Preferential Void Swelling Along Cellular Walls in Additively Manufactured 316L Stainless Steel Under Fe Self-Ion Irradiation

Additively manufactured (AM) 316L stainless steel fabricated by laser powder bed fusion was irradiated with 5 MeV Fe self-ions to 50 peak displacements per atom at 600 °C to evaluate its swelling resistance in reactor-relevant environments. Cross-sectional transmission electron microscopy (TEM) revealed large voids with diameters up to 140 nm that were preferentially aligned rather than randomly distributed. The spacing between adjacent void strings was approximately 500 nm, matching the cellular wall structure present prior to irradiation. It is hypothesized that the high dislocation density associated with these walls acts as a strong yet biased sink for interstitials, promoting local vacancy accumulation and enhanced void formation. This facilitated nucleation also reduces the influence of injected interstitials, resulting in a wider safe analysis zone than typically observed during heavy-ion irradiation. Swelling measurements extracted from this region show an approximately linear dependence on local damage level, corresponding to a swelling rate of ~1% per dpa with little or no incubation period. The results indicate that the cellular structures play an important role in void swelling. The results demonstrate that the retained cellular structure plays a dominant role in void swelling evolution in AM 316L stainless steel. Therefore, post-build stress-relief treatments should be carefully optimized to reduce cellular dislocation density and enhance the swelling resistance of AM 316L for nuclear reactor applications.