Semi-Empirical Parameterization of Interatomic Interactions, Which is Based on Statistical-Thermodynamic Analysis of Data on Phase Equilibriums in BCC-Fe-Co Alloy. I. Primary Ordering

Using the statistical-thermodynamic methods within the scope of the self-consistent field approximation, an atomic ordering of the substitutional b.c.c.-Fe-Co alloy and the magnetic contribution of both its components (Fe and Co) in the interatomic interaction are investigated, and the temperature-concentration dependences of atomic order and magnetism parameters of Fe-Co alloys are calculated. The model of pair-wise interatomic interactions is applied. Magnetism of the alloy is taken into account within the scope of the mean ('molecular') field approximation. Parameterization of interatomic interactions ('integrals' of the exchange interaction, mixing energies of substitutional (magnetic) atoms of Co and Fe at sites of the b.c.c. lattice) is based on analysis of experimental data on phase equilibriums in b.c.c.-Fe-Co alloy. Results of their optimization are in agreement with available experimental data, primarily, on the concentration dependences of Curie and Kurnakov temperatures. The determined parameters enable the calculation of thermodynamic characteristics of an alloy at different temperatures and concentrations. Thus, the total configuration-dependent free energy of both the B2-type structure (with b.c.c.-FeCo stoichiometry) and the D0(3)-type structure (with b.c.c.-Fe3Co stoichiometry) are estimated taking into account the magnetism of both components in the configuration internal energy and entropy of the alloy. Equilibrium values of the atomic-order parameters and magnetizations of both magnetic components in the B2- and D0(3)-type structural states of an alloy are determined taking into account the necessary conditions of thermodynamic equilibrium by means of the sets of three or four transcendental equations, respectively. For example, their temperature dependences as a result of the order disorder phase-transformation point for b.c.c. Fe0.5Co0.5 and Fe0.6Co0.4 alloys are investigated numerically.