Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are out of position in the crystal structure. Dislocations are generated and move when a stress is applied. The motion of dislocations allows slip – plastic deformation to occur.
Sources of dislocation
Dislocation density in a material can be increased by plastic deformation; three mechanisms for dislocation formation are homogeneous nucleation, grain boundary initiation, and interfaces between the lattice and the surface, precipitates, dispersed phases, or reinforcing fibers.
The creation of a dislocation by homogeneous nucleation is a result of the rupture of the atomic bonds along a line in the lattice. A plane in the lattice is sheared resulting in two oppositely faced half planes or dislocations. These dislocations move away from each other through the lattice.
Irregularities at the grain boundaries in materials can produce dislocations which propagate into the grain. The steps and ledges at the grain boundary are an important source of dislocations in the early stages of plastic deformation.
Due to the small steps on the surface of most crystals, stress in some regions on the surface is much larger than the average stress in the lattice. This stress leads to dislocations In single crystals, the majority of dislocations are formed at the surface.
| Edge Dislocation |
Screw Dislocation |
| If one of the plane is not continuous from top to bottom or vice versa and ends part away within the crystal, it is called edge dislocation,Edge dislocation are of two types positive (_l_) and negative (T) dislocation |
If plane of atoms follows dislocation in a helical or screw path then it is called as screw dislocation.,Screw dislocation are of two types clockwise and anticlockwise dislocation |
| Relation between dislocation line and Berger vector (B),is perpendicular to each other |
Relation between dislocation line and Berger vector (B) is parallel to each other |
| Direction of dislocation line movement relative to Berger vector is parallel. |
Direction of dislocation line movement relative to Berger vector is perpendicular |
| Edge dislocation can be removed from slip plane by dislocation climb mechanism |
Screw dislocation can be removed from slip plane by cross slip mechanism |
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