HRTEM Study of Oxide Nanoparticles in Fe-16Cr ODS Ferritic Steel Developed for Fusion Energy

Crystal and interfacial structures of oxide nanoparticles in Fe-16Cr ODS ferritic steel fabricated by mechanical alloying (MA), hot extrusion, and heat treatment have been examined using high-resolution transmission electron microscopy (HRT EM) techniques. A partial crystallization of oxide nanoparticles was frequently observed in as-fabricated ODS steel. The crystal structure of crystalline oxide particles is identified to be mainly Y4Al2O9 (YAM) with a monoclinic structure. Orientation relationship between the oxide and the matrix are dependent on the particle size. Large nanoparticles with a diameter larger than 20 nm tend to be incoherent and have a nearly spherical shape, whereas small nanoparticles with a diameter smaller than 10 nm tend to be coherent or semi-coherent and have faceted interfaces with the matrix. The oxide nanoparticles become fully crystallized after prolonged annealing at 900 °C. These results lead us to propose a three-stage formation mechanism of oxide nanoparticles in ODS steels.