Molecular Dynamics Simulations Of Helium And Hydrogen In Tungsten

Due to its high melting point and good resistance to radiation damage,tungsten has been chosen as the plasma facing materials(PFMs)in fusion reactors.The PFMs are subjected to high flux particles(helium,hydrogen and neutron)bombardment.Impurity atoms can diffuse fastly in tungsten,and have a strong tendency to accumulate in defects and form bubbles.The bubbles can degrade the mechanical properties and affect the service life of the materials.In this paper,the classical molecular dynamics program package LAMMPS has been employed to simulate the behavior of impurity atoms and clusters in tungsten,including in bulk and ?5(310)tilt grain boundary(GB).Firstly,molecular statics method has been employed to study the basic properties of tungsten and formation energies of defects.It is found that influence range of GB is about 3-4 A.The expansion in GB and displacement between GB surfaces provide a good environment for defect formation and trapping.The diffusion behavior of impurity atoms has been modeled by molecular dynamics.The diffusion coefficient for interstitial helium atom has been calculated by using three different potential models.The obtained diffusion active energies agree well with the data calculated by first principles.The helium atoms tend to occupy the tetrahedral interstitial sites at low temperatures,while they turn to the octahedral interstitial sites when the temperature rises.Once the impurities have been trapped by GB,they will hard to escape.The diffusion coefficients for impurities in GB are smaller than that in bulk.Interstitial hydrogen diffuses slower than interstitial helium.Helium can easily meet each other to form symmetrical clusters in tungsten.The pressure of cluster can release by emission<111>crowdion interstitials.The hydrogen will not aggregate in the bulk and GB.One vacancy will not become the nuclear center of hydrogen clusters.In the region that distant from GB about 6-15 A,the helium concentration is the lowest since helium can be absorbed by GB or other nuclear centers.In the GB,helium form clusters while hydrogen exists in the form of single atom.