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6.2 Precipitation in RSR 197
The precipitation of the phases in RSR 197 can be
summarized as follows. Upon quenching, the DO22 phase
precipitates from the Mo supersaturated matrix. The DO22
phase is stabilized by Al. (DO22 has also been shown to be
stabilized by Ta additions to in Ni-Mo binaries. There is
not enough Ta in the matrix phase of RSR 197, however, to
account for the DO22 precipitation in RSR 197). The DO22
phase may nucleate preferentially in at least one of two
ways: 1) under the influence of a strain gradient between
the gamma prime cuboids, or 2) because the distribution of
gamma prime cubes is such that this distribution prohibits
the nucleation of the other two variants between the cubes
in preference for the variant whose c axis is perpendicular
to a gamma prime cube face. Either could explain the
preference of the DO22 to form as a single variant on these
unique 100 faces of the gamma prime cubes. After the DO22
has nucleated, this strain is shown to be about +1.9% in the
direction of the DO22 c axis. Convergent beam electron
diffraction was used to make this measurement.
The N4M0 phase seems to precipitate after the DO22 has
precipitated. The two coarsen and grow in a mutual
relationship along the 100 direction of the cube face,
presumably under the influence of self stress, analogous to
the coarsening behavior of N3AI in Ni-Al binary alloys
(Ardell et al., 1966).