Direct imaging of dislocation core structures by Z-contrast STEM

The development of Z-contrast imaging in the scanning transmission electron microscope has enabled dislocation core structures to be visualized with increasing detail. In single-component systems, core structures have been generally found to be as expected from simple considerations or theoretical predictions. In more complex structures, however, cores are generally seen to take on configurations that were not previously anticipated, utilizing reconstructions or non-stoichiometry to lower the elastic strain field. We present an overview of these developments in a variety of materials. In suitable circumstances, impurity segregation sites have been directly imaged at dislocation cores. More generally, electron energy loss spectroscopy gives detailed, atomic-resolution information on stoichiometry and impurity concentrations. These additional variables introduce additional degrees of freedom into the whole issue of structure/property relationships. We show examples of an impurity-induced core structure transformation and a cooperative doping effect. First-principles density functional calculations are used to link observed core structures to macroscopic properties. We also show that planar dislocation arrays are accurate descriptions of tilt grain boundaries, with the structural units geometrically equivalent to individual dislocation cores.