125
must also appear on the (010) face since this face
contains an 024 plane which is also a plane in the 1/5
(042) variant. In a similar way, the 1/5 (402) variant
will appear on the 001 face, but since it is a different
variant of the Dla, its morphology will not be the same as
the Dla morphology of the 1/5 (042). The 1/5 (402)
variant will also appear on the 100 face, since this
variant also contains a 204 plane. To differentiate the
Dla precipitate on the 001 face from the same variants on
the other 100 faces, it is necessary to compare the DO22
images with the Dla images. The DO22 images appear only
on the (001) faces of the gamma prime.
The DO22 precipitates form in discreet lines along
<100> directions, analogous to the alignment of the gamma
prime described by Ardell et al. (1966). Figure 5.11a
shows this morphology. The individual morphologies of the
DO22 are consistent with the morphologies explained
earlier. The morphology of the Dla phase is also as
predicted from the B = <(001^> images. The variants of
N4M0 Dla, shown in Figures 5.12b and 5.12c, form along
the DO22 precipitates, just as they did in the B = <(00l)>
images. These third axis dark field images show that the
interlacing of the Dla and DO22 phases results in almost
complete order of the gamma phase.