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Imperfections
• So far it has been assumed that perfect order 
exists throughout crystalline materials
• However the arrangement of atoms in all 
materials contains imperfections which 
affect the materials properties
• By controlling these imperfections it is 
possible to create stronger metals and alloys 
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Impurities
• Very difficult to have a pure metal of just 
one type of atom
• Impurity atoms will always be present and 
some exist as point defects as just discussed
• Alloying
– impurity atoms are intentionally added to 
import specific materials properties
• improve mechanical strength and corrosion 
resistance
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Lattice Imperfections
• Three basic types of lattice imperfections
– Point defects
– Linear defects (dislocations)
– Surface Defects
• Defects in the atomic arrangement, not in 
the material itself
• Control these defects through alloying, heat 
treatment to produce improved engineering 
materials
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Point Defects
• Point defects are localised disruptions of the 
lattice involving one or more atoms
• These imperfections may be introduced
– movement of atoms when they gain energy by heating
– during processing
– introduction of impurities
– intentionally through alloy
• Three types of point Defects
– Vacancy
– Interstitial Defects
– Substitutional Defects
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Vacancy
• Simplest type of point 
defect
• A vacancy is produced 
when an atom is 
missing from a normal 
site
• Introduced during
– solidification at high 
temperature as a result 
of atomic vibrations
– or as a consequence of 
radiation damage
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Interstitial Defect
• An interstitial defect is 
formed when an extra 
atom is inserted into the 
lattice structure at a 
normally unoccupied 
position
• smaller than lattice atoms
• surrounding lattice is 
compressed and distorted
• C atoms in steel 
intentionally added
Interfacial defect
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Substitutional defect
• A substitutional defect is 
introduced when a host 
atoms is replaced by a 
different atom
• larger than host atoms
– compress surrounding 
atoms.
• smaller than host atoms
– tension in surrounding 
atoms
• Can be impurity
• Intentional alloying 
addition 
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• Frenkel Defect
--a cation is out of place.
• Shottky Defect
--a paired set of cation and anion vacancies.
Shottky 
Defect:
Frenkel 
Defect
More Point Defects
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Linear Defects
• A dislocation is a linear defect around which some 
of the atoms are misaligned
• Introduced into the material during solidification 
or when the material is deformed
• Useful in explaining deformation and 
strengthening in metals
• Two types of dislocation
–– Edge dislocationEdge dislocation
–– Screw dislocationScrew dislocation
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Edge dislocation
• Extra half plane of 
electrons
• Centres around the line 
along the end of the extra 
half plane of electrons
• Dislocation line
– for an edge location the 
dislocation line is 
perpendicular to the page
• Around the dislocation 
line there is localised 
lattice distortion
b
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Edge dislocation
• Atoms above dislocation 
line are squeezed together, 
while the atoms below are 
pulled apart
• The magnitude and 
direction is of the lattice 
distortion is expressed in 
terms of the Burgers Burgers 
vectorvector bb
• Burgers vector for an edge 
dislocation is 
perpendicular to the 
dislocation line
b
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Screw dislocation
• Spiral stacking of 
crystal planes around 
the dislocation line
• Formed by a shear 
stress 
• Burgers vector is 
parallel to the 
dislocation line
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Mixed dislocation
• Contain edge and screw 
components
• Burgers vector is neither 
perpendicular or parallel, 
but will be the same at all 
points along its line
• For metallic materials the 
the Burgers vector will 
point in a crystallographic 
direction and will have a 
magnitude equal to the 
interatomic spacing
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† Plastic deformation refers to irreversible deformation or 
change in shape that occurs when the force or stress 
that caused it is removed.
† Elastic deformation - Deformation that is fully recovered 
when the stress causing it is removed.
† Dislocation density - The total length of dislocation line 
per cubic centimeter in a material.
Significance of Dislocations
• Because of the random 
orientation of the 
grains, the direction of 
slip varies from one 
grain to the next.
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Surface defects
• Surface defects are boundaries or planes 
that separate a material into regions 
– each region having the same crystal structure, 
but different orientations
• Material surface
• Grain Boundaries
• Twin Boundaries
• Stacking Faults
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Material surface
• Exterior surface the lattice abruptly ends
• Each atom at the surface
– no longer has the proper co-ordination number
• not bonded to the maximum # of nearest neighbours
– atomic bonding is disrupted
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Grain boundary
• Microstructure consists 
of many grains
– a grain is a portion of 
the material where the 
arrangement of atoms 
is identical
• However the orientation 
of the atomic 
arrangement is different 
for each adjoining grain
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(a) The atoms near the boundaries of the three grains 
do not have an equilibrium spacing or arrangement. 
(b) Grains and grain boundaries in a stainless steel 
sample. 
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Grain boundaries
• A small angle grain 
boundary is an array of 
dislocations that produces a 
small misorientation 
between adjoining lattices
• Small angle grain 
boundaries formed by 
– edge dislocation
• tilt dislocations
– screw dislocations
• twist dislocations
D
b
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Stacking Faults
• Stacking faults that occur in FCC metals represent 
an error in the stacking sequence of close packed 
planes
• Perfect stacking sequence for an FCC metal
– ABCABCABC
• Stacking Fault
– ABCABABC
• Stacking faults interfere with the slip process
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Twin Boundaries
• A twin boundary is a plane 
across which there is a 
special mirror image 
misorientation of the 
lattice structure
• Can be produced by a 
shear force acting along 
the twin boundary causes 
the atoms to shift out of 
position
• Interferes with slip
• Occurs during heat 
treatment or deformation 22
Summary
• Imperfections
– Point
• vacancy, interstitial atoms, substitutional
– Linear (dislocations)
• edge, screw
– Surface
• surfaces, grain boundaries, stacking faults, twin 
boundaries