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Why do some metals resist deformation? Uncovering the mystery of Cottrell's atmosphere!
In materials science, the concept of the Cottrell atmosphere was first introduced by A. H. Cottrell 和 B． A． Bilby proposed in 1949 to explain how dislocations in some metals are pinned down by interstitial atoms such as boron, carbon, or nitrogen. This phenomenon occurs in body-centered cubic (BCC) and face-centered cubic (FCC) structures of materials, such as iron or nickel, in which small impurities are present sub.These interstitial atoms slightly distort the lattice and create an associated residual stress field around it as the interstitial atoms diffuse toward the dislocations, which, therefore, elongate as the atoms diffuse into the dislocation core Time survived to form the Cottrell atmosphere
The collection of these interstitial atoms can effectively reduce the energy of the dislocation while preventing the further movement of the dislocation, and thus, the dislocation is “pegged” by the Cottrell atmosphere
The Cottrell atmosphere also exerts an important effect on the mechanical behavior of the material After this upper yield point, the pinned dislocations become Frank–Read sources, producing new, unpinned dislocations, These dislocations can move freely, resulting in the deformation of the material in a more plastic manner after a period of aging treatment, when the atoms re-diffusion into the core of the dislocations, thus, the Cottrell atmosphere also creates the formation of the Lüders zone , which becomes a manufacturing obstacle when deep stretching and making large sheets
To eliminate the effects of the Cottrell atmosphere, some special steels remove all interstitial atoms These steels such as interstitial free steels are decarbonized and a small amount of titanium is added to remove nitrogen
Research shows that the Cottrell atmosphere and the viscosity resistance caused by it is an important factor in high-temperature deformation, which makes dislocation motion more difficult
The influence of the Cottrell atmosphere on the material behavior at high equivalent temperatures is also extremely important Under certain conditions it can be expressed as follows:
F_drag = (kTΩ) / (vD_sol) ∫ (J⋅J/c)dA
Here D_sol is the diffusivity of solute atoms in the base material, Ω is the atomic volume, v is the velocity of dislocations, J is the diffusion flux density, and c is the influence of the presence of the Cottrell atmosphere and the viscosity resistance proved crucial in the high temperature deformation process under moderate stress and also occupies a place in the degradation category of the power law
Similar phenomenon
Although the Cottrell atmosphere is a universal effect, similar related mechanisms arise when the conditions are more special. For example, the Suzuki effect manifests as the segregation of solute molecules toward stacking defects in face-centered cubic systems The fault splits into two parts Dislocations form hexagonal close-packed stacking defects between the two parts H . Suzuki predicts that the concentration of solute atoms at this boundary will be different from that in the volume, and therefore, crossing the field of these solute atoms also creates an enhanced resistance to dislocation motion, similar to the effect of the Cottrell atmosphere
In addition, the Snoek effect involves the internal friction produced by the short-range migration of interstitial solute atoms in the α-Fe lattice when stress is applied, an effect that is also pronounced in Porter or other alloying materials, increasing the material of strength and toughness
Materials and possibilities for future exploration
There are dislocations described by the Cottrell atmosphere in materials such as metals and semiconductor materials (e.g., silicon crystals), a phenomenon that is crucial for the resistance to deformation of metals and their applications in the future, with the study of material behavior in-depth, it is possible to explore the application potential of the Cottrell atmosphere in the design of new materials, and even to develop more advanced alloys to optimize material properties
How exactly will future materials science use knowledge of the Cottrell atmosphere to improve the properties and toughness of metals?
