Phase transformations in iron-nitride compound layers upon low-temperature annealing:: Diffusion kinetics of nitrogen in ε- and γ′-iron nitrides

The epsilon/gamma'-iron nitride (epsilon-Fe3N1+x, gamma'-Fe4N) compound double layers with thicknesses of about 10 mu m were grown on pure alpha-Fe, by gas nitriding at 823 K, followed by quenching. The specimens were subsequently annealed at significantly lower temperatures, in the range of 613 to 693 K, for different periods of time. These heat treatments led to a redistribution of N, within the compound layer as well as between the compound layer and the adjacent ferrite, inducing thickness changes in the epsilon-and gamma'-sublayers. The microstructure and sublayer-thickness changes were analyzed by light microscopy and X-ray diffraction (XRD). The experimentally observed time and temperature dependences of the layer-thickness changes were compared with the results obtained from numerical simulations, by adopting a model based on volume diffusion in the epsilon- and gamma'-phases and on local equilibrium at the phase interfaces. In this manner, the intrinsic diffusion coefficient of N in the E-phase and the. integral diffusion coefficient of N in the gamma'-phase were determined for the applied range of annealing temperatures.