Cottrell cloud
A Cottrell cloud ( English Cottrell atmosphere ) is a concept from solid state physics and materials science to explain the hindrance of the dislocation movement by interstitial atoms (carbon or nitrogen). The theory was described by Alan Cottrell and B. Bilby in 1948.
properties
Cottrell clouds occur in body-centered cubic (krz) metals such as iron and chromium that are contaminated by small interstitial atoms (e.g. carbon and / or nitrogen). Since these foreign atoms are a little larger than the interstitial spaces and smaller than the iron atoms, they easily distort the crystal lattice surrounding them and an internal stress field arises around them, which is associated with an increase in the distortion energy. This stress can be compensated for by the distortion stress in the immediate vicinity of a dislocation core. The foreign atoms therefore prefer to move towards dislocations, as shown in the figure above, in order to reduce the overall distortion energy of the system.
Once a foreign atom has attached itself to the dislocation, it stays there. If an external tension is applied to the body, for example under tensile load, additional force is necessary to initiate the dislocation movement because of the dislocations pinned by the Cottrell clouds. This manifests itself as a so-called pronounced yield point or upper yield point (breakaway stress) in a stress-strain diagram . After the dislocations have freed themselves from the foreign atom clouds, the stress drops to a lower value, the so-called lower yield point, from which the material can freely plastically deform.
If the material ages after liberation at room temperature or at higher temperatures, the C and N atoms diffuse back to the dislocation nuclei, and a pronounced yield point can be observed again. Cottrell clouds make deep-drawing and forming processes of sheet metal more difficult and lead to the formation of Lüders bands (" flow figures " / "orange peel") on the surfaces, which can then be rejected.
In order to avoid the Cottrell effect, some steels are developed so that they do not contain any interstitial impurities, e.g. B. IF steels (interstitial free), which therefore have particularly good forming properties. Another possibility is the skin pass of sheet metal before deep drawing. This is understood to mean a slight pre-deformation (0.5 to 2%) of the steel intended for deep drawing, in order to remove the dislocations from the Cottrell clouds caused by aging.
literature
- AH Cottrell, B. Bilby: Dislocation theory of yielding and strain aging of iron. In: Proc. Phys. Soc. London Sect. A, 1949, vol. 62, pp. 49-62.