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 (∼2 × 10⁻⁸ dpa/s) to a neutron fluence of 6.9 × 10²² n/cm² (E > 0.1 MeV) at 371–389 °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 (∼50 nm in diameter) and lower in number density (2.58 × 10²¹ 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 × 10²¹ m⁻³. In 304 SS and X-750, cavities are of ∼20 nm and ∼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 γ’ precipitates found in several previous studies. In X-750, L1₂-structured γ’ precipitates were found, and multivariate statistical analysis confirmed the 3:1 stoichiometry (Ni₃(Ti,Al)) of the γ’ precipitates and the superlattice reflections confirmed the stability of the crystal structure of these γ’ precipitates, indicating higher-than-expected precipitate stability under high-dose neutron irradiation.