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Co-Fe-Gd (Cobalt-Iron-Gadolinium) V. Raghavan

[1990Ati] determined an isothermal section for this system at 1050 ⬚C (reviewed by [1992Rag]). Recently, [1998Su] employed the tie-line compositions determined by the diffusion-couple technique by [1995She] to optimize the thermodynamic interaction parametes. They computed six isothermal sections that agree reasonably well with the limited experimental results available.

Binary Systems The Co-Fe phase diagram was reviewed by [1993Nis]. The fcc phase ␥ forms through a peritectic reaction at 1499 ⬚C between bcc ␦ and liquid. The Co and Fe form a continuous fcc solid solution (␥) over a wide range of temperature, which transforms to bcc (␣) below 985 ⬚C. An order-disorder transformation occurs in the equiatomic ␣ phase at 730 ⬚C and the CsCl-type ordered phase exists over a wide composition range below this temperature. The Co-Gd phase diagram was calculated by [1995Liu]. There are seven intermetallic phases in this system: Co17Gd2, Co5Gd, Co7Gd2, Co3Gd, Co2Gd, Co3Gd4, and CoGd3. With the exception of Co17Gd2, they form through peritectic reactions. The final solidification is through a eutectic reaction at 645 ⬚C and 63.2 at.% Gd. The Fe-Gd phase diagram was reassessed by [1998Zha]. There are four line compounds in this system: Fe17Gd2,

Fe23Gd6, Fe3Gd, and Fe2Gd. All of them form through peritectic reactions. The final solidification is through a eutectic reaction at 832 ⬚C and 73.6 at.% Gd.

Ternary Isothermal Sections The isothermal section at 1050 ⬚C determined by [1990Ati] (reviewed in [1992Rag]) shows that the pairs Fe17Gd2-Co17Gd2, Fe3Gd-Co3Gd, and Fe2Gd-Co2Gd form continuous solid solutions. These solutions are denoted here by M17Gd2, M3Gd, and M2Gd, respectively. [1995Liu] evaluated the thermodynamic properties of this ternary system, using the experimental results of [1990Ati]. [1995She] studied the phase equilibria in this system by the diffusion-couple technique and presented five isothermal sections at 1200, 1050, 1000, 950, and 900 ⬚C. Tie-line compositions were determined for the pairs ␥-M17Gd2, M17Gd2M3Gd, M3Gd-M2Gd, M17Gd2-Co5Gd, Co5Gd-Co7Gd2, and Co7Gd2-M3Gd. Using the experimental data of both [1990Ati] and [1995She], [1998Su] optimized the thermodynamic properties of the ternary system and recalculated the ternary equilibria at 1200, 1050, 1000, 950, 900, and 427 ⬚C. The calculated and experimental tie-line compositions agree reasonably well. The computed isothermal sections at 1200, 900, and 427 ⬚C are redrawn in Fig. 1, 2, and 3. The liquid phase region shrinks from Fig. 1 to Fig. 2 and is not

Fig. 1 Co-Fe-Gd computed isothermal section at 1200 ⬚C [1998Su]

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Journal of Phase Equilibria Vol. 22 No. 5 2001

Phase Diagram Evaluations: Section II

Fig. 2 Co-Fe-Gd computed isothermal section at 900 ⬚C [1998Su]

Fig. 3 Co-Fe-Gd computed isothermal section at 427 ⬚C [1998Su]

present at 427 ⬚C. Both Co5Gd and Co7Gd2 are stable at 1200 and 900 ⬚C. The ␣ phase is present at 900 ⬚C. At 427 ⬚C (Fig. 3), Co5Gd is not stable; at high Gd contents, Co3Gd4 a