Metalurgia

Chapter 5

Dislocation theory
Subjects of interest
• Introduction/Objectives • Observation of dislocation • Burgers vector and the dislocation loop • Dislocation in the FCC, HCP and BCC lattice • Stress fields and energies of dislocations • Forces on dislocations and between dislocations

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Tapany Udomphol

May-Aug 2007

Chapter 5

Dislocationtheory
Subjects of interest (continued)
• Dislocation climb • Intersection of dislocations • Jogs • Dislocation sources • Multiplication of dislocations • Dislocation-point defect interactions • Dislocation pile-ups

Suranaree University of Technology

Tapany Udomphol

May-Aug 2007

Objectives

• This chapter emphasises the understanding of the effects of dislocation behaviour on FCC, BCCand HCP crystal structures. • This includes the interaction of dislocations such as climb, jogs, intersection and multiplication of dislocations and the roles of dislocations on plastic deformation of metals.

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Tapany Udomphol

May-Aug 2007

Introduction
Dislocations introduce imperfection into the structure and therefore these could explain how realmaterials exhibit lower yield stress value than those observed in theory.

• Lower the yield stress from theoretical values.
Produce imperfection in crystal structures
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• Produce plastic deformation (strain hardening). • Effects mechanical properties of materials.

Dislocations

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Tapany Udomphol

May-Aug 2007

Observation of dislocationsA variety of techniques have been used to observe dislocations in the past 20 years to aid the better understanding of dislocation behaviour.

Chemical (etch–pit) technique
• Using etchant which forms a pit at the point where a dislocation intersect the surface. • Preferential sites for chemical attack are due to strain field around dislocation sites (anodic). • Can be used in bulk samples butlimited in low dislocation density crystal (104 mm-2).
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Note: Pits are 500 Ao apart and with the dislocation density of 108 mm-2.
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Etch pits on slip bands in alpha brass crystals
May-Aug 2007

Tapany Udomphol

Decoration of dislocation technique
A small amount of impurity is added to form precipitates after suitable heat treatment to giveinternal structure of the dislocation lines.

• Hedges and Mitchell first used photolytic to decorate dislocation in AgBr. • Rarely used in metals but in ironic crystals such as AgCl, NaCl, KCl and CaF2.
Hexagonal network of dislocations in NaCl detected by a decoration technique.
Suranaree University of Technology
Tapany Udomphol

May-Aug 2007

Transmission electron microscope (TEM)
TEM isthe most powerful technique used to study dislocations. • A thin foil of 100 nm is prepared using electropolishing from a ~1 mm thick sheet. • This thin foil is transparent to electrons in the electron microscope and this makes it possible to observed dislocation networks, stacking faults, dislocation pile-ups at grain boundaries. • By using the kinematic and dynamic theories of electrondiffraction it is possible to determine the dislocation number, Burgers vectors and slip planes.
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Dislocation network in cold-worked aluminium.

Note: The sampling area is small therefore the properties observed cannot represent the whole materials.
Suranaree University of Technology
Tapany Udomphol

May-Aug 2007

X-ray microscopy
• Using an X-ray technique to detect dislocationstructure. • The most common techniques are the Berg-Barret reflection method and the Lang topography method. • The resolution is limited to 103 dislocations/mm2.

Suranaree University of Technology

Tapany Udomphol

May-Aug 2007

Burgers vector and the dislocation loop
Burgers vector is the most characteristic feature of a dislocation, which defines the magnitude and the direction of slip....