In Situ TEM Research On Plastic Behaviors Of Nano-scale Au

The mechanical properties, including ductility, of nanocrystalline (NC) materialsare controlled by their unique deformation mechanisms. The plastic behaviors of NCmetals is not only important for the fundamental understanding of the deformationmechanism of small sized materials, providing theoretical guidance for fabricationmaterials with high-strength, but also providing theoretical guidance for the stability ofthose nano materials used in micro/nano device.In this dissertation, with our self-developed TEM double tilt tensile device. The insitu plastic deformation tests of Au NC and Au single-crystal were conducted in highresolution transmission electron microscopy (HRTEM). Their plastic deformationbehaviors and the microstructure evolution process were investigated. The mainachievements are following:NC Au system:(1) Using the bimetallic technique, the plastic deformation processof NC gold thin films were in situ observed. For the NC with grain size d≤~15nm, thegrain boundary (GB) behaviors mediates the plastic deformation, such as grain growth,GB sliding/migration and grain rotation.(2) The tensile tests showed that the NC Au nanowires exhibit~48%homogeneouselongation ability. The in situ TEM observations indicate that the ultra-large plasticdeformation is realized through the coordination of GB sliding and GB migration.During the deformation process, the GB sliding provides the large plastic deformationability, which was at the mean time accommodated by the GB migration.Single-crystal Au system:(1) For a Au nanowire with diameter d~60nm, the insitu TEM tensile experiment showed that the nanowire exhibit of~249%super-plasticity. Large number of in situ observation indicates that there is a size effect on thedislocation types: the type of dislocations transform from full dislocations into partialdislocations as the diameter of the nanowires decrease.(2) For a Au nanowire with d~30nm, in situ TEM tensile experiment found thatthe plastic deformation is mediated by partial dislocations. The plastic deformation isaccommodated by partial dislocations glide in the two intersecting {111} planes,stacking fault ribbons and partial dislocations nucleation, moving and annihilation.(3) The in situ observation indicated that the dislocation density is sensitive to the strain rate. At high strain rate, the dislocation density is high. While at low strain rate,the dislocation density is low.(4) The deformation twinning that results from partial dislocation were observedat atomic-scale. The steps formation on the twin boundaries and the tips of twinmigration that results from partial dislocations glide were also observed.(5) The in situ observation for the nanowire with different length/width ratioindicated that the plastic ability has size effect. For small ratio, the plastic ability is large.While for large ratio, the plastic ability is little.