Structure and properties of clusters of self-interstitial atoms in fee copper and bcc iron

Static and molecular dynamics simulations have been used with different types of interatomic potentials to investigate the structure, properties and stability of self-interstitial atom (SIA) clusters produced during irradiation. In α-iron (Fe), faulted clusters of (110) dumbbells are unstable for all the potentials. The most stable SIA clusters are sets of parallel (111) crowdions. Large clusters of this type form perfect dislocation loops with Burgers vector b = 1/2 (111). Small clusters (less than 9 SIAs) of (100) crowdions are stable at OK, but transform into a set of (111) crowdions on annealing. Larger (100) clusters are stable and form perfect dislocation loops with b = (100). Both types of loops are glissile. In copper (Cu), clusters of parallel (100) dumbbells and (l 10) crowdions are stable. Large clusters of these types form faulted and perfect dislocation loops with b = 1/3(lll) and 1/2 (110) respectively. Small faulted clusters (less than 7 SIAs) of irregular shape can transform into a set of (110) crowdions during annealing. Larger faulted clusters are stable as hexagonal 1/3(111) Frank loops at temperatures of about up to 1050 K for a period of several hundred picoseconds. All faulted clusters are sessile. Clusters of (110) crowdions and 1/2(110) perfect loops are glissile and stable at all temperatures. When large enough (more than 49-64 SIAs) they can dissociate on their glide prism. Symmetric three-dimensional clusters of (100) dumbbells are stable at OK but during annealing they transform into sets of (110) crowdions. The results for both iron and copper are discussed and compared with experimental data and provide a basis for investigating and explaining the observed differences in radiation damage accumulation behaviour between fee and bcc metals.