Hydrogen diffusion in tungsten: A molecular dynamics study

The diffusion behavior of hydrogen (H) in tungsten (W) has been studied by molecular dynamics simulations. The diffusivities from 200 K to 3000 K are calculated and the diffusion equation is fitted to D = 5.13 × 10^-8exp(-0.21 eV/kT) m²/s. The diffusion equations in different temperature ranges of 200-500 K, 500-2400 K and 2400-3000 K are also given separately and the results imply different H diffusion modes in different temperature ranges, which is proved by analyzing equilibrium H positions at low and high temperatures. The H atom jumps between different tetrahedral interstitial sites (TISs) at lower temperatures, but the transition to octahedral interstitial sites (OISs) is also observed at high temperatures. Moreover, with a vacancy present in the W system, vacancy trapping of H is observed, and it is shown that the vacancy trapping reduces with the increasing temperature. The H binding energy to monovacancy is obtained using three different methods including NEB and fitting both H diffusivity and time for H to detrap from the vacancy, which provides more information of the H behaviors with the existence of defects.