| The low-dimensional metallic nanomaterials, including nanoparticles and nanowires have exhibited their potential applications in the nanotechnology, e.g. nanodevice design, nanomaterial manufactory and nanothermite detonation. Thus, the investigation of these nanomaterials are necessary and important for the future application in realizing enhanced strength of nanodevices, better understanding the physics of nanomaterials, and tunable energy release rate and ignition sensitivity in energetic materials.Based on molecular dynamics and local crystal structural analysis, the mechanical behavior and material deformation mechanisms of low-dimensional single crystal metallic nanomaterials are investigated, under extreme loading conditions such as ultra-high laser sintering, high strain rate loading and impact loading. A computational pre-processing modeling application—Space Lattice Construction Nanobuilder v1.2is developed in this work, for the purpose of facilitating the atomistic simulation. By utilizing this application, a two-nanosphere model for sintering, a nanowire/nanorod model under combined loads and longitudinal impact, a particle-beam model and a beam-beam model under transverse impact are constructed.First, a comparative study of both solid and hollow spherical silver nanoparticles with different sizes under different heating rates of laser sintering is conducted systematically in this dissertation. The solid state sintering of both solid and hollow nanoparticles can occur spontaneously at the room temperature, with the partial dislocation activities and lattice slips (i.e., either stable or transient stacking faults) as an important mechanism in the fast neck growth stage. During the laser sintering process, an interesting phenomenon is observed in which all the hollow nanoparticle pairs show an inverse trend of the neck width growth at an ultrahigh heating rate. This phenomenon is quite different from that known in the solid nanoparticle cases, which implies that besides the size and heating rate, the nanoparticle geometry could also play an important role in the sintering process. The plastic deformation induced by the lattice sliding in the early neck growth stage, as well as the transformation from the fcc to hcp structure in the later premelting stage, is found to be the important mechanisms for the hollow nanoparticles at lower heating rates. In contrast, at ultrahigh heating rates, the surface diffusion and premelting from both the outside and inside, facilitated by introducing the inner free surfaces in hollow nanoparticles, make the entire structure more unstable and easier to collapse at a lower temperature level.Second, the loading path effect on the mechanical behavior of single crystal copper nanowires is investigated in this dissertation. Different loading conditions such as pre-tensile torsion and pre-torsional tension at different temperatures are considered. The results show that elastic pre-loading conditions can induce a distinct weakening on the resistance of plastic deformation under later applied loads. Meanwhile, coupled thermal and pre-loading effect can facilitate the transformation from elasticity to plasticity. Formations of fivefold twins are observed in nanowires subjected to the pre-torsional tension loading path. These fivefold twinning all occur at the necking stage before fracture, and are pre-torsion-and size-dependent but insensitive to the change in temperature and cross-sectional shape. The results indicate that the loading path effect on the mechanical behavior plays an important role in the formation of some special microstructures such as multiple twins in metallic nanowires.Third, the influence of system size on wave propagation and deformation patterns in (initially)<100>/{100} copper nanobars are studied with square cross section under symmetric longitudinal impact loading. It was found that the wave propagation speed increases with increasing cross-sectional area and eventually approaches the value obtained for a quasi-infinite sample. Extensive plasticity occurs across the entire length of the nanobars, whereas the quasi-infinite samples remain in the elastic regime and exhibit a vibrating (ringing) behavior. The deformation pattern in the nanobars is strongly dependent on the cross-sectional area. For the smallest nanobar with h=10a the material fully reorients from<100>/{100} to <110>/{111} with few stacking faults and twins. Material in the medium nanobar with h=20a does not reorient completely; the local crystal deformation is mediated mainly by partial dislocation activity leading to predominantly non-intersecting stacking faults and twins. The largest nanobar with h=40a exhibit behavior similar to that for the h=20a case but with greater propensity for intersecting stacking faults.Finally, the transverse impact-induced bending response of single crystal and five-fold twinned copper nanobeam is investigated, to understand the effects of impact velocity and aspect ratio. It is found that bending vibration of the nanobeams could happen with certain values of impact velocity or aspect ratio. At relatively low impact velocity and aspect ratio, only minor defects are formed in the confined impact area, and the nanobeams can exhibit the property of bending vibration as observed at continuum level. By further increasing the impact velocity or aspect ratio, the vibration phenomenon becomes less obvious, severe plastic deformation can occur with necking formation and ultimate breakage at very large impact velocity or aspect ratio. As compared, the bending vibration of five-fold twinned... |
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