Introduction,types

1. Defects in crystal
Dr.R.Vidhya

2. Introduction
• Crystalline solids – The constituent atoms or molecules
are arranged in an orderly fashion throughout in a
three dimensional pattern. The atoms are arranged
like soldiers on a parade ground in a well defined
columns and rows.
• Crystal defect – The irregularities in the structure of
crystal.
2

3. Types of defects in crystals
Point defects
(0D)
• Vacancies
• Interestitial
defects
• Schottky
defect
• Frenkel
defect
Line defects
(1D)
• Edge
dislocation
• Screw
dislocation
Surface defects
(2D)
• Grain
boundaries
• Twin
boundaries
• Stacking fault
Volume
defects (3D)
• Voids

4. Point defect – 0D
• Point defect
 Imperfect packing of atoms during crystallisation.
 Imperfect point like regions in the crystal.
(a) Vacancies
 Simplest point defect in a crystal.
 Missing of atom or a vacant atom site.
 May be single or divacancies or trivacancies.
4
Vacancy

5. (a) Interstitial defect
 Extra atom occupy interstitial position.
 Atom is displaced from its normal position within the crystal to an
interstitial position.
 Foreign atom substitutes the position of parent atom in the crystal
lattice.
5
Interstital defect

6. (c) Schottky defect
6
 A pair of positive and negative ions are missing in a crystal.
 Pair of ion vacancies – Schottky defect.
 Crystal is electrically neutral in this type of defect.

7. (d) Frenkel defect
7
 An atom may leave its regular site and may occupy nearby
interstitial site giving two defects – Vacancy and interstital.
 Two defects together – Schottky defect.
 Common in ionic crystals.

8. Line defect – 1D
• Line defect
 Due to dislocation or distortion of atoms along a line in some direction.
 Dislocation is a boundary between slipped and unslipped region.
 Types – Edge dislocation
Screw dislocation
8

9. (a) Edge dislocation
9
 Due to insertion of an extra half plane of atoms.
 Extra plane of inserted atoms is above the slip plane – Positive dislocation
represented by
 Extra plane is inserted below the slip plane – Negative dislocation
represented by T
 Burger vector – indicates the direction and magnitude of the shift of the
lattice on the slip plane.

10. (b) Screw dislocation
10
 Displacement of atoms in one part of a crystal relative to the rest of crystal
forming a spiral ramp around dislocation line.
 Region of lattice disturbance extends in two separate planes at right
angles to each other.
 Burger vector – indicates the direction and magnitude of the shift of the
lattice on the slip plane.
 Burger vector is parallel to the dislocation line.
Screw dislocation

11. Surface defect – 2D
• Surface defect
 Takes place on the surface of a material.
 Due to imperfect placing of atoms during crystallization or defective
orientation of the surface.
 Types - Grain boundary
- Twin boundary
- Stacking fault
11

12. (a) Grain boundary
12
 Grain boundary – Defects which separate grains of different orientation
from each other in a polycrystalline material.
 Orientation difference between two crystals is greater than 10-15° - High
angle grain boundary
 Orientation difference between two crystals is less than 10° - Low angle
grain boundary.
Grain boundary

13. (b) Twin boundary
13
 Boundaries in the grains at which the atomic arrangement on one side of
the boundary is the mirror image of the atoms on the other side.
 Volume of material which has orientation similar to the mirror image of
the matrix operation – Twin
 Mirror – Twinning plane.

14. (c) Stacking fault
14
 Stacking fault – planar surface imperfections and are caused by fault in
the stacking sequence of atomic planes in crystals of FCC and HCP
materials.

15. Volume defect – 3D
• Volume defect
 Cracks may arise when there is only small electrostatic dissimilarity
between the stacking sequences of close packed planes in metals.
 Whenever a cluster of atoms are missing large vacancy or void is got.
 Foreign particle inclusions, large voids or non crystalline regions which
have the dimensions of the order of 20 Å - volume imperfection.
15