DFT study of the solubility of hydrogen and carbon in Ni₃Nb-D0_a and Ni₃Nb-D0₂₂ systems

Highlights: • Solubility of H atoms in Ni₃Nb-D0_a and -D0₂₂ compounds is investigated. • Solubility of C atoms in Ni₃Nb-D0_a and -D0₂₂ is also investigated. • The influence of Nb concentration in Ni on the H and C solubility is evaluated. • The DFT results and experimental findings on the H solubility in Ni-based superalloys are in good agreement. - Abstract: The influence of the local environment on C and H atoms was investigated in Ni₃Nb-D0_a (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 D0_a 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 D0_a and D0₂₂. The solubility energy in D0_a (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