Study On Helium Capture Behavior By Irr Adiation-Induced Micro-Defects In W And W Alloys

Fusion energy is considered to be an effective way to solve energy problems in the future,but the commercialization of fusion reactors is still facing many challenges,mainly due to the safety and stability problems of first-wall structural materials.The extreme operation environment of the fusion reactor will degrade the performance of structural materials.The main reasons included irradiation damage,which interaction with transmutation element（He and H）induced swelling,embrittlement,and surface stripping failure by thermal evolution.The key point to study the macroscopic degeneration in structural materials is to explore the evolution mechanism of their micro-structure and to elucidate the formation of irradiation defects with the interaction of helium.This paper mainly adopts high-energy helium ions to irradiate tungsten and tungsten based alloys（W-Re,W-Ni-Fe heavy alloy）.Positron annihilation spectroscopy（PAS）,transmission electron microscopy（TEM）and scanning electron microscopy（SEM）were used to characterize the changes in defect types in samples irradiated by different temperatures and doses,and to explore the interaction between defects and helium atoms under various irradiation environments.In addition,thermal aging and thermal desorption were used to study the stability and the thermodynamic behavior of micro-defects with helium in tungsten heavy alloy.What’s more,the formation mechanism of nano-scale protrusions（fuzz）structure on the surface of the material was further explored by the annealing.The paper’s research results are summarized as follows:（1）Temperature is a key factor that causes the change of the interaction between the defect and helium atoms in W and W-5Re alloys irradiated by helium ions.A large number of vacancy defects were produced at room temperature,and vacancies captured helium atoms to form stable He_mV₁complexes.The single vacancies recovered,and He_mV₁complex captured helium atoms to form He_mV_ncomplex clusters with the temperature increases.He_mV_ncomplexes continue to aggregate and form helium bubbles,which gather on the surface of the material and rupture to form a multi-layer porous structure.It is found that the vacancy acts as a carrier in the binding of vacancy to helium atoms,and accelerates the diffusion of helium atoms and helium bubbles at high temperature.What’s more,the second phase element Re inhibited vacancy migration obviously.（2）The vacancy become the nucleus sites of helium atoms,which trapping helium atoms to form He_mV_ncomplexes at different temperatures in helium ion irradiated W and 97W-2Ni-Fe alloys.The He_mV₁complex combined with helium atoms and vacancies to form He_mV_ncomplex,which diffused to the surface of the samples at higher temperature.What’s more,the element Ni and Fe inhibited vacancy migration unobvious under high temperature irradiation.The concentration of defect type changes from vacancies to He_mV_ncomplex rapidly with the increase of the dose(5×10¹⁷He/cm²),and helium atoms are more easily captured by the vacancy in the diffusion process to form He_mV_ncomplexes.（3）The thermal evolution of the interaction between micro-defects and helium atoms mainly includes the following five processes by annealing at different temperatures in helium ion irradiated 97W-2Ni-Fe alloy.1)The vacancy defects and He_mV₁complexes were stable after annealing at low temperature.2)Vacancies were restored and He_mV₁complexes capture helium atoms to form He_mV_ncomplexes.3)He_mV₁complex decomposed into vacancies and helium atoms,and helium atoms diffused and gathered.4)He_mV_ncomplexes decompose into vacancies and free helium atoms,and helium atoms gathered to form tiny helium bubbles,which diffused to samples’surface.5)He_mV_ncomplex fully disintegrates and mall helium bubbles captured helium atoms to form larger helium bubbles.Further studies show that the fuzz structure component is the element of W,which formed on account of larger helium bubbles burst,and its structural characteristics are related to the elements in the matrix phase.It is found that helium atoms are more likely to bond with Ni and Fe atoms in the matrix,and the high density helium capture point in the matrix accelerates the nucleation of helium bubbles,which intensifies the growth of fuzz structures in the matrix phase.（4）The surface morphology and hardness of W heavy alloys（90W-7Ni-3Fe and 97W-2Ni-1Fe alloy）have no significant changes after thermal aging,but there is an amount of Fe element segregation in the matrix phase.The results of helium desorption are related to the irradiation dose and the content of elements in the matrix phase.The decomposed temperature of the He_mV_nX_y（X is Ni,Fe）complex in the matrix is higher than that of He_mV_ncomplex in the W phase,which leads to serious surface damage in the W phase after desorption.