Abstract.During 350keVAr+ irradiation at 77K, laser-deposited FeAg multilayers first show stress relaxation and demixing processes at the interfaces followed by grain coarsening and a supersaturation of the bcc -Fe phase with Ag due to ballistic mixing. At high fluences, the fcc -Fe(Ag) phase (a=3.65A) is formed, which can be explained by either chemically guided ballistic short-range relocations or by the occurrence of thermal spikes, where all atoms possess sufficient energy to allow collective structural rearrangements, but only during such a short time that a decomposition due to long-range diffusion is suppressed.