The effect of a helium implantation schedule on the evolution of the cavity microstructure in Type 316 stainless steel and Fe-20Ni-15Cr alloy during heavy-ion irradiation was investigated for damage levels up to 100 displacements per atom (dpa) with three helium injection schedules, that is, 15 atomic parts per million (appm) He/dpa, 50 appm He/dpa, and 15 appm He pre-injected prior to Ni⁺ ion irradiation. In the case of the dual-ion irradiated specimens, there was a trend towards a saturation of the swelling with increasing damage level that was coincident with the saturation and a subsequent decrease of the number density of the small cavities (<8 nm diameter for Type 316 stainless steel and <4 nm for Fe-20Ni-15Cr) and the dislocation density. The bi-modal cavity size distribution that was determined for the Fe-20Ni-15Cr alloy at 40 dpa changed to uni-modal distribution at 65 and 100 dpa.