Secondary phase precipitates (SPP) play an important role of controlling mechanical property of HT9, a 12Cr ferritic-martensitic steel aimed for applications in next-generation nuclear reactors. Understanding of irradiation-induced SPP could help predict HT9 performance and is pursued through experimental studies and theoretical modeling. However, there is inconsistency in literature about irradiation-induced SPP phases. Either one or a few SPPs of α’, G-phase, χ, M6C and Laves phases have been reported under similar nominal irradiation conditions, which may be attributed to difference in HT9 heats, thermal history, or irradiation conditions among different facilities. Experimental studies of spatial relationship between SPP and other microstructures are rare. On the other hand, the damage rate of neutron irradiation is orders of magnitude lower than that of ion irradiation. This damage rate difference is likely to shift the temperature range at which neutron and ion irradiation will form specific SPPs. Therefore, an experiment on the same HT9 heat irradiated by the same facilities is needed to achieve a clear understanding of temperature/damage rate effect to microstructure evolution and help link modeling and experiments.
We propose to examine SPP in HT9 (AC0-3 from Los Alamos National Laboratory), including four samples irradiated at BOR-60 fast reactor in Russia at 376 °C, 415 °C, 426 °C, 524 °C, and three samples irradiated at the University of Michigan with Fe and He ions at 445 °C, 460 °C and 570 °C to 15 – 20 dpa. The neutron and ion irradiated samples chosen for this work were irradiated to similar levels of damage, but the ion irradiated materials were irradiated at higher temperatures, as based on Mansur’s temperature shift theory, to produce comparable microstructures between them. Transmission electron microscopy in conjunction with x-ray energy dispersive spectroscopy study will be employed to measure SPPs, cavities, dislocation loops, and irradiation-induced segregation at grain boundaries. Because the samples are already available, we expect to start the characterization experiments in September to October 2019, analyze the data, conduct the second experiment to complete the data acquisition in January to February 2020. The result will be a systematic data set of microstructures in HT9. It may help answer two fundamental questions about SPP: the temperature shift under irradiation, and whether there is spatial correlation between SPP and other microstructures.