Ion-irradiation effects on dissimilar friction stir welded joints between ODS alloy and ferritic stainless steel

•FSSW has successfully been used in the welding of dissimilar materials.•The irradiation causes different degrees of hardening in the welding zones.•The formation of He bubbles at precipitates was found in the dissimilar joints.•The hardening effect is due to formation of He-filled vacancies. Friction stir spot welding (FSSW) is an advanced technique for the joining of materials to prevent agglomeration of fine oxide particles, grain coarsening, and stress corrosion cracking etc. In this study, the dissimilar FSSW joint of stainless steel 430/ODS was irradiated with a Fe+/He+ dual ion beam. Irradiation damage can cause deterioration in the mechanical properties especially in the welding zones. The joint quality therefore plays a decisive role in the life expectancy of nuclear reactors. The effect of irradiation on different zones in the joint (the thermo-mechanically affected zone, the heat affected zone and the base material) was investigated by TEM and nanoindentation. Irradiation causes a hardness increase in all welding zones with a characteristic hardness maximum. The relative hardness increase and the related microstructure are discussed. The formation of He bubbles at chromium carbide precipitates and the homogeneous distribution of He filled vacancies in the mixture region of the 430/ODS FSSW joints was observed.