Dislocation can also be called dislocation. In materials science, it refers to an internal microscopic defect of crystal materials, that is, the local irregular arrangement of atoms (crystal defect). Geometrically speaking, dislocation is a kind of line defect, which can be regarded as the dividing line between the sliding part and the non-sliding part in the crystal. Its existence has a great influence on the physical properties, especially the mechanical properties of materials. The concept of dislocation was first put forward by Italian mathematician and physicist Vito volterra in 1905.

There are two main forms of virtual dislocation: edge dislocations and screw dislocation, which are line defects. Mixed dislocations have the characteristics of the first two. Mathematically, dislocation belongs to a topological defect, sometimes called "soliton" or "soliton". This theory can explain the behavior of dislocations in real crystals: they can move in the crystal, but their types and characteristics remain unchanged during the movement; If two dislocations (Burgess vectors) in opposite directions move to the same point, they will both disappear, or be called "annihilation". If they do not interact with other dislocations or move to the crystal surface, then any single dislocation will not "disappear" on its own (that is, the Burgess vector will be conserved forever).

Dislocation lines must be continuous, either starting and ending at the crystal plane (or grain boundary), or forming a closed loop (dislocation loop), or connecting with other dislocations at the junction. Because of the depth of field, we usually get a little error in high resolution or using double beams.