Formation mechanism and the role of nanoparticles in Fe-Cr ODS steels developed for radiation tolerance

Structures of nanoparticles in Fe-16Cr-4.5Al-0.3Ti-2W-0.37Y(2)O(3) (K3) and Fe-20Cr-4.5Al-0.34Ti-0.5Y(2)O(3) (MA956) oxide dispersion strengthened (ODS) ferritic steels produced by mechanical alloying (MA) and followed by hot extrusion have been studied using high-resolution transmission electron microscopy (HRTEM) techniques to understand the formation mechanism of nanoparticles in MA/ODS steels. The observations of Y-Al-O complex-oxide nanoparticles in both ODS steels imply that decomposition of Y(2)O(3) in association with internal oxidation of Al occurred during mechanical alloying. While the majority of oxide nanoparticles formed in both steels is Y(4)Al(2)O(9), a few oxide particles of YAlO(3) are also observed occasionally. These results reveal that Ti (0.3 wt %) plays an insignificant role in forming oxide nanoparticles in the presence of Al (4.5 wt %). HRTEM observations of crystalline nanoparticles larger than similar to 2 nm and amorphous or disordered cluster domains smaller than similar to 2 nm provide an insight into the formation mechanism of oxide nanoparticle in MA/ODS steels, which we believe from our observations involves solid-state amorphization and recrystallization. The role of nanoparticles in suppressing radiation-induced swelling is revealed through TEM examinations of cavity distributions in ion-irradiated Fe-14Cr and K3-ODS ferritic steels. HRTEM observations of helium-filled cavities (helium bubbles) preferably trapped at nanoscale oxide particles and clusters in ionirradiated K3-ODS are presented.