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Crystal Defects

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Defects in Solids There is no such thing as a perfect crystal.

• What are these Defects?

• Why are they important?

• Many of the important properties of materials are due

to the presence of imperfections.

• The properties of some materials are profoundly

influenced by the presence of imperfections.

• It is important to have knowledge about the types of

imperfections that exist and the roles they play in

affecting the behavior of materials.

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Atom Purity and Crystal

Perfection

• Anything that deviated from this concept or intruded in

this uniform homogeneity would be an imperfection.

1. There are no perfect crystals.

2. Many material properties are improved by the

presence of imperfections and deliberately

modified (alloying and doping).

• Image“borrowed”from

http://comp.uark.edu/~pjansma/geol3513_25_defmechs1_04.ppt who got it from

“Davis & Reynolds 1996.”

Defects in Crystals

It's easy to think of real crystals as having these ideal

structures.

In fact, no crystals are perfect; all crystals have defects.

Crystals can have:

• The "right" atoms in "wrong" places.

• "Wrong" atoms in "right" or "wrong" places.

• Missing atoms.

• Etc.

One type of defect is the point defect.

Crystal

Defects

Point Defects

Line Defects

Surface Defects

Volume Defects

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Point Defects

Vacancy

Substitutional impurity

Self-Interstitials

Interstitial Impurity

Frenkel imperfection

Schottky imperfection

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Point Defects • Vacancy: A Vacancy refers to an atomic site

from where the atom is missing.

Tensile Stress

Fields ?

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• Vacancies:

-vacant atomic sites in a structure.

Point Defects

Vacancy

distortion

of planes

Point Defects • Substitutional impurity: A Substitutional

impurity refers to a foreign atom that has replaced a

parent atom.

Tensile Stress

Fields Compressive stress

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• Self-Interstitials:

-"extra" atoms positioned between atomic sites.

Point Defects in Metals

self-

interstitial

distortion

of planes

Point Defects • Interstitial Impurity: An Interstitial impurity

refers to small sized atom occupying the void space in the

parent crystal without replacing the parent atoms

Compressive

Stress

Fields

Relative

size

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Point Defects • Frenkel imperfection: An ion displaced from

a regular site to an interstitial site is called frenkel

imperfection.

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Point Defects

• Schottky imperfection: A pair of cation and

anion missing from an ionic crystal resulting in a pair of

vacant ion sites is called schottky imperfection

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Point Defects

• 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.

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Line Defects • These are one dimensional imperfections in the

geometrical sense.

• These are also known as linear defects or Dislocations.

• Dislocations are areas were the atoms are out of position

in the crystal structure.

• These are generated when stress is applied.

• The motion of dislocations allows slip – plastic

deformation to occur.

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Line Defects

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 planes does not extend to the full length but

ends in between within the crystal it is called edge

dislocation.

• Edge dislocation are caused by the termination of a plane

of atoms in the middle of the crystal.

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• Edge dislocation moves parallel to the direction of

stress.

• It can be easily visualized as an extra half-plane of

atoms in a lattice.

• The locus of defective points produced in the lattice by

the dislocation lie along a line.

• The inter-atomic bonds are significantly distorted only

in the immediate vicinity of the dislocation line.

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edge dislocation

edge dislocation

http://www.techfak.uni-kiel.de/matwis/amat/def_en/kap_5/illustr/a5_1_1.html

Screw Dislocation

• It results from a displacement of the atoms in one part of

a crystal relative to the rest of the crystal forming a spiral

ramp around the dislocation line.

• The motion of a screw dislocation is a result of shear

stress.

• But the defect line movement is perpendicular to direction

of the stress and the atom displacement, rather than

parallel.

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Screw

Dislocation Slip Planes

Slip Direction

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• The atoms represented by the

blue have not yet moved from

their original position.

• The atoms represented by the

green are in the process of

moving.

• The atoms represented by the

red have moved to their new

position in the lattice & have

reestablished metallic bonds.

• It can be seen that only a

portion of the bonds are

broken at any given time

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screw dislocation

http://uet.edu.pk/dmems/screw_dislocation.htm

Surface Defects • These are two dimensional defects.

• Also known as interfacial defects.

• Surface defects refer to regions of distortions that lie

about a surface having thickness of a few atomic

diameters

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• Environment of an atom differs at surface level and at

bulk level.

• Coordination number is less.

• So more surface energy.

• This results in

• Decrease in lattice spacing.

• Change in crystal structure.

• To reduce these effects the materials try to reduce the

surface area.

External Surface

• Crystalline solids are made up of grains.

• Grain boundaries are several atoms distances wide,

and there is mismatch of orientation of grains on

either side of the boundary.

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Grain Boundaries (or) Crystal Boundary

Grain Boundaries

• When the orientation difference between two crystals

is less than 10° it is called low angle boundaries.

• If the low grain boundary is formed by edge

dislocations, it is called tilt boundary, and twist

boundary if formed of screw dislocations.

• The orientation difference is usually greater than 10-

15°. For this reason the grain boundaries are also

known as high angle boundaries.

• Grain boundaries are chemically more reactive

because of grain boundary energy.

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Grain Boundaries • The sub grain boundaries are of two types they are

• Tilt boundaries

• Twist boundaries

• An array of edge dislocations is called tilt boundary.

• A cross grid of screw dislocation is called twist

boundaries.

• A twin boundary happens when the crystals on either

side of a plane are mirror images of each other.

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Tilt

boundari

es

Twist

boundarie

s

• Two single crystal sections are miss oriented but

joined together such that one plane is a mirror image

of the other.

• Twin boundaries occur in pairs such that the

orientation change introduced by one boundary is

restored by the other.

• The boundary common to the two planes is a twinned

region. They are formed during the growth of crystal.

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• Twins which forms during the process of

recrystallization are called annealing twins,

• Twins which are formed during plastic deformation

are called deformation twins.

• Twinning occurs on a definite crystallographic plane

and in a specific direction, both of which depend on

the crystal structure.

• Annealing twins are typically found in metals that

have FCC crystal structure.

• While mechanical/deformation twins are observed in

BCC and HCP metals.

• Annealing twins are usually broader and with

straighter sides than mechanical twins.

• Twins do not extend beyond a grain boundary. 37

Stacking Defects • Stacking faults are planar surface imperfections caused by

fault in the staking sequence of atomic planes in crystals

• In FCC crystal we have three different stacking layers ABC

while in HCP stacking we have only two different layers

BC hence when FCC crystal grows we have the stacking

as ………ABCABCABCABC……

• While growing if the plane A indicated by arrow above

missing then we get the sequence

• ………ABCABCBCABC……

• Thus we find that the stacking in the missing region be

comes HCP. This thin region is a surface imperfection and

is called a stocking fault.

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Volume Defects

• Presence of a large vacancy (crakes) or void such as

clusters of atoms missing is also considered as a volume

defect. While crystal is grown there is every possibility of

inclusion of non-crystalline regions of dimensions of at

least 10 to 30°A. This is also called Volume imperfection.

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