The effects of grain size and intragranular dislocation on yield mechanism and subsequent plastic deformation in ultrane-grained (UFG) Al and Cu were investigated by large-scale atomic simulations. Polycrystalline atomic models with and without intragranular dislocation sources were used to elucidate the relationship between mechanical properties and defect texture. It is found that the intragranular dislocation plays a signicant role in both incipient yield and grain boundary mediated dislocation nucleation. In addition UFG Cu yields earlier than UFG Al because partial dislocations in Cu are more likely to activate from grain boundaries, where the partial dislocation leaves deformation twin and secondary dislocation tends to move on twin boundary accompanied by the shift of twin boundary plane.
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