Origin of sub-surface swelling peaks in ion irradiation and their mitigation through dose rate control

Sub-surface swelling, occasionally observed in ion-irradiated metals, refers to a swelling peak near the surface, just beyond a void-denuded zone, and significantly shallower than the damage peak. This study shows that such swelling is an artifact of ion irradiation and should not be used to infer neutron-induced swelling behavior. This phenomenon typically occurs under low temperatures and high dose rates. Pure Fe was selected as a model system and irradiated with 5 MeV Fe²⁺ ions at three temperatures (425, 475, and 525 °C), three damage levels (50, 100, and 150 dpa), and three dose rates (6 × 10⁻³, 1.2 × 10⁻³, and 2 × 10⁻⁴ dpa/s). At 425 °C, sub-surface swelling was observed at the highest dose rate but disappeared at the lowest rate, where swelling more closely followed the displacements per atom (dpa) profile. A multiphysics model reproduced the observed shift in void nucleation from a surface peak to a deeper peak near the damage maximum as the dose rate decreased. This shift is attributed to increased defect loss to sinks relative to recombination, which reduces sensitivity to interstitial excess arising from implanted atoms and the spatial distribution differences between interstitials and vacancies. These findings provide useful best practices to enhance the credibility of ion irradiation as a rapid testing method for emulating neutron damage in reactors.