93
1979). This would of course apply to nonsymmetrical
changes in lattice parameter as well. The magnitude of
these nonsymmetrical changes can be deduced by measuring
the change in HOLZ line positions as the lattice parameter
changes, described in the preceding section.
Again, the easiest way to visualize the effects of
crystal asymmetry is to look at a few examples of CBED
HOLZ patterns to see how this asymmetry affects HOLZ line
position. The direction of shift of the lines will depend
on the orientation of the now noncubic crystal with
respect to the beam direction. For example, if the
expansion or contraction is along the c axis, the c
direction defined as being parallel to the beam, the
symmetry of the pattern changes very little. If the beam
is a parallel to either of the a or b cube axes, the
change in symmetry is very marked, as shown in Figures
4.11a, 4.11b, and 4.11c. The method for calculating these
HOLZ line patterns is explained in Appendix A.
It is not straightforward to differentiate symmetry
changes from lattice parameter changes to arrive at a
measure of both lattice parameter and loss of cubicity.
Ecob et al. (1981) simulated CBED HOLZ line patterns to
measure the lattice parameter differences in gamma/gamma
prime alloys, and to measure the changes in symmetry of
the gamma prime phase after recrystallization. The
simulations were then compared to actual patterns.
Numerous trial and error iterations would usually