Nitriding of Nanocrystalline Iron in the Atmospheres with Variable Nitriding Potential

The properties of nanostructured materials differ significantly from chemically identical bulk materials. The phase transitions in these materials are of a complex nature since the size of the particles is a crucial parameter. In this paper, the series of phase transitions occurring during the gaseous nitriding of nanocrystalline iron is studied, and a detailed analysis of the phase composition and the structure of the material is given. An evolution of the mean crystallite size along the whole process is examined. The influence of variable nitriding potential on the lattice parameters of α-Fe, γ′-Fe₄N, and ε-Fe_xN phases is shown. Unexpectedly high mean crystallite size of the products is observed at the initial stage of each phase transition. The observed concentration of nitrogen in the sample volume is much higher than the one predicted by the classical thermodynamic description of the Fe–N system. This discrepancy is explained in the light of a phase transition mechanism regarding the Gibbs–Thomson effect.