Oscillatory Mechanism of α-Fe(N) ↔ γ'-FeN Phase Transformations during Nanocrystalline Iron Nitriding.

The kinetics of nanocrystalline α-iron nitriding to γ'-iron nitride in an ammonia atmosphere was studied at 598-648 K and at atmospheric pressure. Oscillatory changes in nitriding reaction rates depending on nitrogen concentration in a solid sample were observed. This phenomenon was explained by a gradual change in the iron active surface coverage degree, with nitrogen resulting from a gradual change in the free enthalpy of nitrogen segregation. The α-Fe(N) nanocrystallites' transformation into γ'-FeN went through six metastable FeN states. The continuous function proposed by Fowler and Guggenheim was modified to a stepwise variable function.