Molecular dynamics study of small helium cluster diffusion in titanium

The diffusions of He atom and small He clusters in Ti at different temperatures are simulated by molecular dynamics. The prefactors and the activation energies of diffusion coefficients are calculated. It can be concluded that the diffusion is anisotropic. Simulations show that prefactors of diffusion coefficients are different from those of He species diffusing in different directions, but the activation energies are the same. The result demonstrates that it is insufficient to predict the diffusion behavior of He in metal when only the energy barrier in static lattice is considered. Dynamics calculations are necessary to obtain the correct prefactors. Another counterintuitive observation is that He-dimer migrates more quickly than single He atom does at room temperature. The results emphasize the importance of dynamics simulations in predicting diffusion behavior of He in metals.