Activation energy and saddle point configuration of high-mobility dislocation loops: A line tension model

We report an analytical study of thermally activated motion of perfect dislocation loops with high mobility in terms of an elastic model, where the dislocation loops are assumed to be smooth flexible strings under the influence of a potential barrier. The activation energy and saddle point configuration of the dislocation loops are analytically expressed within the present model. The activation energy monotonously increases with the loop length and converges to a finite value. However, the features of the thermally activated motion remarkably change depending on the loop length. If the dislocation loops are longer than a critical length ${L}_{c}$, the saddle point configuration is the well-known double-kink type. On the other hand, if the dislocation loops are shorter than ${L}_{c}$, the saddle point configuration is the so-called rigid type, that is, the dislocation loops overcome the potential barrier without changing their shapes except for thermal fluctuations. The former is regarded as dislocation-like transport, while the latter is point-defect-like migration. Therefore, as the dislocation loops grow, a transition from point defect to dislocation substantially occurs for the dislocation loops.