First-principles Co database: Energetics of binary Co alloys and compounds

The field of superalloys has received a recent spike in interest with the discovery of metastable $\gamma^{\,\prime}\!-$Co$_{3}$(Al,W) precipitates with the L1$_{2}$ structures. We present density functional calculations for the first and second nearest-neighbor solute-vacancy binding energies of 27 substitutional solutes in fcc-cobalt. As by-products, we also calculate the dilute mixing energy, dilute volume of mixing, and solubility enthalpy. A modest correlation between the solute size and its binding to an accompanying vacancy has been found. Our calculations reveal that a vacancy not only relieves the strain associated with large solutes, but also mediates a weak bonding between the large solute and its next nearest-neighbor atoms, resulting in high solute-vacancy binding energies. We also find that the solute-vacancy binding energy is minimized for a half-filled $d$-band, in the middle of a transition metal series, and varies parabolically with the $d$-band filling. In general, $4d$ and their counterpart $5d$ transition metals have nearly similar solute-vacancy binding energies, but much larger than those of $3d$ transition metals, and the deviation increase by moving away from a half-filled band.