Ab initio investigation of the screw dislocation-hydrogen interaction in bcc tungsten and iron

The interaction of hydrogen with 1/2<111> screw dislocations is investigated in body-centered cubic tungsten and iron using ab initio calculations. Different core reconstructions are evidenced, depending on the number of hydrogen atoms introduced inside the dislocation core. Corresponding interaction energies are highly attractive in both metals, with a significant contribution of zero point energy associated with H vibrations, particularly in Fe. The pinning by hydrogen of the dislocation in its reconstructed core remains efficient for a local atomic fraction of hydrogen as low as 1/5. Other octahedral interstitial sites close to the reconstructed core are also attractive, contrary to the perfect bcc crystal where these sites are unstable and where hydrogen lies in the tetrahedral sites. Hydrogen recovers its bulk behavior only beyond the eighteenth neighbor octahedral sites. A simple pair interaction model is parameterized on ab initio calculations and used with a mean-field approximation to predict the concentration profiles of hydrogen segregated in and around the dislocation core. This segregation model predicts that dislocation core-sites remain completely decorated by hydrogen atoms up to at least 750 K and 200 K in W and Fe, respectively.