Both phases can grow from cuboid to cuboid with

mutual 420 habit planes, as demonstrated by the lattice

image in Figure 5.12. These phases merely represent a

change in the stacking sequence across a common 420 plane.

 It is entirely likely that there is no gamma phase

present in RSR 197 aged at 760 C for 100 hours. The dark

field images in Section 5.4.1 show no volume of

intercuboidal gamma phase not occupied by D022 or Dla.

 When aged at 810 C, above the solvus for the Ni4Mo,

only the D022 phase forms. This is never observed in

Ni-Mo binary alloys. The D022 still has a slight 420

character but the alloy does not seem as well ordered as

at lower temperatures. The D022 at this temperature also

precipitates with its c axis perpendicular to a specific

100 face of the gamma prime.

 In samples air quenched from 870 after aging, short

range order can exist. This short range order gives rise

to dark field diffraction contrast. This means that it

can be imaged as discreet domains. This very strongly

indicates that the type of (1, 1/2, 0) short range order

diffraction effect observed in RSR 197 is due to

microdomains.

 At this highest aging temperature, interfacial

dislocations are formed. If the dislocations are first

allowed to form by aging the alloy at 870 C for one hour

and then the aging is continued at 760 C for 100 hours

without an intermediate quench, the D022 and Dla phases