PHASE-TRANSFORMATIONS IN NITROGEN-IMPLANTED ALPHA-IRON

A systematic study of nitride-phase formation in iron implanted with nitrogen was performed by conversion electron Mossbauer spectroscopy (CEMS). The samples of alpha-iron were implanted at room temperature with 100 keV N2+ ions whose mean range in the iron target was about 55 nm. The implanted dose varied from 5 x 10(16) to 6 x 10(17) N atoms/cm2. After implantation each sample was isothermally annealed in vacuum for 1 h at temperatures from 150 to 500-degrees-C in 50-degrees-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 x 10(17) N atoms/cm2), the nitrogen-containing alpha'-martensite and/or alpha"-Fe16N2 phases appear. For intermediate doses (2 x 10(17)-2.7 x 10(17) N atoms/cm2) epsilon-Fe3N is formed additionally. For doses higher than 3 x 10(17) N atoms/cm2 the dominant phase is epsilon-Fe2N. In the course of annealing, epsilon-Fe2N decomposes to epsilon-Fe3N and gamma'-Fe4N and finally disappears at about 300-350-degrees-C. At 500-degrees-C only the Zeeman sextet of alpha-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.