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Figure 5.17a is a microdiffraction pattern from the
gamma phase. The DO22 reflections are present in this
convergent beam microdiffraction pattern. A dark field
image can be formed if one of these DO22 reflections is
used to form the image. Figure 5.17b is an image taken
using a 1/4(420) reflection. There are two features of
special interest in this image. First, the fine
precipitate is imaged throughout the gamma phase. Second,
as was the case when DO22 was responsible for the (1, 1/2,
0) reflections, one variant of the DO22 predominates along
any single gamma prime cube face, though the other variant
is present as well. Both variants can be plainly seen in
the diffraction pattern. The fine scale of this
precipitate shows that it precipitated during the cool
down from the aging temperature and that it was not
present in the alloy at the aging temperature.
After 100 hours aging, large platelets of equilibrium
delta NiMo have formed throughout the grains. An example
is shown in Figure 5.18a, a low magnification image
showing numerous rod-like and plate-like precipitates.
The diffraction pattern of the needle or edge on plate
shown in Figure 5.18b is shown in Figure 5.18c. The
diffraction pattern is not a low index one, but is close
to a B = <fl00])> NiMo pattern and close to a B = <^100^ gamma
matrix diffraction pattern.