CRYSTAL DEFECTS

1. 1
POINT IMPERFECTIONS
LINE IMPERFECTIONS
SURFACE IMPERFECTIONS
VOLUME IMPERFECTIONS
UNIT-I
Chapter-2
CRYSTAL DEFECTS

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CRYSTAL DEFECTS AND IMPERFECTIONS
An ideal crystal is a perfect crystal in which each atom
has identical surroundings. Real crystals are not perfect.
A real crystal always has a large number of
imperfections in the lattice.
Since real crystals are of finite size, they have a surface
to their boundary.
At the boundary, atomic bonds terminate and hence the
surface itself is an imperfection.
One can reduce crystal defects considerably, but can
never eliminate them entirely.

3. Point Defects
• Vacancies
• Self Interstitial
• Schottky defect
• Frenkel defect
• Electronic defect
• F-Centre defect
3

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CRYSTAL DEFECTS AND IMPERFECTIONS
The study of imperfections has a two fold purpose, namely,
A better understanding of crystals and how they affect the
properties of metals.
Exploration of possibilities of minimizing or eliminating these
defects.
The term “defect” or “imperfection” is generally used to
describe any deviation from the perfect periodic array of
atoms in the crystal.

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CRYSTAL DEFECTS AND IMPERFECTIONS
Crystal imperfections can be classified on the basis of their
geometry as,
Point Imperfections,
Line imperfections
Surface (or) plane imperfections and
Volume imperfections

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POINT IMPERFECTIONS
They are imperfect point- like regions, one or two
atomic diameters in size and hence referred to as
‘zero dimensional imperfections’.
There are different kinds of point imperfections.
VACANCIES
If an atom is missing from its normal site in the
matrix, the defect is called a vacancy defect.
It may be a single vacancy, divacancy or a trivacancy.

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POINT DEFECT-VACANCY

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POINT IMPERFECTIONS
In metals vacancies and created by thermal excitation.
When the temperature is sufficiently high, as the atoms vibrate
around their regular positions, some acquire enough energy to leave
the site completely.
When the regular atom leaves, a vacancy is created.
A pair of one cation and one anion can be missed from an ionic
crystal.Such a pair of vacant ion sites is called Schottky imperfection.
This type of defect is dominant in alkali halides.

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SCHOTTKY IMPERFECTIONS

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SUBSTITUTIONAL IMPURITY
It refers to a foreign atom that substitutes for or
replaces a parent atom in the crystal.
Pentavalent or trivalent impurity atoms doped
in Silicon or Germanium are also substitutional
impurities in the crystal.

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SUBSTITUTIONAL IMPURITY

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INTERSTITIAL IMPURITY
An interstitial defect arises when an atom occupies a
definite position in the lattice that is not normally occupied
in the perfect crystal.
In crystals, packing density is always less than 1.
If a small sized atom occupies the void space in the parent
crystal without disturbing the parent atoms from their
regular sites, then it is called as ‘interstitial impurity’.

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INTERSTITIAL IMPURITY

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INTERSTITIAL IMPURITY
In ionic crystals, an ion displaced from a regular site to an
interstitial site is called ‘Frenkel imperfection’.
As cations are generally the smaller ones, it is possible for
them to get displaced into the void space.
Anions do not get displaced as the void space is too small
compared to the size of the anions.
A Frenkel imperfection does not change the overall electrical
neutrality of the crystal. This type of defect occurs in silver
halides and CaF2.

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DIAGRAM SHOWING THE
IMPERFECTIONS

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ELECTRONIC DEFECTS
Errors in charge distribution in solids are called
‘electronic defects’.
These defects are produced when the composition of
an ionic crystal does not correspond to the exact
stoichiometric formula.
These defects are free to move in the crystal under
the influence of an electric field.

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EFFECT OF POINT IMPERFECTIONS
The presence of a point imperfection introduces distortions in
the crystal.
In the case of impurity atom, because of its difference in size,
elastic strains are created in the regions surrounding the
impurity atom.
All these factors tend to increase the potential energy of the
crystal called ‘enthalpy’.
The work done for the creation of such a point defect is called
the ‘enthalpy of formation’ of the point imperfection.

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LINE IMPERFECTIONS
The defects, which take place due to dislocation or
distortion of atoms along a line, in some direction are
called as ‘line defects’.
Line defects are also called dislocations. In the geometic
sense, they may be called as ‘one dimensional defects’.
A dislocation may be defined as a disturbed region
between two substantially perfect parts of a crystal.
It is responsible for the phenomenon of slip by which
most metals deform plastically.

