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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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www.letstalkaboutscience.wordpress.c
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