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

Abstract

Structures of nanoparticles in $\text{Fe-}16\text{Cr-}4.5\text{Al-}0.3\text{Ti-}2\text{W-}0.37{\text{Y}}_{2}{\text{O}}_{3}$ (K3) and $\text{Fe-}20\text{Cr-}4.5\text{Al-}0.34\text{Ti-}0.5{\text{Y}}_{2}{\text{O}}_{3}$ (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 ${\text{Y}}_{2}{\text{O}}_{3}$ in association with internal oxidation of Al occurred during mechanical alloying. While the majority of oxide nanoparticles formed in both steels is ${\text{Y}}_{4}{\text{Al}}_{2}{\text{O}}_{9}$, a few oxide particles of ${\text{YAlO}}_{3}$ are also observed occasionally. These results reveal that Ti $(0.3\text{ }\text{wt}\text{ }\mathrm{%})$ plays an insignificant role in forming oxide nanoparticles in the presence of Al $(4.5\text{ }\text{wt}\text{ }\mathrm{%})$. HRTEM observations of crystalline nanoparticles larger than $\ensuremath{\sim}2\text{ }\text{nm}$ and amorphous or disordered cluster domains smaller than $\ensuremath{\sim}2\text{ }\text{nm}$ 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.