Transmission electron analysis of dislocation loops in T91 steels from MEGAPIE and MIRE irradiation experiments

T91 steel samples were irradiated in both a neutron and a neutron plus proton irradiation environments, and the resulting microstructure was studied using transmission electron microscopy. Three samples were irradiated with neutrons and protons to a cumulative dose of 0.88, 2.07, and 4.35 dpa at temperatures of 254 °C, 260 °C and 320–350 °C, respectively and two samples were irradiated with neutrons to 0.6 and 1 dpa, at a temperature of 290 °C for comparison. Radiation induced dislocation loops were observed in both groups of samples. In all samples irradiated up to 1 dpa, irrespective of irradiation conditions, dislocation loops were segregated near the line dislocations, forming a clear heterogeneous distribution. In the high dose sample, on the other hand, they were uniformly distributed over the grain interior and the middle dose sample showed a mixed character. Voids of similar average size were only observed in the middle and high dose samples, and their appearance seems to be correlated with the onset of homogeneous loop distribution. The observed defect properties are rationalized on the basis of modeling results.