THE EQUILIBRIUM DIAGRAM OF THE Fe-Ga BINARY SYSTEM

Abstract

The equilibrium diagram of the Fe-Ga system below 1000℃ has been determined mainly by X-ray investigation incorporated with differential thermal analysis. The system consists at room temperature of three intermediate phases ε, χ, and ψ. The ε phase with a very narrow homogeneous range has an ordered face-centred structure corresponding to the ideal formula Fe3Ga. At about 550℃, it transforms into another phase ζ which separates at higher temperature directly from the solid solution of Ga in Fe. The structure of the high temperature phase ζ has not been determined. The homogeneous range of the χ phase extends at room temperature from 55 at. % Ga to 60 at. % Ga. The structure of this phase appears to be very complicated, presumably corresponding to the structural formula Fe4Ga5, Fe3Ga4 or Fe2Ga3. It is formed at about 960℃ by a peritectic reaction. The ψ phase is formed by another peritectic reaction at 820℃. The structure is tetragonal, with a=6.2628? and c=6.5559? at 20℃. The space group is D4h14-P42/mnm, and each unit cell contains four formula units corresponding to FeGa3. No noticeable solubility of Fe in Ga has been observed. There exists an eutectic isothermal ranging from FeGa3 to Ga, the eutectic point being very near to the pure component. There exists also an eutectoid isothermal at about 590℃, the eutectoid point being approximately at 50 at. % Ga, at which the solid solution of Ga in Fe decomposes simultaneously into ζ and χ. The most remarkable feature of this system is the primary solid solution region of Ga in Fe. The solubility at room temperature is 15.2 at. %. It gradually increases with temperature, and above 700℃, it increases suddenly up even to 50 at. %. The structure below 625℃ is body-centred cubic, designated as a in the phase diagram. But above 625℃, it changes into two other structures with their respective phase regions, designated as α1 and α2. The structures of α1 and α2 have not been elucidated yet, but most probably they are derived from a by stacking together the fundamental unit cells with atomic rearrangement or vacancy defect.