cottrell atmosphere
DESCRIPTION
Cottrell AtmosphereTRANSCRIPT
Cottrell atmosphereFrom Wikipedia, the free encyclopediaInmaterials science, the concept of theCottrell atmospherewas introduced by Cottrell and Bilby in 1949[1]to explain howdislocationsare pinned in some metals bycarbonornitrogeninterstitials.
A carbon atom below a dislocation in iron, forming a Cottrell atmosphereCottrell 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. Thisstress fieldcan be relaxed by theinterstitialatomdiffusing 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 astressstraingraph. 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 atroom temperaturefor a few hours, enables the carbon atoms to rediffuse back to dislocation cores, resulting in a return of the upper yield point.Cottrell atmospheres lead to formation ofLders bandsand 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 asinterstitial free steelaredecarburizedand small quantities oftitaniumare added to remove nitrogen.Notes[edit]1. Jump up^Cottrell, A. H. (1949),"Dislocation Theory of Yielding and Strain Ageing of Iron",Proceedings of the Physical Society62(1): 4962,Bibcode:1949PPSA...62...49C,doi:10.1088/0370-1298/62/1/308
Categories: Crystallographic defects
http://en.wikipedia.org/wiki/Cottrell_atmosphere