Study On The Effect Of Y₂Ti₂O₇ Nanoparticles On Microstructure And Properties Of CLAM Steel

Fusion can be the most ideal energy for mankind in the future because of its cleanliness,efficiency,sustainable development and safe operation.However,at present,the research and development of fusion reactor is facing many technical problems,one of which is the first wall material problem of fusion reactor.The low activation nano structure dispersion strengthened steel has excellent thermal conductivity,radiation resistance,helium resistance and high temperature strength,so it will become the first choice material for the first wall material of future fusion reactor.However,the mechanical alloying and other methods used to prepare nano structured oxide dispersion strengthened steel are inefficient and can not meet the large-scale requirements of fusion materials.In this paper,the preparation of nano structure oxide dispersion strengthened steel was made by the process of co-precipitation and traditional melting.Firstly,nano-sized Y2Ti2O7+Fe powders were prepared by co-precipitation method with orthogonal experiment.The Y2Ti2O7+Fe mixed powders were characterized by XRD and field emission scanning electron microscopy,and the best preparation process was determined;then,CLAM steel was prepared using vacuum induction furnace,during which Y2Ti2O7+Fe powder with good comprehensive performances was added to steel.The effects of different Y2Ti2O7 additions on the microstructure and properties of steel are studied.The following main conclusions are obtained:(1)The Y2Ti2O7+Fe prepared by co-precipitation method was analyzed by XRD,and the results showed,that besides Y2Ti2O7,Fe and Fe3O4,there are no Y2O3,TiO2 and other impurity phases in Y2Ti2O7+Fe,indicating that both Y3+ and Ti4+ exist in the form of single pyrochlore Y2TiO7;(2)Y2Ti2O7+Fe mixed powder can be successfully prepared by the co-precipitation method.Combining size analysis,phase analysis and morphology analysis,Y2Ti2O7+Fe(3#sample)prepared at conditions of pH=8.5,10%dispersant,calcination temperature 1000℃,and reduction temperature 380℃)is good overall performance(spherical shape,uniform particle size,good crystallinity and dispersion),meeting the requirements of the added particles in the steel;(3)Metallographic analysis shows that adding Y2Ti2O7 to steel can obviously refine the grains.The grain size after heat treatment increased compared with the previous one,but the increment decreased with the increase of Y2Ti2O7 content,which was 22.3%,18.5%,12.4%and 9.1%respectively;(4)Mechanical properties test results shows that the microstructure and properties of steel before and after tempering have undergone significant changes.Compared with the rolled steel,tempered steel is more uniform organization,strength and hardness decreased significantly,while the plastic,toughness has significantly improved;(5)Scanning electron microscopy analysis shows that second phase particles with morphology of spherical,size of 0.1～0.5 μm and composition of Y-Ti-O-C-Ta-Cr-Fe(or Y-Ti-O-C-Ta-Cr-V-Fe)are found in 2#,3#and 4#steel,and the more Y2Ti2O7 is added,the more second phase particles are.But when the addition of Y2Ti2O7 increases,the second phase particles tend to be agglomerated;(6)Transmission electron microscope observation shows that the micro structure of each furnace steel is lath martensite,in which second phase particles exist,whose shape are spherical,elliptical and elongated with a size range of 50～300 nm.The second phase in 1#steel is mainly carbides(WC)and intermetallic compounds(Fe-Cr or Cr-Mn),while those in 2#,3#and 4#steels are mainly Y-Ti-O-C-Ta-W phase,Y-Ti-O-C-Ta-W-Si phase and Fe-Cr-C-W phase,but no pure Y-Ti-O phase is found.