Study Of Microscopic Defects And Helium Behavior In Tungsten And Tungsten-rhenium Alloys In Plasma Environment

With advantages of high melting point,high thermal conductivity,high sputtering threshold energy,low hydrogen isotope storage rate and low corrosion rate,etc.,tungsten（W）has been selected as the best candidate for plasma-faced materials（PFMs）.However,in the harsh fusion environment,the damage caused by 14 MeV neutrons to PFMs uniformly distributed throughout the bulk,and the effect on the material properties is significant.Neutrons with energy above MeV will produce a large number of transmutation gas elements,such as H,He,etc.,through the nuclear interaction of（n,a）（n,p）.Owing to inertia and low solubility in materials,these transmutation gas atoms will aggregate,bind together,adhere to each other and grow up into bubbles in the materials,which will lead to great changes in the microstructure of the materials and decrease in the macroscopic properties.At the same time,the transmutation solid elements produced by neutron nuclear reaction,such as Re,Ta and other atoms,will accumulate with the prolonging of reactor operation time,and the effect on the microstructure of materials is still not clear.In this paper,tungsten（W）and tungsten-rhenium alloy（W-5wt.%Re,simplified in W5Re）were selected as the research objects.The quasi-uniform distribution of damage in bulk was designed by SRIM program contribution.The defect types of irradiation were studied by positron annihilation lifetime technique.The depth distribution of irradiation damage and the surrounding chemical environment were investigated by slow positron beam technique.Together with the grazing incidence diffraction technique of Beijing Synchrotron Radiation Source（BSRS）,the microstructural changes in the specific irradiated thin layer were exploied.These methods facilitated the study the radiation defects and helium behavior in tungsten and tungsten-rhenium alloys in plasma environment.1)He-ions beams of 16 keV,70 keV and 200 keV were selected from the 0¹0MeV simulated He-ions to respectively and successively irradiate pure tungsten（W）and W-Rhenium alloy（W-5wt.%Re）at room temperature to achieve the average damage level of 1.4 dpa within 500 nm depth range.VASP（Vienna Ab-initio Simulation Package）simulation finds that the charge density is uniform distributed in pure W lattice,in which positrons will be in a delocalized state.When the atoms at the center of lattice are replaced by vacancies,the lattice volume will collapse slightly,and the introduction of impurity He atoms will lead to the uneven force on the lattice atoms.As the number of impurity atoms increases,the charge density redistributes,which affects the positron annihilation probability at the defect sites.2)A considerable number of small vacancy clusters with low helium atom ratios will formed in tungsten when the helium ion influence is below 10²¹ ions m^-2.However,with the increase of irradiation dose,interstitial helium atoms around vacancy-type defects will begin to saturate and this will promote the formation of helium-vacancy complexes clusters.The positron annihilation characteristic parameters of W-4（200 keV+70 keV+16 keV）shows a relatively flat region in the depth range of 27-477 nm.Meanwhile,the S-W curve deviates from W-1（16 keV）and W-2（70 keV）to W-3（200 keV）,and clusters appear in the depth range of 57-230nm.These changes are contributed by the increase of irradiation dose,and the formation of large-size He_nV_m clusters.Continuous He-ions irradiation with different energy can achieve a uniform damage distribution in a certain range of pure tungsten metal,which meets the expectation of this paper.3)When 5wt.%Re is added to pure W,the micro-surface of the material will no longer be the perfect grain morphology.There seems to be one type of defects in W5Re samples irradiated by dose below 1×10²¹ions m^-2.The defect may consist of He_nV_m-Re with a certain proportion of helium atoms.In tungsten-rhenium alloys,Re atoms dispersed in tungsten lattice gap and other places.With the increase of irradiation dose,the density of vacancy defects in materials increase greatly.At the same time,Re element has a strong attraction to vacancies,which promotes the formation of Re-vacancy complexes defects.Under continuous He-ions implantation,the He-vacancy complexes defects will trap the moving interstitial He atoms,causing the defect concentration rapidly increasing.These defects grow up through absorption and emerging,and then induce trapping mutation in the irradiated region.The participation of Re atoms will inhibit the fluidity of these defects,thus causing a greater density of helium around the defects in the irradiated region.So there is an obvious helium effect appears in the coincidence Doppler spectrum.In addition,a large number of He_nV_m clusters with high n/m ratio or high release volume will interfere with the lattice periodicity in both W and W5Re,resulting in the increase of lattice stress and then left shift of diffraction peaks in irradiated samples.