Combination of Continuum and Atomistic Approaches for the Study of Dislocation Nucleation from Atomic Size Surface Defec
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Combination of Continuum and Atomistic Approaches for the Study of Dislocation Nucleation from Atomic Size Surface Defects Sandrine Brochard, Pierre Beauchamp and Jean Grilhé Laboratoire de Métallurgie Physique, UMR 6630 du CNRS, Université de Poitiers, UFR Sciences SP2MI, Téléport 2, Bd. Marie et Pierre Curie, B.P. 30179 86962 Futuroscope Chasseneuil Cedex, FRANCE ABSTRACT Atomistic simulations realized on an f.c.c. crystal containing atomic size surface defects (step and groove) show that the defects are privileged sites for dislocation nucleation. Before nucleation, an elastic shear, precursor of the dislocation, appears in the plane in zone with the step where the dislocation will be nucleated. In order to explain the strong localization of the localized elastic precursor shear, we have analyzed the stress concentration near the surface defects using the continuum point force approach. For the step case, the origin of the localized shear is related to an increase in the interplanar separation due to the stress concentration. INTRODUCTION In nanostructured materials (thin films, whiskers, nanograins) where the Frank-Read source mechanism cannot operate, the dislocation are generally assumed to be nucleated from the free surfaces and / or interfaces [1,2]. To overcome the large image forces which attract the dislocation toward the surface, stress concentrations or surface roughness are necessary for the dislocation to be nucleated [3-13]. We have studied the nucleation of dislocations from two atomic size surface defects, step and groove, using atomistic simulations. Although different from a micro-crack with its characteristic local stress field, for which detailed studies can be found in references [11-13], the groove can be seen as the first stage of a growing crack. The simulations not only show that the surface defects are privileged sites for dislocation nucleation (reducing the nucleation stress by almost one half), but also that the nucleation is preceded by the localization of an elastic shear, precursor of the fully formed dislocation. In this paper, the link between the localized precursor shear and the stress concentration near the surface defects is studied using both atomistic simulation and continuum theory of elasticity. ATOMISTIC SIMULATION The computational procedure used for this atomistic simulation has been described elsewhere and reference [14] should be consulted for full details. The interatomic forces are derived from a semi-empirical many-body potential for aluminum [15]. The geometry of the problem is presented in figure 1 (a). The free surface is (100), the step and the groove lie along a close-packed direction, [01 1], which is the periodic direction, with period equal to the nearest neighbor distance. The (100) surface is free and the other three sides of the computational block are kept fixed. The typical slab used contains approximately 1500 mobile rows. The crystal was submitted to a progressively increasing strain, with a characteristic strain step of 1%, reduced to 0.01% just be
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