. Fig. 29.3  a Wide field scanning (BSE image) of a Si single crystal wafer whose surface is parallel to the (001) plane, thus looking along the [001] pole; E0 = 15 keV. b The traces of two different sets of crystal

a

planes are marked, as well as the parallel channeling bands defined when the scan angle to the planes equals ± θB. Images processed with ImageJ-Fiji CLAHE function

b

29.1.2\ Polycrystalline Materials

Most crystalline materials are not single crystals. During solidification from a molten state, numerous crystals

nucleate randomly in the liquid and grow in size until they encounter each other, producing a three-dimensional microstructure of randomly oriented crystalline units called “grains.” Materials fabrication processes such as

100 µm

. Fig. 29.6  Electron channeling contrast from grains in polycrystalline 75Fe-25Ni; E0 = 15 keV; BSE detector

rolling, extrusion, forging, etc., and subsequent thermal treatment result further modify the crystalline microstructure, resulting in grain sizes ranging from centimeters to nanometers depending on composition and thermomechanical history and often inducing “preferred orientations” where certain crystal directions will align among a subset of the grains. What happens to the channeling contrast 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”) interrupts 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 orientation 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 dislocations 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 contrast images. .  Figure 29.7 shows the channeling contrast from the damaged area around a large diamond stylus hardness 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.