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

Abstract

Molecular-statics and molecular-dynamics (MD) simulations based on the embedded-atom method (EAM) were used to model the energy and mobility of self-interstitial atom (SIA) clusters in bcc α-iron. Isolated SIAs and SIA clusters, directly produced in displacement cascades have significant impact on the microstructural evolution under neutron and high-energy charged particle beam irradiations. The SIA clusters are composed of 〈1 1 1〉 split dumbbells 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) 〈1 1 1〉. As the loops grow, SIAs fill successive jogged edge rows, with minimum free energy cusps found at the ‘magic’ numbers corresponding to un-jogged filled hexagonal shells. The total energy of the clusters is in excellent agreement with continuum elasticity dislocation theory predictions. However, the core region is extended compared to an isolated edge dislocation. The extended regions are preferentially located at the hexagonal corners of the loop, forming intrinsic kinks. As a result of the intrinsic kinks, the SIA clusters are highly mobile and undergo one-dimensional motion on their glide prism. The high cluster mobility is related to the easy motion of the edge segments which propagate the kinks along the loop periphery resulting in increments of prismatic glide. The corresponding activation energy for SIA cluster diffusion is less than 0.1 eV. Linking atomistic point defect cluster calculations dislocation theory provides a powerful tool in understanding radiation damage.