[en] Electron irradiation is used as a means of accelerating the diffusion in FeNi alloys. Moessbauer effect, X-ray and electronic microscopy experiments on samples with 28 to 50% Ni show that the ''Invar character'' disappears after irradiation up to 250⁰C: in particular the variation of the parameter versus temperature becomes linear around room temperature. At the same time, two phases appear obviously in the alloy, one rich in nickel and ordered with a FeNi (AuCu) superstructure, the other rich in iron and probably ordered (Fe₃Ni). The invar state is therefore showed to be a metastable state. A diagram of the iron nickel alloy is proposed