| Nano-structured oxide dispersion strengthened(ODS)ferritic/martensitic steels are the leading candidate materials for the fast reactor fuel cladding application due to their excellent high-temperature strength and radiation tolerance due to their characteristic microstructure of ultra-high density of nanoscale clusters/precipitates,ultrafine grains and high-density of dislocations.At present,the research of nano-structured ODS steel focuses mainly on the optimization of alloy composition,mechanical alloying and heat curing parameters to improve the microstructure and mechanical properties of nano-structured ODS steel.However,the research on hot rolling is limited.In this thesis,two different Cr content of nano-structured ODS steels containing A1 and Zr elements(9Cr-ODS steel and 15Cr-ODS steel)were prepared by mechanical alloying(MA)and hot isostatic pressing(HIP),followed by different hot rolling deformation.The effects of hot rolling on the microstructures,including the types,size and density of nanoscale precipitated phases,before and after hot rolling were studied by electron back scattered diffraction(EBSD),scanning electron microscopy(SEM)and transmission electron microscope(TEM).The mechanical properties were measured too.This research could provide theoretical foundation for the research and application of nano-structured ODS steel microstxuctures and performances optimization.The following results were obtained:(1)The relative densities of 9Cr-ODS steel and 15Cr-ODS steel prepared by mechanical alloying and HIP are both more than 98%,and reach up to?99%after hot rolling.The quantity of rolling deformation has little influence on the relative density.(2)The average grain size before rolling is about 2.33?m,and the average grain size of 9Cr-ODS steel after hot rolling is 0.78?m(TD)and 1.76?m(ND).Hot rolling can significantly refine the grains of ODS steels.The grains are elongated along the rolling direction in TD surface,and significant cubic texture(?1=0°,?°=0°,?2=0°){0 0 1}<1 00>and brass-R texture(?1=0°,?= 55°,?2 =45°){1 1 1}<1 1 0>are observed.(3)High density nanoscale(<10nm)Y4Zr3O12(orthorhombic hexahedral structure)and Y2TiO5(orthogonal structure)precipitates are observed in the HIPed ODS steels,and the density of these precipitates is about 2.0 ×1022/m3.There are also exist large size(>lOnm)Y4Al2O9(monoclinic structure)precipitates,and the density is about 3.0 ×1021/m3 which is lower than the nanoscale precipitates.The density and size of large size(>10nm)precipitates and small size(?10nm)precipitates do not change obviously after hot rolling.(4)The average hardness of ODS steel before hot rolling is about 375HV.Hot rolling can significantly improve the hardness of the 9Cr-and 15Cr-ODS steels,but the effect of rolling reduction on hardness increase is not obvious(5)Hot rolling can also significantly improve the tensile strength of ODS steels.The tensile strength of ODS steels after hot rolling at room temperature is 100MPa higher than the HIPed ODS steels before hot rolling.The tensile strength of ODS steel before and after rolling decrease with the rise of temperature.The total elongation of ODS steel rises first and decreases with the increasing temperature,and reaches the highest at 600?.The fracture mechanism changes from quasi-cleavage fracture to dimple fracture with the increasing temperature.The effect of rolling reduction on the tensile strength and elongation is not obvious.Compared with other ODS steel,the toughness and total elongation of nano-structured ODS steel with A1 and Zr addition is significantly improved without intensity reduced,showing excellent mechanical properties. |
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