Virtual miscibility gap and interdiffusion coefficient in iron-nickel invar alloys

Fe-Ni Invar alloys exhibit anomalies in thermodynamic and physical properties at high temperatures. This is considered to result from the tendency to two-phase separation in the fcc phase. This idea is supported also by interdiffusion experiments in the Fe-Ni system, where the interdiffusion coefficient is anomalously small in the Invar region at lower temperatures. A simple theory of regular solutions based on a pair interaction model is applied to the Fe-Ni system; with the assumption of two gamma states this gives a miscibility gap in the Invar region.