Abstract: Recent advances in the ability to generate extremes of pressure and temperature in dynamic experiments and to probe the response of materials has motivated the need for special materials optimized for those conditions as well as a need for a much deeper understanding of the behavior of materials subjected to high pressure and/or temperature. Of particular importance is the understanding of rate effects at the extremely high rates encountered in those experiments, especially with the next generation of laser drives such as at the National Ignition Facility. Here we use large-scale molecular dynamics (MD) simulations of the high-rate deformation of nanocrystalline tantalum to investigate the processes associated with plastic deformation for strains up to 100%. We use initial atomic configurations that were produced through simulations of solidification in the work of Streitz et al [Phys. Rev. Lett. 96, (2006) 225701]. These 3D polycrystalline systems have typical grain sizes of 10-20 nm. We also study a rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume (isochoric), constant temperature (isothermal) shear deformation over a range of strain rates, and compute the resulting stress-strain curves to large strains for both uniaxial and biaxial compression. We study the rate dependence and identify plastic deformation mechanisms. The identification of the mechanisms is facilitated through a novel technique that computes the local grain orientation, returning it as a quaternion for each atom. This analysis technique is robust and fast, and has been used to compute the orientations on the fly during our parallel MD simulations on supercomputers. We find both dislocation and twinning processes are important, and they interact in the weak strain hardening in these extremely fine-grained microstructures.
Abstract: We utilize a novel computational approach to model the problem of impurity segregation at grain boundaries in nanophase materials. It is based on a parallel MonteCarlo algorithm that places the impurities according to the local chemical potential for the species, following the thermodynamic driving force for segregation. This technique is combined with molecular dynamics techniques to study the role played by Fe impurities in the properties of nanocrystalline Cu grain boundary properties. The impurities were found to improve microstructural stability as studied by high temperature annealing simulations, and grain boundary cohesion as studied via spall resistance high stresses produced by simulated laser irradiation. Virtual tensile tests of samples with and without impurities revealed that the impurities did not affect the high flow stress typical of nanostructured material. We interpret these results in terms of impurity dragging and grain boundary sliding.
Abstract: We have previously reported that the fracture behavior of nanocrystalline (NC) Ni is via the nucleation and coalescence of nano-voids at grain boundaries and triple junctions, resulting in intergranular failure mode. Here we show in large-scale molecular dynamics simulations that partial-dislocation-mediated plasticity is dominant in NC Cu with grain size as small as ~ 10 nanometers. The simulated results show that NC Cu can accommodate large plastic strains without cracking or creating damage in the grain interior or grain boundaries, revealing their intrinsic ductile properties compared with NC Ni. These results point out different failure mechanisms of the two face-centered-cubic (FCC) metals subject to uniaxial tensile loading. The insight gained in the computational experiments could explain the good plasticity found in NC Cu not seen in Ni so far.
Abstract: By means of finite element method which is based on the conventional theory of mechanism-based strain gradient plasticity, cohesive interface model is used to study the intergranular fracture in polycrystalline metals with nanoscale and ultra-fine grains. A systematical study on the overall strength and ductility of polycrystalline aggregates which depend on both grain interiors and grain boundaries for different grain sizes is performed. The results show that the overall strength and ductility of polycrystalline aggregates with nanoscale and ultra-fine grains are strongly related to the competition of grain boundaries deformation with that in grain interiors. Finally, the deformation and failure behavior of nanocrystalline nickel are described by using the computational model.
Abstract: The grain size effect on blunting of cracks in nanocrystalline and ultrafine-grained materials (UFG) is theoretically described. Within our description, lattice dislocations emitted from cracks are stopped at grain boundaries. The stress fields of these dislocations suppress further dislocation emission from cracks in nanocrystalline and UFG materials, and the suppression depends on grain size. The dependences of the number of dislocations emitted by a crack on grain size (ranging from 10 to 300 nm) in Cu and 3C-SiC (the cubic phase of silicon carbide) are calculated which characterize the grain size effect on crack blunting that crucially influences ductility of these materials.
Abstract: This paper reviews our recent studies on the effect of twin boundary (TB) on the deformation behavior in Cu with nanoscale growth twins. In situ straining transmission electron microscopy investigations on TB migration, TBs and twin ends acting as dislocation emission sources, and the interactions between dislocations and TBs are highlighted. Results provide some useful understanding of why Cu with nanoscale twins leads to a combination of ultrahigh strength and high ductility.
Abstract: Ultra-large compressive plasticity at room temperature has recently been observed in electrodeposited nanocrystalline nickel (nc-Ni) under micro-scale compression (Pan, Kuwano, Fujita and Chen: Nano Lett. Vol. 7 (2007), p. 2108). With aid of a TEM sample preparation technique employing focused ion beam (FIB), TEM observations on deformed nc-Ni evidenced deformation-induced microstructural evolution of nc-Ni at a variety of strain levels: Whilst the deformation increases, substantial grain growth is uncovered in the nc-Ni. No apparent ex situ evidence of intragranular dislocation activities is found in the deformed sample. As thermal diffusion plays an insignificant role in the deformation in nc-Ni at room temperature (~0.17Tm), this premium plasticity is achieved in accommodation with the grain-boundary-mediated deformation, with assistance of extensive grain growth that is mainly driven by high stresses at steady plastic flow.
Abstract: A summary of experimental results from nanoindentation, strain rate-controlled tension, in-situ bending and high pressure torsion on bulk electrodeposited nanocrystalline nickel, focusing on the effects of grain size on the mechanical behaviour and deformation mechanisms is presented. The interaction between dislocations and grain boundaries was locally examined by studying the dependence of nanohardness on grain size and indentation size; this is done by always performing nanoindents in the center of individual grains and by varying the grain size and indentation depth systematically. The grain size effects on the different deformation mechanisms of nanocrystalline nickel were revealed by strain rate-controlled tension and nanoindentation experiments, which show that with decreasing grain size the strain rate sensitivity increases and the activation volume decreases, indicating increased grain boundary mediated deformation processes in nanocrystalline nickel. Creep experiments at room temperature revealed that in nanocrystalline nickel grain boundary sliding or diffusion along the interface may dominate at lower stress levels, but with increasing stresses the deformation process is mainly controlled by dislocation creep. In-situ bending experiments in an atomic force microscope revealed directly that grain boundary mediated deformation processes play a significant role in nanocrystalline nickel, which is also supported by the observation of grain coarsening and softening of nanocrystalline nickel caused by high pressure torsion.
Abstract: This paper reports on the influence of strain rate on the onset of mechanical softening of nanocrystalline gold at room temperature. Micro-tensile testing was performed with applied strain rates on the order of 10−4 s−1 to 10−6 s−1. Our results defined a threshold strain rate, whereby plastic deformation at larger rates was dominated by dislocation processes and at smaller rates by one or more other deformation mechanisms.