19. Slip Not-yet-slipped
Boundary between slipped and unslipped parts
on the slip plane
Dislocation Line (One-Dimensional Defect)

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LINE IMPERFECTIONS
The two types of dislocations are,
Edge dislocation
Screw dislocation

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EDGE DISLOCATION
In perfect crystal, atoms are arranged in both vertical and
horizontal planes parallel to the side faces.
If one of these vertical planes does not extend to the full
length, but ends in between within the crystal it is called ‘edge
dislocation’.
In the perfect crystal, just above the edge of the incomplete
plane the atoms are squeezed and are in a state of compression.
Just below the edge of the incomplete plane, the atoms are
pulled apart and are in a state of tension.

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The distorted configuration extends all along the edge into the
crystal.
Thus as the region of maximum distortion is centered around
the edge of the incomplete plane, this distortion represents a line
imperfection and is called an edge dislocation.
Edge dislocations are represented by ‘’ or ‘‘ depending on
whether the incomplete plane starts from the top or from the
bottom of the crystal.
These two configurations are referred to as positive and
negative edge dislocations respectively.
EDGE DISLOCATION

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EDGE DISLOCATION

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25. Lattice strains
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BURGERS VECTOR
The magnitude and the
direction of the displacement are
defined by a vector, called the
Burgers Vector.
In figure (a), starting from the
point P, we go up by 6 steps, then
move towards right by 5 steps,
move down by 6 steps and finally
move towards left by 5 steps to
reach the starting point P.Now the
Burgers circuit gets closed.
When the same operation is
performed on the defect crystal
(figure (b)) we end up at Q
instead of the starting point.

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BURGERS VECTOR

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BURGERS VECTOR
So, we have to move an extra step to return to P, in order to close
the Burgers circuit.
The magnitude and the direction of the step defines the Burgers
Vector (BV).
The Burgers Vector is perpendicular to the edge dislocation line.

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SCREW DISLOCATION
In this dislocation, the
atoms are displaced in two
separate planes
perpendicular to each other.
It forms a spiral ramp
around the dislocation.
The Burgers Vector is
parallel to the screw
dislocation line.
Speed of movement of a
screw dislocation is lesser
compared to edge
dislocation. Normally, the
real dislocations in the
crystals are the mixtures of
edge and screw dislocation.

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SCREW DISLOCATION

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33. Edge vs. Screw Dislocation
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35. Glide Motion of
an edge
dislocation

39. Surface Step is
created when
dislocation
moves out.

40. Movement of an Edge Dislocation
From
W.D. Callister
Materials
Science
and Engineering

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43. Glide of LH screw dislocation
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Glide of LH screw dislocation

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Glide of LH screw dislocation

46. Cross slip of a screw dislocation
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47. Obstacle to screw dislocation
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48. Cross-slip
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49. Cross slip movement of
dislocation in second plane
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52. Atomistic mechanism of climb
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53. Atomistic mechanism of climb
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Atomistic mechanism of climb

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Atomistic mechanism of climb

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Atomistic mechanism of climb

57. Climb of an edge dislocation
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58. • The stress required to cause the dislocation to move increases exponentially with
the length of the Burgers vector. Thus, the slip direction should have a small
repeat distance or high linear density. The close-packed directions in metals and
alloys satisfy this criterion and are the usual slip directions.
• The stress required to cause the dislocation to move decreases exponentially with
the interplanar spacing of the slip planes. Slip occurs most easily between planes
of atoms that are smooth (so there are smaller ‘‘hills and valleys’’ on the surface)
and between planes that are far apart (or have a relatively large interplanar
spacing). Planes with a high planar density fulfill this requirement. Therefore the
slip planes are typically close-packed planes or those as closely packed as possible.
• Dislocations do not move easily in materials such as silicon or polymers, which
have covalent bonds. Because of the strength and directionality of the bonds, the
materials typically fail in a brittle manner before the force becomes high enough
to cause appreciable slip. In many engineering polymers dislocations play a
relatively minor role in their deformation.
• Materials with ionic bonding, including many ceramics such as MgO, also are
resistant to slip. Movement of a dislocation disrupts the charge balance around
the anions and cations, requiring that bonds between anions and cations be
broken. During slip, ions with a like charge must also pass close together, causing
repulsion. Finally, the repeat distance along the slip direction, or the Burgers
vector, is larger than that in metals and alloys. 58

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SURFACE IMPERFECTIONS
Surface imperfections arise from a change in the stacking
of atomic planes on or across a boundary.
The change may be one of the orientations or of the
stacking sequence of atomic planes.
In geometric concept, surface imperfections are two-
dimensional. They are of two types external and internal
surface imperfections.

