Editing Dislocation (section)

==== Homogeneous nucleation ====
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 <math> \frac {\tau_{\text{hom}}}{G}=7.4\times10^{-2}</math>, where <math>G</math> is the shear modulus of copper (46 GPa).  Solving for <math>\tau_{\text{hom}} \,\!</math>, 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.