B2 FeCo has the highest saturation magnetization of any material, but has zero room temperature ductility in the ordered state that somewhat increased in the disordered state. Brittleness of FeCo has long been a puzzle given its high-symmetry B2 structure, ½<111>{110} slip, and low ordering temperature—all features of intrinsically ductile intermetallics. Employing first-principles calculations and statistical mechanics, a study was made of the structural stability, point defects and order-disorder transition of B2 FeCo , and suggested a mechanism potentially leading to its intrinsic brittleness. It was found that B2 FeCo was 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 was very sensitive to the change in local atomic environment: structural instability and transformation into low-symmetry L10 structure or sheared L10 structure could be caused by reduced dimensionality or applied shear stress, respectively. It was suggested that the internal stress (e.g., near to dislocation cores) could be closely connected with the B2 FeCo intrinsic brittleness, since it was likely to induce local B2→L10 structural transformations.

First-Principles Study of the Structural, Defect and Mechanical Properties of B2 FeCo Alloys. C.L.Fu, M.Krčmar: Physical Review B, 2006, 74[17], 174108 (8pp)