Hollow-cone dark-field transmission electron microscopy for dislocation density characterization of trimodal Al composites

This paper describes a methodology based on hollow-cone dark-field (HCDF) transmission electron microscopy (TEM) to study dislocation structures in both nano- and micro-crystalline grains. Although the conventional approach based on a two-beam condition has been commonly used to acquire weak-beam dark-field (WBDF) TEM images for dislocation structure characterization, it is very challenging to employ this technique to study nanocrystalline materials, especially when the grains are less than 100 nm in diameter. Compared to the conventional two-beam approach, the method described in this paper is more conducive for obtaining high-quality WBDF-TEM images. Furthermore, the method is suitable for studying samples with both nanocrystalline and coarse-grains. A trimodal Al metal-matrix-composite (MMC) consisting of B4C particles, a nanocrystalline Al (NC-Al) phase, and a coarse-grained Al (CG-Al) phase has been reported to exhibit an extremely high strength and tailorable ductility. The dislocations in both NC-Al and CG-Al phases of the trimodal Al MMCs at different fabrication stages were examined using the HCDF method described. The influence of the dislocation density in both NC-Al and CG-Al phases on the strength and ductility of the composite is also discussed.