Dislocation
In materials science a dislocation is a type of crystallographic defect, or irregularity, in crystal structure that gives rise to many of the properties of real materials. The theory was originally developed by Vito Volterra in 1905.
Dislocations are caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side. The analogy with a stack of paper is apt: if a half a piece of paper is inserted in a stack of paper, the defect in the stack is only noticeable at the edge of the half sheet.
There are two major types of dislocations:
- Edge dislocations; and
- Screw dislocations.
Edge dislocations
Edge dislocations are formed by adding an extra half-plane of atoms to a perfect crystal, so that there is a defect in the regular crystal structure along the line where the extra half-plane ends (Figure 1).
Screw dislocations
Screw dislocations are formed by inserting a "parking garage ramp" that extends to the edges of the garage into an otherwise perfectly layered structure.
Burgers vector
The orientation and magnitude of a dislocation is characterised by its Burgers vector, perpendicular to the dislocation line. (b in Figure 1)
Dislocations, slip and plasticity
Until the 1930s, one of the enduring challenges of materials science was to explain plasticity in microscopic terms. A naive attempt to calculate the shear stress at which neighbouring atomic planes slip over each other in a perfect crystal suggests that, for a material with metals is typically within the range 20 000 to 150 000 MPa, this is difficult to reconcile with shear stresses in the range 0.5 to 10 MPa observed to produce plastic deformation in experiments.
In 1934, Egon Orowan, Michael Polanyi and G. I. Taylor, roughly simultaneously, realised that plastic deformation could be explained in terms of the theory of dislocations. Dislocations can move if the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge. Even a simple model of the force required to move a dislocation shows that shear is possible at much lower stresses than in a perfect crystal.
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