30
higher probability of finding a B atom next to an A atom
than there is of finding a B atom next to another B atom.
This model is the statistical-mechanical model originally
proposed by Clapp and Moss (1966, 1968a, 1968b). It is
derived from classical descriptions of SRO.
The scattered intensities can also be derived from a
model in which very small long range ordered (LRO) regions
within the normally disordered matrix diffract to produce
the diffuse maxima. This is the microdomain model,
originally proposed by Spruiell and Stansbury (1965).
They used x-ray diffraction to study the phenomenon in
Ni-Mo alloys. Ruedl et al. (1968) used dark field
electron microscopy to image these small LRO domains.
They found, as did Das and Thomas (1974), Okamoto and
Thomas (1971), and Das et al. (1973), that the microdomain
model could explain the very fine precipitate that they
were able to image using the (1, 1/2, 0) diffuse
reflections. Similarly, deRidder et al. (1976) proposed a
cluster model which describes clusters of atoms with
simple polyhedral arrangements. The polyhedral clusters
so described are actually prototypes of long range order,
though they can also be considered as most probable
arrangements and hence statistical. In addition to
explaining diffuse maxima at the (1, 1/2, 0) positions,
these clusters can explain other diffuse maxima in
electron diffraction patterns of Ni-Mo binary alloys.