| The disadvantages of Zr-alloy fuel cladding in resisting serious accidents have been revealed in Fukushima nuclear disaster happened in 2011.In order to improve the safety performance and thermal efficiency of nuclear power plants,it is necessary to develop advanced nuclear fuel cladding materials with excellent comprehensive properties for the replacement of the existing Zr-alloy claddings,which would be operated in Pressurized Water Reactors(PWRs)and Super Critical Water Reactors(SCWRs).Stainless steels(SSs)are expected to be used as fuel cladding materials under different operating conditions since they exhibit excellent mechanical properties,oxidation-and corrosion-resistances at high temperatures(HTs),as well as good neutron radiation-resistance.However,it was found that one underlying issue for stainless steels is their microstructural stabilities at HTs,that is to say,the properties would be deteriorated sharply after a long term service at HTs,resulting in a premature failure.Therefore,the present work aims at improving the HT microstructural stabilities of candidate stainless steel claddings through adding minor alloying elements into the base alloys.Specifically,the present work would adopt the cluster formula approach to design multi-component alloy compositions,based on which,both the addition rule of minor-alloying elements and thermodynamic calculations are also used.The effects of minor-alloying elements on the microstructural stabilities and mechanical properties of Fe-Cr-Ni,Fe-Cr-Al,and Fe-Cr-Ni-A]series of stainless steels have been investigated systematically.The alloying principle have been revealed and the relationship among the composition,microstructure and properties have been established in these three types of stainless steels in this work.The main research contents and conclusions are as follows:1)Fe-Cr-Ni series of austenitic stainless steels(ASSs)have attracted more attention as fuel cladding materials of SCWRs due to their excellent corrosion-resistance and mechanical properties at HTs.Such kind of ASSs are based on the composition of Fe-25Cr-20Ni wt.%,named as 310S.In order to further improve their microstructural stabilities and suppress the precipitation of brittle Cr23C6 and a phases at 600?700?,the influences of the ratios of strong carbide-forming elements M(M=Nb,Ti,Ta,Zr)to C,the species of M,and the solid-solution strengthening elements(Mo,W)on the microstructural stabilities and mechanical properties of 310S ASSs have been investigated systematically.All the experimental results indicated that the ratio of M to C should be strictly maintained as M:C=1:1(in molar ratio)in these modified alloys.A higher ratio with M:C>1:1 could accelerate the formation of brittle a-FeCr phase,otherwise,the coarse Cr23C6 particles could be precipitated easily when M:C<1:1.Based on the ratio of M:C=1:1,the microstructural stabilities of M-modified 310S alloys are also affected by the species of M.Specifically,the additions of both Ta(0.34 wt.%)and Zr(0.35 wt.%)can promote a large amount of MC-type nanoparticles precipitated into the austenitic matrix uniformly,which inhibits the formation of brittle Cr23C6 and a phases on grain boundaries effectively.It was evidenced by the fact that the volume fraction(f)of these coarse particles is lower than 2.0%after 700?/408 h aging,and the elongation of 408 h-aged alloys is higher than 40%.Unfortunately,the addition of Ti could deteriorate the HT microstructural stability of 310S sharply,leading to the volume fraction of coarse Cr23C6 and a up to 15%after 700?/408 h aging,and the elongation of alloys lower than 25%.Compared with the Mo element,the addition of W can improve the microstructural stability effectively,while the excessive Mo(>0.7 wt.%)promotes the precipitation of a phase significantly,and accelerates the transformation of Cr23C6 to a.The MoZr-modified Fe-25Cr-22Ni-0.05C-0.37Mo-0.35Zr and MoNbTaW-modified Fe-25Cr-22Ni-0.05C-0.73Mo-0.18Nb-0.34Ta-0.70W(wt.