Cottrell atmosphere
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2008-01-26T23:04:24Z
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In [[materials science]], the concept of the '''Cottrell atmosphere''' was introduced by Cottrell and Bilby in 1949 <ref>A Cottrell and B Bilby. Proc. Phys. Soc. 1949</ref> to explain how [[dislocations]] are pinned in some metals by [[carbon]] or [[nitrogen]] [[Interstitial_defect|interstitials]].
[[Image:Cottrellatmosphere.png|thumbnail|right|'''Figure 1:''' A carbon atom below a dislocation in iron, forming a Cottrell atmosphere]]
Cottrell atmospheres occur in body-centered cubic (BCC) 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_%28physics%29|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.
Once a dislocation has become pinned, a small extra force is required to unpin the dislocation prior the yielding, producing an observed upper yield point in a [[Stress-strain_curve|stress-strain]] graph. After unpinning, dislocations are free to move in the crystal, which results in a subsequent lower yield point, and the material will deform in a more plastic manner.
Leaving the sample to age, by holding it at room temperature for a few hours, enables the carbon atoms to rediffuse back to dislocation cores, resulting in a return of the upper yield point.
Cottrell atmosphere's lead to formation of [[Luders' Bands]] and large forces for deep drawing and forming large sheets, making them a hindrance to manufacture. Some steels are designed to remove the Cottrell atmosphere effect by removing all the interstitial atoms. Steels such as [[Interstitial Free Steel]] are [[Decarburizing|decarburized]] and small quantities of [[titanium]] are added to remove nitrogen.
==Notes==
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[[Category:Materials science]]
[[de:Cottrell-Wolke]]