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-0.37 Y₂ O₃ (K3) and Fe-20Cr-4.5Al-0.34Ti-0.5 Y₂ O₃ (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 Y₂ O₃ in association with internal oxidation of Al occurred during mechanical alloying. While the majority of oxide nanoparticles formed in both steels is Y₄ Al₂ O ₉, a few oxide particles of YAlO₃ 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 ∼2 nm and amorphous or disordered cluster domains smaller than ∼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 ion-irradiated K3-ODS are presented.