%)alloys are developed after the composition optimization in the present work.Both exhibit the prominent microstructural stabilities at 650? after the repeated verification of 50 Kg-grade samples,as evidenced by the fact that only a few coarse particles(Cr23C6 and ?)with f<1.0%are precipitated from the matrix even after aging for 3000 h.Therefore,these two optimized alloys possess excellent impact toughness with the impact absorbed energy Akv>140 J/cm2,which is two times higher than the commercial HR3C alloy(Fe-25Cr-20Ni-0.4Nb-0.08C-0.2N,Akv=64 J/cm2).Moreover,the ultimate tensile strengths of these two alloys are all above 300 MPa at 650?,which can reach the requirements of mechanical properties of the fuel cladding materials in SCWRs.2)Fe-Cr-Al series of ferritic stainless steels(FSSs)are good potential candidate materials for accident-tolerant-fuel(ATF)claddings in PWRs due to their excellent oxidation-resistance in the water vapor.In order to solve the mechanical properties degradation issue that is caused by the dissolution of the second phases and the coarsening of the matrix grains,the effects of Laves-phase forming elements(Nb,Ti,Ta,Zr)on the microstructural stabilities and mechanical properties of Fe-Cr-Al alloys have been studied systematically based on the Fe-13.5Cr-4.5Al-2.0Mo-1.0Nb(wt.%).It was found that the thermal stability of Laves-phase is dependent on the solid solubility of forming elements in the ferritic matrix,in which the smaller solid solubility of M corresponds to the higher thermal stability of Laves-phase.Therefore,the Ta and Zr are selected to be as the main alloying elements in Fe-Cr-Al alloys.The addition of Ta can promote the uniform precipitation of nano-sized Laves-phase particles in the ferritic matrix.The amount of Laves precipitates is proportional to the amount of Ta addition,which is consistent with the thermodynamic calculation results.Moreover,with the increase of Ta content to 0.65 wt.%,the microstructural stabilities of Fe-Cr-Al alloys can be improved significantly.For instance,the grain size of 0.65 wt.%Ta-modified alloy is still maintained at 50 ?m even after 1100?/1h retreatment,which is induced by the existence of Laves phase particles.It would result in a high ultimate tensile strength(up to 700 MPa)finally.Besides the Fe2Zr-Laves phase,Zr element can also induce the formation of Fe23Zr6 phase when the amount of Zr increases to 0.33 wt.%and the thermal stability of Fe23Zr6 is higher than that of Fe2Zr,which would be used as a new strengthening phase in Fe-Cr-Al alloys.Moreover,the recrystallization temperature of Fe-Cr-Al alloys can be increased remarkably by Zr addition,which would suppress the grain coarsening at HTs effectively.The Ta-modified Fe-13.51Cr-4.73Al-2.08Mo-0.67Nb-0.65Ta(wt.%)alloy developed in the present work shows great microstructural stabilities,in which the volume fraction of Laves phase is two times higher than that in the base alloy,while the grain size of the former is less than one-half of the latter base alloy.3)Alumina-forming ASSs possess the advantages of both Fe-Cr-Ni and Fe-Cr-Al series of alloys,which would be applied as the ATF cladding materials in SCWRs.In order to balance the contradiction between the creep-resistance and oxidation-resistance of such alloys,based on the Fe-20Ni-18Cr-2.5Al-2.5Mo-0.86Nb-0.08C(wt.%)alloy,the present work investigated the effects of strong carbide-forming elements M(M=Ti,V,Ta)on the phase precipitation and oxidation resistance at 800? of the base alloy.It was found that the addition of both V and Ta can promote the diffusion of Al3+in the austenitic matrix,resulting in the enhanced oxidation resistance of alloys by the formation of compact Al2O3 protective scales.However,the inner nodular oxides were observed in the Ti-modified alloy,which has a detrimental effect on the oxidation resistance in the water vapor circumstance.Moreover,the formation of B2-NiAl phase accelerates the precipitation of a-FeCr through changing the local chemical composition,which further deteriorates the oxidation resistance of alloys.The ratios of M to C should be strictly maintained in the range of 1.0?2,0(in molar ratio)in these modified alloys,thus,the coarse Cr23C6 and ? could be inhibited effectively.The Fe-18Cr-20Ni-2.5Al-2.3Mo-0.44Nb-0.89Ta(wt.%)alloy developed in the present work exhibits the prominent oxidation resistance,in which the mass gain is reduced by 25%after oxidized at 800? for 500 h when compared with the base alloy. |
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