12.48, and c = 7.10. All of these phases and the relevant

Ni-Ta phases are summarized in Table 2.1.


2.2.3 Domain Variants/Antiphase Boundaries


 The phases just described exhibit wide variability in

both the crystallographic habits which they can take and

in the interfaces that result from domain impingement.

When these different variants come into contact out of

phase, domain boundaries are created. These interfaces

are known as antiphase boundaries. Common to all of the

ordered precipitates previously described are 1)

translational antiphase boundaries, 2) antiparallel twin

boundaries, 3) perpendicular twin boundaries, and 4)

dissociated antiphase boundaries. The permissible

variants and three of the four antiphase boundary types

are described in the following sections.

2.2.3.1 Ni4x (Dla)


 Thirty different variants can form in this structure

(Harker, 1944). First, the tetrad (c) axis can be

parallel to any one of the three cube axes of the parent

lattice. Second, the a axis of the Ni4x lattice can be

rotated clockwise or counterclockwise relative to the FCC

cube axis. Third, the origin can be shifted, allowing

five independent variants (one x and 4 Ni) to exist.

There are thus 3x2x5 = 30 domain orientations. Ruedl et

al. (1968) have reviewed the three domain boundaries that