Alternating bright/dark anomalous subunitcell contrast
in HREM images along or near the close-packed direction of 6H-SiC,
Ti5Si3, α-Ti, and 4H-SiC, all of which
are hexagonal, was examined using computer-generated crystal
models, HREM image simulations, and digital diffractograms from
the corresponding experimental images. The primary variables
were crystal tilt and thickness. Crystal model projections showed
that the scattering potential was smeared anisotropically within
the unit cells by small crystal tilts, which reproduced the
experimentally observed anomalous subunit-cell contrast modulations
in the corresponding simulations. The effect increased with
thickness, but it did not occur in exact zone axis
simulations for any crystal thickness. Structural considerations
indicated that the contrast resulted from tilt-induced violations
of Gjonnes-Moodie dynamical extinctions and excitation of
kinematically forbidden reflections in the imaging zone. Digital
diffractograms from experimental HREM images confirmed their
presence in the imaging zone diffraction patterns. These effects
were absent in HREM images from cubic crystals in this material
system because the structurally induced requisite kinematically
forbidden reflections do not occur in the imaging zone.
Multislice image simulations of sheared needle‐like precipitates in an Al‐Mg‐Si alloy
