Computer simulation of fundamental behaviors of interstitial clusters in Fe and Ni

Atomistic features of small interstitial clusters in Fe and Ni have been investigated by computer simulation. The gradual change from an interstitial cluster to a dislocation loop was observed in detail from three points of view, (i) strain distribution along crowdion axis for each crowdion in a cluster, (ii) distribution of Burgers vector of peripheral dislocation line of a cluster, and (iii) recombination behavior of a vacancy of with a cluster. It is found that strain distribution was spread into two opposite directions on a crowdion axis with the increase of a cluster size. The irregularity on the curve of the distribution of Burgers vector of a smaller loop gradually disappears with the increase of a loop size. Recombination between a crowdion and a vacancy occurs on the peripheral position of smaller clusters, but this does not occur in larger clusters. These aspects show that the gradual change from an interstitial cluster to a dislocation loop occurs at a certain width of cluster size. Dynamic behavior was also investigated under cylindrical shear stress and the Peierls stress was obtained as a function of loop size. The results show that the Peierls stress decreases with increasing loop size down to the value of a straight edge dislocation. Activation energies of one atomic jump of these small dislocation loops were also calculated and small values of about 0.2 eV were found for loops of about 200 crowdions for both Fe and Ni.