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Martin (1982) has studied the nucleation of the DO22
phase. (Mishra (1979), Chevalier and Stobbs (1979), and
Nesbit and Laughlin (1978) studied nucleation in NiMo
binaries using electron microscopy, but were only able to
study the nucleation of N4M0 Dla and N2M0 Pt2Mo. The
DO22 is not stable in the binary alloy.) In Martin's
N4M0 A1 containing ternary (this ternary does not contain
gamma prime) the DO22 and Pt2Mo were shown to form more
or less simultaneously when aged at 600 C. The Dla formed
sluggishly and was present after 2000 hours. The DO22 and
Pt2Mo were still present after this long aging time.
Pt2Mo was the predominant phase, very similar to alloy RSR
209.
In the RSR 197 alloy, the DO22 phase nucleates during
the slow quench from solution heat treatment temperature.
The streaks that appear to emanate from the DO22
reflections towards the Dla reciprocal lattice positions
shown in Figure 5.3a could be better described as
emanating from N4M0 nodes. This would imply that the
N4M0 Dla has formed as very thin platelets, possibly as
faults in the N3M0 crystal structure. This could be
determined by lattice imaging. Figure 5.3c shows both the
DO22 phase and the bright faults that would correspond to
the N4M0.
In the RSR. 197 alloy quenched and aged at 870 C, two
(1, 1/2, 0) microstructures have formed. In the sample