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CHAPTER TWO

METALLOGRAPHY & MICROSCOPY

1. INTRODUCTION:1. INTRODUCTION:

•• Materials characterisation has two main aspects:Materials characterisation has two main aspects:

�� Accurately measuring the physical, mechanical and chemical Accurately measuring the physical, mechanical and chemical

properties of materialsproperties of materials

�� Accurately measuring (determining) the structure of a materialAccurately measuring (determining) the structure of a material

�� Atomic level structuresAtomic level structures

�� Microscopic level structuresMicroscopic level structures

•• A critical part of materials science & technology is to seek relationships A critical part of materials science & technology is to seek relationships

between:between:

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PerformancePerformance isis thethe ultimateultimate endend useuse functionfunction ofof thethe materialmaterial andand isis thethe

resultresult fromfrom aa properproper setset ofof propertiesproperties achievedachieved byby optimisingoptimising bothboth atomicatomic

levellevel andand microstructuralmicrostructural levelslevels ofof thethe structurestructure ofof thethe materialmaterial preparedprepared

andand fabricatedfabricated usingusing carefullycarefully controlledcontrolled andand optimisedoptimised processingprocessing

techniquestechniques..

PERFORMANCEPERFORMANCE

PROCESSING

PROPERTIESSTRUCTURE

What is metallography (Materiallography)?What is metallography (Materiallography)?

•• The study of structure of materialsThe study of structure of materials

•• It includes the techniques used to:It includes the techniques used to:

�� Prepare specimens for examination, Prepare specimens for examination,

�� Examine the specimens and Examine the specimens and

�� Interpreting the structures.Interpreting the structures.

What is a microstructure?What is a microstructure?

•• Most engineering materials of great importance are based on metals, Most engineering materials of great importance are based on metals, they are crystalline in the solid form.they are crystalline in the solid form.

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•• Metals also are normally polycrystalline, meaning they consist of Metals also are normally polycrystalline, meaning they consist of

many small crystals, called grains.many small crystals, called grains.

•• In some cases, these grains can be viewed with the naked eye and In some cases, these grains can be viewed with the naked eye and

these structures are called these structures are called MACROSTRUCTURESMACROSTRUCTURES

•• The structural features of small grains are observed using an optical The structural features of small grains are observed using an optical

microscope, or an electron microscope. These structures are called microscope, or an electron microscope. These structures are called

MICROSTRUCTURESMICROSTRUCTURES..

•• TheThe structuralstructural featuresfeatures presentpresent inin aa materialmaterial areare aa functionfunction ofof thethe

compositioncomposition andand formform ofof thethe startingstarting material,material, andand anyany subsequentsubsequent heatheat

treatmenttreatment andand /or/or processingprocessing treatmenttreatment thethe materialmaterial receivesreceives..

•• MicrostructuralMicrostructural analysisanalysis isis usedused toto provideprovide informationinformation onon howhow thethe

materialmaterial waswas producedproduced andand thethe qualityquality ofof thethe resultingresulting materialmaterial..

•• MicrostructuralMicrostructural features,features, whichwhich areare ofof greatgreat concernconcern toto usus includeinclude:: graingrain

sizesize,, phasephase volumevolume fractionfraction,, precipitateprecipitate sizesize,, defectsdefects (porosity(porosity andand

cracks)cracks)

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WhatWhat isis MicrostructuralMicrostructural AnalysisAnalysis usedused for?for?

�� MacrostructuralMacrostructural andand MicrostructuralMicrostructural examinationexamination techniquestechniques areare usedusedinin areasareas suchsuch asas::

�� QualityQuality ControlControl:: AnalysisAnalysis isis usedused toto determinedetermine whetherwhether thethe structuralstructuralparametersparameters areare withinwithin specificationsspecifications:: aa criteriacriteria forfor ACCEPTANCEACCEPTANCE ororREJECTIONREJECTION ofof productsproducts

�� FailureFailure AnalysisAnalysis:: toto determinedetermine thethe causecause ofof failurefailure.. FailureFailure occuroccur duedue totoseveralseveral factorsfactors (incorrect(incorrect materialmaterial selection,selection, improperimproper processingprocessingtreatment,treatment, poorpoor qualityquality control)control)..FailureFailure analysisanalysis providesprovides informationinformationaboutabout thethe causecause ofof failurefailure

�� ResearchResearch StudiesStudies:: isis usedused toto determinedetermine thethe microstructuralmicrostructural changeschangesthatthat occuroccur asas aa resultresult ofof varyingvarying parametersparameters suchsuch asas composition,composition, heatheattreatmenttreatment oror processingprocessing.. TheThe researchresearch studiesstudies developdevelop thethePROCESSINGPROCESSING -- STRUCTURESTRUCTURE -- PROPERTIESPROPERTIES relationshipsrelationships..

