Study of the Radiation Hardening Mechanisms of Heat-Resistant Oxide Dispersion Strengthened Steels in Accelerated Tests Using Ion Irradiation and Ultramicroscopic Analysis

Nanoscale mechanisms of radiation hardening of oxide dispersion-strengthened (ODS) heat-resistant steels EP-450 ODS and EP-823 ODS have been investigated after irradiation with 5.6-MeV Fe2+ ions with varying doses of radiation damage up to 100 dpa and temperatures in the range 350–500°C. The microstructure of the original and irradiated materials has been studied by transmission electron microscopy (TEM) and atom probe tomography (APT). The strengthening of the radiation-modified layer of irradiated samples has been studied by the dynamic indentation method. Initial state analysis of the steels has showed that EP-450 ODS steel contains a larger amount of small oxide particles (up to 20 nm) compared to EP-823 ODS steel. In addition, the density of nanosized Y–Ti–Cr–O clusters in EP-450 ODS steel is ~1023 m–3, which is two orders of magnitude higher than that in EP-823 ODS steel. At low irradiation doses, EP823 ODS steel demonstrates a higher radiation hardening rate at low irradiation temperatures compared to EP-450 ODS steel, and the rate of embrittlement decreases with increasing temperature. This is largely due to the formation of nanosized radiation-induced Ni–Mn–Si clusters with a high density in EP-823 ODS steel under the impact of irradiation. Other radiation-induced changes such as the rearrangement of the system of oxides and Y–Ti–Cr–O clusters, the formation of clusters predominantly enriched in Cr, and the formation of dislocation loops have been detected. It has been found that the radiation hardening of EP-450 ODS steel increases with the irradiation dose. In general, both steels demonstrate similar hardening at high irradiation doses.