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CRYSTAL DEFECTS
ABSTRACT
A perfect crystal, with every atom of the same type in the correct position, does not exist.
All crystals have some defects. Imperfection in the regular geometrical arrangement of
the atoms in a crystalline solid is called a defect. Defects contribute to the mechanical
properties of metals. In fact, using the term “defect” is sort of a misnomer since thesefeatures are commonly intentionally used to manipulate the mechanical properties of a
material. There are four basic classes of crystal defects point, linear, planar and volumedefects.
INTRODUCTION
!o far it has been assumed that perfect order exists throughout crystalline materials on an
atomic scale. "owever ,such an idealised solid does not exist all contain large number of various defects or imperfections. #y “crystalline defect” is meant a lattice regularity
having one or more of its dimensions on the order of an atomic diameter. The properties
of a material are sensitive to the deviations from crystalline perfection the influence isnot always adverse, and often specific characteristics are deliberately fashioned by the
introduction of controlled amounts or numbers of particular defects. Adding alloying
elements to a metal is one way of introducing a crystal defect. The presence of defects
also affects the color of the crystal
These imperfections result from deformation of the solid, rapid cooling from high
temperature, or high$energy radiation %&$rays or neutrons' stri(ing the solid. )ocated at
single points, along lines, or on whole surfaces in the solid, these defects influence itsmechanical, electrical, and optical behaviour.
In general, there are many other (inds of possible defects, ranging from simple and
microscopic, such as the vacancy and other structures shown in the illustration, tocomplex and macroscopic, such as the inclusion of another material, or a surface.
#roadly the defects are classified on the basis of geometry and dimensionality as*
• +oint* which are places where an atom is missing or irregularly placed in the
lattice structure.
• )inear* which are groups of atoms in irregular positions. They are commonly
called dislocations.
• +lanar* which are interfaces between homogeneous regions of the material. +lanar
defects include grain boundaries, stac(ing faults and external surfaces.
• olume* occur on a much bigger scale than the rest of the crystal defects.
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POINT DEFECTS
+oint defects are defects which are not extendedin space in any dimension. +oint defects are where
an atom is missing or is in an irregular place in the
lattice structure. +oint defects include self interstitial atoms, interstitial impurity atoms,
substitutional atoms and vacancies as shown in
figure -.
A self interstitial atom is an extra atom that hascrowded its way into an interstitial void in the
crystal structure. A substitutional impurity atom is
an atom of a different type than the bul( atoms,which has replaced one of the bul( atoms in the
lattice. !ubstitutional impurity atoms are usually
close in sie %within approximately -/0' to the bul( atom. An example of substitutional impurity
atoms is the inc atoms in brass. In brass, inc
atoms with a radius of 1.-22 nm have replaced some of the copper atoms, which have a
radius of 1.-34 nm.
VACANCIES
acancies are sites which are usually occupied by an atom but which are unoccupied. If a
neighboring atom moves to occupy the vacant site, the vacancy moves in the oppositedirection to the site which used to be occupied by the moving atom. The stability of the
surrounding crystal structure guarantees that the neighboring atoms will not simply
collapse around the vacancy. In some materials, neighboring atoms actually move away
from a vacancy, because they can form better bonds with atoms in the other directions.
INTERSTITIES
Interstities are atoms which occupy a site in the crystal structure at which there is usuallynot an atom. They are generally high energy configurations. !mall atoms in some crystals
can occupy interstices without high energy, such as hydrogen in palladium. Interstitialimpurity atoms are much smaller than the atoms in the bul( matrix. Interstitial impurity
atoms fit into the open space between the bul( atoms of the lattice structure. An example
of interstitial impurity atoms is the carbon atoms that are added to iron to ma(e steel.5arbon atoms, with a radius of 1.16- nm, fit nicely in the open spaces between the larger
%1.-37 nm' iron atoms.
FRENKEL DEFECT
8lectroneutrality is the state that exists when there are e9ual numbers of positive and
negative charges from the ions. :ne such defect involves a cation$vacancy and a cation$interstitial pair. This is called a Frenke !efect. It might be thought of as being formed by
a cation leaving its normal position and moving into an interstitial site. There is nochange in the density of the crystalline solid.
