Characterization of nanosized precipitates in 9Cr-ODS steels by SAXS and TEM

Oxide dispersion strengthened (ODS) steels are the leading candidate structural materials for fast reactor and fusion reactor application due to excellent radiation tolerance and high temperature creep strength. High number density nanoscale oxides play a key role in controlling microstructure and properties. Atomized alloy powders with different ball-milling times were employed to produce 9Cr-ODS steels by hot isostatic pressing (HIP). Nanosized precipitates in 9Cr-ODS steels with different ball-milling times were characterized by synchrotron small angle X-ray scattering (SAXS) together with high resolution transmission electron microscopy (HRTEM). Grain morphology and size were observed by electron backscatter diffraction (EBSD). The effects of nanosized precipitates on grain size and mechanical properties were analyzed. SAXS and TEM results indicated that the size of Y-Ti-O-rich nano-clusters in 9Cr-ODS steels decreases with the increasing milling time, while the distribution density increases. The maximum value of distribution density is about 2.93×10²³ m^-3 in 9Cr-ODS steel ball milled for 20 h. The maximum value of distribution density of pyrochlore structure Y₂Ti₂O₇ is the highest (1.03× 10²² m^-3) in 9Cr-ODS steel ball milled for 8 h. Some large-scale Ti-Al-O-rich precipitates are observed and show core/shell structure. Their distribution density increases with ball milling time. With increasing ball milling time, the grain size decreases and the yield strength increases. The contribution of Y-Ti-O-rich nanosized precipitates to yield strength is dominated.