Three nickel alloys, Inconel X750, Inconel 600, and Inconel 625, and a 20 percent cold-worked Type 316 stainless steel were bombarded with 5-MeV nickel ions at 650°C (1202°F) to damage levels up to 90 displacements per atom. Post bombardment examination was carried out using 1-MeV transmission electron microscopy to image the entire range of ion damage. The evolution of the microstructure as a function of depth and displacement damage was determined by stereo microscopy. Despite variations in base composition and initial microstructure the three nickel alloys behaved in a similar fashion and exhibited an order of magnitude less swelling than the Type 316 stainless steel. In Inconel X750 containing a dispersion of γ′, loss of coherency of the γ′ particles occurred at very low doses at 550, 600, and 650°C (1022, 1112, and 1202°F). At 600 and 650°C the irradiation induced loop structure was controlled by the initial distribution of γ′, and further precipitation of fine γ′ occurred in the rest of the matrix.