Helium bubble distributions in a nanostructured ferritic alloy

► A 14YWT NFA was implanted with He ions to fluences of 6.75 × 1021 He m−2 at 400 °C. ► He bubbles were observed in the ferrite matrix and on grain boundaries and dislocations. ► A polydisperse distribution of bubble sizes was observed in the ferrite matrix. ► He bubbles were found on the surfaces of nanoclusters and Ti(C,N) precipitates. ► Microstructural defect “traps” might reduce the availability of He, and susceptibility to He embrittlement.