http://bzuiam.webs.com

1. LINE DEFECTS

2. Muhammad Umair Bukhari
Engr.umair.bukhari@gmail.com
www.bzuiam.webs.com
03136050151

3. • 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.

4. TYPES OF DISLOCATION
• Edge Dislocation
• Screw Dislocation
• Mixed dislocation

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

6. 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.

7. This is a line defect called an
EDGE DISLOCATION

8. 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.

9. 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.

10. NEXT MEMBER

11. Screw Dislocation

12. 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.

13. 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.

14. t
b || t
b

15. 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.

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

17. BURGERS VECTORS

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

19. S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
F 1
9
2
8
3
7 A closed
6 Burgers 4
5 Circuit in an 5
4 ideal crystal 6
7
3
8
2
1 9
16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

20. Burgers vector in
edge dislocation

21. 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.

22. 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.

23. BURGERS VECTOR IN SCREW
DISLOCATION

24. BURGERS VECTOR IN SCREW DISLOCATION
• Consider the atom to
atom sequence
MNOPQ in a crystal
having a dislocation as
shown in
(Fig.a)

25. 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.

26. CONTD…..
• The burgers vector in a screw dislocation is
parallel to the dislocation line.

27. t
b || t
b