The microstructure evolution and irradiation hardening in 15Cr-ODS steel irradiated by helium ions

Oxide dispersion strengthened (ODS) steels with excellent irradiation swelling resistance and high temperature creep resistance are regarded as one of the priority candidate materials for the first wall/blanket materials in fusion reactors. Microstructure evolution and irradiation hardening in 15Cr-ODS steel irradiated by helium ions at R.T. and 400 °C respectively have been studied by transmission electron microscopy and nanoindentation technique. Three main oxides were identified, including high-density dispersed Y-Ti-O nanoclusters, tens of nanometers Y2TiO5 and a few hundred nanometers Y2Ti2O7 particles with the largest size but the least density. Helium bubbles were homogeneously distributed in the matrix in both irradiated specimens and the aggregation state of bubbles near GBs was related to the nature of GB. The average size of bubbles increased and the density decreased in the steel irradiated at elevated temperature. A defect-depleted zone was observed near a GB with a Y2TiO5 precipitate imbedded and a large helium bubble was present at the interface of the Y2TiO5 particle/GB. It suggests that the GBs combined with Y2TiO5 can enhance the aggregation of helium bubbles since GBs can act as defect sinks as well as diffusion channels and Y2TiO5 has a high ability of trapping helium atoms. Significant irradiation hardening was obtained at both irradiation temperatures. The relationship between microstructure and the irradiation hardening was discussed.