Physical Principles of Electron Microscopy: An Introduction to TEM ...

TEM Specimens and Images 113
core, in order to accommodate the extra “half-plane” of atoms. Because the
Bragg angles for electron diffraction are small, it is likely that, at some
places within this strained region, the bending causes the angle between an
atomic plane and the incident beam to become approximately equal to the
Bragg angle �B . At such locations, electrons are strongly diffracted, and
most of these scattered electrons will be absorbed at the TEM objective
aperture. Consequently, the dislocation appears dark in the TEM image, as
shown in Fig. 4-15.
Because they are mobile, dislocations play an important role in the
deformation of metals by applied mechanical stress. Although they had been
predicted to explain a discrepancy between the measured and theoretical
strengths of metals, TEM images provided the first direct evidence for the
existence and morphology (shape) of dislocations; see Fig. 1-11 on p. 14.
� i
P
Figure 4-14. Bending of the atomic planes around an edge dislocation, whose direction runs
perpendicular to the plane of the diagram. At points P, the angle of incidence �i is equal to the
Bragg angle of the incident electrons, which are therefore strongly diffracted.
Figure 4-15. Dislocations and a stacking fault in cobalt metal. The dislocations appear as
curved dark lines running through the crystal, whereas the stacking fault is represented by a
series of almost-parallel interference fringes. (The specimen thickness is slightly less on the
left of the picture, resulting in a reduction in the projected width of the stacking fault.)
Courtesy of Prof. S. Sheinin, University of Alberta.
P