Quantitative characterization of microstructural defects in up to 32 dpa neutron irradiated EUROFER97

The microstructure of the neutron-irradiated reduced activation ferritic/martensitic (RAFM) steel EUROFER97 was evaluated by transmission electron microscopy (TEM). Emphasis was put on analyzing the influence of the irradiation dose on the evolution of size and density of microstructural defects like dislocation loops and voids at low irradiation temperatures of 330–340 °C. To study the dose dependence, samples irradiated to 15 and 32 dpa were analyzed. The weak-beam dark-field (WBDF) technique was applied to analyze dislocation loops and small defect clusters using different diffraction conditions. The average densities and sizes of the defects increase slightly from 1.4 × 1022 m−3 and 3.4 nm at 15 dpa to 1.7 × 1022 m−3 and 4.8 nm at 32 dpa. Through-focus series also revealed the presence of small voids in the material, but with a density at least one order of magnitude lower than that of the dislocation loops. In order to correlate the irradiation induced changes in the microstructure to the changes in the mechanical properties, the obtained quantitative data was used to estimate dose-dependent hardening with the dispersed barrier hardening model. The estimation of hardening by using recent literature results on the loop obstacle strength shows, that alone the defects visible in the TEM are not sufficient to explain the hardening quantified in the post-irradiation tensile tests.