What Matteo and Jaime said are all correct. But there is still one additional piece of information I want to add and discuss. Namely, the energy of a dislocation dipole is much lower that the energy of dislocations apart. Since the crystal prefers to keep its energy as small as possible, dislocations in load-free crystal normally exist in bounded state in form of dipoles, so we do not see any macroscopic plastic deformation. Only if the applied stress exceeds some critical threshold, dislocations of positive and negative signs move apart dissolving the dipoles and creating geometrically necessary dislocations (GND) with non-zero net Burgers vector. Thus, we see that the dislocation dipoles serve as the source of GND (in addition to the well-known Frank-Read source). Interesting thing happens near the free-boundary of crystal: there we have the dipoles of dislocations and image dislocations. If the resolved shear stress achieves maximum near the free surface (like in the case of bending of a beam), the dislocation dipoles are dissolved: real dislocations move toward the neutral axis of the beam, their images move apart. See for instance
https://www.researchgate.net/publication/245536640_On_bending_of_single_crystal_beam_with_continuously_distributed_dislocations
Article On bending of single crystal beam with continuously distribu...
In my view during the dislocation there charge particles displacement occurs and hence the dipoles and multipoles arises.
before asking or before answering, did you guy even tried to find and open a book on dislocations (e.g. Nabarro or Hirth and Lothe or Hull and Bacon...) and have a look for a possible answer ? alternatively, did you try googling for this? trying to reinvent on this has little sense....
Dislocation dipoles have nothing to do with charged or non charged particles. A dipole is simply an arrangement of two dislocations of opposite sign on parallel planes and at a very close distance.
To my knowledge arrangement of atoms or ions in a periodical manner is the crystal structure and dislocation is a type of defect in the crystal structre.
So if we consider Matteo Leoni's comment which is based on that there is nothing to do with charged or non charged particles.Then can you please tell me that what actually happens in dislocation ?
What i understand in dislocation the defect occurs in a certain plane inside the crystal structure like the edge dislocation which occurs due to sliding between two planes.
Then can you please tell me if not atom or ion , sliding of which things actually do occur ?
Please kindly do add your comment dear Matteo Leoni....
Waiting for ur answer
Also please simplify this sentence.....
A dipole is simply an arrangement of two dislocations of opposite sign on parallel planes and at a very close distance.
forthwhich i will be thankful 2 u
Dear Choi,
Kindly refer book "Introduction to Dislocations" by Hull and Bacon . They describe the formation of dipole and multipole and their role in the deformation process with necessary schematics. Some excerpts for you- not complete "edge dislocations of
opposite sign gliding past each other on parallel slip planes tend
to form stable dipole pairs...Dislocationdipoles are a feature of the early stages of plastic deformation, when slip is confined to one set of planes....Dislocation sources operate under resolved shear stress τ, and dislocations of one types interact with dislocations of opposite sign to form an array of dipoles known as a multipole". The dipoles and multipoles is generally described as part of dislocation jogs. good luck
Dear Seongho,
Matteo's definition of a dipole is the correct one but let me just add some additional physical meaning to it. The key about dislocation dipoles and multipoles is that the net Burgers vector (which is the analog of 'charge' in crystal plasticity) of the entire arrangement is zero. That is, globally, they introduce no net slip into a crystal. Multipoles (and dipoles in particular) are very interesting and useful configurations because they preserve the periodicity of a crystal lattice, and are the preferred configuration to use in atomistic calculations of dislocation structures.
Other than that, dipoles are seen often during plastic deformation when dislocation loops having one fast orientation and one slow orientation expand under applied stress, leading to the formation of oppositely signed long segments, i.e. dipoles.
What Matteo and Jaime said are all correct. But there is still one additional piece of information I want to add and discuss. Namely, the energy of a dislocation dipole is much lower that the energy of dislocations apart. Since the crystal prefers to keep its energy as small as possible, dislocations in load-free crystal normally exist in bounded state in form of dipoles, so we do not see any macroscopic plastic deformation. Only if the applied stress exceeds some critical threshold, dislocations of positive and negative signs move apart dissolving the dipoles and creating geometrically necessary dislocations (GND) with non-zero net Burgers vector. Thus, we see that the dislocation dipoles serve as the source of GND (in addition to the well-known Frank-Read source). Interesting thing happens near the free-boundary of crystal: there we have the dipoles of dislocations and image dislocations. If the resolved shear stress achieves maximum near the free surface (like in the case of bending of a beam), the dislocation dipoles are dissolved: real dislocations move toward the neutral axis of the beam, their images move apart. See for instance
https://www.researchgate.net/publication/245536640_On_bending_of_single_crystal_beam_with_continuously_distributed_dislocations
Article On bending of single crystal beam with continuously distribu...
Khanh Chau,
good point, the elastic energy of a single dislocation is unbounded (increases logarithmically) and its stress field decays as 1/r (long range), while the elastic energy of a dipole (multipole) is finite and its field decays as 1/r^n, where n is the order of the multipole (n=2 for a dipole), i.e. much faster. That makes multipoles, as you mention, much more energetically favored.
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