Effects Of Alloying Elements On The Nano-oxides And Thermal Age-hardening Behaviors Of ODS Steels For Generation ? Nuclear Reactors

Fe Cr Al oxide dispersion strengthened(ODS)steel is one of the most promising candidate cladding materials for Generation IV nuclear energy systems due to the favorable combinations of excellent high-temperature creep strength,superior irradiation resistance and outstanding corrosion resistance etc.The macroscopic properties of ODS steel depend mainly on the nano-oxides dispersed in the matrix.Up to now,there has been no systematic research about the effects of alloying elements and their contents on the phases and corresponding number fractions,dispersion morphology and interfacial structures of nano-oxides in Fe Cr Al ODS steel.To optimize the alloy composition and improve the overall properties of Fe Cr Al ODS steel,the effects of alloying elements and their contents on nano-oxides have been investigated in this work.Additionally,in order to improve the stability of microstructure and mechanical property of Fe Cr Al ODS steel during service at high temperature,the effects of alloying elements on the microstructure and hardening behavior of ODS steel under thermal ageing conditions have been systematically studied in the present research.Firstly,the effects of Al,Ti and Zr and their contents on the phases and corresponding number fractions,dispersion morphology(the shape,size and spatial distribution)and interfacial structure(including orientation correlations and coherency)of nano-oxides in 15Cr ODS steel have been investigated by transmission electron microscopy(TEM)and high resolution transmission electron microscopy(HRTEM).Statistical information about the phases and corresponding number fractions,dispersion morphology and coherency of oxides was obtained.Secondly,the hardness and microstructure&chemistry variations of 12Cr,12Cr-7Al and 18Cr-9Al ODS steels after early-stage thermal ageing(475°C,300 h,the same with below)have been analyzed and characterized by hardness measurement and atom probe tomography(APT)and,consequently,the effects of Cr and Al contents on the phase separation,phase transformation and age-hardening behavior of ODS steel have been investigated.The hardness and microstructure&chemistry variations of 15Cr,15Cr-7Al and15Cr-7Al-0.4Zr ODS steels after early-stage thermal ageing have been analyzed and characterized by hardness measurement and APT and,consequently,the effects of Al and Zr additions on the phase separation,phase transformation and hardening behavior of 15Cr ODS steel have been investigated.According to the results of early-stage thermal ageing research,the microstructure&chemistry and mechanical property variations of 15Cr and 15Cr-5Al(with a low Al content)ODS steels after long-term thermal ageing(475°C,9000 h,the same with below)have been analyzed and characterized by APT,TEM,energy-filtered transmission electron microscopy(EFTEM)and tensile test and,consequently,the effects of Al,Ti and Y₂O₃ on the phase separation,phase transformation and hardening behavior of ODS steel have been investigated.The main concluding remarks are:(1)The nano-oxides in 7Al-0.5Ti-0.4Zr ODS steel are mainly composed of trigonal?-phase Y₄Zr₃O₁₂(?44.8%),cubic Y₂Zr₂O₇(?15.7%),orthorhombic YAl O₃(?14.2%)and orthorhombic Y₂Ti O₅(?11.2%),while in 4Al-0.1Ti-0.6Zr ODS steel they are mainly composed of trigonal?-phase Y₄Zr₃O₁₂(?54.3%),cubic Y₂Zr₂O₇(?23.5%)and orthorhombic Y₂Ti O₅(?6.2%).The formation of Y–Al complex oxides are intensely suppressed by Zr addition and,moreover,the suppressing effect is enhanced with increasing the content ratio of Zr/Al in the ODS steel.(2)For 7Al-0.5Ti-0.4Zr and 4Al-0.1Ti-0.6Zr ODS steels,the average diameter,number density and inter-particle spacing of nano-oxides,as well as the number fraction of coherent nano-oxides are 8.4 nm,4.67×10²² m^-3,50.5 nm,1.45%and?78.4%,respectively,for the former,while they are 5.2 nm,8.22×10²² m^-3,48.4 nm,0.61%and?90.2%,respectively,for the latter.When the contents of Al,Ti and Zr were changed from 7 wt.%,0.5 wt.%and 0.4 wt.%to 4 wt.%,0.1 wt.%and 0.6 wt.%,respectively,the dispersion morphology and coherency of nano-oxide are greatly improved.(3)The increments in Vickers hardness of 12Cr,12Cr-7Al and 18Cr-9Al ODS steels were measured to be 4 HV,45 HV and 34 HV,respectively,after early-stage thermal ageing.There was no new phase formed in 12Cr ODS steel,while significant formation of intermetallic compound?'phase(i.e.,cubic Fe₂Al Ti)occurred in12Cr-7Al and 18Cr-9Al ODS steels,indicating that the early-stage thermal age-hardening of 12Cr-7Al and 18Cr-9Al ODS steels should be attributed to?'phase.Additionally,no?'phase was detected in 18Cr-9Al ODS steel,indicating that the addition of 9 wt.%Al could completely suppress the?–?'phase separation in 18Cr ODS steel.(4)The increments in Vickers hardness of 15Cr,15Cr-7Al and 15Cr-7Al-0.4Zr ODS steels were measured to be 7 HV,32 HV and 44 HV,respectively,after early-stage thermal ageing.Only?'phase newly formed in 15Cr ODS steel,indicating that the less significant hardening of 15Cr ODS steel is due to the formation of?'phase.No?–?'phase separation but significant formation of?'phase occurred in both 15Cr-7Al and15Cr-7Al-0.4Zr ODS steels,indicating that the addition of 7 wt.%Al could completely suppress the?–?'phase separation in 15Cr ODS steel and,therefore,the early-stage thermal age-hardening of 15Cr-7Al and 15Cr-7Al-0.4Zr ODS steels should be attributed to?'phase.(5)For 15Cr and 15Cr-5Al ODS steels,the increments in yield strength,the average radius of?'phase,average Cr content of?'phase and Cr-compositional amplitude between?and?'phases are 99 MPa,1.55 nm,76.1 at.%and 64.1 at.%,respectively,for the former,while for the latter they are 145 MPa,2.50 nm,81.0 at.%and 70.3 at.%,respectively,indicating that 4.5 wt.%Al addition should enhance the?–?'phase separation in 15Cr ODS steel.Additionally,significant formation of?'phase occurred in 15Cr-5Al ODS steel and,therefore,the much more significant age-hardening of 15Cr-5Al ODS steel,relative to 15Cr ODS steel,is a consequence of the strengthening from both the enhanced?–?'phase separation and?'phase.(6)A large number of nano-particles with core/shell structure,which is composed of oxide core and Cr-rich shell,occurred in both 15Cr and 15Cr-5Al ODS steels after long-term thermal ageing,indicating that the matrix/oxide interface may act as trapping sites for Cr atoms,which could delay or even imped?-?'phase separation by reducing the diffusivity and the remaining amount of Cr in the matrix.The total elongation of both ODS steels decrease only by?2%after long-term thermal ageing,indicating that they have very low susceptibility to thermal age-embrittlement.