Crystal and metal/oxide interface structures of nanoparticles in 15Cr–2W–0.1Ti–4Al–0.6Hf–0.35Y2O3 ODS steel

Abstract

FeCrAl oxide dispersion strengthened (ODS) steel is one of the most promising candidate cladding materials of generation IV nuclear fission reactors because of its excellent resistance to not only corrosion but also creep and irradiation due to the ultrahigh density nanometer-scale oxides. Crystal and metal/oxide interface structures of the nanoparticles in FeCrAl-ODS steel with Hf addition, i.e., Fe–15Cr–2W–0.1Ti–4Al–0.6Hf–0.35Y2O3, have been studied by high resolution transmission electron microscopy (HRTEM). The characterization of crystal structure was accomplished on up to 153 particles, which have diameter of 2–10 nm and peak number fraction and, therefore, represent the oxides contributing most significantly to the macroscopic properties of the ODS steel, and, subsequently, the proportions of particles having crystal structures consistent with various types oxides were determined by statistical analyses. Relative to FeCrAl-ODS steel without Hf addition, i.e., Fe–15.5Cr–2W–0.1Ti–4Al–0.35Y2O3 ODS steel, the coherency of the oxide nanoparticles was considerably improved. The crystal structure of about 51% of the nanoparticles were found to be consistent with Y2Hf2O7 oxide with anion-deficient fluorite structure while the crystal structures of only ∼32% of the nanoparticles are consistent with Y–Al complex oxides, which indicates that the addition of 0.6 wt.% Hf into FeCrAl-ODS steel inhibits the formation of Y–Al complex oxides remarkably while prompts the significant occurrence of Y–Hf complex oxides. The crystal structures of about 17% of the nanoparticles are consistent with Y–Ti complex oxides, which indicates even a very small amount of Ti could promote the significant formation of Y–Ti complex oxides. The crystallographic orientation correlations of the oxides and matrix were determined. The formation mechanisms of various kinds of oxides and, moreover, the reasons of the unusual irradiation tolerance and thermal stability of the ODS steel are discussed based on the results.