Dislocation Generation from Grain Boundaries in Nanosized Cu-beams

As the dimensions of a structure are decreased towards the nanometer scale, the mechanical response to loading deviates from what is observed macroscopically. This is due to a change in material properties stemming from the relative increase in number of surface atoms as compared to bulk atoms. Surface atoms lack some of their neighboring atoms, leading to a reorganization of the electron clouds and, thereby, to energy states differing from those of bulk atoms. Also at a grain boundary the atomic lattice is disturbed, and bonds between atoms in regions of different lattice orientations must be formed. The energy states of atoms close to such a disorder differ from the states for bulk atoms. All disturbed atomic regions are prone to dislocation interactions under mechanical loading. This should be especially true in the vicinity of both a surface and a grain boundary. Here we investigate tensile loading of nanosized Cu-beams, of length 100a0, with a0 denoting the lattice parameter, and with square cross sections with side length between 6a0 and 48a0 by molecular dynamic simulation. A grain boundary, normal to the loading direction, is introduced at the center of the beam to create two grains. The grain boundaries investigated are between the lattice orientations [100], [110] and [111], i.e. in all three combinations. Under loading dislocations are generated from the grain boundaries and extend into the weakest of the two grains. No dislocations pass a grain boundary or extend into both grains. It is also found that the yield stress decreases in the presence of a grain boundary as compared to a single grain beam. The stress-stain curves themselves are ragged and a correlation between the development of the dislocation density and the raggedness is observed.