First-principles Investigations Of Point Defects In Tungsten Grain Boundaries

Macroscopic radiation damage effects of materials served in nuclear devices are consequences of two types of interactions between projectile particles and atoms included in the irradiated materials,that is,atomic displacements which will result in lattice defects vacancies and self-interstitials,and nuclear reactions which can create foreign elements.The SIAs may gather to form dislocation loops,and the Vs usually gather to form voids.The hydrogen and helium atoms tend to assemble in vacancies,dislocations and GBs,and form helium bubbles.These defects contribute to materials swelling,hardening,amorphization and embrittlement,and may accelerate material failure under irradiation.As point defects sinks,GBs can enhance radiation resistance of materials.However,the sink strengths of point defects might depend on GB structures.In order to provide a rational guidance for the GB engineering to design favorite GBs to enhance radiation resistance of materials,it is of fundamental importance to explore the relationship between the solutions and segregations of point defects at GBs and the GBs structures.Tungsten(W)is one of the promising candidates for plasma facing materials(PFMs)of the magnetic confinement fusion reactor and the spallation target served in Accelerator Driven sub critical System(ADS).Therefore,the relatitionships between the segregation behaviors of V,SIA,H and He in GBs in W and the GB structures have been systematically investigated through first-principles calculations in this paper.The main results are given as follows:1.The abilities to trap Vs and SIAs of GBs have been investigated.On one hand,vacancy formation energy rapidly increases then slowly decreases as the hard-sphere radius of the vacancy increases.The vacancy formation energy is the largest when hard-sphere radius is about 1.38 ?,which is the hard-sphere radius of W atom in equilibrium single crystal tungsten.That is,any denser or looser atomic configuration around GBs than that in bulk is helpful to form vacancy.On the other hand,SIA formation energy at GBs decreases with increasing hard-sphere radius of the interstitial sites,which indicates that GBs with larger interstitial sites have stronger ability to trap SIAs.Based on the data obtained for GBs investigated in this study,it is found that the ability to trap Vs increases as the GB energy increases,and the capability of trapping SIAs linearly increases as the excess volume of GB increases.Due to its lowest GB energy and smallest excess volume among all GBs studied,twin GB ?3(110)[111] has the weakest capability to trap both Vs and SIAs.Therefore,compared other GBs,the probability of recombination between Vs and SIAs is lowest at GB ?3(110)[111].In other word,GB ?3(110)[111] has the lowest ablity to recombine Vs and SIAs.2.Hydrogen solutions and segregation at eight symmetric tilt GBs have been investigated.It is found out that the favorite distance between H atom in interstitial sites and the first nearest W atoms is 1.95?.The hydrogen solution energy decreases as the hard-sphere radius of interstitial site increases.And the trend significantly slow down when the hard-sphere radius is more than 0.57 ?(the distance between H and the first nearest W is 1.95 ? in the interstitial site).This is because the mechanical contribution to solution energy is about zero as the hard-sphere radius is more than 0.57?.In addition,twin GB ?3(110)[111] has the weakest capability to trap H atoms.Analysing the relationship btween the separation energy of GB and the total hydrogen concentration,It is found out twin GB ?3(110)[111] is the most resistant to hydrogen induced intergranular fractures.3.Helium solutions and segregation at eight symmetric tilt GBs have been investigated.For each GB,helium atom prefers to dissolve in interstitial sites with the lowest charge density.Interactions between the helium atom and the GBs atoms are rather localized for all eight GBs,and the helium solution explicitly relates to the local environment of the interstitial site.It is found out that the helium solution energies decrease as the Voronoi volumes of interstitial sites increase,and they can be quantitatively determined by helium solution energy in regular tungsten clusters.Based on this quantitative relationship,it is easy to estimate the helium solution energy in various interstitial sites in tungsten without massive first-principles calculations.In addition,the helium segregation energy decreases as the excess volume of GB increases.Due to its smallest excess volume,twin GB ?3(110)[111] has the weakest capability to trap He atoms.