Solute-solute interactions in dilute Nb-X-O alloys from first principles

Recent efforts to develop Refractory Multi-Principal Element Alloys (RMPEAs) for high temperature applications have led to a renewed interest in elucidating fundamental properties of refractory metals. These metals can dissolve high concentrations of interstitial oxygen to the detriment of their mechanical properties and oxidation resistance. Strategies are therefore sought to control the oxygen solubility of refractory alloys. Even in simple dilute refractory alloys, the underlying mechanisms driving binding energies between substitutional solutes and oxygen is not fully understood. We have used first principles calculations of Nb-X-O alloys to examine the interactions between interstitial oxygen and dilute substitutional solutes X = Al, Si, Sc, Ti, V, Cr, Y, Zr, Mo, Ru, Hf, Ta, W, and Re in BCC Nb. Electronegativity differences drive charge transfer between the Nb matrix and both the substitutional and interstitial solutes. This transfer of charge results in strong electrostatic interactions at short distances that play a key role in determining X-O binding energies. The solute misfit volume, which leads to local straining of the surrounding BCC Nb lattice, is also found to have a strong effect on the X-O binding energies. Finally, we identify a repulsive interaction, of significance for a subset of X-O pairs, that arises from a closed-shell hybridization between localized orbitals on solutes.