The impact of irradiation temperature on the microstructure of F82H martensitic/ferritic steel irradiated in a proton and neutron mixed spectrum

F82H low-activation martensitic steel was irradiated in the Swiss Spallation Neutron Source (SINQ) Target-III to irradiation doses of 10–12 dpa in a temperature range of 140–360 °C. Transmission electron microscope observations have been performed to study the radiation effects on the microstructure at different irradiation temperatures. The results show: (1) High-density helium bubbles of size ⩾1 nm are observed in the samples irradiated at temperatures ⩾175 °C. The size of bubbles increases with increasing irradiation temperature, but the density remains almost constant. (2) The irradiation temperature is the controlling parameter for the amorphization of the precipitates in F82H steel. Present work indicates that the amorphous temperature for typical precipitates (M23C6) is about 235 °C, corresponding to about 0.3Tm (Tm is the absolute melting point temperature) of the steel. (3) With increasing irradiation temperature, the mean size of the defect clusters remains almost constant below 235 °C, but increases rapidly at higher temperatures. The density of defect clusters shows no significant change up to 255 °C, and then decreases rapidly above this temperature.