31
The former statistical model implies that the short
range ordered structure will probably not be the same as
the structure of the long range ordered phases that will
ultimately precipitate. In the microdomain model, the
short range ordered structure may be the same as the final
long range ordered structure since the model structure is
in fact merely a microcell of the final long range ordered
cell.
2.2.5 Ordering Reactions and Kinetics
2.2.5.1 Binary Alloys
Ni-20% Mo. This composition corresponds to
stoichiometric N4M0. Saburi et al. (1969) used electron
microscopy to study the ordering kinetics of a Ni-20% Mo
binary alloy quenched from solid solution and aged at 800
C. They conclude that the ordering process is
heterogeneous. Long range ordered domains of N4M0
nucleate in the matrix and grow with time. Domain
impingement is characterized by numerous perpendicular
twin plates. Chakravarti et al. (1970) used both TEM and
FIM (Field Ion Microscopy) to study the ordering of Ni-20%
Mo from solid solution. At 700 C they report that the
transformation is wholly homogeneous. A fine, mottled
"tweed" structure develops with aging times of up to and
less than three hours. After three hours, heterogeneous
precipitation of N4M0 is observed along grain
boundaries. Ling and Starke (1971) used x-ray line