View source for Lomer–Cottrell junction

In [[materials science]], a '''Lomer–Cottrell junction''' is a particular configuration of [[dislocation]]s that forms when two perfect dislocations interact on interacting slip planes in a crystalline material.<ref name="hirth17">{{cite book |last1=Anderson |first1=P.M. |last2=Hirth |first2=J. P.|last3=Lothe |first3=J. |year=2017 |title=Theory of Dislocations |publisher= Cambridge University Press.}}</ref>

The sessile or immobile nature of the Lomer–Cottrell dislocation forms a strong barrier to further dislocation motion. Trailing dislocations pile up behind this junction, leading to an increase in the stress required to sustain deformation. This mechanism is a key contributor to [[work hardening]] in ductile materials like [[Aluminium|aluminum]] and [[copper]].<ref name="hirth17"/>

==Formation Mechanism==

When two perfect dislocations encounter along a slip plane, each perfect dislocation can split into two [[Partial dislocations|Shockley partial dislocations]]: a leading dislocation and a trailing dislocation. When the two leading [[Partial dislocations|Shockley partials]] combine, they form a separate dislocation with a burgers vector that is not in the slip plane. This is the Lomer–Cottrell dislocation. It is sessile and immobile in the slip plane, acting as a barrier against other dislocations in the plane. The trailing dislocations pile up behind the Lomer–Cottrell dislocation, and an ever greater force is required to push additional dislocations into the pile-up.

==Example in FCC Crystals==
For an FCC crystal with slip planes of the form {111}, consider the following reactions:
* Dissociation of dislocations:
:<math>\frac{a}{2}[\text{0 1 1}] \rightarrow \frac{a}{6}[\text{1 1 2}] + \frac{a}{6}[\text{-1 2 1}]</math>
:<math>\frac{a}{2}[\text{1 0 -1}] \rightarrow \frac{a}{6}[\text{1 1 -2}] + \frac{a}{6}[\text{2 -1 -1}]</math>

* Combination of leading dislocations:
:<math>\frac{a}{6}[\text{1 1 2}] + \frac{a}{6}[\text{1 1 -2}] \rightarrow \frac{a}{3}[\text{1 1 0}]</math>

The resulting dislocation lies along a crystal direction that is not a slip plane at room temperature in FCC materials. This configuration contributes to immobility of the Lomer-Cottrell junction. 

==References==
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[[Category:Crystallographic defects]]


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