WhatWhat InformationInformation cancan bebe ObservedObserved

•• QualityQuality ofof specimenspecimen preparationpreparation isis aa determiningdetermining factorfactor inin thethe valuevalue ofof

examinationexamination

Effect of cold workEffect of cold work

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SPECIMENSPECIMEN PREPARATIONPREPARATION

•• SpecimenSpecimen preparationpreparation isis anan importantimportant partpart ofof metallographymetallography

•• AA specimenspecimen mustmust bebe appropriatelyappropriately preparedprepared toto ensureensure correctcorrect

observationobservation andand interpretationinterpretation ofof thethe microstructuremicrostructure..

•• SpecimenSpecimen preparationpreparation requirementsrequirements

�� DeformationDeformation--freefree specimenspecimen

�� FlatFlat specimenspecimen

�� NoNo thermalthermal damagedamage

�� NoNo scratchesscratches

Sample SelectionSample Selection

SectioningSectioning

MountingMounting

Grinding Grinding

Polishing Polishing

Etching Etching

Sample ExaminationSample Examination

Specimen preparation consists of :Specimen preparation consists of :

The number, location and orientation of the samples examined are important The number, location and orientation of the samples examined are important

parameters in selection samples.parameters in selection samples.

Specimens are Specimens are examined using optical examined using optical

and /or electron and /or electron microscopesmicroscopes

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SectioningSectioning

•• Abrasive cutting is the most common cutting method Abrasive cutting is the most common cutting method

•• The cutting tool (disc) is made of silicon carbide (SiC) The cutting tool (disc) is made of silicon carbide (SiC)

of Diamond particlesof Diamond particles

•• use coolant fluid (lubrication) to avoid overheating of use coolant fluid (lubrication) to avoid overheating of

specimen and possible change in material structurespecimen and possible change in material structure

Electric Discharge Machining Electric Discharge Machining

(EDM) cutting(EDM) cutting

Mounting: Mounting: Provides a safe and efficient mean of handling samples (particularly Provides a safe and efficient mean of handling samples (particularly

small and irregular specimens)small and irregular specimens). There are two types of mounting:. There are two types of mounting:

1.1. Hot mounting: Hot mounting: requires compression pressure and heat and is done using requires compression pressure and heat and is done using

mounting machinesmounting machines

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MountingMounting

2.2. Cold mounting: Cold mounting: uses epoxy resin hardened at room temperatureuses epoxy resin hardened at room temperature

Grinding and PolishingGrinding and Polishing•• Grinding removes the damage on the specimen surface produced by sectioningGrinding removes the damage on the specimen surface produced by sectioning

•• Grinding is done using abrasive paper with different grit size Grinding is done using abrasive paper with different grit size

•• After grinding is complete, the specimen is polishedAfter grinding is complete, the specimen is polished

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Grinding and PolishingGrinding and Polishing

Water rinse

Polishing Polishing

Water rinse

6 µm0.25 µm 1 µm

Grinding Grinding

Wate

r rinse

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Grinding and PolishingGrinding and Polishing

EtchingEtching

Techniques

� Swab

� Immersion

� Electrolytic

Purpose

� Grain boundaries

� Flow lines

� Constituents or phases present in structure

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QUANTITATIVE METALLOGRAPHYQUANTITATIVE METALLOGRAPHY

�� AnAn imageimage isis quantifiedquantified byby describingdescribing::

�� Size,Size, Shape,Shape, Distribution,Distribution, andand QuantityQuantity

�� TheThe measurementsmeasurements areare mademade manuallymanually ((linearlinear interceptintercept methodmethod,,

pointpoint countingcounting),), oror byby computerisedcomputerised automatedautomated methodsmethods onon digitallydigitally

acquiredacquired imagesimages ((ImageImage analyseranalyser))

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TEMTEM SEMSEM OMOM

Grain 1Grain 1

Grain 2Grain 2

Atomic

XRD

Optical Microscopy (OM)Optical Microscopy (OM)

Old & Modern Optical MicroscopesOld & Modern Optical Microscopes

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Inverted MicroscopeInverted MicroscopeUpright MicroscopeUpright Microscope

• •

u v

fa

Object

Image

• •u

b

v

Object

Image

• •v

c

uImage

Object

Ray diagrams illustrating the formation of an image by a single lens of focal length, f.

