Atomistic simulations of pipe diffusion in bcc transition metals

Diffusion along dislocations, the so-called pipe diffusion (PD), may significantly contribute to self-diffusion in plastically deformed materials. In this work, we carry out a comprehensive investigation of PD mechanisms in several representative body-centered cubic transition metals by means of large-scale atomistic simulations. We find that screw and edge dislocations exhibit distinct intrinsic PD mechanisms associated with dynamical formation and migration of kink pairs and bounded Frenkel pairs, respectively. Different atomic structures of both core types are decisive for the character of the migration events, resulting in a very fast 1D diffusion along the screw dislocations and a slower 3D diffusion along the edge dislocations. The predicted PD coefficients are several orders of magnitude greater than the bulk diffusion coefficients, indicating that the PD contribution needs to be taken into account when interpreting diffusion measurements in deformed bcc metals at temperatures below half of the melting temperature.