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 strain 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).
Prismatic looppunching
Precipitate with dislocations
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.
The two precipitates ("A" and "B") are seen as dark shapes; their nature is unclear, but they are probably SiO2

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go to 6.3.2 Examples and Case Studies for Dislocations

go to 6.3.3 Stacking Faults and Other Defects

go to Precipitates and Other Defects as Seen with Preferential Etching

© H. Föll (Defects - Script)