Cottrell atmosphere

A Cottrell cloud (English Cottrell atmosphere ) is a concept in solid-state physics and materials science to explain the hindrance of dislocation motion by interstitial atoms ( carbon or nitrogen). The theory was described by Alan Cottrell and B. Bilby 1948.

Properties

Cottrell clouds occur in body-centered cubic ( bcc ) metals such as iron and chromium, which by small interstitial atoms (eg, carbon and / or nitrogen) are contaminated. Since these impurities are a little larger than the interstitial sites and smaller than the iron atoms, they distort the surrounding crystal lattice easily and around them results in a residual stress field which is connected with an increase in the strain energy. This voltage can be compensated by the distortion power in the vicinity of a core region. The foreign atoms move therefore preferred to transfers made to, as shown in Figure 1, in order to reduce the total strain energy of the system.

Once again a foreign atom is attached to the transfer, it stays there. When an external voltage is applied to the body, for example, under tensile load because of the festgepinnten by the Cottrell cloud displacements, additional force required to initiate movement of dislocation. This manifests itself as so clearly defined yield point or upper yield point ( Losreißspannung ) in a stress-strain diagram. After the dislocations have liberated from the impurity clouds, the voltage drops to a lower value, called the lower yield point, from which the material can deform plastically free.

If the material ages after the liberation at room temperature or at higher temperatures, the C and N atoms diffuse back to the dislocation cores, and one can observe a clearly defined yield point again. Cottrell clouds make it difficult deep drawing and forming processes of sheet metal, and lead to the formation of Lüders bands ( " flow lines " / "orange skin") on the surfaces, which may be the Committee.

To avoid the Cottrell effect, some steels are designed so that they do not include interstitial impurities such as the IF steels ( interstitial free ), which thus have particularly good formability. A further possibility is the temper rolling of metal sheets prior to thermoforming. This refers to a small, about 5% pre-deformation of certain steel for deep drawing, to release the dislocations from Cottrell clouds caused by aging.