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EXTERNAL SURFACE IMPERFECTIONS
They are the imperfections represented by a boundary. At the
boundary the atomic bonds are terminated.
The atoms on the surface cannot be compared with the atoms
within the crystal. The reason is that the surface atoms have
neighbours on one side only. Where as the atoms inside the crystal
have neighbours on either sides. This is shown in figure in next
slide. Since these surface atoms are not surrounded by others,
they possess higher energy than that of internal atoms.
For most metals, the energy of the surface atoms is of the order
of 1 J/m2.

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EXTERNAL SURFACE IMPERFECTIONS

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INTERNAL SURFACE IMPERFECTIONS
Internal surface imperfections are the imperfections which
occurred inside a crystal.
It is caused by the defects such as, grain boundaries. tilt
boundaries, twin boundaries and stacking faults.

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GRAIN BOUNDARIES
They are the imperfections which separate crystals or grains of
different orientation in a poly crystalline solid during nucleation or
crystallization.
It is a two dimensional imperfection. During crystallization, new
crystals form in different parts and they are randomly oriented with
respect to one another.
They grow and impinge on each other.
The atoms held in between are attracted by crystals on either side
and depending on the forces, the atoms occupy equilibrium
positions.

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GRAIN BOUNDARIES
These positions at the boundary region between two crystals
are distorted.As a result, a region of transition exists in which
the atomic packing is imperfect.
The thickness of this region is 2 to 10 or more atomic
diameters.
The boundary region is called a crystal boundary or a grain
boundary .
The boundary between two crystals which have different
crystalline arrangements or different compositions, is called as
interphase boundary or commonly an interface.

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GRAIN BOUNDARIES

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TILT BOUNDARIES
This is called low-angle boundary as the orientation
difference between two neighbouring crystals is less than 10°.
The disruption in the boundary is not so severe as in the
high-angle boundary. In general low-angle boundaries can be
described by suitable arrays of dislocation.
Actually a low-angle tilt boundary is composed of edge
dislocation lying one above the other
The angle or tilt will be
where b = Burgers vector and
D = the average vertical distance between dislocations.
D
b



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TILT BOUNDARIES

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TWIN BOUNDARIES
If the atomic arrangement on one side of a boundary is a
mirror reflection of the arrangement on the other side, then it is
called as twin boundary.
As they occur in pair, they are called twin boundaries. At one
boundary, orientation of atomic arrangement changes.
At another boundary, it is restored back. The region between
the pair of boundaries is called the twinned region.
These boundaries are easily identified under an optical
microscope.

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TWIN BOUNDARIES

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STACKING FAULTS
Whenever the stacking of atomic planes is not in a proper
sequence throughout the crystal, the fault caused is known as
stacking fault.
For example, the stacking sequence in an ideal FCC crystal
may be described as A-B-C-A-B-C- A-B-C-……. But the
stacking fault may change the sequence to A-B-C-A-B-A-B-A-
B-C. The region in which the stacking fault occurs (A-B-A-B)
forms a thin region and it becomes HCP.
This thin region is a surface imperfection and is called a
stacking fault.

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STACKING FAULTS

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A twin boundary is a reversal in the crystal lattice, such as ABC | BA-CBA-CBA.
(The | represents the point of the twin boundary, where the stacking order
reverses).

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In an antiphase region, a portion of material shifts over. For example: ABC-A|ABC-
ABC-ABC|C-ABC. The bolded portion has shifted over by one spot, which creates
antiphase boundaries so that two of the same layers are stacked on top of each
other.

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STACKING FAULTS

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STACKING FAULTS

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VOLUME IMPERFECTIONS
Volume defects such as cracks may arise in crystals when
there is only small electrostatic dissimilarity between the
stacking sequences of close packed planes in metals. Presence
of a large vacancy or void space, when cluster of atoms are
missed is also considered as a volume imperfection.
Foreign particle inclusions and non crystalline regions which
have the dimensions of the order of 0.20 nm are also called as
volume imperfections.

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