A comparison study of change in hardness and microstructures of a Zr-added FeCrAl ODS steel irradiated with heavy ions

FeCrAl oxide dispersion strengthened (ODS) steel is regarded as one of the most promising candidate materials for nuclear fission reactors such as supercritical pressurized water reactor (SCPWR), sodium-cooled fast reactor (SFR), and lead bismuth-cooled fast reactor (LFR) and so on, since it has excellent irradiation resistance and good corrosion resistance of super-critical water and Pb–Bi coolants. In the present work, irradiation response of a Zr-added FeCrAl ODS steel (15Cr–4Al-0.1Ti-0.6Zr) together with an Al-free ODS (16Cr-0.1Ti) steel and a non-ODS steel T91 was studied. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the microstructures including grains, dislocations, and oxides in the pristine specimens and irradiation defects in the irradiated specimens. Nano-indentation tests were used to evaluate the change in hardness after irradiation. The oxides in the 15Cr–4Al-0.1Ti-0.6Zr specimen are slightly larger than those in the 16Cr-0.1Ti specimen, but with a similar number density. After irradiated with 9.45 MeV 83Bi35+ to 26 dpa at ambient temperature, hardening was observed in all the specimens. Irradiation hardening of the 15Cr–4Al-0.1Ti-0.6Zr specimen is slightly higher than the 16Cr-0.1Ti specimen but significantly lower than the non-ODS steel T91. The irradiation defects in the ODS steel specimens are apparently smaller than those in T91 with a similar number density. By calculating sink strength of the initial microstructures such as grains, dislocations and oxides, the evolution of irradiation defects was discussed. A linear relationship between the irradiation hardening and the sink strength was established. A similar linear relationship between the calculated irradiation hardening using the TEM data (based on dispersion barrier hardening model (DBH) and Friedel-Kroupa-Hirsch (FKH) model, respectively) and sink strength was also found. The calculated irradiation hardening is relatively lower than the experimental value, indicating the possible contribution of Cr segregation to hardening which needs further study.