Metalurgia

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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

Suranaree University of Technology

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.

Suranaree University of Technology

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

Suranaree University of Technology

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.
Suranaree University of Technology

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....
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