Investigation On Preparation, Microstructure And Mechanical Properties Of Nano-structured9Cr-ODS Steel

Development of high-performance structural materials for fusion reactor first wall, which will be exposed to high fluxes of high energy (14MeV) neutrons from the deuterium-tritium fusion, is one of the major challenges in materializing future fusion reactors. Nano-structured oxide dispersion strengthened (Nano-structured ODS, ODS) steels are promising candidate materials for such reactors because they exhibit the excellent properties, such as superior high temperature strength and outstanding swelling resistance. This thesis is based on the9Cr-Nano-structured ODS steels as the research object. Firstly Fe-9Cr-1.5W-0.4Mn-0.1Ta-0.2V-0.3Ti-0.3Y3O2alloy powder is prepared by mechanical alloying (MA), In order to determine the best milling time parameters, effects of milling time on the microstructural properties, the particle size and solution of as-milled powders are investigated; Then MA powders were consolidated by hot isostatic pressing (HIP) to produce9Cr-ODS steels with extremely high density of nano-scale oxide dispersion strengthened precipitates. Microstructure, mechanical property and high-temperature stability of9Cr-ODS steels are investigated, the following conclusions are obtained:1. At the beginning of the MA, the shape of powder is irregular, particle size increases gradually. When milling to1h, MA powders have serious cold welding phenomenon and particle mean size increases to a maximum of36.4um. With the increase of milling time, the powder particle size decreases and the shape tends to be regular globular form. After50h milling, the powder particle size is about7.7μm, which does not change any more. It can be considered that fracturing and welding reach dynamic balance.2. The XRD calculations show that the grain size decrease with the increase of the milling time. And the grain size at50h is7.2nm and reach a dynamic balance. The analysis of XRD, BSEI and EDS show solution is complete at50h, so the best milling time is50h.3. The microstructure of9Cr-ODS steel has a variety of separation phase:(1)High density of non-stoichiometric fine Y-Ti-O rich clusters, which are coherent with the matrix. The density of clusters is4.4X1022/m3. After heat treatment, the density is4.0X1022/m3. Nano-clusters and precipitates are very stable after heat treatment, almost they have the same size, distribution and density with HIPed9Cr-ODS steels.(2)A few precipitates of Y2Ti2O7(fcc, ao=1.009nm).(3) A few a rich CrMn oxide precipitation phase, The oxide precipitation phase is probable Cr2MnO4(bcc).4. The average microhardness of HIP sample is389HV, and the average microhardness of heat treatment sample is190HV.9Cr-ODS steel shows high strength at room temperature and high temperature tensile test. Tensile strength at room temperature,550℃and650℃are1124MPa,697MPa and386MPa respectively. As the stretching temperature increases, tensile fracture surface of9Cr-ODS steel change from mainly brittle fracture characteristics to mainly ductile fracture characteristics.