57Fe Mössbauer Study on Distribution and Interaction of N Interstitials in γ-FeN Austenite and α′-FeN Martensite

N atoms occupy interstitial sites in α- and γ-Fe and affect the physical and chemical properties of Fe. In order to clarify the distribution of N atoms in α- and γ-Fe and the influence upon Fe, 57Fe Mössbauer measurements have been performed for Fe–N austenite and martensite. In γ-FeN austenite, there are three kinds of Fe atoms with zero, one and two N atoms as near neighbors. An Fe atom and two near neighbor N atoms in γ-Fe constitute a N–Fe–N 180°-linear chain, although such a configuration cannot be observed in Fe–Ni–C austenite. From the amounts of these components, it was confirmed that the distribution of N atoms in γ-Fe is not random and the repulsive force acts between near neighbor N atoms. By comparison with Fe–Ni–C austenite, it became clear that the repulsive force between N atoms is weaker than that between C atoms. It was also confirmed that the martensitic transformation from γ- to α′-phase can be induced by applying the magnetic field. This phenomenon occurs at the critical magnetic field which depends on N content and increases with N content. The 57Fe Mössbauer spectrum of α′-FeN martensite can be analyzed by four magnetic components which correspond to Fe as the first, second, third and further near neighbors of the interstitial N atom. Among them, the third near neighbor Fe shows a large hyperfine magnetic field, 36 T at room temperature, but the mean hyperfine magnetic field of α′-FeN martensite is nearly equal to that of α-Fe. Although the amounts of the first and second near neighbor Fe atoms can be explained by assuming that N atoms randomly occupy the Oz sites in the bct lattice, the amounts of the third and the further near neighbors Fe cannot. This may be because of the diffusion of N atoms to form the N-rich region in the matrix at room temperature.