The states of nitrogen atoms in the processing of Ni-Cr alloy surface nitriding modification

The energies of supercells of Ni-Cr alloy containing an isolated N atom or a pair of N atoms in the octahedrons with 0-3 Cr atoms were obtained by the first principle calculations. In supercells with an isolated N atom and a pair of N atoms, the energy of supercells decreases with increasing of the number of Cr atoms nearest neighboring to a N atom while the increment is about 0.4 eV per Cr atoms and almost independent on the distribution of the Cr atoms. According to the energy data, the interaction between neighboring N atoms was deduced to be in a narrow range of 2.37-2.56 eV. In local equilibrium approximation, it was found that the energy of N dependent on the N concentration linearly, and the calculated non-retractable nitrogen was about 18 at.% in sublattice which was in agreement with experimental result.