Mossbauer spectroscopy study of the aging and tempering of high nitrogen quenched Fe-N alloys: Kinetics of formation of Fe16N2 nitride by interstitial ordering in martensite

The distribution of nitrogen atoms in austenite and during the different stages of aging and tempering of martensite is studied by Mossbauer spectroscopy, X-ray diffraction, and transmission electron microscopy (TEM). Transmission Mossbauer spectroscopy (TMS) and conversion electron Mossbauer spectroscopy (GEMS) are used for studying the austenite phase where the distribution of nitrogen atoms is found to depend on the nitriding method, gas nitriding in our case, or ion implantation. Conversion electron Mossbauer spectroscopy, which concerns a depth predominantly less than 200 nm, reveals a nitrogen atom distribution different from that found in the bulk by TMS. The identification and kinetics of the stages of aging and tempering of martensite are followed by TMS measurements, and the phase characterization is confirmed by X-ray diffraction and TEM. The major stages are the early ordering of nitrogen atoms, which leads to small coherent precipitates of (alpha ''-Fe16N2; the passage by thickening to semicoherent precipitates of alpha ''-Fe16N2; the dissolution of alpha '' Fe16N2 with the concomitant formation of gamma'-Fe4N; and the decomposition of retained austenite by tempering. The three first stages correspond to activation energies of 95, 126, and 94 kJ/mole, respectively, consistent with the nitrogen diffusion for the first and third stages and the dislocation pipe diffusion of iron for the second.