ordered b.c.c. phase, α′ (B2), are considered to constitute the equilibrium phases. The Gibbs energy descriptions of these five phases are given below and evaluated thermodynamic parameters are listed in Table 2, Table 3.

Phase equilibria between the α (A2), α′ (B2) and γ (A1) phases

As shown in the present result on the phase equilibria between α (A2), α′ (B2) and γ (A1) phases in Fig. 9, the $α (A2)+γ (A1)$ two-phase region extends with decreasing temperature. Below the intersection with the α (A2)/α′ (B2) boundary, the equilibrium turns into $α′ (B2)+γ (A1)$ and further enlargement of the two-phase region is observed. Compared with the previous assessment of the phase diagram of the Fe–Co system by Fernandez Guillermet [5], the $b.c.c./b.c.c.+γ (A1)$ boundary significantly deviates

Conclusion

Phase equilibria between the α (A2), α′ (B2) and γ (A1) phases in the Fe–Co binary system were determined using thin film specimens at temperatures between 400 and 800°C. It was found that the equilibrium state can be attained even at low temperature due to the enhanced diffusion through surface and grain boundary of thin-film specimens. The limiting temperature for obtaining the equilibrium was estimated to be about 0.4 T_m, where T_m represents the melting temperature. The α/α′/γ phase

Acknowledgements

The authors wish to thank Mr Y. Abe for his help with the experimental work. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.

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¹: Present address: National Institute of Advanced Industrial Science and Technology, 1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan.

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Phase equilibria in the Fe–Co binary system

Abstract

Introduction

Section snippets

Experimental procedure

Lattice parameters of b.c.c. and f.c.c. phases

Thermodynamic assessment

Phase equilibria between the α (A2), α′ (B2) and γ (A1) phases

Conclusion

Acknowledgements

CALPHAD

Acta metall mater

CALPHAD

CALPHAD

Phys Lett

Phys Rep

Acta mater

CALPHAD

J Chem Thermodyn

J Magn Magn Mater

Z Metallkd

High Temp-High Pressures

Trans ASM

Trans Jpn Inst Met

Phys Rev B

JOM

Z Metallkd

J Appl Phys

Proc Roy Soc A

Z Metallkd

Ann Phys Ser 10