Microstructure Control And Performance Study Of 14Cr Oxide Dispersion Strengthened Ferritic Steel

14Cr oxide dispersion strengthened（ODS）ferritic steel has become the candidate structural material for the advanced fission reactor and future fusion reactor,due to its outstanding high-temperature and anti-radiation performances.In order to further improve its service temperature,microalloying,heat treatment and thermal deformation were tried in this work to regulate the nano-oxides,second phases and microstructure,aiming at optimize the performances.Based on the evolution behaviors of Y₂O₃ under the experimental condition in this work,Al and Ti were added to the 14Cr ODS steel.The results indicate that Ti is more effective than Al to strengthen the 14Cr ODS steel,and the 14Cr ODS steel alloyed with Ti were then chosen as the research object.The inhibition mechanism of Ti on the formation of lath martensite in the steel was also investigated.Besides,the tensile properties are optimized by regulating the second phases and microstructure through heat treatment.On the basis of this research,the hot deformation behavior of the optimized steel was investigated,as well as the corrosion behavior in supercritical water（SCW）.Firstly,the evolution behavior of Y₂O₃ during the milling and annealing process was analyzed.After short time milling,the perfect crystal structure of Y₂O₃ was broken.Shape of Y₂O₃ transformed from spheroidal to flake,and large amount of amorphous structure was formed within the grains.The rod-like particles may form during the milling or the TEM preparation process.The destructive Y₂O₃ particles gradually recrystallized and spheroidized during the annealing process,and the higher annealing temperature promoted recrystallization process more obviously.Besides,the optimized milling speed and duration time are 400 rpm and 30 h,respectively,based on the analysis of mechanically alloyed powders with addition of Al/Ti.All the 14Cr ODS steels with different compositions possess bimodal structure,which results from the uneven grain size of the pre-alloyed powders and the partial non-uniform distribution of the nanoparticles.The addition of Ti is effective to refine the size of grains and the nanoparticles,as a result,the hardness of 14YTi ODS（14Cr ODS steel alloyed with Ti）steel is higher than that of 14Y（14Cr ODS steel without Al/Ti）and 14YAl ODS（14Cr ODS steel alloyed with Al）steels.The nanoparticles in14Y and 14YTi are mainly stable Y₂O₃ and Y₂Ti₂O₇.The as-sintered 14YAl contains two types of Al₂O₃,i.e.γphase andαphase,while after the heat treatment,the Al₂O₃particle is mainly theα-Al₂O₃ with large size.During the heat treatment process,partialγ-Al₂O₃ reacts with Y₂O₃ to form the Y-Al-O nanoparticles,and partialγ-Al₂O₃transforms toα-Al₂O₃.The formation of Y-Al-O with different crystal structures depends on the solid state reaction betweenγ-Al₂O₃ and Y₂O₃ and the inherent stability.The inhibition mechanism of Ti on the formation of lath martensite in 14Cr ODS steel was investigated.In the MP（milled pre-alloyed powders）,MX（mixed powders of MP powders and ODS powders with the mass ratio of 1:1）and ODS powders,flake structure and pores are all found,while there are big differences in the particle size and morphology,of which ODS powders show the smallest particle size due to the introduction of Ti and Y₂O₃.Martensite transformation occurred during the consolidation of MP steel,while no lath martensite was found in MX and ODS steels.M₂₃C₆ particles in the MP steel show a chain distribution along the grain boundary,while the formation of TiC in MX and ODS steel effectively inhibits the precipitation of M₂₃C₆,thus eliminating the chain distribution.Therefore,the Cr-depleted phenomenon is weakened and the formation of lath martensite was inhibited.In addition,the diffusion rate of Y is too low to obtain the homogeneous distribution in MX steel,while Ti distributes uniformly in MX steel in the form of TiC or Y₂Ti₂O₇.Accordingly,it is considered that it is not Y₂O₃ but Ti that plays a key role in restraining the formation of the lath martensite in 14Cr ODS.The microstructure and tensile properties of 14Cr ODS have been optimized by heat treatment.The recrystallization process was promoted when annealed at 800?C and 1000?C,and the fully recrystallized microstructure can only be obtained after annealing at 1200?C.But large numbers of pores formed along the grain boundary of the 1200?C-annealed sample,leading to the obvious reduction of density.The second phases in the as-sintered ODS steel include a large number of M₂₃C₆ particles,TiC and Y₂Ti₂O₇.The annealing process promotes the dissolution of M₂₃C₆ particles and the precipitation of TiC and Y₂Ti₂O₇.The size and content of M₂₃C₆ gradually decreases with the increasing annealing temperature,and the content of TiC and Y₂Ti₂O₇ particles increases simultaneously.M₂₃C₆ particles dissolve completely after annealing at 1200?C.The second phases in the 1000?C-annealed ODS steel are effectively optimized without reduction of density,thereby possessing the optimal tensile properties.Both microstructure and density should be considered adequately when choosing the appropriate heat treatment process.The hot deformation behavior of 14Cr ODS steel was investigated under the condition of 1050¹200?C and 1⁰.001 s^-1,and the deformation activation energy was obtained,as well as the constitutive equation.Both the increase of temperature and the decrease of strain rate can promote the recrystallization process of 14Cr ODS steel.The fully recrystallized can be obtained under the deformation condition of1200?C,0.001 s^-1 and 1200?C,0.01 s^-1.Dislocation tangles,bow out of grain boundaries and sub-grains are found in the deformed microstructure,implying the combination of different recrystallization mechanisms during hot deformation.Nanoparticles in the 14Cr ODS steel possess good dimension stability during deformation,and are coherent with the matrix.Hot deformation is an effective way to control the microstructure of ODS steel without weakening the strengthening effect of nanoparticles.The corrosion behavior of 14Cr ODS steel in SCW were also investigated,and the formation mechanism of the different corrosion morphologies were analyzed.The corrosion layer of the 14ODS steel consists of the transition layer,inner oxides and outer oxides.The inner oxide is FeCr₂O₄,and is stable during the whole corrosion process.The outer oxide changes from Fe₃O₄ to Fe₂O₃ when the corrosion time is more than 400 h.This phenomenon is related to the changing concentration of O₂ in the interface between corrosion layer and SCW.Both uniform corrosion and nodular corrosion occur during corrosion in SCW.The uniform corrosion is controlled by different reaction processes,and can be divided into two phases based on the different outer oxides.The formation of the nodular corrosion section is mainly attributed to the evaporation of Cr,which sharply promotes the diffusion process.The structure and evolution of nodular corrosion layer is same as that of uniform corrosion.