The effects of cascade remnants on Freely Migrating Defects (FMD) were studied by measuring Radiation-Induced Segregation (RIS) in Cu-l%Au at 400°C during simultaneous irradiation with 1.5-MeV He and (400-800)-keV heavy ions (Ne, Ar or Cu). The large RIS observed during 1.5-MeV He-only irradiation was dramatically suppressed under simultaneous heavy ion irradiation. For Cu simultaneous irradiation, the suppression disappeared immediately after the Cu irradiation ceased, while for simultaneous inert gas (Ne or Ar) irradiation, the suppression persisted after the ion beam was turned off. These results demonstrate that the displacement cascades created by heavy ions introduce additional annihilation sites, which reduce the steady-state FMD concentrations. As the cascade remnants produced by Cu ions are thermally unstable at 400°C, the RIS suppression occurs only during simultaneous irradiation. On the other hand, the inert gas atoms which accumulate in the specimen apparently stabilize the cascade remnants, allowing the suppression to persist.