Microstructural evolution of three potential fusion candidate steels under ion -irradiation

The present paper studies the gas bubble distribution in triple beam irradiated (Fe+, He+, H+) EUROFER97, ODS-EUROFER and a 13%Cr ODS-steel. All materials were irradiated at 350 °C, 450 °C and 550 °C up to a displacement damage of 40 dpa, a helium concentration of ∼12.5 appm He/dpa and a final hydrogen content of ∼50 appm H/dpa, respectively. While most of the bubbles, with diameters up to 107 nm, in the 13%Cr ODS-steel were found to be attached to the ODS-particles at all temperatures, they showed a homogenous distribution in EUROFER97 at 350 °C. At 450 °C however, many small clusters with mainly facetted cavities were observed, whereas at 550 °C most of the bubbles were localized at microstructural sinks like grain boundaries, dislocations or precipitates. The largest bubble found in EUROFER97 had a diameter of ∼19 nm. In both materials, the largest bubbles were located in the area where, according to SRIM calculations, superposition of maximum displacement damage and highest gas concentration occurred. This observation suggests that synergetic effects between helium and hydrogen strongly influenced the bubble growth but not the bubble nucleation. Despite exhaustive analysis, no bubbles could be identified in any of the three ODS-EUROFER lamellae. Apparently, bubbles have formed only below the TEM resolution limit or the helium was evenly distributed within the grains or at the particle-matrix interface. In addition, electron energy loss spectroscopy (EELS) was used to confirm the existence of helium and hydrogen inside closed gas bubbles. © 2020