Dislocation - Sources of Dislocations

Sources of Dislocations

Dislocation density in a material can be increased by plastic deformation by the following relationship: . Since the dislocation density increases with plastic deformation, a mechanism for the creation of dislocations must be activated in the material. 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 2 oppositely faced half planes or dislocations. These dislocations move away from each other through the lattice. Since homogeneous nucleation forms dislocations from perfect crystals and requires the simultaneous breaking of many bonds, the energy required for homogeneous nucleation is high. For instance the stress required for homogeneous nucleation in copper has been shown to be, where G is the shear modulus of copper (46 GPa). Solving for, we see that the required stress is 3.4 GPa, which is very close to the theoretical strength of the crystal. Therefore, in conventional deformation homogeneous nucleation requires a concentrated stress, and is very unlikely. Grain boundary initiation and interface interaction are more common sources of dislocations.

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.

The surface of a crystal can produce dislocations in the crystal. Due to the small steps on the surface of most crystals, stress in certain regions on the surface is much larger than the average stress in the lattice. The dislocations are then propagated into the lattice in the same manner as in grain boundary initiation. In monocrystals, the majority of dislocations are formed at the surface. The dislocation density 200 micrometres into the surface of a material has been shown to be six times higher than the density in the bulk. However, in polycrystalline materials the surface sources cannot have a major effect because most grains are not in contact with the surface.

The interface between a metal and an oxide can greatly increase the number of dislocations created. The oxide layer puts the surface of the metal in tension because the oxygen atoms squeeze into the lattice, and the oxygen atoms are under compression. This greatly increases the stress on the surface of the metal and consequently the amount of dislocations formed at the surface. The increased amount of stress on the surface steps results in an increase of dislocations.

• The stresses produced by a dislocation source may be visualized by photoelasticity in a gamma-irradiated LiF single crystal. The tensile stress along the glide plane is red. The compressive stress is dark green:

• Dislocation source (schematic)

• Stresses in a dislocation pile-up