Self-diffusivity of nitrogen in γ'-iron nitride (“Fe4N”)

The growth of γ'-iron nitride (“Fe₄N”) on pure iron plate samples was followed by a thermogravimetric technique in the temperature range of 450–550 ° C at various nitrogen activities (fixed by H₂/NH₃ gas mixtures). The growth kinetics were characterized by an initial stage of nucleation and growth control followed by nitrogen diffusion controlled growth. At 450 ° C, the self-diffusivity of nitrogen D^* is independent of nitrogen activities. At 500 ° C, D^* decreases with increasing nitrogen activities. At 550°C, D^* decreases to a minimum and then increases again with increasing nitrogen activities. It is suggested that the nitrogen diffusion mechanism in γ'-iron nitride is strongly related to the non-stoichiometry of this phase. At 450 and 500 ° C, the growth kinetics can be explained by a mechanism that involves the mixed control of two processes: (i) nitrogen diffusion via a small number of vacant regular sites ($\text{1}\text{2}$, $\text{1}\text{2}$, $\text{1}\text{2}$) and (ii) nitrogen diffusion via a large number of disordered sites ($\text{1}\text{2}$, 0, 0), (1, $\text{1}\text{2}$, 0), (0, 0, $\text{1}\text{2}$) as interstitials. At 550°C, the diffusion mechanism is controlled by nitrogen diffusion via the vacant regular sublattice sites at low nitrogen activities. At high nitrogen activities, the diffusion mechanism again becomes mixed control of the two diffusion processes. At the nitrogen activities of 100 and 174, D^* can be represented, respectively, by: D^* = (5.81 × 10⁻⁷cm²/s) · exp((−19.05 $\text{kcal}\text{mole}$)/RT) and D^* = (2.45 × 10⁻⁹cm²/s) · exp((−11.16 $\text{kcal}\text{mole}$)/RT) It appears that the activation energy of diffusion for nitrogen diffusion via the disordered sites is higher than that via the regular sites.