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developed during the extrusion process. This phase was
taken into solution during the solution heat treatment.
The larger gamma prime precipitates along the grain
boundaries result from accelerated coarsening of gamma
prime at the high angle grain boundaries (Funkenbusch,
1983). This coarsening presumably takes place during the
air quench from the solution heat treatment temperature.
The major features of the microstructure include the
gamma prime cuboids and the surrounding gamma matrix.
Many of the gamma prime cuboids are impinging, as shown in
Figure 5.2b. Some precipitation of DO22 phase has
occurred within the gamma matrix. The selected area
diffraction pattern shown in Figure 5.3a clearly shows the
presence of the DO22 superlattice reflections (cf. Figure
2.20b). There is very strong streaking associated with
each DO22 reflection. The streaks are not visible on the
fluorescent screen of the microscope and can be seen only
after long photographic exposure. Figure 5.3b is a g =
1/4 (420) dark field image of the DO22 phase. The beam
direction is approximately along a B = 001 direction.
The fine DO22 phase is distributed uniformly throughout
the gamma matrix. A high magnification g=l/4(420),
B=<001> image of this DO22 phase is shown in Figure 5.3c.
Note the angularity of the precipitate and the presence of
what appear to be many unidirectional faults in the DO22
precipitates.