of the diffraction pattern. The figure shows clearly that

the c axis of the D022 phase is perpendicular to a 100

face of the gamma prime. The third variant will thus

appear only in regions of overlapping gamma and gamma

prime, regions where the DO22 axis is parallel to the

beam, regions B, C, D, and E in Figure 5.4. The

diffraction spot from which a dark field image of variant

A (Figure 2.18b) would be formed corresponds to the same

spot from which an L12 dark field image would be formed.

Both the D022 and L12 crystals have a superlattice

reflection at g = (110). Imaging the third variant, then,

is rendered impossible by the presence of the gamma prime.

 A comparison of the images in Figures 5.6b and 5.6f

with the schematic in Figure 5.9 also shows that the (420)

habit of the D022 phase can easily be reconciled with the

(420) Mo planes of the D022. The crystallography of the

D022 would seem to determine the morphology of the D022
precipitate. The DO22 precipitates can be seen to be

comprised exclusively of either combinations of the (420}

planes or combinations of (420) planes and the limiting

(100 planes of the L12 interfaces. For example, many of

the D022 precipitates are triangular in projection. This

is consistent with four {420) planes truncated by a t1001,

e.g., an (024), (024), (420), and (420) plane truncated by

an (010) plane. This pyramid would appear as a triangle

in either an [001] or an [010] projection. Two such