Abstract
Ultrafine-grained Al exhibits remarkable tension/compression asymmetry of yield stress. Large-scale atomistic simulations and dislocation theory reveal that the yield event is not related to intragranular dislocations but caused by dislocation nucleation from the grain boundaries (GBs). Dislocation core associated with the stacking fault energy is strongly affected by the external stress in Al; the dissociation of perfect dislocation is stabilized by high tensile stress. These dislocations are more likely to be nucleated from GBs with lower yield stress. The mechanism, which is completely different from well-known mechanisms for nanocrystalline and amorphous metals, is unique to high-strength UFG metals.
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