A dislocation, in the materials science
sense, is a linear, or one-dimensional, defect
in a crystal structure
. Around a dislocation the atoms are misaligned
. Virtually all crystalline
materials contain dislocations, that are formed during solidification
, during plastic deformation
or as a result of thermal stress
during rapid cooling.
There are several kinds of dislocation:
- The edge dislocation. You can visualize this one as follows: imagine a stack of sheets of paper. These represent the planes of atoms in the crystal. Now insert one sheet of paper halfway into the stack: this is a half-plane of atoms. Along the sheets, the atoms are aligned, as well as between the sheets, except for the region around the edge of the extra half-plane. The edge of the half-plane is called the dislocation line.
- A screw dislocation can be visualized with the same stack of paper sheets (without the half-plane!). Now make a cut through the sheets, from the middle to one of the sides. At one side of the cut, lift the stack so that each sheet on that side of the cut is aligned with the sheet above on the other side. Now if you were to take a circular walk on the atom planes, around the edge of the cut, you would go up (or down, depending on which way you turn) the planes in a spiral. The edge of the cut is the dislocation line for the screw dislocation.
- Many dislocations exhibit components of both types mentioned above and are logically called mixed dislocations.
The magnitude and direction of the lattice
distortion around a dislocation is expressed with the Burgers vector
. This vector is found by taking an imaginary walk around the dislocation, so that in a normal lattice you would end up at the starting point, for example: six atoms up, five to the right, six down and five to the left. If you do this around the dislocation line, you will not end up where you started and the vector
pointed from where you ended up to where you were supposed to end up is the Burgers vector. For an edge dislocation, Burgers vector and dislocation line are perpendicular
, for a screw dislocation they are parallel
and for a mixed dislocation the dislocation edge isn't straight so it varies.
Dislocations are important because they play a large role in the plastic deformation and work hardening of crystalline materials, and because as a result of the misalignment of atoms around a dislocation, diffusion of atoms along a dislocation line is easier and quicker than through the bulk material. Thus, the amount of dislocations present in a material has a large influence on the properties of that material.