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2.3.3 SRO (1, 1/2, 0) Scattering
In Ni-Mo binary alloys, SRO is characterized by
diffuse scattering at the (1, 1/2, 0) positions. (All g
vectors will subsequently be defined in FCC coordinates.)
Diffraction patterns of (1, 1/2, 0) SRO for B = [100] and
B = [112] are shown in Figure 2.19a. Characteristic of
this scattering is the absence of any true superlattice
reflection, for example, the (100) and (011) reflections
of either the DO22 or the LI2 superlattices. Because
gamma prime phase is always present in the alloys studied
here, any selected area diffraction pattern will always
contain LI2 superlattice reflections. Discriminating
between the DO22 superlattice reflections and scattering
at (1, 1/2, 0) SRO positions with superimposed LI2
superlattice reflections is difficult. There are two ways
to differentiate SRO from the LI2 and DO22 superlattices,
both of which are discussed in Chapter 5.
2.3.4 Variant Imaging
In section 2.2.3 of this chapter, a plethora of
possible variants for each precipitate were given. As
should be readily apparent from the reciprocal lattice
constructions of this section, only certain of these
variants are imagable with the electron microscope. Many