First-principles study of the structural, defect, and mechanical properties of B2FeCo alloys

B2 FeCo has the highest saturation magnetization of any material, but has zero room temperature ductility in the ordered state that somewhat increases in the disordered state. Brittleness of FeCo has long been a puzzle given its high-symmetry B2 structure, 1/2 < 111 >{110} slip, and low ordering temperature-all features of intrinsically ductile intermetallics. Employing first-principles calculations and statistical mechanics, we study the structural stability, point defects and order-disorder transition of B2 FeCo, and suggest a mechanism potentially leading to its intrinsic brittleness. We find that B2 FeCo is marginally stable, weakly ordered with a high density of antisite defects, and low anti-phase boundary energies for < 111 > slip on {110} and {112} planes. Most importantly, this system is very sensitive to the change in local atomic environment: structural instability and transformation into low-symmetry L1(0) structure or sheared L1(0) structure can be caused by reduced dimensionality or applied shear stress, respectively. We suggest that the internal stress (e.g., near the dislocation cores) may be closely connected with the B2 FeCo intrinsic brittleness, since it is likely to induce local B2 -> L1(0) structural transformations.