Deformation twinning in nanocrystalline Al by molecular-dynamics simulation
ABSTRACT We use a recently developed, massively parallel molecular-dynamics code for the simulation of polycrystal plasticity to elucidate the intricate interplay between dislocation and GB processes during room-temperature plastic deformation of model nanocrystalline-Al microstructures. Our simulations reveal that under relatively high stresses (of 2.5 GPa) and large plastic strains (of ~12%), extensive deformation twinning takes place, in addition to deformation by the conventional dislocation-slip mechanism. Both heterogeneous and homogeneous nucleation of deformation twins is observed. The heterogeneous mechanism involves the successive emission of Shockley partials from the grain boundaries onto neighboring slip planes. By contrast, the homogeneous process takes place in the grain interiors, by a nucleation mechanism involving the dynamical overlap of the stacking faults of intrinsically and/or extrinsically dissociated dislocations. Our simulations also reveal the mechanism for the formation of a new grain, via an intricate interplay between deformation twinning and dislocation nucleation from the grain boundaries during the deformation. The propensity for deformation twinning observed in our simulations is surprising, given that the process has never been observed in coarse-grained Al and that the well-known pole mechanism cannot operated for such a small grain size. It therefore appears that the basic models for deformation twinning should be extended with particular emphasis on the role of grain-boundary sources in nanocrystalline materials.
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ABSTRACT: Deformation twins were widely observed in polycrystalline Cu with grain sizes varying from micrometers to nanometers during the process of equal channel angular pressing at room temperature and low strain rate (∼10−2 s−1). The microstructures of deformation twins were characterized by a transmission electron microscope (TEM) and a high-resolution TEM. It was found that deformation twinning in coarse-grained Cu occurred mainly in shear bands and their intersections as a result of the very high local stress resulted from the severe plastic deformation, and followed the well known pole mechanism. With a decrease in the grain size down to submicrometer (<1 μm) and nanometer (<100 nm) dimensions, twinning was observed to take place via partial dislocation emission from grain boundaries and grain boundary junctions, which is different from the pole mechanism operating in coarse-grained Cu. These observations are consistent with the predictions of recent molecular dynamic simulations for nanocrystalline face-centered cubic materials. The deformation conditions required for twinning and the formation mechanism of deformation twins varying with grain size in Cu are discussed.Acta Materialia.