Thermodynamics of the Fe-N and Fe-N-C Systems: The Fe-N and Fe-N-C Phase Diagrams Revisited

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INTRODUCTION

THE Fe-C, Fe-N and Fe-N-C systems are highly relevant for Fe-based components, in particular if these are subjected to technically applied nitriding and nitrocarburizing treatments.[1] The Fe-N-C equilibrium phases considered in the present work have been listed in Table I, as assembled from References 2 and 3. The binary Fe-C system is the basis of all technically applied steels.[4] The stable phases in the binary Fe-C system[5] are the terminal interstitial solid solution phases a (ferrite) and c (austenite), the liquid solution phase and graphite. However, due to kinetically obstructed precipitation of graphite, at carbon contents of up to 25 at. pct, the iron carbide cementite, h-Fe3C1d , occurs in metastable equilibria. Recently, the non-stoichiometry of h in equilibrium with a and c with positive values of d, has been quantified.[6] On this basis, a new thermodynamic description for the cementite phase has been presented,[7] which, in contrast to the previous descriptions,[8–11] recognizes and well describes its non-stoichiometric character. HOLGER GO¨HRING, Ph.D. Student, is with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), Heisenbergstraße 3, 70569 Stuttgart, Germany. Contact e-mail: [email protected] OLGA FABRICHNAYA, Research Scientist, is with the Institute of Materials Science, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany. ANDREAS LEINEWEBER, formerly Research Scientist with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), is now Professor with the Institute of Materials Science, TU Bergakademie Freiberg. ERIC JAN MITTEMEIJER, Director, is with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), and also Professor with the Institute for Materials Science, University of Stuttgart, Stuttgart, Germany. Manuscript submitted March 16, 2016. Article published online October 3, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A

The constitution of compound layers developing upon nitriding of Fe can be predicted by the Fe-N phase diagram,[12] assuming local equilibrium in the solid state, featuring the interstital solid solution phases a (ferrite) and c (austenite) and the iron nitride phases c0 -Fe4N1x and e-Fe3N1+z. In order to identify such local equilibria, the Fe-N system to be considered, as discussed above for the Fe-C system, represents metastable equilibrium states, corresponding to suppression of the formation of N2 gas. In genuine equilibria, iron-nitride phases such as c0 and e do not occur. Metastable equilibria in the Fe-N system can be investigated by gas-nitriding experiments using NH3/H2 atmospheres, defining the chemical potential of N in the gas phase.[1] For data obtained from such gas-nitrided specimens, furthermore the establishment of a steady state instead of a local equilibrium at the surface of the specimens, i.e., equality of the rate of N dissolution and recombination instead of equal

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