Phase transformations in nitrogen-implanted α-iron

A systematic study of nitride-phase formation in iron implanted with nitrogen was performed by conversion electron Mössbauer spectroscopy (CEMS). The samples of α-iron were implanted at room temperature with 100 keV N+2 ions whose mean range in the iron target was about 55 nm. The implanted dose varied from 5×1016 to 6×1017 N atoms/cm2. After implantation each sample was isothermally annealed in vacuum for 1 h at temperatures from 150 to 500 °C in 50 °C steps. The CEMS measurements allowed identification of the iron nitride phases formed by implantation and after annealing. At low doses (up to 1.5×1017 N atoms/cm2), the nitrogen-containing α′-martensite and/or α″-Fe16N2 phases appear. For intermediate doses (2×1017–2.7×1017 N atoms/cm2) ε-Fe3N is formed additionally. For doses higher than 3×1017 N atoms/cm2 the dominant phase is ε-Fe2N. In the course of annealing, ε-Fe2N decomposes to ε-Fe3N and γ′-Fe4N and finally disappears at about 300–350 °C. At 500 °C only the Zeeman sextet of α-Fe remains in the spectra. Considerable nitrogen release as detected by nuclear reaction analysis accompanied the phase evolution during annealing. These measurements lead to a new phase diagram for the implanted FeN system.