Metallurgical and Materials Transactions A

, Volume 27, Issue 4, pp 1063–1071

An evaluation of the Fe-N phase diagram considering long-range order of N atoms in γ′-Fe4N1-x and ε-Fe2N1-z

Authors

  • Bart J. Kooi
    • Materials Science Centre, Department of Applied PhysicsUniversity of Groningen
  • Marcel A. J. Somers
    • Laboratory of Materials ScienceDelft University of Technology
  • Eric J. Mittemeijer
    • Laboratory of Materials ScienceDelft University of Technology
Physical Chemistry

DOI: 10.1007/BF02649775

Cite this article as:
Kooi, B.J., Somers, M.A.J. & Mittemeijer, E.J. MMTA (1996) 27: 1063. doi:10.1007/BF02649775

Abstract

The chemical potential of nitrogen was described as a function of nitrogen content for the Fe-N phases α-Fe[N], γ′-Fe4N1-x, and ε-Fe2N1-z. For α-Fe[N], an ideal, random distribution of the nitrogen atoms over the octahedral interstices of the bcc iron lattice was assumed; for γ′-Fe4N1-x. and ε-Fe2N1-z, the occurrence of a long-range ordered distribution of the nitrogen atoms over the octahedral interstices of the close packed iron sublattices (fcc and hcp, respectively) was taken into account. The theoretical expressions were fitted to nitrogen-absorption isotherm data for the three Fe-N phases. The α/α+ γ′, α +γ′/γ′, γ′/γ′ + ε, andγ′ + ε/ε phase boundaries in the Fe-N phase diagram were calculated from combining the quantitative descriptions for the absorption isotherms with the known composition of NH3/H2 gas mixtures in equilibrium with coexisting α andγ′ phases and in equilibrium with coexistingγ′ and ε phases. Comparison of the present phase boundaries with experimental data and previously calculated phase boundaries showed a major improvement as compared to the previously calculated Fe-N phase diagrams, where long-range order for the nitrogen atoms in theγ′ and ε phases was not accounted for.

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© The Minerals, Metals & Material Society and ASM International 1996