Segregation of hydrogen on screw dislocations in tungsten and its impact on dislocation mobility

We investigate the interaction between hydrogen atoms and 1 2 〈 111 〉 screw dislocations in tungsten combining density functional theory (DFT) calculations and molecular dynamics (MD) simulations using a newly developed machine learning (ML) potential trained on ab initio data. Our study reveals two distinct hydrogen-decorated dislocation core structures: the previously known hard core configuration and a newly characterized easy core configuration. DFT calculations indicate comparable dislocation-hydrogen interaction energies for both cores, suggesting a potential competition between these configurations. Ising models, parameterized on DFT data are employed to predict hydrogen segregation profiles for both core configurations, and the results are corroborated by MD simulations. Free energy analyses and finite-temperature MD simulations indicate that the hydrogen-decorated hard core is thermodynamically favored over a wide range of temperatures and hydrogen concentrations. The mobility of this stable hydrogen-decorated hard core structure is assessed by evaluating energy barriers through DFT calculations and performing MD simulations under applied stress. Both approaches consistently show that hydrogen segregation on the hard core structure significantly reduces dislocation mobility, leading to a locking of screw dislocations.