Dislocation

LINE DEFECTS

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• A line defect is also called dislocation.

• A dislocation is a Crystallographic defect or irregularity, within a crystal structure.

• The presence of dislocations strongly influences many of the properties of materials. The theory was originally developed by Vito Volterra in 1905.

TYPES OF DISLOCATION

• Edge Dislocation

• Screw Dislocation

• Mixed dislocation

EDGE DISLOCATION

• An edge dislocation is a defect where an extra half-plane of atoms is introduced mid way through the crystal.

EDGE DISLOCATION

• It distort nearby planes of atoms. When enough force is applied from one side of the crystal structure.

• The extra half plane will break its existing bonds and form new bonds with its neighbor opposite of the dislocation motion.

If a

plan

e en

ds

abru

ptly

inside

a cr

ysta

l we

have

a d

efec

t.The w

hole of abruptly

ending

plane is not a

defect

Only the edge of the plane

can be considered as a

defect

This is a line defect called an EDGE DISLOCATION

Edge Dislocation

• The figure shows shear force is applied on its upper and lower surfaces.

• Consequently an extra half plane is introduced.• Due to this extra half plane the crystal is badly distorted.

This step is repeated in many discreet steps until the dislocation has moved entirely through the lattice.After all deformation, the extra half plane forms an edge that is one unit step wide.

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Screw Dislocation

SCREW DISLOCATION

• The motion of a screw dislocation is also a result of shear stress.– Motion is perpendicular to direction of stress,

rather than parallel (edge).• . Term Screw for such defect is derived from

the fact that lattice planes of the crystal spiral the dislocation line.

CONTD….

• The upper front portion in the following figure has been sheared by one atomic distance to the right relative to the lower front portion.

• In figure t represents the dislocation line.

Screw D

isloca

tion Li

ne

b

t

b || t

12

3

CONTD…….

• If circuit is clock wise then it is called RIGHT HAND screw dislocation.

• If circuit is anti-clock wise, then it is called LEFT HAND screw dislocation.

Mixed Dislocation

• Most dislocations can exhibit both edge and screw characteristics. These are called mixed dislocations.

BURGERS VECTORS

DEFINATION

The magnitude and the direction of the slip is represented by a vector b called the Burgers vector.

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A closed Burgers

Circuit in an ideal crystal

SF

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Burgers vector in edge dislocation

Burgers vector in edge dislocation

• Burger circuit forming a loop MNOPQ contains a dislocation.

• If same atom to atom connection is made in a defect free crystal (fig.b) and the circuit will not close.

Contd..

• The vector required to complete the circuit is called burgers vector.

• In an edge dislocation ,the burgers vector lies perpendicular to the line of dislocation.

BURGERS VECTOR IN SCREW DISLOCATION

BURGERS VECTOR IN SCREW DISLOCATION

• Consider the atom to atom sequence MNOPQ in a crystal having a dislocation as shown in

(Fig.a)

CONTD...• Now if the same atom to

atom connection is made in a defect free crystal (fig b) the circuit will not close.

• The closure failure is due to absence of a dislocation.

CONTD…..• The burgers vector in a screw dislocation is

parallel to the dislocation line.

Screw D

isloca

tion Li

ne

b

t

b || t

12

3