Molecular Dynamics Simulations Study On The Deformation Mechanisms Of Fivefold Twinned Silver Nanowires

Silver nanowires are considered as the most likely alternatives to indium tin oxide (ITO), and are envisioned to be key components of flexible optoelectronic devices. Silver nanowires synthesized in laboratory typically contain specific fivefold twin structure, although past experiments and simulations strongly confirmed that nanoscale twinning can be an effective means to improve the overall performance of nanomaterials, the effects of such specific fivefold twin structure on the mechanical behaviors of one-dimensional metallic nano-materials are still obscure. In this thesis, the mechanical behaviors, especially the deformation mechanisms, of single crystal and fivefold twinned silver nanowires under different load, including tension, compression, bending, and torsion, are investigated by molecular dynamics simulations. The main conclusions are:1. In tension, the plastic deformation in single crystal is controlled by partial dislocation slip and twinning, fivefold twin boundary effectively impedes the motion of dislocations. Dislocations are trapped inside the fivefold twinned nanowires and interact with each other, leading to abundant sessile dislocations. Twin boundaries gradually lose their coherent property and smoothness during plastic deformation, even are accompanied by the appearance of voids at the intersection of five twin boundaries to release stress concentration caused by dislocation pile-ups.2. In compression, the plastic deformation in single crystal is controlled by full dislocation slip, dislocations can escape from nanowire, leading to "dislocation starvation" state. In fivefold twinned nanowire, dislocations inevitably interact with twin boundaries. When non-screw full dislocation reacts with twin boundary, they transmit onto a atypical{100} glide plane, with increasing deformation, they eventually cross-slip onto{111} plane that parallel to loading axis. Due to the dislocation-twin and dislocation-dislocation interaction, the dislocation density increases significantly during plastic deformation in fivefold twinned nanowire.3. Upon bending, the plastic deformation in single crystal is controlled by twinning, leading to the reorientation of{100} side surfaces to{110} side surfaces. The plasticity in fivefold twinned nanowire is entirely concentrated within very small range in the middle and two ends. Movements of dislocation are impeded by twin boundaries and a large number of dislocation are piled in the vicinity of twin boundaries. Fivefold twinned nanowire breaks early to release the stress concentration caused by completely localized plasticity and dislocation pile-ups. Compared with the single crystal nanowire, the fivefold twinned nanowire demonstrates significantly lower plasticity.4. The plastic deformation of fivefold twinned silver nanowire under twisting can be divided into two stages:for a moderately twisted nanowire, the plastic deformation is entirely controlled by twinning partial dislocation slip. When unloading at this stage, the plastic deformation can be fully restored, the fivefold twinned nanowire demonstrates pseudo-elastic behaviors. When increasing deformation, coaxial Shockley partial dislocations appear within each grain, these dislocations expand along the axis from nucleation end to the other end, and are also recoverable upon unloading. Meanwhile, some irreversible non-screw Shockley partial dislocations appear in the vicinity of loading end, preventing the restoration of nanowire to initial state upon loading.