Abstract: Oxide-dispersion-strengthened (ODS) steels are promising candidates for advanced nuclear energy systems owing to their outstanding high-temperature creep resistance and irradiation tolerance. Herein, the effects of nano-oxide particles on the low-cycle fatigue behavior and microstructural evolution of low-activation 9Cr-ODS steels are investigated at 700 °C under strain amplitudes ranging from ±0.30 % to ±0.50 %. Two types of 9Cr-ODS steels, namely PM-ODS and NPM-ODS, are prepared to incorporate different oxide particle characteristics. PM-ODS contains oxide particles with an average size of 12.6 nm and a number density of 1.7 × 10²²/m³, while NPM-ODS contains finer particles with an average size of 6.4 nm and a higher number density of 2.6 × 10²³/m³ before fatigue testing. NPM-ODS exhibits a nearly stable cyclic response at all strain amplitudes, while cyclic softening occurs in PM-ODS at ±0.50 %. The peak stress in NPM-ODS is 90-120 MPa higher than that in PM-ODS across all strain amplitudes. Notably, the fatigue life of NPM-ODS at ±0.30 % is nearly an order of magnitude longer than that of PM-ODS. This improvement is attributed to its one order of magnitude higher particle number density, 50 % smaller particle size, and more uniform oxide distribution. These characteristics effectively restrict dislocation motion and suppress grain/subgrain coarsening, thereby enhancing back stress and fatigue resistance. Alternatively, PM-ODS shows considerable microstructural recovery owing to the weaker pinning effect of oxides, resulting in a much shorter fatigue life. These results suggest that the high-temperature fatigue performance of ODS steels can be effectively improved by optimizing oxide particles.

Key words: ODS steels, LCF, Cyclic softening, Nano-oxide particles, Back stress