DFT study of the solubility of hydrogen and carbon in Ni3Nb-D0a and Ni3Nb-D022 systems

The influence of the local environment on C and H atoms was investigated in Ni3Nb-D0a (space group 59) and Ni3Nb-D022 (space group 139) crystals. The interstitial positions of these atoms and their solubility energies were studied by analyzing the results of first-principles calculations based on density functional theory. The results were compared with those obtained in pure and Nb alloyed fcc Ni. For C atoms, several possible interstitial sites were found, but only one (per structure) is slightly more stable than the others: the 4d sites for D0a and the 4c sites for D022. The C atom is slightly soluble and its solubility energy is equivalent in the two intermetallic structures: approximately equal to 1.06 eV, which is significantly larger than in fcc Ni (0.55 eV). In the case of hydrogen, other interstitial sites were identified: the 4e positions for both D0a and D022. The solubility energy in D0a (approximately 0.12 eV, without the zero-point energy correction) was found to be smaller than that in D022 (0.17 eV) but larger than that in nickel (0.09 eV). With the zero-point energy correction, the solubility in both Ni3Nb alloys is therefore equivalent. These results were finally used to assess the fraction of light elements in Ni3Nb relative to that in fcc Ni and that in Ni doped Nb.