An Evaluation of the Fe-N Phase Diagram Considering Long-Range Order of N Atoms in γ'-Fe4N1-x and ε-Fe2N1-z

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, and γ' + 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 and γ' phases and in equilibrium with coexisting γ' 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 the γ' and ε phases was not accounted for

Thermodynamics and Long-Range Order of Interstitials in an h.c.p. Lattice: Nitrogen in ε-Fe2N1-z

The thermodynamics of nitrogen in the ε-Fe2N1-z phase were evaluated. To this end absorption isotherms for nitrogen in ε-Fe2N1-z, depicting the dependence of the nitrogen content on the applied chemical potential of nitrogen, were determined in the temperature range 673-823 K by equilibrating iron foils in gaseous ammonia/hydrogen mixtures. The absorption isotherms could be described very well by a Gorsky-Bragg-Williams approximation for long-range order (LRO) of nitrogen atoms on the hexagonal nitrogen sublattice, formed by the octahedral interstices of the h.c.p. sublattice of iron atoms. The actual occurrence of LRO was evidenced by diffraction analysis. Mössbauer spectra were interpreted as composed of spectra of iron atoms surrounded by 1-3 nitrogen atoms. The relative amounts of the iron environments as determined from the Mössbauer spectra agreed very well with the corresponding probabilities predicted by the LRO model

Thermodynamics and Long-Range Order of Nitrogen in γ'-Fe4N1-x

Models are given for the description of the chemical potential of nitrogen in γ'-Fe4N1-x. In previous work, γ'-Fe4N1-x was treated as a (sub)regular solution, thereby assuming that the N atoms are distributed randomly on the sites of their own sublattice. However, in γ'-Fe4N1-x, long-range ordering occurs of the N atoms over the sites of their own sublattice. Then, the expression for the configurational entropy should account for the occurrence of ordering. In the present article, the descriptions adopted and tested for γ'-Fe4N1-x are based on a Langmuir-type approach, the Wagner-Schottky (WS) approach, and the Gorsky-Bragg-Williams (GBW) approach. Application of the various models to data of nitrogen-absorption isotherms for the γ' iron-nitride phase shows that the subregular solution concept fails to describe the experimental data satisfactorily, whereas a very good agreement between theory and experiment is obtained for the WS and GBW approaches. It is shown that, in particular, accounting for the occupation of disorder (octahedral) sites by N atoms is necessary to obtain an accurate description of the chemical potential of nitrogen in γ'-Fe4N1-x.