Electron irradiation effects on iron-nickel invar alloys

Electron irradiation was used as a means of accelerating the diffusion in Fe-Ni alloys. Mössbauer effect, X-ray and electron microscopy experiments on samples with 28 to 50% Ni show that the Invar character disappears after irradiation up to 250°C, in particular the variation of the lattice parameter versus temperature becomes linear around room temperature. At the same time, two phases appear in the alloy, one rich in nickel and ordered with FeNi (AuCu) superstructure, the other rich in iron and probably ordered (Fe 3Ni). The Invar state is therefore shown to be a metastable state. A diagram of the Fe-Ni alloy is proposed.