| Nano-structured oxide dispersion strengthened (ODS) steels exhibit excellent mechanical property and irradiation resistance, coming mainly from their high-density of Y-Ti-O-rich clusters/precipitates, which are regarded as the leading structural material candidates for first wall application in future fusion reactors. Many kinds of Y-Ti-O-rich clusters/precipitates are reported in the literature, such as, non-stoichiometric Y-Ti-O nano-cluster, stoichiometric Y2Ti2O7, Y2TiO5 and Y2O3 phases, which depending on composition and manufacture processing. The ODS steels is still in the very early stage of development, many basic problems are not well understood yet, for example, the formation mechanism of Y-Ti-O clusters. In this study,9Cr-ODS steels with different Y/Ti atom ratios were prepared by MA (mechanical alloying) and HIP (hot isostatic pressing). Scanning electron microscopy (SEM) and x-ray diffraction analysis (XRD) were applied to study the changes of powder morphology, grain size and dissolution during MA. The microstructure of the ODS steels, such as the grain size and the types and distribution of precipitates after HIP, were studied by transmission electron microscope (TEM), electron back scattering diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM). Hardness and tensile properties were tested. The following results were obtained:(1) The relative density of both ODS steels with different Y/Ti ratio reaches up to 98% or above.(2) EBSD results showed that the average grain sizes of both ODS steels with Y/Ti=0.14 and Y/Ti=0.989 are below 500nm and there is no obvious anisotropy in the microstructure.(3) In the ODS steel with Y/Ti=0.14, high-density of Y-Ti-O nano-clusters and a few Y2Ti207 precipitates are found. The density and size of Y-Ti-O nano-clusters are 1.25×1023/m3 and 2-3 nm, respectively. The size of Y2Ti2O7 precipitates is about 10 nm. In the ODS steel with Y/Ti=0.14, the density of Y-Ti-O nano-clusters decreases to about 7.0×1022/m3, And the size and density of Y2Ti2O7 precipitates increase. The Cr, Mn, Ti-riched oxides are found.(4) The tensile strength of both ODS steels reaches to 1200MPa at room temperature. The ODS steel with Y/Ti=0.14 shows lower tensile strength and better plastic compared to the ODS steel with Y/Ti=0.989. |
PDF Full Text Request