The effect of hydrogen atoms on the screw dislocation mobility in bcc iron: A first-principles study

We investigate the effect of hydrogen on the mobility of a screw dislocation in body-centered cubic (bcc) iron using first-principles calculations, and show that an increase of screw dislocation velocity is expected for a limited temperature range. The interaction energy between a screw dislocation and hydrogen atoms is calculated for various hydrogen positions and dislocation configurations with careful estimations of the finite size effects, and the strongest binding energy of a hydrogen atom to the stable screw dislocation configuration is estimated to be $256\pm32$ meV. These results are incorporated into a line tension model of a curved dislocation line to elucidate the effect of hydrogen on the dislocation migration process. Both the softening and hardening effect of hydrogen, caused by the reduction of kink nucleation enthalpy and kink trapping, respectively, are evaluated. A clear transition between softening and hardening behavior at the lower critical temperature is predicted, which is in qualitative agreement with the experimental observation.