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

Hollow-Cone Dark-Field Transmission Electron Microscopy for Dislocation Density Characterization of Trimodal Al Composites. B.Yao, H.Heinrich, C.Smith, M.van den Bergh, K.Cho, Y.Sohn: Micron, 2011, 42[1], 29-35