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MAGNIFICATION IN LIGHT MICROSCOPEMAGNIFICATION IN LIGHT MICROSCOPE

•• Using the thin lens equation:Using the thin lens equation:

•• Figures above show that, by similar triangles, the magnification M Figures above show that, by similar triangles, the magnification M

produced by the single lens is given by v/uproduced by the single lens is given by v/u

vuf

111+=

f

fv

fu

fM

−=

−=

�� FromFrom thethe formula,formula, largerlarger MM requiresrequires smallersmaller focalfocal length,length, f,f,

�� However,However, lenseslenses withwith smallersmaller focalfocal lengthslengths areare difficultdifficult toto makemake

�� Instead,Instead, higherhigher magnificationsmagnifications areare achievedachieved byby combiningcombining lenseslenses

�� ForFor example,example, whenwhen usingusing twotwo lenseslenses::

( )( )

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2211

ff

fvfvM

−−=

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RESOLUTIONRESOLUTION ININ THETHE LIGHTLIGHT MICROSCOPEMICROSCOPE

•• ResolutionResolution isis thethe closestclosest spacingspacing ofof twotwo pointspoints thatthat areare visiblevisible asas

distinctdistinct entitiesentities throughthrough thethe microscopemicroscope

•• InIn thethe lightlight microscope,microscope, thethe qualityquality ofof thethe objectiveobjective lenslens playsplays aa majormajor

rolerole inin determiningdetermining thethe resolvingresolving powerpower ofof thethe apparatusapparatus..

•• However,However, thethe resolutionresolution inin aa lightlight microscopemicroscope isis limitedlimited byby aa

diffractiondiffraction ofof lightlight effecteffect

• Diffraction: when light passes through an object the intensity is reduced

depending upon the color absorbed. Thus the selective absorption of

white light produces colored light

•• DiffractionDiffraction inin aa microscopemicroscope occursoccurs whenwhen aa lightlight wavewave passespasses veryvery closeclose

toto thethe edgeedge ofof anan objectobject oror throughthrough aa tinytiny opening,opening, suchsuch asas aa slitslit oror

apertureaperture

Dispersion

Short wavelengths are “bent”

more than long wavelengths

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•• InIn thethe microscope,microscope, diffractiondiffraction ofof lightlight cancan occuroccur atat

thethe specimenspecimen planeplane duedue toto interactioninteraction ofof thethe lightlight

withwith smallsmall particlesparticles oror features,features, andand againagain atat thethe

marginsmargins ofof thethe objectiveobjective frontfront lenslens oror atat thethe edgesedges

ofof aa circularcircular apertureaperture withinwithin oror nearnear thethe rearrear ofof thethe

objectiveobjective..

•• ItIt isis thisthis diffractiondiffraction ofof lightlight thatthat makesmakes itit possiblepossible

toto observeobserve magnifiedmagnified imagesimages ofof specimensspecimens inin thethe

microscopemicroscope..

•• However,However, itit isis alsoalso diffractiondiffraction thatthat limitslimits thethe sizesize

ofof objectsobjects thatthat cancan bebe resolvedresolved (limit(limit thethe

resolution)resolution)

micro.magnet.fsu.edu/primer/lightandcolor/diffraction.htmmicro.magnet.fsu.edu/primer/lightandcolor/diffraction.htm

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micro.magnet.fsu.edu/primer/lightandcolor/diffraction.htmmicro.magnet.fsu.edu/primer/lightandcolor/diffraction.htm

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r = d/2r = d/2

α

Rayleigh’sRayleigh’s criterioncriterion:: whenwhen thethe maximummaximum ofofintensityintensity ofof anan AiryAiry discdisc coincidescoincides withwith thethefirstfirst minimumminimum ofof thethe second,second, thenthen thethe twotwopointspoints cancan bebe distinguisheddistinguished

Aperture Aperture

RESOLUTIONRESOLUTION ININ THETHE LIGHTLIGHT MICROSCOPEMICROSCOPE

•• FromFrom thethe diffractiondiffraction theory,theory, thethe resolutionresolution isis givengiven byby Abbe’sAbbe’s equationequation::

•• λλ isis thethe wavelengthwavelength ofof light,light, µµ isis thethe refractiverefractive indexindex forfor thethe mediummedium

throughthrough whichwhich thethe lightlight passespasses (air(air == 11,, waterwater == 11..3333,, oiloil == 11..44))

•• TheThe refractiverefractive indexindex isis == ratioratio ofof thethe speedspeed ofof lightlight inin aa vacuumvacuum toto thatthat inin aa

secondsecond mediummedium ofof greatergreater densitydensity

NA

dr

λ

αµ

λ 61.0

sin

61.0

2===

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•• µµ sinsinαα isis calledcalled thethe numericalnumerical apertureaperture (NA)(NA)

•• InIn orderorder toto obtainobtain higherhigher resolutionresolution (smallest(smallest r),r), itit isis possiblepossible toto

decreasedecrease λλ oror increaseincrease µµ oror αα..

