3. Extrinsic dislocations and segregation

Extrinsic dislocations possess long-range elastic deformation fields and therefore constitute preferential sites of attraction for solute atoms in the joint. As in a matrix, these form "Cottrell clouds" or "Cottrell atmospheres". When the local content of segregated elements reaches its maximum value, the dislocation stabilizes; the dislocation is said to be trapped by the Cottrell atmosphere.

Figure 23 shows an example of yttrium overconcentration in the immediate vicinity of two stabilized extrinsic dislocations in a general alumina grain boundary.