Investigation On Preparition9Cr Nano-structured ODS Steel By Mechancial Alloying And Spark Plasma Sintering

Nano-structured oxide dispersion strengthened (ODS) steels are the leading candidate materials of fusion reactor first wall due to their excellent high temperature strength, radiation tolerance and helium embrottlement resistance which come from their characteristic microstructure, i.e. ultra-high density of Y-Ti-O-rich nano-scale clusters/precipitates and dislocations. The problems, such as the anisotropy of microstructure and properties in ODS steels produced by hot extrusion (HE), the coarser grain in ODS steels produced by hot isostatic pressing(HIP) were existed in the past decade, which lead to the properties of materials can’t completely meet the design requirements of fusion reactor first wall and fast reactor fuel claddings. Spark Plasma Sintering (SPS) was employed in this thesis instead of the traditional HE and HIP process to make9Cr nano-structured ODS steel with ulta-fine grain size (100-200nm) and without obvious anisotropy. This study is meaningful to develop the new type of first wall materials for future fusion reactor application.In this paper, the effect of mechanical allying (MA) parameters on the morphology, size and structure of powders were studied; the mechanical properties of the ODS steel sintered under different SPS conditions were measured. The microstructure of the SPSed ODS steels was analyzed. The following conclusions were obtained in this study:(l)This study uses high energy planetary ball miller for MA of mixed powders under the condition of230r/min. The morphology and size of powder particles with different milling times were studied by SEM and laser particle size analyzer. The results showed that at the beginning of the MA, the cold welding is dominant and the deformed particles are welded together, and the average particle size increases. With the increase of milling time, the fragmentation becomes dominant and the average particle size decreases gradually. The cold welding and fragmentation get balance after ball milling for70hours and the average particle size keeps unchanged. XRD diffraction and EDS analysis showed that the dissolution of alloying elements is basically completed after20h milling The grain size decreases with the increasing of time. The grains size is about10μm after70h milling.(2)The effects of different SPS sintering time and temperature were studied. The density of the ODS steel sintered at900℃for8min is92.70%. The tensile strength at room temperature and700℃is1442MPa and215MPa respectively..(3)The microstruture of the ODS steels sintered at900℃for3min,8min and10min were studied by TEM. The results showed that the grain size in the three steels is small (100-200nm). With the increase of the holding time, the grain size does not increase obviously.(4)There are two kinds of the precipitates found in ODS steels subjected different heat treatment at900℃,1000℃,1100℃,1200℃for2h and1250℃for8h. One is the high density of Y-Ti-O-rich nanoclusters which is coherent with the matrix. The size of clusters is about several nanometers. The nanoclusters are found in all the heat treatment temperature thus they are very stable in such temperature range. The other is found at1200℃for2h which is determined to be Y2Ti2O7precipitates with the size of about10nm. At1250℃for8hours, the number of Y2Ti2O7precipitates increases and the size becomes the larger.