A previously published correlation between the compositional dependence of neutron-induced swelling at high temperatures in Fe-Ni-Cr alloys and the development of compositional micro-oscillations has also been found in this study of swelling induced by 5-MeV Ni⁺ ion irradiation. The tendency toward decomposition is strongest in the 35 to 45% nickel range and diminishes in the range of 60 to 75% nickel. Chromium appears to play no role other than as a surrogate for iron. The scale of the micro-oscillation appears to be more sensitive to temperature than displacement rate. One consequence of this decomposition is the occasional formation of precipitate phases upon cooling after irradiation. In Fe-35Ni, cellular martensite is found to form after irradiation at 625, 675, and 725°C. In Fe-60Ni-15Cr irradiated at 625°C an ordered phase that may be Ni³(Fe,Cr) is sometimes formed. The fortuitous formation of these phases allows the visualization of the size and spacing of the compositional modulations.