Microstructure, hardness and defect structure of the He irradiated ODS ferritic steel

In this paper, the microstructure, hardness and defect structure of the oxide dispersion strengthened Fe-16Cr-2W-0.5Ti-0.4Y2O3 (designated as ODS-16Cr) and Fe-16Cr-2W-0.5Ti-0.4Y2O3-4Al-1Zr (ODS-16Cr-Al/Zr) steels irradiated with different doses of He ions were investigated by means of TEM, nanoindentation and positron annihilation spectroscopy. In the both alloys, nano-scale bubbles appeared in the vicinity of the precipitates as well as the grain boundaries. High density dislocation networks were observed as well. Moreover, the hardness of the irradiated steels increased distinctly with increasing He implantation. The irradiation hardening in the ODS-16Cr-Al/Zr alloy is more prominent in comparison with the ODS-16Cr alloy. The positron annihilation results illustrated that concentration of vacancy clusters of ODS-16Cr-Al/Zr steel is higher in the near surface under the larger dose, which is one reason of stronger irradiation hardening below the surface of 300-400 nm. The higher concentration of vacancies or vacancy clusters in ODS-16Cr-Al/Zr samples can be attributed to the fact that the solute Al atoms can effectively trap interstitials, which suppresses the annihilation of interstitials with vacancies.