Length-scale effects in the nucleation of extended dislocations in nanocrystalline Al by molecular-dynamics simulation

The nucleation of extended dislocations from the grain boundaries in nanocr ystalline aluminum is studied by molecular-dynamics simulation. The length of the stacking fault connecting the two Shockley partials that form the ex tended dislocation, i.e., the dislocation splitting distance, r(split) depe nds not only on the stacking-fault energy but also on the resolved nucleati on stress. Our simulations for columnar grain microstructures with a grain diameter, d, of up to 70 nm reveal that the magnitude of r(split) relative to d represents a critical length scale controlling the low-temperature mec hanical behavior of nanocrystalline materials. For r(split)>d, the first pa rtials nucleated from the boundaries glide across the grains and become inc orporated into the boundaries on the opposite side, leaving behind a grain transected by a stacking fault. By contrast, for r(split)<d two Shockley pa rtials connected by a stacking fault are emitted consecutively from the bou ndary, leading to a deformation microstructure similar to that of coarse-gr ained aluminum. The mechanical properties of nanocrystalline materials, suc h as the yield stress, therefore depend critically on the grain size. (C) 2 001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights rese rved.