Elevated temperature irradiation damage in CANDU spacer material Inconel X-750

Heavy ion irradiation induced damage in Inconel X-750 at low temperatures (60–400 °C) has been reported in our previous study. In the current investigation, the microstructure evolution and phase change during heavy (1 MeV Kr2+) irradiation at elevated temperatures (500 °C and 600 °C) were characterized under in situ observation of intermediate voltage electron microscope (IVEM) at Argonne National Laboratory. For each temperature, defect analyses using the weak beam dark field method were carried out at several doses, up to 5.4 dpa. Small defects (<5 nm) yielded from high temperature irradiation comprise mainly stacking fault tetrahedras (SFTs), small ⅓ 〈1 1 1〉 and ½ 〈1 1 0〉 type dislocation loops. Large interstitial Frank loops were observed and a clear characteristic for growth of loops was video-captured. Unfaulting of interstitial Frank loops was observed. The number density of the defects saturated at a relatively low dose of 0.68 dpa. No obvious change of defect fraction was found with increasing dose, but more complex dislocation structures formed at higher doses. In contrast to low temperature irradiation, the primary strengthening phase γ′ was found to be stable during irradiation at temperatures >500 °C and was not disordered up to 5.4 dpa. No cavities were observed after the irradiation even at 600 °C.