Partial dislocation

In materials science, a partial dislocation is a decomposed form of dislocation that occurs within a crystalline material. An extended dislocation is a dislocation that has dissociated into a pair of partial dislocations. The vector sum of the Burgers vectors of the partial dislocations is the Burgers vector of the extended dislocation. A dislocation will decompose into partial dislocations if the energy state of the sum of the partials is less than the energy state of the original dislocation. This is summarized by Frank's Energy Criterion: Shockley partial dislocations generally refer to a pair of dislocations which can lead to the presence of stacking faults. This pair of partial dislocations can enable dislocation motion by allowing an alternate path for atomic motion. In FCC systems, an example of Shockley decomposition is: Which is energetically favorable: The components of the Shockley Partials must add up to the original vector that is being decomposed: Frank partial dislocations are sessile (immobile), but can move by diffusion of atoms. In FCC systems, Frank partials are given by: For FCC crystals, Thompson tetrahedrons or Thompson notation are an invented notation for more easily describing partial dislocations. In a given unit cell, mark point A at the origin, point B at a/2 [110], point C at a/2[011], and point D at a/2[101]--these points form the vertices of a tetrahedron. Then, mark the center of the opposite faces for each point as α, β, γ, and δ, respectively. With this, the geometric representation of a Thompson tetrahedron is complete. Any combination of Roman letters describes a member of the {111} slip planes in an FCC crystal. A vector made from two Roman letters describes the Burgers vector of a perfect dislocation. If the vector is made from a Roman letter and a Greek letter, then it is a Frank partial if the letters are corresponding (Aα,Bβ,...) or a Shockley partial otherwise (Aβ, Aγ,...). Vectors made from two Greek letters describe stair-rod dislocations.