6.2 Precipitation in RSR 197


 The precipitation of the phases in RSR 197 can be

summarized as follows. Upon quenching, the D022 phase

precipitates from the Mo supersaturated matrix. The D022

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 D022 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 D022

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 Ni4Mo phase seems to precipitate after the D022 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 Ni3Al in Ni-Al binary alloys

(Ardell et al., 1966).