# Precipitates and Dislocations

| 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.