Analysis of Anomalous Thermal Expansion Coefficient of Fe–Ni Invar Alloys

Thermal expansion coefficients of Fe–Ni alloys were analyzed by using the Grüneisen equation. The anomalous thermal expansion coefficient term, α a , was calculated by subtracting the lattice vibration term described by Grüneisen theory from the experimental data. The anomalous volume change obtained by integrating α a was analyzed in terms of the two-states model of iron, the energy separation of which is assumed to be a function of the exchange field and temperature. The temperature dependence of the energy splitting is in good agreement with that of the spontaneous magnetization.