Effect of friction stir welding and self-ion irradiation on dispersoid evolution in oxide dispersion strengthened steel MA956 up to 25 dpa

The effect of ion irradiation on the microstructure of oxide dispersion strengthened (ODS) MA956 steel, before and after friction stir welding (FSW), was studied. Both the base material (BM) and welded stir zone (SZ) were irradiated with 5 MeV Fe++ ions at 450 °C up to 25 displacements per atom (dpa). Characterization was performed using scanning transmission electron microscopy (STEM) and atom probe tomography (APT), with particular emphasis on the YAlO dispersoid characteristics and dislocation microstructures. After irradiation, the dispersoids in the BM increased in diameter and decreased in number density, which was explained by an Ostwald ripening mechanism. FSW caused significant coarsening and agglomeration of the dispersoids. After irradiation, both the diameter and number density of the SZ dispersoids increased, which was explained by an irradiation-enhanced diffusion mechanism. Dislocation loop and network behavior was also characterized and large dislocation loops of ≈20 nm diameter formed by 1 dpa in both the BM and SZ samples, whereas the network density remained nearly constant with irradiation.