HRTEM study of oxide nanoparticles in K3-ODS ferritic steel developed for radiation tolerance

Crystal and interfacial structures of oxide nanoparticles and radiation damage in 16Cr–4.5Al–0.3Ti–2W–0.37 Y2O3 ODS ferritic steel have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. Oxide nanoparticles with a complex-oxide core and an amorphous shell were frequently observed. The crystal structure of complex-oxide core is identified to be mainly monoclinic Y4Al2O9 (YAM) oxide compound. Orientation relationships between the oxide and the matrix are found to be dependent on the particle size. Large particles (>20 nm) tend to be incoherent and have a spherical shape, whereas small particles (<10 nm) tend to be coherent or semi-coherent and have a faceted interface. The observations of partially amorphous nanoparticles and multiple crystalline domains formed within a nanoparticle lead us to propose a three-stage mechanism to rationalize the formation of oxide nanoparticles containing core/shell structures in as-fabricated ODS steels. Effects of nanoparticle size and density on cavity formation induced by (Fe8+ + He+) dual-beam irradiation are briefly addressed.