Atom probe tomography characterization of high-dose ion irradiated MA957

Because the microstructure of oxide dispersion strengthened (ODS) alloys largely determines its mechanical properties in nuclear reactors, it is critical to understand the irradiation response of microstructure after high-dose radiation damage for safety and performance concerns. This paper presents a comprehensive characterization of microstructural responses to ion irradiation of the ODS ferritic alloy MA957 using atom probe tomography and compares the observations to current theories on mechanisms for microstructural evolution during irradiation. The MA957 specimens were irradiated by 1.8 MeV Cr3+ ions to doses and at temperatures relevant to advanced fast reactors (100 dpa and 500 dpa at 400–500°C). The stability of YTiO particles, formation of alpha prime (α′), and radiation-induced segregation were quantified and analyzed. The results show that the evolution of YTiO particles with respect to dose and temperature is consistent with the Nelson-Hudson-Mazey rate theory model that considers the competing effects of recombination and ballistic dissolution. The formation of Cr-rich precipitates (αʹ) was observed in specimens irradiated at temperatures up to 450°C, and it shows a strong dependence on temperature. Irradiation-induced segregation on grain boundaries was investigated in the 100 dpa specimens. In addition to the commonly studied element Cr, this study also analyzed segregation of Y and Ti. While the magnitude of segregation varies due to intrinsic variability of grain boundary character, clear trends with respect to irradiation temperature were observed. Overall, these results revealed relatively small but distinct effects of irradiation on microstructure evolution to 100 or 500 dpa, demonstrating the excellent radiation tolerance of ODS alloys.