Microstructural evolution of neutron irradiated ultrafine-grained austenitic stainless steel

• This paper examines the evolution of microstructure at varying length scales of ultrafine-grained 316 stainless steels prior to and following neutron irradiation to 3.9 dpa under representative irradiation conditions in a light water reactor. The investigation of different length scales was conducted using electron backscatter diffraction, transmission electron microscopy, and atom probe tomography. • A comparative analysis of the microstructure conducted before and after irradiation indicated that neutron irradiation at 365 °C resulted in an increase in grain size. • Intragranular segregation, as evidenced by the number density of frank loops, cavities, and clusters, was observed to be lower in the ultrafine-grained than in the coarse-grained 316 stainless steels. • Radiation-induced segregation at the grain boundaries of Cr, Ni, and si was found to be slightly higher in the ultrafine-grained than in the coarse-grained 316 stainless steels after neutron irradiation. Austenitic stainless steels utilized in-core components of pressurized water reactors are prone to radiation-induced segregation, which leads to the degradation of microstructure and mechanical properties. To improve irradiation resistance, one possible solution is to increase the number density of point defect sinks, such as grain boundaries. For this purpose, ultrafine-grained or nanostructured microstructures are recommended due to their high density of grain boundaries. This paper investigates the microstructural changes in ultrafine-grained 316 austenitic stainless steel exposed to neutron radiation up to 3.9 dpa in irradiation conditions representative of light water reactors. The microstructure at different length scales was analyzed using electron backscattered diffraction, transmission electron microscopy, and atom probe tomography before and after neutron irradiation. The study compares its findings with those of existing literature on coarse-grained austenitic stainless steels to evaluate the benefit of ultrafine-grained 316 austenitic stainless steels regarding irradiation ageing in representative conditions of light water reactors.