Microstructural evolution during creep of 9Cr-ODS steels

9Cr-ODS (oxide dispersion strengthened) steels are being developed for structural materials of fast breeder reactors and fusion reactors. The steels can be characterized by small Y–Ti–O complex oxide particles dispersed in the matrix. Thus, many researches have studied creep properties by focusing on oxide particles and dislocations with transmission electron microscopy (TEM) observations, but such studies are still insufficient to clarify the creep mechanism. In this work, a potential microstructural feature affecting creep properties, prior particle boundary (PPB) cavities formed during creep and aligned along the extrusion direction, was studied. Field emission gun-scanning electron microscopy (FE-SEM) observations on the crept specimens indicated PPB cavities enhanced creep crack propagation. The effect of PPB cavities on creep properties were quantitatively studied by consolidating two different sizes mechanical alloying (MA) powders (>90 and <45 μm). PPB cavities decreased when using the larger MA powder and creep properties parallel to the extrusion direction were improved. It is concluded that PPB is one of the important microstructural features affecting creep properties, where a reduction in PPB cavities signifies an improvement.