04 Crystal Defects

7/27/2019 04 Crystal Defects

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Crystal Defects

• Crystals are rarely found to be perfect

• Atoms vibrate due to thermal effect and

electrons change their position

• Type of defects

1. Point defects

2. Line defects

3. Surface defects

4. Volume defects



Point defects

The defects that take place due to imperfect packing of atoms during crystallisation, are known as point defects

Point defects also take place due to vibration of atoms athigh temperatures.

Types of point defects are1.Vacancies

2.Interstitial defects

3.Frenkel defect

4.Substitutional defect5.Schottky defect

6.Phonon



Vacancy Defect

• Whenever one or more atoms are missing

from a normally occupied position, as in fig

below defects caused is known as vacancy.



Vacancy Defect

• Vacancy defects can be a result of imperfect

packing during the formation of crystals.

•They also arise from thermal vibration of the

atoms at high temperatures.

• It may be noted that there may be a single

vacancy, di vacancy, tri vacancy and so on



Interstitial Defects

• Whenever extra atom occupies interstitial

position in the crystal system, without

dislodging the parent atom the defect caused

is called as interstitial defect.



Interstitial defects

1. The atom that occupies the interstitial

position is generally smaller than the parent

atom.

2. In close packed structure the largest size of

atom that can fit in the voids have radius

22.5% of radii of parent atom.

3. If atom of larger size fits into the voids they

produce distortion in atoms.



Interstitial Defects



Point defects



Frenkel defect

• When a missing atom (responsible for

vacancy) occupies interstitial position

(responsible for interstitial defect) the defect

caused is frenkel defect



Substitutional defect

Foreign atom (i.e. other than the parent atom)

occupies a position, which was initially meant

for parent atom.

The size of replaced atom may not be same as

parent atom



Schottky defect

If a pair of +ve and –ve ions is missing from a

crystal, it is schottky defect.

The crystal is electrically neutral.



Point defects

(1) vacancy

(2) self-interstitial

(3) interstitial impurity

(4) Small Substitutional

impurity

(5) Large substitutional

impurity.



Phonon

• Whenever a group of atoms is displaced fromits ideal location, the defect is known asphonon

• Such defect is caused by thermal vibrations.• When atoms interact with one another they

vibrate in synchronism like the waves on

ocean.• This defect affects the electrical and magneticproperties



Line defects

The defects that take place due to dislocation ordistortion of atoms along a line, in some direction, arecalled as line defects.

Line defects also take place when a central portion of acrystal lattice slips without effecting the outer portion.

Types of Line defects:-1.Edge dislocation

2.Screw dislocation

3.Mixed dislocation



Edge dislocation

• When half plane of atom is inserted between

the planes of atoms in a perfect crystal, the

defect is edge dislocation



Edge dislocation

Dislocation

line



┴

┴

+ve edge dislocation -ve edge dislocation

• Symbolically +ve edge dislocation is represented as ┴ and the - ve edge dislocation is represented as ┬

http://en.wikipedia.org/wiki/Image:Dislocation_edge_d2.jpg








Edge Dislocation

• If extra half plane in the upper plant of the

crystal, atoms in the upper half experience

compressive stress & atoms in the bottom half

experience tensile stress.

• The dislocation line is simply the edge of extra

half plane in the crystal and hence the name

of edge dislocation



Edge Dislocation Glide



Edge Climb

Positive climb

Removal of a row of atoms

Negative climb

Addition of a row of atoms



Perfect crystal

Edge Dislocation



Extra Half plane of atomsBurgers or Slip vector

Edge Dislocation



Burgers Vector

Crystal with edge dislocation

Edge Dislocation



Burger’s circuit

• A dislocation in a crystal can be identified by consideringan atom-to-atom path known as Burgers circuit aroundthe dislocation line

• A burger circuit always closes in a perfect crystal & in an

imperfect crystal containing a dislocation, the circuit failsto close .

• The vector that is required to close the circuit around adislocation line is known as Slip vector or Burger’s vector

(closure-failure distance) & is denoted by• The burgers vector , describes both the magnitude &

direction of dislocation



• If an imaginary circuit is drawn, atom to atom arounda dislocation line, an extra step is needed tocomplete the circuit, compared to a similar circuit ina perfect crystal . This extra step is called the Burger’s

vector or slip vector .

• An edge dislocation is one that has its Burger’s vector

at right angles to the dislocation line

• In a screw dislocation, the Burger’s vector is parallel to the dislocation line, and the arrangement of atomsaround the dislocation is like a spiral staircase



Screw dislocation

• When 2 separate planes are shifted due to

application of shear force we get screw

dislocation

• The arrangement of atoms appear like a that

of a screw or helical surface.



Dislocation Line

Burgers vector b

Screw Dislocation



Screw dislocation from above



Mixed dislocations

Pure EdgePure screw

Edge dislocation Screw Dislocation



Edge dislocation Screw Dislocation

Burger vector b is perpendicular to

dislocation line Burger vector b is parallel to

dislocation line

Number of atoms involved in edge

dislocation are less Atoms involved in formation of

screw dislocation are more Force required for formation &

movement of edge dislocation is

less relatively Relatively more force is required in

formation & movement of screw

by dislocations Edge dislocation may be +ve ┴ or -

ve ┬ Screw dislocation may be right

handed or left handed High speed of movement of edgedislocation

Speed of movement is less An edge dislocation moves in the

direction of Burgers vector (slip

direction) in the slip plane. A screw dislocation moves in a

direction ┴ to Burgers vector (slip

direction) in the slip plane



Surface defects

• Defects that take place on the surface of a

material are known as surface defects or plane

defects.

• Surface defects take place either due to imperfectpacking of atoms during crystallisation or

defective orientation of the surface.

• Types of surface defects1. Grain boundary

2. Twin boundary

3. Stacking fault



Grain Boundary

• Whenever grains of different orientation

separate the general pattern of atoms and

exhibits a boundary the type of defect is grain

boundary.

• It usually takes place during solidification of

liquid metal



Grain Boundary



Twin boundary

• When the boundaries in which the atomic

arrangement on one side of the boundary is mirror

image of the arrangement of atoms of the other side

defect is called as twin boundary• The region in which a twin boundary defect occurs is

between the twinning planes.



Stacking fault

• When the stacking of atoms is not in proper

sequence throughout the crystal the fault is

called as stacking fault.

• The region where stacking fault occurs forms a

thin region of hexagonal close packing in a FCC

crystal.



Volume defects

1. Pores - can greatly affect optical, thermal,

mechanical properties

2. Cracks - can greatly affect mechanical

properties

3. Foreign inclusions - can greatly affect

electrical, mechanical, optical properties



Why study crystal defects.?

• Many of the engineering properties of solidsare structure-sensitive.

• They vitally depend on the presence orabsence of defects

• The presence of defects also play a very

important role in – Plasticity, Annealing, Precipitation, Diffusion,

Sintering, Oxidation… etc.



Effects of defects on properties of materials

• Point defects such a vacancies and interstitials

influence properties like electrical conductivity,

mechanical strength and diffusivity.

• Line defects or Dislocations influence the

mechanical properties of crystalline materials.

The modification of mechanical properties (such

as strength level, ductility, toughness) is closelyrelated to dislocation theory.



Effects of defects on properties of materials

• Point defects and line defects affect elevated

temperature mechanical properties like creep.

• Bulk defects are of interest in context of

solidification, deformation process and

powder metallurgy, hardening process, etc.

04 Crystal Defects

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