ICAMS / Interdisciplinary Centre for Advanced Materials Simulation


Dislocations and grain boundaries: how do nanomaterials deform?

Date: 06.01.2009
Place: International Conference on Plasticity, St. Thomas (VI), USA

Alexander Hartmaier

Large-scale molecular dynamics simulations have been performed to study the mechanical behavior of a bio-inspired nanocomposite that is expected to exhibit a high strength together with a high toughness. As model system we consider hard nanosized Ni platelets embedded in a soft Al matrix. We study plastic deformation under quasi-static loading conditions and compute the macroscopic strength under geometrical variation of the nanostructure. The investigation is restricted to materials with quasi two-dimensional columnar nanostructures, which allows us studying scale dependent material properties over a wide range of length scales. It is found that interfacial sliding contributes significantly to the plastic deformation despite a strong atomic bonding across the interface. The composite materials furthermore show a high strength due to confinement effects on dislocation generation and motion. Similar results are found in an investigation of the hardness close to grain boundaries in tungsten. In this coupled experimental and numerical effort, nanoindentations are performed to measure the hardness as a function of the distance to a small-angle grain boundary. The results clearly show that the grain boundary contributes to the plastic deformation, while it also hardens the material by obstructing dislocation motion. These results are compared with the findings in the literature. Finally, a model is sketched, how the results of the atomistic simulation can be incorporated into a dislocation dynamics model of the plastic deformation of nanocrystalline metals. In this model, stresses and elastic strains are described in an elastic continuum, while plastic deformation is based on the motion of individual dislocations. For nanocrystalline metals special care has to be taken, when modeling the interaction of dislocation with grain boundaries. Furthermore, as seen in the atomistic simulations discussed above, the grain boundary itself will contribute to the plastic deformation, an effect that cannot be neglected on the nanoscale.

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