04 Crystal Defects
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Transcript of 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
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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
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Vacancy Defect
• Whenever one or more atoms are missing
from a normally occupied position, as in fig
below defects caused is known as vacancy.
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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
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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.
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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.
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Interstitial Defects
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Point defects
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Frenkel defect
• When a missing atom (responsible for
vacancy) occupies interstitial position
(responsible for interstitial defect) the defect
caused is frenkel defect
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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
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Schottky defect
If a pair of +ve and –ve ions is missing from a
crystal, it is schottky defect.
The crystal is electrically neutral.
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Point defects
(1) vacancy
(2) self-interstitial
(3) interstitial impurity
(4) Small Substitutional
impurity
(5) Large substitutional
impurity.
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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
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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
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Edge dislocation
• When half plane of atom is inserted between
the planes of atoms in a perfect crystal, the
defect is edge dislocation
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Edge dislocation
Dislocation
line
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┴
┴
+ve edge dislocation -ve edge dislocation
• Symbolically +ve edge dislocation is represented as ┴ and the - ve edge dislocation is represented as ┬
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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
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Edge Dislocation Glide
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Edge Climb
Positive climb
Removal of a row of atoms
Negative climb
Addition of a row of atoms
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Perfect crystal
Edge Dislocation
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Extra Half plane of atomsBurgers or Slip vector
Edge Dislocation
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Burgers Vector
Crystal with edge dislocation
Edge Dislocation
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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
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• 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
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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.
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Dislocation Line
Burgers vector b
Screw Dislocation
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Screw dislocation from above
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Mixed dislocations
Pure EdgePure screw
Edge dislocation Screw Dislocation
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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
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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
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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
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Grain Boundary
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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.
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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.
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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
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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.
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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.
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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.
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