Structural and stability of B2 FeCo1-XVX and Fe1-XCoVX systems: Cluster expansion approach

Fe-Co alloys are considered good candidates for high-temperature applications due to their high saturation magnetisation and Curie temperature. However, these alloys show low levels of ductility at room temperature. In this study, cluster expansion was employed to probe the thermodynamic stability of the FeCo1-XVX and Fe1-XCoVX alloys. Ten new stable structures were found from both FeCo1-XVX and Fe1-XCoVX systems. Their stability was observed by deducing the heats of formation, and it was found that VFeCo2 and VFe2Co (P4/mmm) are the most thermodynamically stable phases. The results also showed that vanadium prefers the Co-site rather than the Fe-site substitution. The calculated Pugh’s ratio and Poisson’s ratio confirm that alloying with V effectively improved the ductility. It was also found that VFeCo2, VFe2Co, VFe4Co3 and FeCo showed a positive shear modulus condition of stability for the structures. The ternary addition of V in the FeCo system resulted in enhanced magnetic properties. Thus, ternary systems with vanadium addition enhance the ductility of the Fe-Co systems, and these alloys could be used to develop future magnets.