Prototype calculations of B2 miscibility gaps in ternary b.c.c. systems with strong ordering tendencies

Prototype calculations in ternary ordering systems based on the b.c.c. lattice have been carried out with the Cluster Variation Method (CVM) in the irregular tetrahedron approximation including tetrahedron interactions. The systems under investigations were characterized by strong ordering tendencies (i.e. with large, negative first neighbour interactions in all binary sub-systems) which resulted in the opening of a miscibility gap inside the B2 single-phase field. This miscibility gap is shown to be produced by frustration of the B2 cluster by a calculation using a hypothetical system with identical first neighbour interactions in the three binary sub-systems. This phase diagram presents as a key feature a central three-phase miscibility gap, which transforms into an ordinary two-phase miscibility gap involving two B2 phases after the symmetry of the interactions is broken. The results are discussed in connection with the experimental Fe–Ti–Rh phase diagram.