Study On Toughening-strengthening And Resistance Tritium Properties Of N-ODS Steel For Fusion Reactor

Fusion reactor cladding is the core component of thermonuclear fusion reactor.Its structural materials are faced with harsh service environment such as high dose neutron irradiation?high service temperature and stress load?deuterium-tritium fuel permeation/retention etc.Oxide dispersion strengthened steels(ODS)have been considered as one of the best candidate structural materials for fusion reactor claddings due to its excellent high temperature strength and radiation resistance.However,ODS steel is faced with the problem of high brittleness,and the permeation/retention of deuterium-tritium fuel also hinders its further application.If the brittleness of ODS steel can be improved and the ability of hydrogen(deuterium-tritium)resistance can be improved,it will be of great significance to the design and research of the cladding structural materials for fusion reactor and the development of fusion reaction system.In this paper,two schemes of composition optimization and layered structure design are tried to improve the strength and toughness of ODS steel and the deuterium-tritium permeability resistance,so as to prepare new structural composites with the integration of toughening and deuterium-tritium resistance.In this paper,on the basis of 9Cr-ODS steel(Fe-9.96Cr-2.82W-1.42Mn-0.25V-0.11C-0.3Ti-0.35Y₂O₃,wt%),the effect of N element on the mechanical properties of ODS steel was investigated by introducing N element into high-energy ball milling.Advanced characterization methods,such as transmission electron microscopy(TEM)and atomic probe tomography(APT),were used to analyze the precipitates in the matrix of N-ODS steel,and the strengthening and toughening mechanism was described.Meanwhile,the hydrogen isotope permeation/retention behavior of N-ODS steel was studied,and the influence mechanism of the surface and precipitation relative hydrogen isotope permeation/retention behavior of N-ODS steel was discussed.Based on N-ODS steel,the precipitation refining process was further explored.At the same time,the N-ODS/Ta layered composite structure was designed,and its properties and structure were characterized.The influencing factors of the layered interfacial transition layer and the growth kinetics equation were studied.The main conclusions are as follows:(1)The addition of appropriate N element can improve the tensile strength and elongation at break of ODS steel at the same time,and realize the strengthening and toughening of ODS steel,but excessive N will deteriorate the properties of ODS steel.Through XRD characterization,it was found that excessive N element could not be dissolved into the matrix.In the subsequent sintering process,the coarse precipitated phase will be formed to deteriorate performance.By observing the microstructure of N-ODS steel,it is found that the introduction of N can refine the grain,and the strength and toughness of the material can be improved by the fine grain strengthening.At the same time,the N element refined the oxide precipitates in the matrix and formed the high density nano(Ti,V)N and other MX phases and the"core/shell"nitride-coated oxide complex precipitates in the matrix.The existence of"shell"nitride-coated oxides can improve the wetting property of the interface between oxides and matrix,which is conducive to improving the brittleness of ODS steel.This may also be the strengthening and toughening mechanism of N-ODS steel.(2)The introduction of N element can improve the deuterium-tritium permeability resistance of N-ODS steel.The permeation/retention behavior of hydrogen isotopes in N-ODS steel was studied by using high temperature deuterium permeation and thermal desorption platform,and the results show that the deuterium permeability of N-ODS steel is lower than that of RAFM steel,even lower than that of austenitic 316L stainless steel at high temperatures.By the characterization of the precipitated phases on the surface and matrix of N-ODS steel,it is found that the addition of N causes the precipitation of high density nano phase MX(M=Ti?V;X=N)in N-ODS steel.Using the interfacial"hydrogen trap"effect,the dissolution rate of the material into the surface,the diffusion in vivo and the dissociation rate of the exudated surface can be effectively reduced,thus reducing the hydrogen isotope permeability.At the same time,the deuterium retention of N-ODS steel is increased to 500ppm due to more dispersed precipitates.(3)Two-step hydrogen reduction process can refine Mx precipitates.The strengthening effect of precipitates is closely related to particle size.N-ODS powders are treated with two-step reduction to refine the precipitates.According to the test results of TG-DTA and XRD,it can be found that at the low temperature stage,the N element introduced in the ball milling process and solid solution into the matrix will precipitate Ti N and other Mx phases again.According to the TEM observation of the precipitated phase,the size of the precipitated Ti N phase is small,indicating that the two-step reduction heat treatment can obviously refine the precipitated phase.(4)N-ODS/Ta layered design significantly improves the deuterium-tritium permeability resistance of ODS steel,and dense Ta(C,N)thin films are formed in situ at the layered interface.By SEM observation and XRD characterization of the layered interface,the results show that a layer of dense Ta(C,N)composite phase thin film is formed at the layered interface,and both the source of N and the content of N are related with the thickness of the transition layer of N-ODS/Ta layered interface.Through diffusion kinetics study,it is proved that the thickness of N-ODS/TA layered interfacial transition layer increases in the form of a parabola with holding time.According to the result of the high temperature gas phase deuterium penetration testing,deuterium in 2Ti N-ODS/Ta layer permeability for?=6.33 x10^-10 exp(-63527(J/mol^-1)/RT),the permeability significantly lower than the RAFM steel,even two orders of magnitude lower than austenitic stainless steel,and it is indicated that the dense Ta(C,N)film formed at the lamellar interface can block the deuterium atom diffusion,thus improving the deuterium permeability resistance of ODS steel.