•• TheThe higherhigher thethe NANA thethe greatergreater thethe resolutionresolution

•• TheThe limitslimits ofof thethe objectiveobjective lenslens areare thatthat αα cannotcannot bebe greatergreater thanthan 9090oo,,

andand thatthat thethe objectobject spacespace cancan onlyonly reachreach anan NANA == 11..44

Material NANA

Air

Water

Immersion oil

Glass

Zircon

Diamond

1.0003

1.33

1.4

1.52

1.92

2.42

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N.A.N.A. 0.130.13 0.510.51 0.950.95

D D -- separation distanceseparation distance

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DEPTHDEPTH OFOF FIELDFIELD

•• TheThe depthdepth ofof filedfiled isis defineddefined asas thethe

distancedistance fromfrom thethe nearestnearest partpart ofof thethe

specimenspecimen toto thethe farthestfarthest partpart ofof thethe

specimenspecimen thatthat isis inin focusfocus whenwhen thethe

picturepicture isis beingbeing takentaken

hh

dd

αα

AA

ApertureAperture

Plane of Plane of optimum optimum

focusfocus

DEPTHDEPTH OFOF FIELDFIELD

•• TheThe depthdepth ofof fieldfield cancan bebe

estimatedestimated fromfrom thethe Figure,Figure, whichwhich

showsshows raysrays convergingconverging atat thethe

specimenspecimen.. SimpleSimple geometrygeometry

givesgives::

ααµ

λ

tansin

61.0=h

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Light Microscope vs Electron MicroscopeLight Microscope vs Electron Microscope

�� AsAs λλ increases,increases, rr increasesincreases (low(low resolution)resolution)

�� BestBest resolutionresolution isis whenwhen λλ isis lowestlowest

�� SinceSince electronelectron microscopesmicroscopes useuse electronelectron asas thethe sourcesource ofofillumination,illumination, λλ isis shorter,shorter, thusthus resolutionresolution isis higherhigher

SEMSEM

Depth of Field Depth of Field

OMOM

CONSIDERATIONS IN MICROSCOPYCONSIDERATIONS IN MICROSCOPY

•• MagnificationMagnification

•• ResolutionResolution

•• High magnification without high resolution is “High magnification without high resolution is “EMPTYEMPTY” magnification” magnification

•• RESOLVING LIMITRESOLVING LIMIT

•• EyeEye 101055 nm (0.1 mm)nm (0.1 mm)

•• Light MicroscopeLight Microscope 200 nm (0.2 200 nm (0.2 µµm)m)

•• SEMSEM 3 nm3 nm

•• TEMTEM 0.2 nm0.2 nm

This data is outdated!!!This data is outdated!!!

Check the latest Check the latest

machinesmachines

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LENS ABERRATIONSLENS ABERRATIONS

•• Image quality, in all optical instruments, is limited by distortions which Image quality, in all optical instruments, is limited by distortions which

arise from optical defects called aberrationsarise from optical defects called aberrations

•• Lens aberrations include:Lens aberrations include:

�� Monochromatic (spherical) aberrationMonochromatic (spherical) aberration

�� Chromatic aberrationChromatic aberration

�� AstigmatismAstigmatism

�� DiffractionDiffraction

SPHERICAL ABERRATIONSPHERICAL ABERRATION

�� TheseThese aberrationsaberrations occuroccur whenwhenlightlight waveswaves passingpassing throughthrough thetheperipheryperiphery ofof aa lenslens areare notnotbroughtbrought intointo identicalidentical focusfocus withwiththosethose passingpassing closercloser toto thethe centrecentre

�� ElectronsElectrons furtherfurther fromfrom axisaxis arearefocusedfocused closercloser thanthan thethe electronselectronsclosercloser toto axisaxis

Ideal lensIdeal lens

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CHROMATIC ABERRATIONCHROMATIC ABERRATION

�� TheseThese aberrationsaberrations areare thethe resultresult ofofthethe factfact thatthat whitewhite lightlight isiscomposedcomposed ofof numerousnumerouswavelengthswavelengths..

�� WhenWhen whitewhite lightlight passespasses throughthrough aaconvexconvex lens,lens, thethe componentcomponentwavelengthswavelengths areare refractedrefractedaccordingaccording toto theirtheir frequencyfrequency..

Ideal lensIdeal lens

ASTIGMATISMASTIGMATISM

�� TheThe offoff--axisaxis imageimage ofof aa specimenspecimen

pointpoint appearsappears asas aa discdisc oror blurredblurred

lineslines insteadinstead ofof aa pointpoint

�� DependingDepending onon thethe angleangle ofof thethe offoff--

axisaxis raysrays enteringentering thethe lens,lens, thethe

lineline imageimage maymay bebe orientedoriented eithereither

tangentiallytangentially oror radiallyradially