Magnetic cluster expansion model for bcc-fcc transitions in Fe and Fe-Cr alloys

An ab initio-based magnetic-cluster-expansion treatment developed for body- and face-centered cubic phases of iron and iron-chromium alloys is applied to modeling the alpha-gamma and gamma-delta phase transitions in these materials. The Curie, Neel, and the structural phase-transition temperatures predicted by the model are in good agreement with experimental observations, indicating that it is the thermal excitation of magnetic and phonon degrees of freedom that stabilizes the fcc gamma phase. The model also describes the occurrence of the gamma loop in the phase diagram of Fe-Cr alloys for a realistic interval of temperatures and Cr concentrations.