Elucidating Three-Dimensional Microstructural Evolution in Neutron Irradiated HT-UPS Steel

High temperature-ultrafine precipitate strengthened (HT-UPS) steel has the potential to be used as a structural material in advanced nuclear reactors. However, the response of the HT-UPS steel to neutron irradiation is not very well known. Hence, this work investigates the three-dimensional (3D) microstructural evolution of the HT-UPS steel specimens neutron irradiated to 0.003 displacements per atom (dpa), 0.03 dpa, and 0.3 dpa at 600℃.

Various neutron-irradiation-induced effects were examined via synchrotron X-ray techniques such as the X-ray absorption near edge structure spectroscopy (XANES), micro-computed tomography (µ-CT), and high-energy diffraction microscopy (HEDM). The physical and chemical stability/instability of the chemical constituents such as the Fe, Cr, and Ni was studied following neutron irradiation via XANES. Novel pre- and post-irradiation precipitate distribution evolution study for the same HT-UPS steel specimen was conducted to observe nucleation, growth, and/or ballistic dissolution of Cr₂₃C₆ precipitates via µ-CT.

Similar novel studies were also performed to understand the grain characteristics evolution via HEDM. Studies of HT-UPS steel specimens for pre- and post-annealing at 600℃ were utilized to differentiate annealing effects from irradiation effects. Overall, this research provides one of the first insights into the microstructural changes in HT-UPS steel with low fluence neutron irradiation (< 0.3 dpa), which can be used to predict the material behavior at higher fluences.