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EFFECT OF CRYSTALDEFECTS ON

MECHANICAL

PROPERTIES OFMATTER


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Made

By -:

AkshayAgarwal

DTU/2K12/ME/022

INTRODUCTION

A perfect crystal, with every atom of

the same type in the correct position,does not exist. All crystals have some

defects. Defects contribute to the

mechanical properties and corrosion of

materials. In fact, using the term

defect is sort of a misnomer sincethese features are commonly

intentionally used to manipulate the

mechanical properties of a material.


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Adding alloying components to a

material is one way of introducing a

crystal defect. Nevertheless, the termdefect is used.

Types Of Crystal

Defects

There are basic classes of crystal

defects:

1 Point Defects- Which are places

where an atom is missing or irregularly

placed in the lattice structure. Pointdefects include lattice vacancies, self-

interstitial atoms, substitution impurity

atoms, and interstitial impurity atoms.


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2 Linear Defects- Which are groups of

atoms in irregular positions. Linear

defects are commonly calleddislocations.

3 Planar Defects- Which are interfaces

between homogeneous regions of the

material. Planar defects include grain

boundaries, stacking faults andexternal surfaces.

4 Bulk or Volume defects- These

include pores,

cracks, foreign inclusions and other

phases. These defects are normally

introduced during processing and

fabrication steps.

The plastic deformation in a material

occurs due to the movement of

dislocations (linear defects). Millions of

dislocations result for plastic forming

operations such as rolling and


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extruding.

For both scientific and practical

reasons, much of the research oncrystal defects is directed toward the

dynamic properties of defects under

particular conditions, or defect

chemistry. Much of the motivation for

this arises from the often undesirableeffects of external influences on

material properties, and a desire to

minimize these effects. Examples of

defect chemistry abound, including

one as familiar as the photographicprocess, in which incident photons

cause defect modifications in silver

halides or other materials. Properties

of materials in nuclear reactors is

another important case.

Effect of Crystal


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Defects On Corrosion

Corrosion is the chemical reaction of a

molecule (in this case, in the lattice),

with elements in the environment.

Oxidation is a prime example, and one

of the most common types ofcorrosion. This usually happens at the

surface of the material, or at grain

boundaries, where the atoms aren't in

an equilibrium state (bonded to one

another ideally, with zero net charge).

This is why nano-crystalline materials

tend to corrode at a much higher rate

than materials with a larger grain size

(nano-crystalline materials have much

more grain boundary volume).

When there's a defect, it means that


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the lattice is no longer perfect, the

order has been broken. It can be taken

from this that there might now beareas in which atoms will exist with a

certain charge, making them more

susceptible to forming chemical bonds

with elements in the environment.


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Effects Of Dislocations

On MechanicalProperties

The simplest extended structural

defect is the dislocation. An edge

dislocation is a line defect which maybe thought of as the result of adding

or subtracting a half-plane of atoms. A

screw dislocation is a line defect which

can be thought of as the result of

cutting partway through the crystal

and displacing it parallel to the edge

of the cut.

Dislocations are of great importance in

determining the mechanical properties

of crystals. A dislocation-free crystal isresistant to shear, because atoms

must be displaced over high-potential-

energy barriers from one equilibrium


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position to another.

It takes relatively little energy to move

a dislocation (and thereby shear the

crystal), because the atoms at the

dislocation are barely in stable

equilibrium. Such plastic deformation

is known as slip.


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Miscellaneous Effects

Of Crystal Defects On

Properties Of MatterThe presence of point defects or

disorder in a crystal can profoundly

alter the character of the normal

modes of vibration of the crystal, and

consequently those crystalline

properties in which the lattice

vibrations play the dominant role.

Natural crystals always contain

defects, due to the uncontrolledconditions under which they were

formed. The presence of defects which

affect the color can make these


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crystals valuable as gems, as in ruby

(Cr replacing a small fraction of the Al

in Al2O3). Crystals prepared in thelaboratory will also always contain

defects, although considerable control

may be exercised over their type,

concentration, and distribution.

The importance of defects dependsupon the material, type of defect, and

properties which are being considered.

Some properties, such as density and

elastic constants, are proportional to

the concentration of defects, and so asmall defect concentration will have a

very small effect on these. Other

properties, such as the conductivity of

a semiconductor crystal, may be much

more sensitive to the presence of

small numbers of defects.


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Indeed, while the term defect carries

with it the connotation of undesirable

qualities, defects are responsible for

many of the important properties of

materials, and much of solid-statephysics and materials science involves

the study and engineering of defects

so that solids will have desired

properties. A defect-free silicon crystal

would be of little use in modernelectronics; the use of silicon in

devices is dependent upon small

concentrations of chemical impurities


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such as phosphorus and arsenic which

give it desired electronic properties.

REFERENCES

1 www.wikipeda.com

2 www.sciencedirect.com

3 www.nptel.com

4 www.berkeley.edu5 www.virginia.edu

6
http://www.wikipeda.com/http://www.sciencedirect.com/http://www.nptel.com/http://www.berkeley.edu/http://www.virginia.edu/http://www.wikipeda.com/http://www.sciencedirect.com/http://www.nptel.com/http://www.berkeley.edu/http://www.virginia.edu/


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