First Principles Calculation Of Defect Evolution And Structural Characteristics Of W-based Alloys For First Wall Materials

Compared with traditional energy sources,the energy released by unit mass during nuclear reactions(especially fusion reactions)is huge.The raw materials needed for nuclear fusion reactions are very abundant in seawater,and the fusion products are low in radioactivity and less polluted.Therefore,nuclear fusion energy is the ultimate clean energy for mankind in the future.Metal tungsten has the advantages of high melting point,low hydrogen retention,high thermal conductivity,and high physical sputtering threshold.It is the most likely plasma-facing material(PFM)to be used in the future fusion reactor.Under severe working conditions,PFM will be affected by triple irradiation such as fusion high-energy neutrons,hydrogen helium plasma and high heat load,and its performance will rapidly deteriorate to shorten its service life.In this paper,the annealing mechanism of the defect caused by irradiation damage in pure tungsten(W)is first discussed by the first principle method.Then the influence of various alloying atoms in W on the self-interstitial atom(SIA)and vacancy(V)defect evolution behavior is studied.The Frenkel pair is a defect pair composed of SIA atoms and vacancies,and is one of the most basic defect structural types caused by radiation damage.This paper first examines the compound mechanism of Frenkel pair.According to annealing experiments and molecular dynamics calculations,the synthesis of Frenkel pair under low temperature conditions is mainly due to the migration of the self-interstitial Atom(SIA)and its composite with the nearest neighbor V.The migration of Vacancies at high temperatures is also a factor that promotes the compound of defects.Based on the contribution of SIA diffusion mechanism and the V diffusion mechanism to the compound,the relevant parameters such as the self-composite and reactive complex regions of the Frenkel under these two mechanisms are obtained.On this basis,the properties of SIA,including its stability,migration and rotation,and its effect on lattice distortion are examined in detail.Alloy is one of the effective ways to change the performance of W-based PFM.Transmutation elements such as Re,Os,Ta,and Hf are also produced in W under fusion high-energy neutron irradiation,and the resulting precipitation meeting seriously affects the service performance of W-based materials.In this paper,some 4D and 5D elements(Zr,Mo,Nb,Hf,Ta,Re,etc.)in the periodic table are selected as alloyed atoms(AE),focusing on the properties of the alloyed W-based alloys and the influence of AE on the evolution of the defect structure.First,the dissolution behavior of AE in W is systematically calculated,and the stability of the alloy structure is determined by its dissolution energy.On this basis,the interaction between AE and V(AE-V composite structure)is studied.Including the influence of AE on V formation energy and diffusion threshold energy,the microscopic structural characteristics of the AE-V complex are obtained.Since irradiation will also produce V clusters in W-based alloys,the interaction between AE and V clusters(Vn-AE composite structures)is further calculated,including the influence of AE on the nucleation,growth,and dissociation process of V clusters.On this basis,the alloy elements that are conducive to the diffusion of vacancies and hinder the cluster of vacancies are selected.Finally,the interaction between AE and SIA(SIA-AE composite structure)is studied.By examining its influence on the stability,migration,and rotation characteristics of SIA structure,the structural evolution of SIA in W-based alloys is obtained.And the effect of defect structure on the behavior of alloying elements.Based on the interaction between the alloy element and the point defect,this paper concludes that the Ta element is a alloying element which is beneficial to defect compound.Through the calculation of the first principle,this paper finds that on the one hand,the Ta element is conducive to reducing the diffusion threshold energy of the V in W,making it spread rapidly,and the diffusion threshold energy decreases with the increase of Ta concentration.On the other hand,due to the repulsive effect between Ta and the empty space,it will hinder the formation of the empty space cluster.Therefore,the addition of the Ta element can make the V in W more widely distributed,which is beneficial to the compound of Frenkel.For the SIA structure,the Ta element can hinder the spread of the SIA and balance the diffusion rate between the V and the SIA structure.Therefore,Ta will be a relatively good doping element in terms of the evolution of radiation spot defects.Experimental studies have found that Re and Os induce precipitation in W under irradiation conditions,while Ta does not.According to the known simulation results,it is confirmed that the V and SIA have the effect of inducing the occurrence of Re and Os,but the presence of Ta can effectively reduce the precipitation behavior of Re and Os.The intrinsic behavior of Ta element in W and the interaction of Ta Atom with V and SIA are also investigated in this paper.By simulating the mechanical properties of W-Ta-alloys with different Ta content,it was also found that Ta_nweakened the strength of its W,but its ductility increase.