When I was attending in the introductory course in materials science and metallurgy during my master of sciences, one particular question had intrigued myself for long time: are dislocations real physical entities?
As a curious student at the time, during the class, I have respectfully raised my hand and asked the professor: — Do dislocations really exist? And of course, the entire audience started to laugh on me. Students very often have such curiosities inside themselves, but are always reluctant to ask questions thinking they are only stupid doubts.
If you look in a materials science textbook, in most of the cases, the concept of dislocation will be properly described, but the figures often induce the reader/student to think that a dislocation is a real physical entity in the sense that they are something external to the crystal structure and which is added to it somehow. The textbooks sometimes give the impression that the dislocation is something real, something like an artefact.
The physical understanding behind the concept of “dislocation” is from 1930s when the mechanisms of plastic deformation of crystals were under deep investigation in Europe. The Royal Society Yarrow Professor Geoffrey Ingram Taylor was one of the pioneers in this field of research. In his paper entitled: “The Mechanism of Plastic Deformation of Crystals – Part 1 – Theoretical” , Taylor defined for the first time what is a dislocation.
Taylor’s early concepts on dislocations were motivated by very interesting experimental observations made by Joffe et al.  in analysing deformed rock salt crystals in nicol prisms (a kind of portable homemade polarized optical microscope). The latter authors noted that in deformed rock salt crystals , a very bright line from side to side of a crystal attracted the attention of that experimentalists whose concluded that such line was a representation of a “crystal breakdown (…) indicating distorted material.”
The theoretical interpretation given by Taylor was made by means of picturing a “crystal block” that under stress, the propagation of a line of slipping atoms within a slip plane would result in a perfectly well ordered crystal structure, but deformed by the unit slip, or the dislocation.
For Taylor a dislocation can be viewed as a kinetic phenomenon of the “passage” of a strain field from side to side of a crystal within a slip plane! This strain field, is of course, caused by the external stresses acting on the crystal. Dislocations do not exist! Dislocation is rather a physical concept that defines a defective region of a crystal structure, not a physical entity.
The movement of dislocations in recrystallised Au recorded in situ at 1000 K using Bright-Field Transmission Electron Microscopy.
When dislocations are viewed in a transmission electron microscope, they often appear in a form of lines or loops whose are of diffraction contrast, but the contrast is generated because in the analysed region of a dislocation, the crystal structure is defective (it has an extra plane of atoms). Therefore, the contrast does not exhibit something that is external to the crystal lattice itself, but it only shows that there is a defect in the local atomic arrangement which was supposed to be periodic and ordered.
Coming back to my class in the introductory course of materials science and metallurgy, I have asked this question to my professor at the time, he said to me that dislocations were real indeed, but I should be concerned in to study for the tests rather than asking stupid questions.
- Dark-Field Transmission Electron Microscope (DFTEM) micrograph colourised in a computer showing dislocation lines in recrystallised Au at ~ 1000 K
 Taylor, Geoffrey Ingram. “The mechanism of plastic deformation of crystals. Part I. Theoretical.” Proceedings of the Royal Society of London. Series A 145.855 (1934): 362-387.
 Joffe, Abram Fedorovich, and Leonard Benedict Loeb. “Physics of crystals.” (1928).