121
of the diffraction pattern. The figure shows clearly that
the c axis of the DO22 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 LI2 dark field image would be formed.
Both the DO22 and LI2 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 DO22 phase can easily be reconciled with the
(420) Ho planes of the DO22 The crystallography of the
DO22 would seem to determine the morphology of the DO22
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
(lOo) planes of the LI2 interfaces. For example, many of
the DO22 precipitates are triangular in projection. This
is consistent with four {420} planes truncated by a {l00},
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