Effect of the interaction between the chemical and the magnetic ordering on the phase equilibria of iron-base alloys

The effect of interaction between the chemical and magnetic ordering reactions on the phase equilibria between the b.c.c. phases, A2, B2 and DO₃, and between the f.c.c. phases, A1, L1₀ and L1₂, was studied by using the two- or four-sublattice split Compound Energy Formalism (s-CEF), taking into account the magnetic contribution to the Gibbs energy. An interaction between the chemical and magnetic ordering arises when the magnetic properties of the ordered phase deviate from those of the disordered phase. The deviations are included in the calculation by the s-CEF ordering dependent terms. An enhanced interaction arises from the higher deviation of the magnetic properties of the ordered state and promotes a two-phase separation between the ferro- and para-magnetic B2 phases. A lower deviation causes a depressed interaction which results in a convexity in the Gibbs energy curve in the vicinity of the intersection point between the Curie temperature and the order-disorder transition temperature. In consequence, two-phase separations between either para- or ferro-magnetic A2 and para-magnetic B2 phases are introduced. The effect of the interaction on the order-disorder phase equilibria in the Fe-X (X=Co, Al and Ni) systems was investigated in detail based on thermodynamic analysis.