| Nuclear fusion not only provides a large amount of clean energy,but also has a relatively small impact on the environment.Therefore,fusion energy is considered to be one of the most promising new energy resources.At present,one of the most critical challenges in magnetic confined fusion design is the selection of nuclear fusion reactor structural materials.Due to low coefficient of thermal expansion,moderate irradiation swelling resistance and technological maturity,reduced activation ferritic/martensitic(RAFM)steels are foreseen as the primary candidate structural materials for future fusion reactors.An important consequence of the D-T fusion reaction is the generation of 14 MeV neutrons.These energetic neutrons induce significant atomic displacements(vacancies and interstitials)and large amounts of helium(He)impurities by(n,a)transmutation reactions will be generated in RAFM steels.He can be deeply trapped by vacancies,and the aggregation of He atoms and vacancies can further form He bubbles and voids,resulting in He embrittlement and swelling.First-principles calculations based on density functional theory(DFT)models can provide accurate information about the structure of nanoscale defects produced by irradiation and about the nature of short-range interaction between radiation defects,clustering of defects,and their migration pathways.RAFM steels are mainly referred to a nominal 9 wt.%Cr content,largely because this can minimize the radiation-induced ductile-brittle transition temperature(DBTT)shift.In this study,first-principle calculations was performed to investigate the behavior of vacancy,He impurity and He-vacancy complexes in four different Fe-9Cr(wt.%)alloys and VC precipitate within RAFM steels.Due to the complex atomic site occupation,the formation of vacancy and He impurity with variant local environment and related magnetism change in Fe-9Cr alloys is investigated firstly.The study lays a good foundation for further study of He-vacancy complexes.The average magnetic moment per atom for Fe-9Cr alloys is about 0.3μB smaller than that for bcc Fe.The more surrounding Cr atoms,the smaller formation energies of both substitutional He and vacancy will be.We speculate that with the increase of Fe vacancies in Fe-9Cr alloy,vacancy-induced segregation of Cr atoms may be more and more obvious.Unlike in bcc Fe and dilute Fe-Cr alloys,octahedral interstitial He does not exist in Fe-9Cr alloys at all,even when there is no Cr atom nearby the interstitial He.The formation of tetrahedral interstitial sites He with Inn Cr is more difficult than without.The more obvious the magnetic moment of the surrounding metal atoms is disturbed,the more difficult the corresponding defects are formed in the system.The formation of He-vacancy complexes under irradiation is crucial for the nucleation and early-stage growth of He bubbles in RAFM steels.The energetics of HenVAm(VA represents vacancy)complexes(n and m=0-4)in Fe-9Cr alloy models and bcc Fe are investigated.The interaction of He-He indicates that self-trapping of He is easier in Fe-9Cr alloys than in bcc Fe.However,the existence of Cr suppresses multiple He trapping in the vacancy to some extent.Lower formation energy of HenVm complexes and stronger binding of single He atom to HenVAm complexes in Fe-9Cr alloys as compared to those in bcc Fe indicate that density of these small complexes may be higher in alloys.These complexes can provide more defect annihilation sites for irradiation-induced Frankel pairs.When m/n>1,aggregation of a vacancy to HenVAm complexes becomes more difficult in Fe-9Cr alloys than in bcc Fe.It is well known that the irradiation induced void swelling is related to the aggregation of small vacancy clusters or HenVAm complexes.The results indicate that the addition of Cr atoms to the Fe matrix is beneficial to the irradiation swelling resistance aided by dispersed He-VA complexes.MC-type precipitates play a significant role in normal RAFM steels and especially castable nanostructured alloys(CNAs)under irradiation.In order to illuminate whether the MC-type precipitate phase(such as VC)can play a positive role in suppressing the swelling of the matrix induced by He bubbles,here the behaviors of intrinsic point defects and He impurities in VC is investigated.The results indicate that the formation of carbon(C)vacancy is quite easy and vacancy makes covalent interaction of neighboring C and V atoms more obvious.Besides,the diffusion of vacancy in VC is significantly slower than that in Fe matrix.It’s worth noting that the interaction between carbon vacancies is repulsive which means that the formation of large vacancy clusters maybe difficult in VC.Apart from vacancy,He impurities are also investigated.Interstitial He is less stable and diffuses more easily inside VC precipitate than inside TaC,M23C6 and Fe matrix.Besides,vacancy can trap much more He in VC than in Fe matrix.The result means that vacancy-abundant VC particle may act as He bubble trapping site.Due to the repulsion of vacancies,we suggest that the formation of large voids and He bubbles in VC is difficult.That is to say,under neutron irradiation,He bubbles with small size may have a dispersive distribution in VC precipitate.The present theoretical calculations provide a basic picture that helps to understand the formation and growth of small He-vacancy complexes,voids and He bubbles in matrix and carbides precipitate in normal RAFM steels and CNAs. |
