A transmission electron microscopy study of EBR-II neutron-irradiated austenitic stainless steel 304 and nickel-base alloy X-750

The microstructure of EBR-II neutron-irradiated austenitic stainless steel 304 and nickel-base alloy X-750 was investigated. Both alloys were irradiated at low dose rates (similar to 2 x 10(-8) dpa/s) to a neutron fluence of 6.9 x 10(22) n/cm(2) (E> 0.1 MeV) at 371-389 degrees C. Different types of defects, including Frank loops, cavities, and precipitates were characterized. The Frank loops in Type 304 stainless steel (SS) are larger in size (similar to 50 nm in diameter) and lower in number density (2.58 x 10(21) m(-3)), compared to most previous higher dose rate neutron irradiation studies. The Frank loops in X-750 have an average size 26.0 nm of and a number density of 9.44 x 10(21) m(3). In 304 SS and X-750, cavities are of similar to 20 nm and similar to 14 nm in diameter, respectively. The swelling of both alloys was found to be insignificant. In 304 SS, Ni and Si were found enriched at the cavity surfaces and Ni,Si-rich precipitates were also found. Multivariate statistical analysis using non-negative matrix factorization reveals that these Ni,Si-rich precipitates contain only -5.7 at.% Si, differing from the Ni3Si gamma' precipitates found in several previous studies. In X-750, L1(2)-structured gamma' precipitates were found, and multivariate statistical analysis confirmed the 3:1 stoichiometry (Ni-3(Ti,AI)) of the gamma' precipitates and the superlattice reflections confirmed the stability of the crystal structure of these gamma' precipitates, indicating higher-than-expected precipitate stability under high-dose neutron irradiation. (C) 2019 Elsevier B.V. All rights reserved.