Application of slow positron beam for study of commercial oxide-dispersion-strengthened steels

Microstructure of four different commercial oxide-dispersion-strengthened steels (MA 956, ODM 751, MA 957 and ODS Eurofer) with different chromium content was studied in term of a presence of vacancy defects and their accumulation due to irradiation. The radiation damage (∼45 dpa) was simulated by an implantation of helium ions with kinetic energy of 500 keV and the implantation depth around 1.2 μm. Positron Doppler broadening spectroscopy with a slow positron beam was applied for detection of a defect depth profile (in layers up to 1.6 μm) and observation of radiation resistance of the investigated steels. Samples of the steels were also measured by positron annihilation lifetime spectroscopy in order to find out some change in ODS structure due to the radiation damage, although a sensitivity of this technique is very low in the implanted zone. Results of the Doppler broadening spectroscopy visible demonstrated a defect accumulation during the implantation. It showed that ODS Eurofer is the most radiation resistant from the group of all investigated steels. This was also indicated by a mean lifetime of the positron annihilation lifetime spectroscopy, which further demonstrated alloys MA 956 and ODM 751 as the most radiation affected materials. The results from the positron techniques (mostly Doppler broadening spectroscopy) can indicate an existence of a slight relation of the radiation damage and the content of the most significant alloying element in the samples – chromium. The radiation damage expressed as an increase of S parameters and an increase of positron mean lifetimes demonstrated a small growth along with the increase of the chromium content.