The Fe-Ni system: Thermodynamic modelling assisted by atomistic calculations

The Fe-Ni system is a key subsystem for several alloys with important applications. It has been thermodynamically assessed according to the CALPHAD methodology and using 0 K enthalpies of formation of the ordered phases resulting from ab-initio atomistic calculations. This allowed us to model both stable and metastable fcc-based ordered phases (L12 Fe3Ni, L10 FeNi and L12 FeNi3) in the framework of the compound energy formalism (CEF) by using a 4-sublattice model. The combined ab-initio and CALPHAD approach enabled us to predict low-temperature stable equilibria which are experimentally not accessible due to an extremely sluggish kinetics. A similar 4-sublattice model has also been used for the bcc-based ordered phases (D03 Fe3Ni, B2 and B32 FeNi, D03 FeNi3), which are metastable in the Fe-Ni system, but need to be reliably modelled in order to enable extrapolations to higher order systems such as Al-Fe-Ni. Magnetic ordering, which is particularly important in this system, has also been thermodynamically described and the influence of magnetism on phase equilibria evidenced.