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precipitate and grow just as in the RSR 197 alloy aged
only at 760 C, as discussed in Section 5.1.4.1. The high
temperature aging should reduce any interfacial strain and
thus presumably any intercuboidal strain. The DO22 and
Dla phases still form as previously in the sample aged
only at 760 C for 100 hours. This very strongly indicates
that it is not a strain gradient between the gamma prime
precipitates that leads to preferential nucleation, but
rather the strain induced by the nucleus that will prevent
the nucleation of the variants of DO22 whose c axis is not
perpendicular to the 100 faces of the gamma prime.
The N4M0 Dla phase is not observed as having the
same crystallographic constraints as the DC>22* Along
either cube face, either or both visible variants are
imageable. Based on the observations made in Section
5.4.1, only four N4M0 variants should ever exist between
any two gamma prime faces. If the other two variants were
present, perpendicular twin boundaries would result
between these impinging Dla precipitates. This is the
highest energy antiphase boundary in Dla (Ling and Starke,
1971). These other two variants would also not exhibit
the same crystallographic relationships with the DO22
phase that were described earlier in Section 5.1.4.1.