Activation energy for nucleation of partial dislocation from grain boundaries

The nudged elastic band method has been used to calculate the activation energies for the nucleation of the leading partial dislocation from grain boundaries in pure f.c.c. copper under tensile loading at 0 K. Two planar grain boundaries, Σ5{2 1 0} and Σ7{5 3 0}, and a Σ5{2 1 0} grain boundary containing a ledge (stepped grain boundary) as a grain boundary defect have been studied. The activation energy for the nucleation of the partial dislocation is found to be significantly lower from the grain boundary containing the defect and that there is no real correlation between the activation energy and the change in interfacial energy of the interface during the nucleation process. This investigation shows that classic molecular dynamics simulations cannot be used to determine activation energies and additionally, and more importantly, that dislocation nucleation is much more likely to occur at defect sites. However, it is found that the activation energy from planar grain boundaries increases sharply as the external load is decreased away from the critical stress, and that the critical stress for partial dislocation nucleation can be found simply with molecular dynamics simulations.