Lomer-Cottrell Junction
In materials science, a Lomer–Cottrell junction is a particular configuration of dislocations that forms when two perfect dislocations interact on interacting slip planes in a crystalline material.
Formation Mechanism
When two perfect dislocations encounter along a slip plane, each perfect dislocation can split into two Shockley partial dislocations: a leading dislocation and a trailing dislocation. When the two leading 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:
|leading| |trailing|
- Dissociation of dislocations:
- Combination of leading dislocations:
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.
Significance
The sessile 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 aluminum and copper.
References
- Anderson, P.M., Hirth, J. P., & Lothe, J. (2017). Theory of Dislocations. Cambridge University Press.
- Anderson, P.M., Hirth, J. P., & Lothe, J. (2017). Theory of Dislocations. Cambridge University Press.
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