Cottrell atmosphere
A. Bilby in 1949[1] to explain how dislocations are pinned in some metals by boron, carbon, or nitrogen interstitials.
This stress field can be relaxed by the interstitial atom diffusing towards a dislocation[citation needed], which contains a small gap at its core (as it is a more open structure), see Figure 1.
[citation needed] The collection of solute atoms around the dislocation core due to this process is the Cottrell atmosphere.
These 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.
[5] The existence of the Cottrell atmosphere and the effects of viscous drag have been proven to be important in high temperature deformation at intermediate stresses, as well as contributing to the power-law breakdown regime.
[6] While the Cottrell atmosphere is a general effect, there are additional related mechanisms that occur under more specialized circumstances.
The Suzuki effect is characterized by the segregation of solutes to stacking fault defects.
H. Suzuki predicted that the concentration of solute atoms at this boundary would differ from the bulk.
Moving through this field of solute atoms would therefore produce a similar drag on dislocations as the Cottrell atmosphere.
Just as the Cottrell atmosphere provided a force against dislocation motion, the Suzuki effect in the stacking fault will lead to increased stresses for recombination of partials, leading to increased difficulty in bypassing obstacles (such as precipitates or particles), and therefore resulting in a stronger material.
Under an applied stress, interstitial solute atoms, such as carbon and nitrogen can migrate within the α-Fe lattice, a BCC metal.
At room temperature, the solubility of carbon and nitrogen in solid solutions is exceedingly small.
[10] By raising, the temperature beyond 400oC and cooling at a moderate rate, it is easy to keep a few hundredths of a percent of either element within the solution, while the remainder is supersaturated.
[11] By preparing samples containing a larger amount of carbon or nitrogen in solid solution, magnetic and elastic phenomena are greatly enhanced.
[10] The study of the Snoek effect on annealed irons provides a reliable mechanism for calculating the solubility of carbon and nitrogen in α-iron.
[12] A sample in a mixture of hydrogen and ammonia (or carbon monoxide) is mixed and heated until a stationary state was reached, where the mass of carbon and nitrogen taken up during the process can be found by estimating the changes in the weight of the sample.
In BCC metals, interstitial sites of an unstrained lattice are equally favorable.
[14] In the new, relaxed solute configuration, more energy is therefore required to break a dislocation from this order.
[13] The interstitials that occupy the normal sites in an unstressed lattice will promote internal friction.
[13] Substituted solute atoms and interstitials in strain fields of a dislocation or at grain boundaries have their internal friction changed.
[13] Therefore, the Snoek effect can measure carbon and nitrogen concentration in BCC alpha-Fe and other solutes present in ternary alloys.
