On the Phase Mixtures Produced by Annealing of Long Duration in the γ-Phase Fe–Ni Invar Alloys

Abstract

As well known, the two-phase transformation γ→α+γ is extremely difficult to cccur in the Fe–Ni system, leading to an enormous thermal hysteresis. The nature of this unusual process as well as of the 36%Ni–Fe Invar alloy were studied by annealing the γ-phase Invar alloys in the two-phase region for long duration. A “tail part” in the magnetization(M)-temperature(T) curve was found to grow through three stages; first, a reversible tail becomes large, and then it begins to show a thermal hysteresis, finally it tends to disappear in the third stage. The X-ray spectra suggest that at first an imperfect γ→α lattice transformation occurs locally, and then the diffusion of concentration seems to start, where the distortion of the lattice becomes large due to local transformation. It is likely that the 36%Ni–Fe alloy contains a region corresponding to the tail in the M–T curve.