Experimental and Theoretical Investigations on Phase Diagrams of Fe Base Ternary Ordering Alloys

Abstract

Phase decompositions in several Fe-Al (or Si)-X (X=V, Co, Ge, Ni) ordering alloy systems were investigated mainly by means of transmission electron microscopy and the phase diagrams were thereby constructed. Many types of phase diagram were obtained. In Fe-Si-V system a wide composition range of the phase separation into disordered A2 and ordered D0₃ phases is observed at temperatures lower than Curie temperature, while at the higher temperatures the A2+D0₃ two-phase field becomes narrow. In Fe-Si-Co alloy, the phase separation into B2 and D0₃ occurs in two separate composition regions; one is the region connecting with the B2+D0₃ region in the Fe-Si binary system, and the other exists separately near the Fe-14.3 at%Si-28.3 at%Co alloy. In Fe-Al-V system the phase separation of A2+D0₃ occurs in a large region of Fe-rich corner. For Fe-Al-Ge and Fe-Si-Ge systems the experimental phase diagrams are also proposed.
The free energies of A2 phase and B2 and D0₃ ordered phases in the ternary systems are evaluated by a statistical approach employing the Bragg-Williams-Gorsky approximation: the magnetic interactions are taken into account up to the second nearest neighbour spins of atoms as well as the usual atomic interactions up to the second nearest neighbours. The equilibrium phase diagrams and the Curie-temperature theoretically given by the present calcuation are almost consistent with the diagrams experimentally obtained. The influences of the magnetic energy on the structural stability of ordered phases are also demonstrated. The theoretical description proposed here is useful for the thermodynamic implication of the equilibrium phase diagram.