Deformation mechanism in nanocrystalline FCC metals studied by atomistic simulations

Polycrystalline materials with crystallite diameters below hundred nanometer exhibit extraordinary strength which goes along with a decrease in ductility. In order to tailor tough materials, which combine strength and ductility, the underlying deformation mechanisms have to be understood. Molecular dynamics suggested partial dislocation mediated plasticity in nanocrystalline face-centred cubic (FCC) metals in terms of 1) dislocation nucleation at the grain boundaries, 2) dislocation propagation through the nano-grain, and 3) eventual dislocation absorption in the neighbouring grain boundary network. The partial dislocation nucleation sites were seen to exhibit stress concentrations such as in triple junctions. Moreover, it has been shown that the partial dislocation nucleation is energetically related to the so-called general stacking fault energies (GSFEs). Transmission electron microscopy, in-situ X-ray diffraction, and deformation experiments can be interpreted in terms of the aforementioned dislocation processes. On basis of this knowledge this thesis utilizes atomistic simulation methods to extend the understanding of the fundamental deformation mechanisms which mediate the plasticity in bulk nanocrystalline FCC metals. In the first part of this thesis, ab-initio simulations are exploited to investigate the effect of the superimposed strains in Ni, Al, and Cu on the GSFEs, which describe the energy penalty for rigidly shearing of the crystal. The energy curves are discussed in terms of a) the second and third order elastic constants b) the energy difference between hexagonal closed packed and face centred cubic structures, c) the GSFE / unstable-twinning-fault-energy interdependence and c) the implication on dislocation nucleation. The second part explores the strain rate, temperature, applied stress, and grain size dependence of nanocrystalline Al utilizing classical molecular dynamics simulations. The deformation behaviour is examined on the basis of the underlying deformation mechanism. The higher the strain rate the more the onset of dislocation mediated plasticity is delayed. This can manifest itself into a stress overshoot that diminishes as the strain rate is lowered. The associated critical resolved shear stresses are seen to cause the strain rate sensitivity in the flow stress. In conjunction with the rate dependence of the critical resolved shear stresses, the temperature dependence of the flow stresses supports the picture of thermally activated dislocation processes which control the deformation of nanocrystalline Al in molecular dynamics. Moreover, the strain rate simulations evidence that a subset of dislocation processes operate at the athermal limit which indicates, once more, the difficulty to extrapolate from the high strain rate regime of molecular dynamics towards common experimental conditions. Furthermore, using an iterative strain/relaxation method, athermal critical stresses are calculated for an isolated nano-grain w