First-principles study of the structural, defect, and mechanical properties ofB2FeCoalloys

Abstract

$\mathrm{B}2\phantom{\rule{0.3em}{0ex}}\mathrm{FeCo}$ 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 $\mathrm{FeCo}$ has long been a puzzle given its high-symmetry $\mathrm{B}2$ 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 $\mathrm{B}2\phantom{\rule{0.3em}{0ex}}\mathrm{FeCo}$, and suggest a mechanism potentially leading to its intrinsic brittleness. We find that $\mathrm{B}2\phantom{\rule{0.3em}{0ex}}\mathrm{FeCo}$ 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 $\mathrm{L}{1}_{0}$ structure or sheared $\mathrm{L}{1}_{0}$ 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 $\mathrm{B}2\phantom{\rule{0.3em}{0ex}}\mathrm{FeCo}$ intrinsic brittleness, since it is likely to induce local $\mathrm{B}2\ensuremath{\rightarrow}\mathrm{L}{1}_{0}$ structural transformations.