Thermodynamics and long-range order of interstitials in a hexagonal close-packed lattice

Abstract

Statistical thermodynamics was applied to describe long-range order (LRO) of interstitial atoms in a hexagonal close-packed (hcp) host lattice. On the basis of the Gorsky-Bragg-Williams (GBW) approximation and a division of the interstitial sublattice into six interpenetrating sublattices, all the possible ordered configurations were derived for this assembly. Special attention was devoted to two of the possible ordered configurations of interstitial atoms, viz., the two ground-state structures that have been indicated for ε-Fe2N1-z. A description of the order-disorder transition was obtained, and the evolution of the occupancies of the different types of interstitial sites on changing the total interstitial content was given. Composition-temperature regions of stability for the two ordered configurations were given in phase diagrams for different combinations of pairwise interaction energies. The results are compatible with observations for ε-Fe2N1-z as reported in the literature. The advantages of the present treatment were discussed relative to an earlier one, which a priori excluded nearest neighboring interstitial sites from simultaneous occupancy.