| Nano-structured oxide dispersion strengthened (ODS) steels are the leading structural materials for the first wall/blanket applications in future fusion reactors, because of their excellent irradiation tolerance and improved high temperature strength. These superior properties come mainly from the extremely high-density nano-size Y-Ti-O enriched oxides in ODS steels. However, ODS steels are still in the early stage of development. Many basic problems are not yet to be understood comprehensively, for example, the formation mechanism and irradiation stability of nano-oxides, et al. Hot isostatic pressing (HIP) and spark plasma sintering (SPS) accompany with mechanical alloying (MA) were used for preparing ODS steels in this study. The microstructural evolution of powders during MA and the microstructure of different ODS steels were characterized; Helium ions were implanted into the ODS steels to study the behavior of helium bubbles; Heave ions were irradiated into the ODS steels to study the swelling resistance and the irradiation stability of nano-oxides. The conclusions were obtained as follows:(1) Planetary ball mills (P5) and shaker mills (SPEX8000) were employed for MA. Two kinds of mills exhibited the similar MA processes. Evolution of the powders as the milling time was divided into four stages:(1) Cold welding was dominated at this stage. The average size of powders increased rapidly, and the characteristic lamellar structures were formed. Elements and Y2O3 started to dissolve into the matrix. (2) Fracture was dominated over cold welding. The lamellar structure fractured into the small pieces, the average size of powders decreased rapidly. (3) There were nearly balance between cold welding and fracture. The powder size decreased slowly, and the elements were dissolved homogenously in the matrix. The MA was achieved at this stage. (4) Small powders trended to agglomerate together, which increased the average size of powders. The extra contamination was introduced with the long MA time. Due to the higher impact energy, the milling time in SPEX8000 is about 20h, much shorter than 50h in P5.(2) A new nano-structured ODS steel was prepared by MA and SPS. Its relative density was sensitive to the consolidation temperature due to the short holding time and low sintering temperature. The optimal sintering temperature was 950℃. The lower sintering temperature leaded to the lower density, and the higher temperature leaded to the overbuming. Ultra-fine grains and coarse grains were formed in the SPSed ODS steels due to the inhomogeneous temperature distribution in the MA powders during SPS.(3) In SPSed ODS steels, most of nano-oxides in ultra-fine grains were nanoclusters. The average diameter of oxides was 5.9 nm, and the number density was 1.75×1022/m3. Most of nano-oxides in coarsen grains were pyrochlore structured Y2Ti2O7. The average diameter of nano-oxides was 15.0 nm, and the density was 1.3×1021/m3.(4) The relative densities of HIPed ODS steels were not sensitive to the consolidation temperature (900℃-1200℃), and were all more than 98%. The grain size of ODS steels increased with the increasing HIP temperature.(5) Three kinds of precipitates/clusters were observed in HIPed ODS steel. The first was the non-stoichiometric Y-Ti-O nanoclusters, which were dispersed homogenously in the matrix. The nanoclusters were coherent with the matrix. The diameter of Y-Ti-O nanoclusters was normally smaller than 5 nm. The second was the pyrochlore structured Y2Ti2O7, which was semi-coherent or non-coherent with the matrix. The diameter of Y2Ti2O7 was normally between 5-20 nm. The third was the large precipitates with the size ranged from dozens nanometers to hundreds nanometers, which were normally observed at grain boundaries. These large precipitates were spinel structured Mn(Ti)Cr2O4 in Ti/Mn added ODS steels. Cr2O3 was also formed in Ti-free ODS steels. And Cr-Ti-O phases were observed in Mn-free ODS steels.(6) There were great influence of Y/Ti atom ratio and consolidation temperature on the formation of Y-Ti-O-rich nanoclusters/precipitates. Four kinds of ODS steel were prepared with different Y/Ti ratios,0.14、 0.4、 0.989 and Ti-free. When Y/Ti was 0.4, the average size of Y-Ti-O-rich nanoclusters/precipitates was the smallest (3.0nm), the density was the highest (1.3×1023/m3), and the percentage of Y-Ti-O nanoclusters was the biggest. The average sizes of nanocluster/precipitates increased with the increasing sintering temperature, while the density decreased. It was preferred to form coherent Y-Ti-O nanoclusters in the lower sintering temperature, but Y2Ti2O7 in the higher temperature.(7) 400keV helium ions were implanted into three kinds of steels at 400℃ to a fluence of 4×1017He/cm2. The helium bubbles were apparently smaller in two ODS steels (SPEX-9Cr ODS (#1), SPS-9Cr ODS (#2)) than that in non-ODS F/M steel (Eurofer 97), and the helium bubbles in SPEX-9Cr ODS steel were smaller than that in SPS-9Cr ODS steel. It is because the interfaces between high-density of nanoclusters/precipitates and the matrix provided a great number of trapping sites for helium atoms, that the smaller size and the higher density of nanoclusters/precipitates, the smaller of helium bubbles.(8) Compared to the obvious swelling in F/M steel and 316 austenite steel, no observable swelling were found in the SPS-9Cr ODS steels (#2) pre-implanted 10 appm and 100 appm He, repectively, then irradiated at 460℃ by 5MeV Fe ions to the fluence of 4.6×1017/cm2 and in the 0,14-Y/Ti 9Cr ODS steels (#4) irradiated at 350℃-550℃ by 9MeV Au ions to the fluence of 7.6×1016/cm2. This means that the nano-structured ODS steels had the excellent swelling resistance. The interfaces between the matrix and nanosize oxides promoted the recombination of vacancies and interstitial atoms induced by irradiation, and minimized the; partitioning of vacancies to large size voids. In the other hand, the solute atoms in the oxides dissolved into the matrix after irradiation. The radiation-induced vancancies were annihilated by combining with the solute atoms, thus promoting "self-healing" in ODS steels.(9) SPS-9Cr ODS steels (#2) were irradiated at 460℃ by 5MeV Fe ions to the fluence of 133 dpa,188 dpa and 300 dpa. The size of nano-oxides decreased, and the density increased with the increasing dose.0.14-Y/Ti 9Cr ODS steels (#4) were irradiated at various temperature (350℃-550℃) by 9MeV Au+ to 200 dpa. With the increasing temperature, the size, density and volume fraction of nano-oxides decreased (350-450℃), and then increased (500-550℃). The results showed that the nano-oxides became unstable after high dose irradiation, which was affected by the mechanism of "dissolve-resolution". At the same irradiation temperature, the higher dose, the smaller size and the higher density of nano-oxides. In the same dose, the irradiation stabilities of nano-oxides depended on irradiation temperatures. At about 0.45Tm (Tm is the melting temperature), the irradiation stability of nano-oxides was worse. |
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