Nitrogen diffusion in lattice: a trapping diffusion process

X-ray diffraction, thermoconductivity and nuclear magnetic resonance experiments have been carried out in order to study the diffusion mechanism of N atoms in the lattice. These experiments reveal that a small number of N atoms which enter the lattice, but do not occupy the octahedral interstitial sites, are mobile at the nitrogenation temperature, while most of the N atoms, which enter the octahedral sites, are immobilized. Based on the characteristics of nitrogenation observed in (were R is the rare-earth element) systems, a tripping diffusion model has been proposed and discussed in detail. The previously proposed free diffusion model is the high-temperature approximation of the trapping diffusion model. The experiments and theoretical analysis show that it is the N - lattice interaction, instead of N - N interaction, that leads to the formation of the observed nitrided - unnitrided configuration, causes a tremendous decrease of the apparent diffusion frequency factor and is responsible for the N uptake stability and irreversibility. Also, this work shows how the N - lattice interaction affects the N uptake in various nitrogenation conditions.