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rolling, extrusion, forging, etc., and subsequent thermal
treatment result further modify the crystalline microstruc-
ture, resulting in grain sizes ranging from centimeters to
nanometers depending on composition and thermome-
chanical history and often inducing “preferred orienta-
tions” where certain crystal directions will align among a
subset of the grains. What happens to the channeling con-
trast image as the grain size is reduced from the large single
crystal case?. Fig. 29.5 shows a coarse polycrystal with
grain dimensions in the millimeter range, again viewed
with a large scan field at low magnification. The scan angle
is nearly the same as for. Fig. 29.3, but the presence of
boundaries between the crystals (“grain boundaries”) inter-
rupts the channeling pattern so that we can no longer
observe enough of the pattern to determine the orientation.
We can, however, detect the grains themselves because of
the pattern discontinuities. In. Fig. 29.6, the grain size has
been further reduced, so that the magnification must be
increased so that the angular change across each grain is
now very small. The grains in this 75Fe-25Ni alloy are seen
with nearly uniform shades of gray because each grain ori-
entation effectively provides an easy, intermediate, or hard
orientation for channeling resulting in low, intermediate, or
high BSE emission. How far down in scale can crystalline
features be observed? With special high resolution SEMs
and energy selecting BSE detectors, the angular distortions
introduced by the strain fields of individual crystal disloca-
tions have been seen (Morin et al. 1979 ; Kamaladasa and
Picard 2010 ).
Finally, the long-range effects of plastic deformation,
which introduces defects and residual stress patterns into
ductile metals, can be directly observed in channeling con-
trast images.. Figure 29.7 shows the channeling contrast
from the damaged area around a large diamond stylus hard-
ness indent in the same 75Fe-25Ni alloy specimen shown in. Fig. 29.6. A similar effect around a hardness indentation in
polycrystalline nickel is shown in. Fig. 29.8, revealing the
extent of plastic deformation around the indent.
1 mm. Fig. 29.5 Electron channeling contrast from a coarse grain size in
polycrystalline Fe-3.2Si
100 μm. Fig. 29.6 Electron channeling contrast from grains in polycrystal-
line 75Fe-25Ni; E 0 = 15 keV; BSE detector
100 μm. Fig. 29.7 Electron channeling contrast from grains in polycrystal-
line 75Fe-25Ni deformed by a diamond scribe impact; E 0 = 15 keV; BSE
detector
29.1 · Imaging Crystalline Materials with Electron Channeling Contrast