Editing Dislocation (section)

===Screw===
A ''screw dislocation'' can be visualized by cutting a crystal along a plane and slipping one half across the other by a lattice vector, the halves fitting back together without leaving a defect. If the cut only goes part way through the crystal, and then slipped, the boundary of the cut is a screw dislocation. It comprises a structure in which a [[Helix|helical]] path is traced around the linear defect (dislocation line) by the atomic planes in the crystal lattice. In pure screw dislocations, the Burgers vector is parallel to the line direction.<ref>{{cite book|author=James Shackelford|title=Introduction to Materials Science for Engineers|year=2009|publisher=Pearson Prentice Hall|location=Upper Saddle River, NJ|isbn=978-0-13-601260-3|pages=110–11|edition=7th}}</ref>
An array of screw dislocations can cause what is known as a twist boundary. In a twist boundary, the misalignment between adjacent crystal grains occurs due to the cumulative effect of screw dislocations within the material. These dislocations cause a rotational misorientation between the adjacent grains, leading to a twist-like deformation along the boundary. Twist boundaries can significantly influence the mechanical and electrical properties of materials, affecting phenomena such as grain boundary sliding, creep, and fracture behavior<ref>{{cite book|author=James Shackelford|title=Introduction to Materials Science for Engineers|year=2009|publisher=Pearson Prentice Hall|location=Upper Saddle River, NJ|isbn=978-0-13-601260-3|pages=110–11|edition=7th}}</ref>
The stresses caused by a screw dislocation are less complex than those of an edge dislocation and need only one equation, as symmetry allows one radial coordinate to be used:<ref name="rhill"/>

::<math> \tau_{r} = \frac {-\mu \mathbf{b}} {2 \pi r} </math>

where <math>\mu</math> is the [[shear modulus]] of the material, <math>\mathbf{b}</math> is the Burgers vector, and <math>r</math> is a radial coordinate.
This equation suggests a long cylinder of stress radiating outward from the cylinder and decreasing with distance. This simple model results in an infinite value for the core of the dislocation at <math>r=0</math> and so it is only valid for stresses outside of the core of the dislocation.<ref name="rhill"/> If the Burgers vector is very large, the core may actually be empty resulting in a [[micropipe]], as commonly observed in [[silicon carbide]].