| The disordered core region of dislocations has long been assumed to be a channel through which fast unidimensional diffusion, known as pipe diffusion, takes place. In this work, we present a first attempt to model this important phenomenon with Atomistic Kinetic Monte Carlo (AKMC) where we consider carbon diffusion in the core of dislocations in bcc iron. AKMC allows to achieve time scales far beyond what can be done with molecular dynamics, therefore it is specially suitable to the investigation of diffusion in the solid state. The energy barriers for carbon jumps in the dislocation cores were obtained with the Nudged Elastic Band (NEB) method. It should be pointed out that the energy barriers presented a large variation (from 0.14 eV to 1.55 eV). As a consequence, in the core of dislocations, one can find states separated by low energy barriers which together form a trap for the carbon atom. Diffusion, in turn, is ruled by the slow transitions associated with high energy barriers. This low barrier problem prevents the simulation of carbon diffusion in the core of dislocations with AKMC at low temperatures (~400 K in the core of an edge dislocation and ~700 K in the core of a screw dislocation), because many AKMC steps are wasted with the fast (non-diffusive) transitions, which are more likely to be chosen by the AKMC algorithm. In order to overcome this limitation, the fast transitions and the slow transitions have to be treated in separate. |
|