Dislocation loop structure, energy and mobility of self-interstitial atom clusters in bcc iron

Molecular-statics and molecular-dynamics (MD) simulations based on the embe dded-atom method (EAM) were used to model the energy and mobility of self-i nterstitial atom (SIA) clusters in bcc alpha-iron, Isolated SIAs and SIA cl usters, directly produced in displacement cascades have significant impact on the microstructural evolution under neutron and high-energy charged part icle beam irradiations. The SIA clusters are composed of [111] split dumbbe lls and crowdions bound by energies in excess of 1 eV. The clusters can be described as perfect prismatic dislocation loops with Burgers vector b = (a /2)[111]. As the loops grow, SIAs fill successive jogged edge rows, with mi nimum free energy cusps found at the 'magic' numbers corresponding to un-jo gged filled hexagonal shells. The total energy of the clusters is in excell ent agreement with continuum elasticity dislocation theory predictions. How ever, the core region is extended compared to an isolated edge dislocation. The extended regions are preferentially located at the hexagonal corners o f the loop, forming intrinsic kinks. As a result of the intrinsic kinks, th e SIA clusters are highly mobile and undergo one-dimensional motion on thei r glide prism. The high cluster mobility is related to the easy motion of t he edge segments which propagate the kinks along the loop periphery resulti ng in increments of prismatic glide. The corresponding activation energy fo r SIA cluster diffusion is less than 0.1 eV. Linking atomistic point defect cluster calculations dislocation theory provides a powerful tool in unders tanding radiation damage. (C) 2000 Published by Elsevier Science B.V. All r ights reserved.