SC"OTTKY DEFECT
A cation vacancy$anion vacancy pair is (now as schott(y defect. This defect is created by
removing one cation and one anion from the interior of the crystal and then placing them
both at an external surface. !ince both cations and anions have same charge and for every
anion vacancy there exists a cation vacancy, the charge neutrality is maintained.
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LINEAR DEFECTS
)ine defects, or dislocations, are lines along which whole rows of atoms in a solid arearranged anomalously. The resulting irregularity in spacing is most severe along a line
called the line of dislocation. Dislocations are generated and move when a stress is
applied. The motion of dislocations allows slip ; plastic deformation to occur .)inedefects can wea(en or strengthen solids.
There are two basic types of dislocations, t#e e!$e !isoc%tion and t#e scre&
!isoc%tion.
ED'E DISLOCATION
The edge defect can be easily visualied as anextra half$plane of atoms in a lattice. The
dislocation is called a line defect because the
locus of defective points produced in the lattice
by the dislocation lie along a line. This line runs
along the top of the extra half$plane. The inter$atomic bonds are significantly distorted only in
the immediate vicinity of the dislocation line.The figure 3 below shows the atom positions
around an edge dislocation extra half $plane of
atoms.
SCRE( DISLOCATION
The screw dislocation is being formed by a shear stress that is applied to produce the
distortion. As shown in the figure* the upper front region of the crystal is shifted one
atomic distance to right relative to the bottom portion. The atomic distortion associatedwith the screw dislocation is also linear and along a dislocation line A# in figure 7. The
screw dislocation derives its name from the spiral or helical path or ramp that is traced
around the disclocation line by the atomic planes of atoms.
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PLANAR DEFECTS
+lanar defects also (now as interfacial defects are boundaries that have two dimensionsand normally separate regions of the materials that have crystal structure. These
imperfections include e)tern% s*rf%ces, $r%in +o*n!%ries, t&in +o*n!%ries and p#%se
+o*n!%ries.
E,TERNAL SURFACES
:ne of the most obvious boundaries is the external surface, along which the structure
terminates. !urface atoms are not bonded to the maximum number of nearest neighbors,
and are therefore in a higher energy state than the atoms at the interior positions. Toreduce this energy, materials tend to minimie the total surface area if at all possible.
'RAIN BOUNDARIES
!olids generally consist of a number of
crystallites or grains. =rains can range insie from nanometers to millimetersacross and their orientations are usually
rotated with respect to neighboring
grains. >here one grain stops andanother begins is (now as a grain
boundary. =rain boundaries limit the
lengths and motions of dislocations.
T(IN BOUNDARIES
It is a special type of grain boundary
across which there is a specific mirror
lattice symmetry that is, atoms on oneside of the boundary are located in the
mirror$image positions of the atoms on
the other side. The region of material between these boundaries is termed as
t&in.
STACKIN' FAULTSA change in the stac(ing se9uence over a few atomic spacings produces a st%ckin$ f%*t.
A stac(ing fault is a one or two layer interruption in the stac(ing se9uence of atom
planes. !tac(ing faults occur in a number of crystal structures, but it is easiest to see in
close pac(ed structures. !tac(ing faults are found in ?55 metals when there is aninterruption in the A#5A#5A#5@ stac(ing se9uence of closed$pac(ed planes.
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BULK DEFECTS
#ul( defects occur on a much bigger scale. Theseinclude pores, crac(s, foreign inclusions, and other
phases. They are normally introduced during processing
and fabrication steps. oids are regions where there area large number of atoms missing from the lattice.
Another type of bul( defect occurs when impurity
atoms cluster together to form small regions of a
different phase. The term phase refers to that region of space occupied by a physically homogeneous material.
These regions are often called precipitates.
CONCLUSION
A crystal is never perfect a variety of imperfections can affect the ordering. A defect is a
small imperfection affecting a few atoms. All solid materials contain large numbers ofimperfections or deviations from crystalline perfection. Defects contribute to the
mechanical properties of metals. It also affects the colours of the crystals. Impurities in
solids may exist as a point defects.
REFRENCES-
%-' 5allisters i(ipedia.com %8ncyclopedia'
%7' ndt$ed.org %8ducation Besources'
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