Simulations of dislocation nucleation from atomic size surface steps and grooves

The stress field in the neighborhood of a surface step and a surface groove in a stressed crystal has been evaluated, in the frame of elasticity, using the point force model. This analysis shows that in the groove case the stress concentration induces a shear stress in the glide planes in zone with the groove direction, which can directly favor nucleation of dislocations. On the contrary, in the step case, there is no local shear stress in the glide planes in zone with the step. Atomistic simulations show that before dislocations nucleation, a local shear strain appears in the glide planes where nucleation will occur for both structures. This shear, precursor of the dislocation formation, is well explained by the stress concentration for the groove. For the surface step, the origin of the shear is more indirectly linked to the stress concentration: by increasing the interplanar separation, the stress concentration makes shear easier in some planes. The assumption of an effect of the interplanar distance on the shear localization has been checked using an analysis based on the Frenkel model.