An evaluation of the Fe-N phase diagram considering long-range order of N atoms in gamma'-Fe4N1-x and epsilon-Fe2N1-z

The chemical potential of nitrogen was described asa function of nitrogen content for the Fe-N phases alpha-FE[N], gamma'-Fe4N1-x, and epsilon-Fe2N1-x. For alpha-Fe[N], an ideal random distribution of the nitrogen atoms over the octahedral interstices of the bcc iron lattice was assumed; for gamma'-Fe4N1-x and epsilon-Fe2N1-x, 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 alpha/alpha + gamma', alpha + gamma'/gamma', gamma'/gamma' + epsilon, and gamma' + epsilon/epsilon 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/H-2 gas mixtures in equilibrium with coexisting alpha and gamma' phases and in equilibrium with coexisting gamma' and epsilon 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 gamma' and epsilon phases was not accounted for

Thermodynamics and long-range order of nitrogen in gamma'-Fe4N1-x

Models are given for the description of the chemical potential of nitrogen in gamma'-Fe4N1-x. In previous work, gamma'-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 gamma'-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 gamma'-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 gamma' 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 gamma'-Fe4N1-x