Microstructure of the F82H martensitic steel irradiated in STIP-II up to 20 dpa

The F82H martensitic steel was irradiated in the STIP-II experiment in SINQ up to 20 dpa in a temperature range of 110–400 °C. Transmission electron microscope observations have been performed to study the effects of radiation on the microstructure. The results show that at ⩽350 °C, the size of defect clusters or dislocation loops increases with both dose and temperature. The density of defect clusters or dislocation loops increases with increasing dose but decreases with temperature. A high-density of small He bubbles are visible in samples irradiated at ⩾165 °C with ⩾750 appm He. The bubble size increases while the density decreases with increasing irradiation temperature. These results agree with the previous observations of STIP-I specimens. In a specimen irradiated to 20.3 dpa with 1800 appm He at 400(±50) °C, large voids up to 50 nm in size were detected. The bubbles showed a bimodal size distribution. The martensite lath structure was almost completely removed by radiation. Meanwhile, the M23C6 precipitates along prior martensite lath and austenite boundaries disappeared and new M23C6 precipitates were formed in the matrix of new grains. In all cases, no obvious segregation of bubbles on grain boundaries was observed. However, it was detected that helium bubbles locate preferentially at pre-existing dislocations.