crystal defects.doc

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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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