Composition dependence of the low-temperature magnetic ordering and the hyperfine interactions in Fe–N austenite

Abstract

The electronic structure of two model structures of stoichiometry Fe8N with different types of ordering of the nitrogen atoms are calculated using the ab initio FLAPW method. To first approximation these structures can be treated as a prototype of Fe–N austenite. The influence of the nitrogen atoms on the electronic structure and magnetic properties of γ iron and on the components of the Mössbauer spectrum is investigated, permitting a detailed interpretation of the experimental data. The dependence of the exchange integral on the concentration of nitrogen atoms is obtained, clarifying the influence of nitrogen on the magnetic properties of fcc iron. The effective interatomic interaction potentials of nitrogen atoms in fcc iron in six coordination spheres is calculated using the ab initio FLAPW method and the cluster expansion technique. The potentials thus obtained are used in a Monte Carlo investigation of the temperature dependence of the short-range order in the Fe–N system. The temperature dependence of the heat capacity and short-range order parameters is calculated. It is established that a transition from the disordered state to the nitride Fe4N is observed at 830 K.