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pyramids could share a common [100} face. The resulting

TEM image would be a diamond. A third possibility would

be two parallel sets of (420) planes truncated by two

parallel {1003 planes, perhaps two facing {100) gamma

prime faces. The resulting image would be a rhombus. All

of these features can be readily seen in the dark field

images, Figures 5.6f and 5.6b, of the D022 Ni3Mo.

 Only two Dla variants are imagable along any B =

<100> direction. Both of these variants are associated

closely with the D022 morphologies just described. The

two visible Dla variants (reference the indexing in Figure

5.8) contain the following four 420 planes: the clockwise

variant contains a 420 and a 240. From the previously

described DO22 420 stacking sequence, Figure 5.9, a

clockwise Dla variant could share a D022 420 plane between

010 faces of the gamma prime, or, could share the D022 240

on the 100 faces. Similarly, the counterclockwise variant

could share a D022 420 on the 010 face, or a DO22 420 on

the 100 face. This stacking is schematically represented

in Figure 5.9. Dark field images of the Dla corresponding

to the schematic are shown in Figures 5.6c and 5.6e.

 Because one Dla variant can grow on both the 010 and

the 100 faces of the gamma prime, the Ni4Mo Dla can

readily grow around the edge of the gamma prime cuboids

without introducing a stacking fault. (The D022 would

always form a perpendicular twin boundary when it impinged