TEM examination of the effect of post-irradiation annealing on 7.7 dpa AISI 304L stainless steel

Analytical transmission electron microscopy (ATEM) was used to examine the effect of post-irradiation annealing (PIA) on the material microstructure and radiation-induced segregation (RIS) at the grain boundaries of 7.7 dpa AISI 304L stainless steel. The grain boundary profiles and the irradiation damage were analysed in the as-irradiated state and after PIA at 500°C for 6 hours, at 500°C 25 hours and at 550°C for 25 hours. In as irradiated material radiation induced segregations (RIS) observed on the grain boundaries can be characterised by Cr depletion and Ni enrichment accompanied with slight increase in Si content. The effects of PIA were found to be relatively small after only 6 hours of annealing, while after 25 hours of PIA at both 500 and 550°C, RIS were almost recovered and only marginal deviation in chemical composition could be found near the GB. None of the applied PIA treatments had caused formation of voids, while quite distinct differences were found in microstructure after different PIA treatments. After 6 hours annealing at 500°C dislocation loops start to grow, which is reflected by a few nm increase in evaluated average size of dislocation loops, while dislocation density remains of the same order of magnitude. After annealing for 25 hours at 500°C the average size of dislocation loops remains nearly the same, while dislocation density was reduced almost by one fold. In the areas where dislocation density was found to be the lowest some features, which can most likely be attributed to stacking fault tetrahedral (SFT) were found. Annealing at even higher temperature (550°C) affected the average size of the dislocation loops, making them almost twice as large as well as resulting in a very broad distribution of dislocation sizes. Density of dislocations is also reduced by one fold in comparison to the as irradiated condition and leads to formation of SFT's, which could be found in the specimen consistently.