In the Fe-Be binary system, the solubility of Be in the α-Fe phase deviates significantly from the so-called Arrhenius equation near temperatures of 600°C. The metastable ordering of the bcc structure in this binary system is expected to play a key role in the phase boundary anomaly. Thus, a thermodynamic analysis of the Fe-Be binary system has been performed considering the ordering behaviour of the bcc phase. The total energies of the ordered structures based on the bcc lattice were obtained using ab initio energetic calculations. The cluster expansion method was applied to the results, and the free energies at finite temperatures were calculated for the bcc solid solution. The formation energy of the ζ phase was also calculated using band-energy calculations. The results were analysed together with some experimental data using the sublattice model, and the equilibrium phase diagram was calculated. The results support the formation of a metastable (bcc + B2) two-phase region accompanied by an ordering of the bcc structure. This metastable ordering of the bcc phase was the dominant factor governing the anomalous change in the solubility of Be in the higher temperature range.
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