Microstructural development of a heavily neutron-irradiated ODS ferritic steel (MA957) at elevated temperature

Yamashita, Shinichiro   ; Akasaka, Naoaki; Ukai, Shigeharu  ; Onuki, Somei*

Microstructural observation was done on a neutron-irradiated oxide dispersion strengthened (ODS) ferritic steel, MA957. Since MA957 has been investigated from various viewpoints, special emphases in this study were laid on oxide behaviors including phase stability under irradiation at elevated temperature (973 K). Transmission electron microscopy (TEM) observation of the Y-Ti complex oxide particles showed they were fine (40 nm) whereas the Ti-oxide particles were relatively coarse (300 nm). Dispersion parameters of oxide particles, such as mean size and number density, changed due to irradiation. This fact implies that the recoil resolution of the oxide particles. When irradiated at 973 K, some Y-Ti complex oxides were surviving and interacted with the dislocation structures, which delayed the dislocation recovery and consequently stabilized the elongated grain structures. This is the first evidence showing that oxide particles are effectively functioning as pinning points of dislocations in motion under irradiation to a dose of 100 dpa.