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phase, there is a sixth phase present in this alloy;
NiMo. An image of this precipitate and its diffraction
pattern are shown in Figures 5.7a and 5.7b. The
diffraction pattern is indexed in Figure 5.7c. The phase
was found almost exclusively along grain boundaries.
In order to characterize the Dla and DO22 phases
adequately, a number of variants for each precipitate type
were imaged. This required that images be taken from at
least two different FCC crystal orientations. The two
orientations are the B = [001] and the B = [112]. Figure
5.8 shows these two diffraction patterns and additionally
delineates which diffraction variants were imaged.
As explained in Section 2.3.4.1, there are three
imagable of 12 possible variants in N3M0 D022* All three
of these variants may be imaged along any B = <J.00)>
direction since all three can be present simultaneously in
a B = <^100)> diffraction pattern (Section 2.3.4.1). Figure
5.6b is the dark field image of the 1/4 (420) variant.
This variant does not fill the intercuboidal regions. It
appears along only two faces of the gamma prime. Figure
5.6c is a dark field image of the 1/4 (240) variant. It
appears along the other two gamma prime faces, but not on
the same faces as does the 1/4 (420) variant.
The interpretation of these two dark field images is
straightforward. Figure 5.9 describes the relationship of
the variant orientation with the matrix and interpretation