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 such as the physical and chemical stability/instability of the chemical constituents, precipitate formation and evolution, and the evolution of grain characteristics were examined via synchrotron X-ray techniques. The techniques include X-ray absorption near edge structure spectroscopy, micro-computed tomography, and high-energy diffraction microscopy.

It was found that the Cr atoms are affected by neutron irradiation form radiation enhanced Cr₂₃C₆ precipitates. In contrast, Fe and Ni atoms demonstrated irradiation resistance with no change in local atomic structure. Novel pre- and post-irradiation precipitate distribution evolution study for the same HT-UPS steel specimen demonstrated a reduction in the average precipitate size, predominantly from the nucleation, growth, and/or ballistic dissolution of Cr₂₃C₆ precipitates. These phenomena occurred at very low neutron irradiation fluences, ranging from 0.003 dpa up to 0.3 dpa, which were at least an order of magnitude lower than previous studies. Also, the precipitates were homogenously distributed throughout the sample in HT-UPS steels in both pre- and post-irradiation and pre- and post-annealing conditions.

A unique 3D pre- and post-irradiation study of grain characteristics for the same HT-UPS steel specimen revealed a shift in grain size distribution to a smaller size, the possibility of recrystallized grain formation, and irradiation defect structure induced diffraction spot broadening. Studies of HT-UPS steel specimens for pre- and post-annealing at 600℃ were utilized to differentiate annealing effects from irradiation effects. Annealing reduced plastic strain and mosaicity in the grains indicated by diffraction spot sharpening, whereas the irradiated specimens exhibited diffraction spot broadening indicating the introduction of neutron irradiation induced defects into the grains. Furthermore, the presence of MX (M: Ti, V, Nb; X: C, N) precipitates and dense twin networks were identified in both the pre- and post-irradiation and pre- and post-annealing conditions. In addition to highly efficient defects sinks in the form of Cr₂₃C₆ and MX precipitates, the studies also revealed multiple other defect sinks, including relatively less efficient special boundaries, such as the Σ3 and Σ9 grain boundaries, and relatively highly efficient random grain boundaries. The HT-UPS steel demonstrated resistance to radiation damage due to the chemical and physical stability of Fe and Ni atoms and relative grain structure stability even though the grains indicated the presence of irradiation-induced defects in the form of diffraction spot width broadening with irradiation. Although Cr atoms destabilize and form Cr₂₃C₆ precipitates; the precipitates impart resistance to further damage by acting as defect sinks during irradiation. 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.