| In the present work,we have investigated the defects behavior in several single-phase concentrated solid-solution alloys(SP-CSAs)and tungsten,which are the key members of typical structural metallic materials for nuclear reactor due to their remarkable radiation tolerance.We firstly studied the formation energetics and migration kinetics of vacancy and interstitial defects in Ni,NiFe and NiCo systems using first principles calculations.Secondly,systematic energetics analyses on the shape preference,relative stability and radiation-induced segregation of interstitial loops in nickel-containing SP-CSAs have been conducted using atomistic simulations.In the end,we investigated the segregation,trapping and diffusion of He in a W grain boundary(GB)using combined techniques of ab initio and classical atomistic simulations.The main results are as follows:The higher formation energies of vacancy-tetrahedra in Ni alloys in comparison to Ni indicate that stacking fault tetrahedra is more difficult to nucleate in Ni alloys than in Ni,supporting the experimental observation that SFTs are smaller in Ni alloys than in Ni.The lower formation energies of Co-and Ni-containing interstitials than Fe-containing ones and their lower migration barriers in comparison to vacancies suggest an interstitial-dominated mechanism for Co/Ni enrichment and Fe depletion at interstitial loops,in good agreement with the experimental observations on irradiation-induced segregation.Most importantly,the change of migration barrier disparities between one dimentional(1D)translation and three dimentional(3D)rotation of interstitials imply a shift of interstitial migration from fast long-range 1D motion in Ni to mixed 1D and 3D motion in NiCo,and further to slow short-range 3D,motion in NiFe,thus empowering them the increasing defect recombination rate and the increasingly enhanced radiation tolerance.In SP-CSAs,the preference of the rhombus shape in perfect loops can be explained by the lowest potential energy of the rhombic perfect loops at 0 K,but the preference for elliptical loops in faulted loops is likely due to its low potential energy and possible large configurational entropy.We demonstrate that the energy difference among the dislocation loops with different elements ratio leads to the simultaneous Ni enrichment and Fe/Cr depletion on interstitial loops in NiFe and NiCr,while the smaller atom radius of Fe than Cr and the more negative enthalpy of mixing between Fe and Ni than that between F'e and Cr may lead to the enrichment of Ni/Fe atoms and depletion of Cr in NiFeCr alloy.The decrease of stacking fault energy with increasing compositional complexity provides the energetic driving force for the formation of faulted loops,which,in conjunction with the kinetic reasons,explain the experimental observation that the fraction of faulted loops rises with increasing compositional complexity.Notably,the kinetics is primarily responsible for the fact that faulted loops are absent in nickel-cobalt with low stacking fault energy.More importantly,it is suggested that the mechanical properties and deformation mechanisms of SP-CSAs can be tailored by modifying the compositional complexity.We show that the strong He trapping in a W GB can be attributed to a GB interstitial trapping or a vacancy trapping mechanism,while the extremely high average energy barrier leads to a slow diffusion of He in the GB plane.We further reveal by molecular dynamics simulations that the He diffusion will be dictated by GB migration through the motion of GB disconnections.The present results suggests that the GB does not provide fast transport channel for He.The present study on defects behavior is an imporant step towards clarifying the influence of defects on radiation tolerance in typical structural metallic materials for nuclear reactor.The results also provides useful reference for the possible application of SP-CSAs and polycrystalline W under irradiation in advanced nuclear fusion or fission reactors. |
PDF Full Text Request