Lomer–Cottrell Junction
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In
materials science Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials sci ...
, a Lomer–Cottrell junction is a particular configuration of
dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to sli ...
s that forms when two perfect dislocations interact on interacting slip planes in a crystalline material. 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 Work hardening, also known as strain hardening, is the process by which a material's load-bearing capacity (strength) increases during plastic (permanent) deformation. This characteristic is what sets ductile materials apart from brittle materi ...
in ductile materials like
aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
and
copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
.


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 , consider the following reactions: * Dissociation of dislocations: :\frac
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+ \frac
text Text may refer to: Written word * Text (literary theory) In literary theory, a text is any object that can be "read", whether this object is a work of literature, a street sign, an arrangement of buildings on a city block, or styles of clothi ...
/math> :\frac
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+ \frac
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/math> * Combination of leading dislocations: :\frac
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+ \frac
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\rightarrow \frac
text Text may refer to: Written word * Text (literary theory) In literary theory, a text is any object that can be "read", whether this object is a work of literature, a street sign, an arrangement of buildings on a city block, or styles of clothi ...
/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

Crystallographic defects {{Crystallography-stub