Computer-simulation study of high-temperature phase stability in iron

A pair-potential model describing interatomic interactions in {γ}- and {δ}-iron based on the generalized pseudopotential theory is presented. This potential describes the temperature-dependent stability of different phases and simulates forward and reverse transformations. This, together with the results obtained in the calculation of the thermal expansion, the Gruneisen parameter, and the temperature dependence of the mean-square displacements, for both the fcc and bcc phases, show the reliability of the potential for describing temperature-dependent properties. The stability of fcc and bcc phases has been studied by molecular dynamics. It was found that {γ}-Fe is unstable for temperatures higher than 1820 K and {δ}-Fe is unstable for temperatures lower than 1400 K and higher than 1950 K. In perfect crystallites, when Parinello-Rahman and Nose mechanics are applied, these instabilities lead to the fc→cbcc transformation via Bain distortions and to the bc→cfcc transformation via a Burgers mechanism. The estimated phase-transformation temperature is about 1610 K, which is in good agreement with the experiments. The important role of the vibrational energy in the phase stability has been concluded. The melting temperature of the bcc-Fe was estimated as ≈1880K which is about 70 K higher than the experimental one.