Cottrell atmosphere

In materials science, the concept of the Cottrell atmosphere was introduced by Cottrell and Bilby in 1949 to explain how dislocations are pinned in some metals by carbon or nitrogen interstitials.

Cottrell atmospheres occur in body-centered cubic (BCC) and face-centered cubic (FCC) materials , such as iron or nickel, with small impurity atoms, such as carbon or nitrogen. As these interstitial atoms distort the lattice slightly, there will be an associated residual stress field surrounding the interstitial. This stress field can be relaxed by the interstitial atom diffusing towards a dislocation, which contains a small gap at its core (as it is a more open structure), see Figure 1. Once the atom has diffused into the dislocation core the atom will stay. Typically only one interstitial atom is required per lattice plane of the dislocation.