In Situ TEM Investigation on Mechanical Behaviors of Metallic Nanocrystals at the Atomic Scale

The mechanical reliability of nanoscale metals is a precondition for electronic devices to function well. Deep insights into the mechanical behaviors of metallic nanocrystals are critical for their practical application. In this dissertation, transmission electron microscopy (TEM)-based in situ nanomechanical testing is used to investigate the atomic-scale mechanical behaviors of metallic nanocrystals. Thickening, resulting from pure displacive or diffusive mechanisms, is frequently observed in nanoscale metals during compression. Thus far, the compressive behaviors of metallic nanocrystals, mediated by coupled displacive-diffusive mechanism, remain unexplored. Here, it is found that preexisting dislocations and dislocation slip act as stimuli to activate surface diffusion causing consecutive thinning in silver nanocrystals during compression. As the nanocrystal width reduces to a critical value, surface-diffusion-mediated compression fracture occurs. These findings shed light on atomic-scale diffusion-mediated compressive behavior of metallic nanocrystal. Fivefold twins are frequently observed in various nanostructured metals. However, the formation mechanisms of fivefold twin remain largely unclear, due to the lack of atomic-scale observation of their dynamic formation process. Here, it shows that sequential twinning slip in varying slip systems and the decomposition of high-energy grain boundary account for the fivefold twin formation in a nanoscale gold single crystal under bending as well as the reversible formation and dissolution of fivefold twin in the nanocrystal with preexisting twin under tension and shearing. Moreover, the complex stress state in the neck area facilitates fivefold twin formation in a bi-twinned gold nanocrystal, disobeying Schmid law. These findings provide atomic-scale insights into the formation process of high-order twin structures in nanostructured metals The high-temperature mechanical behaviors of nanoscale metals remain largely unexplored due to the technical limitations. Here, a in situ high-temperature nanomechanical testing method based on electric current-induced Joule heating is proposed. By this method, it is found that deformation twinning, phase transformation and dislocation slip sequentially occur in the tungsten nanocrystal at elevated temperature. Such ductile behavior is related to the loading orientation and the experimental temperature. This dissertation advances the current understanding of the atomic-scale mechanical behaviors of metallic nanocrystals, which is crucial for developing mechanically reliable electronic devices