Effects of neutron irradiation and post-irradiation annealing on the microstructure of HT-UPS stainless steel

Microstructural changes resulted from neutron irradiation and post-irradiation annealing in a high-temperature ultra-fine precipitate strengthened (HT-UPS) stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Three HT-UPS samples were neutron-irradiated to 3 dpa at 500 degrees C, and after irradiation, two of them were annealed for 1 hat 600 degrees C and 700 degrees C, respectively. Frank dislocation loops were the dominant defect structure in both the as-irradiated and 600 degrees C post-irradiation-annealed (PIAed) samples, and the loop sizes and densities were similar in these two samples. Unfaulted dislocation loops were observed in the 700 degrees C PIAed sample, and the loop density was greatly reduced in comparison with that in the as-irradiated sample. Nano-sized MX precipitates were observed under TEM in the 700 degrees C PIAed sample, but not in the 600 degrees C PIAed or the as-irradiated samples. The titanium-rich clusters were identified in all three samples using APT. The post-irradiation annealing (PIA) caused the growth of the Ti-rich clusters with a stronger effect at 700 degrees C than at 600 degrees C. The irradiation caused elemental segregations at the grain boundary and the grain interior, and the grain boundary segregation behavior is consistent with observations in other irradiated austenitic steels. APT results showed that PIA reduced the magnitude of irradiation induced segregations. (C) 2018 Elsevier B.V. All rights reserved.