It is well known that the magnetic properties, such as the Curie temperature, Tc, and the mean magnetic moment, b, of the ordered compounds differ from those of the disordered state. For instance, both Tc and b of the FeCo (B2) and Ni3Fe (L12) ordered compounds are higher than those of the disordered Fe-Co and Fe-Ni alloys. On the other hand, Tc of the Fe3Al (D03) ordered compound is strongly depressed due to the ordering compared with that of the disordered Fe-Al alloy. In consequence, the stability of the ordered phase is enhanced or depressed due to the interaction between the chemical and magnetic ordering caused by the increase or decrease of Tc and b. The purpose of this study is to investigate the interaction effect on the phase equilibria in the Fe-Co-Al and Fe-Co-Ga ternary systems and to evaluate the interaction effect of the FeAl (B2), Fe3Al (D03), FeGa (B2), Fe3Ga (D03) ordering. Furthermore, taking the interaction effect into account, thermodynamic assessment of the b.c.c. Fe-Al and Fe-Ga alloys is carried out.
Tc of b.c.c. phases in A2, B2 and D03 structure, and A2/B2 and B2/D03 transition temperature of Fe-Co-Al and Fe-Co-Ga ternary alloys were determined by DSC and VSM. Measured Tc values of A2 and B2 phases in ternary alloys were extrapolated to Fe-Al and Fe-Ga binary alloys and the virtual Curie temperatures of the A2 disordered and B2 ordered Fe-Al and Fe-Ga binary alloys were evaluated. It was found that the Curie temperature is strongly depressed with B2 and D03 ordering in both systems. Result of thermodynamic analysis suggests that the anomalous A2+B2 two-phase separation of the Fe-Al and Fe-Ga binary systems is caused by the depressive interaction between the chemical and magnetic ordering.