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Lomer–Cottrell junction

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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:
a 2 [ 0 1 1 ] a 6 [ 1 1 2 ] + a 6 [ -1 2 1 ] {\displaystyle {\frac {a}{2}}\rightarrow {\frac {a}{6}}+{\frac {a}{6}}}
a 2 [ 1 0 -1 ] a 6 [ 1 1 -2 ] + a 6 [ 2 -1 -1 ] {\displaystyle {\frac {a}{2}}\rightarrow {\frac {a}{6}}+{\frac {a}{6}}}
  • Combination of leading dislocations:
a 6 [ 1 1 2 ] + a 6 [ 1 1 -2 ] a 3 [ 1 1 0 ] {\displaystyle {\frac {a}{6}}+{\frac {a}{6}}\rightarrow {\frac {a}{3}}}

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

  1. Anderson, P.M., Hirth, J. P., & Lothe, J. (2017). Theory of Dislocations. Cambridge University Press.
  2. Anderson, P.M., Hirth, J. P., & Lothe, J. (2017). Theory of Dislocations. Cambridge University Press.


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