Anomalous Decrease of Thermal Expansion Due to Inhomogeneity in Fe–Ni Invar Alloy

Abstract

In order to investigate the effect of inhomogeneity on the Invar anomaly, the Mössbauer effect, the temperature dependence of magnetization and the thermal expansion were measured for an Invar alloy with various degrees of inhomogeneity which was introduced by heating a foil of 30.3% Ni–Fe, electroplated with nickel, at 1000°C for various periods of time. An analysis of the peculiar shapes of the Mössbauer spectra and the magnetization vs temperature curves reveals that there exists a very large fluctuation of magnetic moment and Curie temperature in the specimen. Furthermore, the thermal expansion is negative or zero in a wide temperature range and exhibits anisotropy. These observations indicate that the inhomogeneity enhances the Invar anomaly. These results are discussed on the basis of the thermodynamical relation between the thermal expansion coefficient and the internal stress piled up in the specimen.