material engineering - thomy dj. vollmer

8/2/2019 Material Engineering - Thomy Dj. Vollmer

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C H A P T E R 4 :I M P ER P EF E CT I O S I N S O LI D

( T H O M Y D J . V O L L M E R )

Material Engineering


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Imperfections in Solid

y Def:

What is imperfection:

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1.

th

e condition or quality of being imperfect 2. a fault or defect


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Topics ofImperfection

y I. Point defects: vacancies and self-interstitials,impurities

y II. Solid solutions: substitutional and interstitial,alloy composition and densities.

y III. Dislocations: edge, screw, dislocation motion,and Burgers vector.

y IV. Grain boundaries: tilt and twist boundaries,surfaces and interfaces, twins boundaries.


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I. Point Defects

y Vacancies and self-interstitials

So, Vacancies is when there is an atom missing.

Resulting to porous Solid lattice.

Self-Interstitials are atoms that are crowded into the little voidspaces among the other atoms.

Results to large distortions in the lattice.

There is no Vacancy/Self-Interstitals free Solid.


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I. Point Defects



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I. Point Defects



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I. Point Defects

y Impurities

Alloys, The impurity in a solid is not always somethingnegative:

Adding the atoms of other Substance, means we can use specificcharacteristic of the metal, too.

Exp: To a soft Silver we can add Copper. So the Silver can beStrong but still corrosion resistant.


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I. Point Defects

We can calculate the equlibrium number of vacancies:

!

kT

QNN vv exp

Nv = No. of vacancies

N = Total No. of lattice sites

k= Boltzmann constant

T = Temperature in Kelvin

Qv = Activation energy to create vacancy


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S U BS T IT U T IO N A L A N D I N T E R ST I T IA L ,A L L O Y C O M P O S I T I O N A N D D E N S I T I E S .

II. Solid solutions


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II. Solid solutions

y Substitutional and Interstitial

This Solid Solution will occur when there are solute atoms thatare added to the host material.

Resulting in same crystal structure but with differentcharacteristics based on the two Solutions.

Features that has to be fulfilled:

Atomic Size, radii not diff. than about 15%

Crystal Structure

Electro negativityValences


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II. Solid solutions


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II. Solid solutions

y Spec. of Composition

We are able to calculate the composition of the alloy with thisequation for the weight percent:


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E D G E , S C R EW, D I S L O C A T I O N M O T I O N , A N DB U R G E R S V E C T O R .

III. Dislocations


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III. Dislocations

y Dislocation, def:

An imperfection line in a crystalline material.

y Edge:

introduced extra half-plane into the crystal


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III. Dislocations

y Screw:

Formed by Shear Stress, which will form distortion.


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III. Dislocations

The combination of these two is usually called mixeddislocations.

The magnitude of the dislocation is stated with (b). Which isalso called the Burgers Vector.

Some Properties:

- Plastic Deformation: Irreversible deformation or change inshape that occurs when the force or stress that caused it is

removed.- Elastic Deformation: Deformation will be fully recovered when

the stress causing it is removed.


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T I L T A N D TWI S T B O U N D A R I E S , S U R F A C E S A N D I N T E R F A C E S , TWI N S B O U N D A R I E S .

IV. Grain boundaries


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y Tilt Boundary:

Having an Angle ofmisalignment:


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Phase Boundary:

Having diff. Phases, eachphase has its own physical/ChemicalProperties.

Twin Boundary:

Atoms on one side of the boundary is the mirror image of theatoms on the other side.


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Bulkor Volume Defects

Pores, Cracks, Foreign Inclusions

Introduced by Processing and Fabrication Steps


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Atomic Vibrations

Can be called imperfection because not all Atoms are vibrating inthe same speed or same energy.

Many Properties are based on this vibration.

Exp: Melting occurs if the vibrations are vigorous enough torupture large numbers ofAtomic Bonds.


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y Change of Properties:

Mech:

Boundaries are good places for fracture to occur

Boundaries interrupt the movement of dislocations

Electrical:

Good places to scatter electrons or inhibit their movement


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References:

Callister - Materials Science and Engineering - An Introduction 7e(Wiley, 2007)

http://www.substech.com/dokuwiki/lib/exe/fetch.php?w=&h=&cache=cache&media=point_defects.png

www.material.itb.ac.idmse.stanford.edu

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