 |
Precipitates usually do not fit into the host
lattice. The growing particle causes considerable stress that can be reduced by
plastic deformation. |
|
 |
If the precipitate fits in one lattice direction,
but not in others (a precipitate with an hexagonal lattice, e.g., may fit
relatively well on the {111} planes of an fcc lattice) a compromise between a
non-spherical shape of the precipitate and a system of dislocation loops in
some direction may produce least stain energy. The precipitate-dislocation
system then has a very specific structure; the process is known as
"prismatic loop punching". An example is shown below on the left
(taken under kinematic bright field conditions). |
|
|
|
|
|
 |
 |
|
|
Precipitate with prismatic loops. An arrangement like that
accounts for the peculiar etch features shown
before |
Plate-like precipitate (the dark grey feature) with
dislocations relieving parts of the stress. |