Carbon clustering and effect on hydrogen trapping in tungsten: A first-principles study

The clustering of carbon (C) in tungsten (W) with and without vacancies and the effects of C content at a vacancy on the hydrogen (H) trapping have been investigated by first-principles computer simulations using density functional theory. The calculations were performed in 128-atom or 432-atom supercells. The results indicate that the nascent formation of tungsten carbide is based on a pair of C atoms located at two neighbor octahedral interstitial sites along the 〈111〉 direction with a distance of 0.284 nm. Small interstitial C atoms prefer to form a zigzag chain on a {110} plane, and the C binding energy increases with increasing number of C until four C atoms and then remains constant at 0.7 eV for larger C clusters. The presence of vacancies enhances the interactions between C and its first nearest neighbor (1NN) W atoms because of strong hybridization between the C-p state and the d state of its 1NN W, strengthening the C trapping at vacancies. Meanwhile, the presence of C increases the stability of the di-vacancy in W. The H binding energies to carbon-vacancy-hydrogen (CmVHx−1) complexes decrease with increasing C contents. For a given C content, the H binding energies to CmVHx−1 complexes basically decrease with increasing number of H. The ultimate H contents that can be trapped by CmV complexes essentially decreases monotonically with increasing C content, and the C effect on H trapping is dependent on temperature.