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We study the combined effects of grain size and texture on the strength of nanocrystalline copper (Cu) and nickel (Ni) using a crystal-plasticity based mechanics model. Within the model, slip occurs in discrete slip events exclusively by individual dislocations emitted statistically from the grain boundaries. We show that a Hall-Petch relationship emerges in both initially texture and non-textured materials and our values are in agreement with experimental measurements from numerous studies. We find that the Hall-Petch slope increases with texture strength, indicating that preferred orientations intensify the enhancements in strength that accompany grain size reductions. These findings reveal that texture is too influential to be neglected when analyzing and engineering grain size effects for increasing nanomaterial strength.

a b s t r a c tIn this work, we explore the microstructural properties that give rise to the plastic anisotropy observed in columnar-grained, nano-twinned Cu. A statistical model for randomly varying source lengths within the grain boundaries of the nanostructure is developed. The model is used to calculate a corresponding critical resolved shear stress for emitting dislocations within a twin lamella on slip systems lying either parallel or inclined from its twin boundary. By incorporating this model into a 3D crystal plasticity finite element model, we can link texture and slip patterns within the twin lamella to anisotropy in the plastic deformation behavior. The model achieves good agreement with flow stress strain evolution and yield data collected over many studies. We show that reducing twin thickness can increase plastic anisotropy as a result of the increase in mean stress to emit dislocations. It is also found that finer twins can lower strain hardening as a consequence of a lower statistical variation in the emission stress.

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It has been shown that unlike its constituent nanocrystalline (NC) phase, a heterogeneous lamella (HL) composite comprising NC and coarse-grain layers exhibits greatly improved ductility. To understand the origin of this enhancement, we present a 3D discrete dislocation, crystal plasticity finite element model to study the development of strains across this microstructure. Here we show that the HL structure homogenizes the plastic strains in the NC layer, weakening the effect of strain concentrations. These findings can provide valuable insight into the effects of material length scales on material instabilities, which is needed to design heterogeneous structures with superior properties. IMPACT STATEMENT Our work reveals that the coarse-grain layer of a severely heterogeneous lamella composite homogenizes deformation in the nanocrystalline layer, resulting in the enhanced ductility for which this composite is renowned.