161
in the W containing alloy, RSR 209. The mismatch between
the gamma prime and gamma lattice parameters in the two
alloys is also different: larger in the RSR 209 than in
the RSR 197. Mismatch is defined as aD gamma prime minus
aG gamma divided by a0 gamma (Sims and Hagel, 1972).
In many cases, Table 5.1 does not list the lattice
parameter of the gamma phase. The measurement of the
exact gamma peak position was not always possible, due to
either insufficient signal, a texture effect, or to the
presence of the metastable Ni-Mo phases. As an example,
the gamma lattice parameter could not be unambiguously
measured for the RSR 197 sample that was aged for 100
hours at 760 C. The diffraction peak in the gamma region
was smeared to such an extent in this spectrum that
locating the exact maximum in the gamma peak intensity was
not possible. Without the TEM result showing significant
NixMo precipitation, interpretation of this x-ray
diffractogram would be difficult.
5.3 Convergent Beam Measurements
Numerous measurements of the gamma prime lattice
parameter were made using the B = <(ll4)> convergent beam
electron diffraction and HOLZ line patterns described
extensively in Chapter 4. The results of these
measurements are listed in Table 5.2. Measurements were
made directly from the negatives using a 10X loupe