Dislocations in Crystal Structures explained
In material science, dislocations are lattice defects within a crystal structure. Dislocations occur when stress is applied and can move from atom to atom, causing plastic deformation. Dislocations play an important role in metallurgy because they strongly influence mechanical properties of materials, such as strength and ductility.
There are two major type of dislocations to be distinguished: edge dislocations and screw dislocations.
Burgers Vector and line direction
The Burgers Vector (b), named after Dutch physicist Dan Burgers, is a vector used to describe the magnitude and direction of the lattice distortion of a dislocation. The Burgers vector is measured by the distance between two atoms the dislocation is moving from.
The line direction is the line within the crystal structure around which the field of the dislocation occurs.
Dislocations are determined by both the burgers vector and the line direction. Depending on which type of dislocation is present, they interact differently with each other.
Edge Dislocations
The edge dislocation is characterized by the extra half-plane of atoms that is introduced to the crystal structure when a significant amount of stress is applied. This disturbs the nearby planes of atoms, causing plastic deformation. The burgers vector and the line direction are perpendicular to each other in edge dislocations.
Dislocations can move from atom plane to atom plane and only affect atoms in the vicinity. Once a dislocation has left an area within the lattice, its structure is completely restored.
Edge dislocations are also referred to as line dislocations because irregularities run along a single line on top of the extra half-plane.
Screw Dislocations
Another type of dislocations are screw dislocations. They are named after their movement, in which the dislocations move along the line direction. The Burgers vector and the line direction are parallel to each other. The movement of screw dislocations can be imagined as a spiral as well.
Screw dislocations and edge dislocations are not mutually exclusive. In practice, mixed dislocations are much more common. The line direction and burgers vector do not necessarily have to be either perpendicular or parallel to each other. Most of the time, their positions are somewhere in between, resulting in a mixed dislocations.
History
Smelting and forging metals has played an important role in human history for thousands of years. Yet, nobody could explain how the plastic deformation could be changed without adding elements to or subtracting from the chemical composition.
It was not until Taylor, Orowan and Polyani discovered the dislocation in 1934 that a scientific explanation was offered for the plasticity of ductile metals. They based their discovery on the theory of dislocations in crystals by Italian scientist Vito Volterra.
The dislocation theory couldn’t be proven until the development of the transmission electron microscope in the 1950s. It was demonstrated that the ductility and strength of metals are determined by dislocations.
