Synchrotron Radiation Study Of Helium Behavior In Pure Tungsten Materials

Tungsten(W)has been considered as one of the most promising plasma facing materials in future nuclear fusion reactors.During operation in the extremely harsh environment in nuclear fusion reactors tungsten surfaces will be exposed to large fluxes of hydrogen(H)isotopes and helium(He)ions with a broad spectrum of energies,in addition to a bombardment of the high flux of 14.1 MeV fusion neutrons and steadystate heat flow.Helium atoms tend to diffuse into the lattice and gather in defects such as vacancies,dislocations,and grain boundaries of tungsten to form helium clusters and helium-vacancy complexes,and evolve into nanosized helium bubbles.The formation of He bubbles is the direct source of helium embrittlement in plasma facing materials.Therefore,a clear understanding on the He ions implantation and evolution behavior in the W-based materials could provide scientific basis for the design of plasma facing components in future nuclear fusion reactors.In this paper,synchrotron radiation grazing incidence X-ray diffraction(GIXRD)and grazing incidence X-ray small angle scattering(GISAXS)were used to study the surface morphology,internal microstructure and strain distribution of He+ irradiated W.The effects of low-energy(100 eV)and high-fluences(6×1023,6×1024 He ions/m2)as well as high-energy(400 keV)and medium-fluences(2×1020,2×1021 He ions/m2)helium ion irradiation at different temperatures(room temperature,400℃)on the surface and sub-surface microstructure evolution of fine-grained and coarse-grained tungsten materials were investigated.The main innovative achievements of this paper are as follows:The evolution of microstructure in sub-surface of fine-grained pure tungsten after low-energy(100 eV)and high-fluence(6X1023,6×1024/m2)He+ implantation was investigated by the S-GIXRD and nanoindentation.Benefiting from the depth dependence of S-GIXRD,it was found that the unimplanted W surface layer was composed of two zones:the compressed zone with a thickness of～56±2 nm introduced by the mechanical polishing,and the unaffected matrix.The helium implanted W was composed of four zones along the direction from the top surface to depth:the compressed zone with a thickness of～46±10 nm,transition zone with a thickness of～146±14 nm,expanded zone,and the matrix.Compared to unimplanted sample,the compressed zone in the He+ implanted W becomes thinner mainly owing to the formation of He-complexes,while the expanded zone is resulted from the helium atoms entering into the tungsten lattice and causing lattice expansion.The transition zone is a result of competition between the expanding and compressing effects which are nonisotropic in nature.At the same time,it is observed by nano indentation that tungsten is hardened after helium implantation,and the hardening effect is mainly distributed in the compression zone and transition zone.The sub-surface microstructure evolution of W samples annealed at different temperatures after He+ irradiation at low energy(100 eV)and high flux(6×1023,6×1024/m2)was studied.It is found from S-GIXRD studies and analysis that the irradiation damage effect is sensitive to the stress state near the surface of materials.The lattice expansion phenomenon was also observed in the annealed samples after irradiation,and the lattice expansion degree of the annealed samples after 1200℃ was the smallest among the samples annealed at 1100～1300℃,indicating that the samples annealed at 1200℃ had the best irradiation resistance,which is attributed to the high density grain boundaries and dislocations.This work illustrated the importance of surface modification in the design of materials against irradiation damage.The fine-grained tungsten was irradiated with medium-fluences(2×1020 ions/m2,2×1021 ions/m2)He2+ at room temperature and 400℃ at high energy(400 keV)and the defect evolution behavior on the surface and near surface was studied by S-GIXRD and TEM.A new method for analyzing GIXRD data at different incident angles for high energy He irradiated W was suggested,that is,it is assumed that the crystalline interplanar spacing of W changes continuously from surface to depth after ion irradiation.The complex GIXRD peak can be well fitted with this method and the microstrain distribution in tungsten after high energy He ion irradiation was evaluated.It is found that there is compressive strain due to mechanical polishing at depths of 0100 nm,while a lattice expansion due to irradiation appears in the depth beyond 100 nm owing to irradiation-induced lattice swelling.The expansion strain reaches a maximum of 0.15%at 600 nm.The strain depth distribution is consistent with the damage and He concentration distribution simulated by SRIM and onfirmed by TEM results.This work provides a reference for GIXRD to study the sub-surface irradiation damage.The microstructure evolution in sub-surface layer of the coarse-grained W after irradiatioi of 400 keV He2+ at different fluence and temperatures was dssessed by SGIXRD and S-GISAXS.At lower fluence(2X1020 ions/m2),blisters appear on the surface and its height decreases from center to edge,while at higher fluence(2×1021 ions/m2)at 400℃,bubbles rupture due to excessive internal pressure.The unirradiated coarse-grain W exhibits a compressive strain state in the near surface,and a lattice swelling is caused by He ion irradiation,which is more severe at higher irradiation temperature and fluence.GISAXS results combined with the TEM analysis indicate that high density dislocation loops with an average radius of about 3.7 nm were formed due to the extrusion of vacancy clusters irradiated at room temperature.In addition to dislocation loops,high density He bubbles with a radius of 1-3 nm were also formed near the surface of the sample irradiated at 400℃.This work provides a reference for the study of sub-surface irradiation damage behavior of plasma facing materials and it has guiding significance for evaluating the damage degree of W materials and predicting service life in the future.