Kink-limited Orowan strengthening explains the brittle to ductile transition of irradiated and unirradiated bcc metals

The line tension model of obstacle hardening is modified to account for the thermally activated, kink-limited glide of 1/2111 screw dislocations, allowing application to the plastic flow of bcc metals. Using atomistically informed dislocation mobility laws, Frenkel-Kontorova simulations, and a simplified dislocation-obstacle model, we identify a size effect for intermediate obstacle densities, where the activation energy for screw dislocation motion halves once the obstacle density falls below a critical value. Our model shows striking agreement with fracture experiments across a wide range of unirradiated and irradiated bcc metals. In particular, we demonstrate that the presence of defects in the crystal lattice can at most double the brittle to ductile transition temperature.