CHAPTER TWO

METALLOGRAPHY & MICROSCOPY

1. INTRODUCTION:

••Materials characterisation has two main aspects:

Accurately measuring the physical, mechanical and chemical properties of materials

Accurately measuring (determining) the structure of a material  Atomic level structures  Microscopic level structures

••A critical part of materials science & technology isis to seek relationships between:

1 STRUCTURE PROPERTIES

PERFORMANCE

PROCESSING

Performance isis thethe ultimate end use function ofof thethematerial and isis thethe result from aaproperproper set ofofproperties achieved bybyoptimising both atomic level and microstructural levels ofof thethe structure ofof thethe material prepared and fabricated using carefully controlled and optimised processing techniquestechniques..

What is metallography (Materiallography)?

••The study of structure of materials ••It includes the techniques used to:

Prepare specimens for examination,

Examine the specimens and

Interpreting the structures.

What is a microstructure?

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

2 ••Metals also are normally polycrystalline, meaning theythey consist of many small crystals, called grains.

••In some cases, these grains can be viewed with thethe naked eye and these structures are called MACROSTRUCTURES

••The structural features of small grains are observeobservedd using an optical microscope, or an electron microscope. These structures are called MICROSTRUCTURES ..

••The structural features present inin aamaterialmaterial areare aafunctionfunction ofof thethe composition and form ofof thethestarting material, and any subsequent heat treatment and /or/or processing treatment thethematerial receives.. receives

••Microstructural analysis isisused totoprovide information ononhow thethe material was produced and thethe quality ofof thethe resulting material.. material

••Microstructural features, which areare ofofgreat concern toto ususinclude:: include grain size ,, phase volume fraction ,, precipitate size ,, defects (porosity and cracks)

3 What isisMicrostructural Analysis used for?

Macrostructural and Microstructural examination techniques areareused inin areas such asas::

Quality Control ::Analysis isisused totodetermine whether thethe structural parameters areare within specifications:: specifications aacriteriacriteria forfor ACCEPTANCE oror REJECTION ofofproducts

Failure Analysis ::totodetermine thethecause ofoffailure..Failure failure Failureoccur due toto several factors (incorrect material selection, improper processing treatment, poor quality control)..Failure control) Failureanalysis provides information about thethecause ofoffailure

Research Studies :: isis used totodetermine thethemicrostructural changes that occur asas aaresultresult ofofvarying parameters such asas composition, heat treatment oror processingprocessing.. The research studies develop thethe PROCESSING --STRUCTURESTRUCTURE --PROPERTIESPROPERTIES relationshipsrelationships..

What Information can bebe Observed ••Quality ofofspecimen preparation isis aadeterminingdeterminingfactor inin thethevalue ofof examination

Effect of cold work

4 SPECIMEN PREPARATION

••Specimen preparation isis ananimportant part ofofmetallography •• AAspecimen must bebe appropriately prepared totoensure correct observation and interpretation ofof thethemicrostructure.. microstructure

••Specimen preparation requirements

DeformationDeformation--freefreespecimen

Flat specimen

NoNothermal damage

NoNoscratches

Specimen preparation consists of :

Sample Selection The number, location and orientation of the samples examined are important parameters in selection samples.

Sectioning

Mounting

Grinding

Polishing

Etching Specimens are examined using optical Sample Examination and /or electron microscopes

5 Sectioning

•• Abrasive cutting is the most common cutting method ••TheThe cutting tool (disc) is made of silicon carbidecarbide (SiC) of Diamond particles ••useuse coolant fluid (lubrication) to avoid overheatoverheatiningg of specimen and possible change in material structure

Electric Discharge Machining (EDM) cutting

Mounting: Provides a safe and efficient mean of handling samples (particularly small and irregular specimens)specimens).. There are two types of mounting: 1.1.Hot mounting: requires compression pressure and heat and is done using mounting machines

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

Grinding and Polishing •• Grinding removes the damage on the specimen surfasurfacece produced by sectioning •• Grinding is done using abrasive paper with differdifferenentt grit size ••After grinding is complete, the specimen is polishepolishedd

7 Grinding and Polishing

Grinding

Water rinse Water rinse Water rinse

Polishing

0.25 µm 1 µm 6 µm

8 Grinding and Polishing

Etching

Techniques

Swab
Immersion
Electrolytic

Purpose

Grain boundaries
Flow lines
Constituents or phases present in structure

9 QUANTITATIVE METALLOGRAPHY

AnAnimage isisquantified bybydescribing:: describing

Size, Shape, Distribution, and Quantity

The measurements arearemade manually ((linear intercept method ,, point counting ),), oror bybycomputerised automated methods onondigitally acquired images ((Image analyser ))

10 XRD TEM SEM OMOM

Grain 1

Grain 2

Atomic

Optical Microscopy (OM)

Old & Modern Optical Microscopes

11 Inverted Microscope Upright Microscope

u v Image a •• Object f

Object

b •• Image u v

v c •• Object u Image

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

12 MAGNIFICATION IN LIGHT MICROSCOPE

••Using the thin lens equation:

1 1 1 = + f u v

••Figures above show that, by similar triangles, thethe magnification M produced by the single lens is given by v/u

f v − f M = = u − f f

From thethe formula, larger MMrequiresrequiressmaller focal length, f,f,

However, lenses with smaller focal lengths arearedifficult totomake

Instead, higher magnifications areareachieved bybycombining lenses

For example, when using twotwolenses:: lenses

(v − f )(v − f ) M = 1 1 2 2 f1 f2

13 RESOLUTION ININTHE LIGHT MICROSCOPE

••Resolution isis thethe closest spacing ofof twotwopoints that arearevisible asas distinct entities through thethemicroscope

•• InIn thethelight microscope, thethe quality ofof thetheobjective lens plays aamajormajor role inin determining thethe resolving power ofof thethe apparatusapparatus..

••However, thethe resolution inin aalight microscope isislimited byby aa diffraction ofoflight effect

• 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

••Diffraction inin aamicroscopemicroscopeoccurs when aalightlightwave passes very close toto thethe edge ofof ananobject ororthrough aatinytinyopening, such asas aaslitslit oror aperture Short wavelengths are “bent” more than long wavelengths

Dispersion

14 •• InIn thethemicroscope, diffraction ofoflight can occur atat thethe specimen plane due totointeraction ofof thethelight with small particles ororfeatures, and again atat thethe margins ofof thethe objective front lens oror atat thetheedges ofof aacircularcircularaperture within orornear thethe rear ofof thethe objectiveobjective..

•• ItIt isisthis diffraction ofoflight that makes ititpossible totoobserve magnified images ofofspecimens inin thethe microscopemicroscope..

••However, itit isisalso diffraction that limits thethe size ofofobjects that can bebe resolved (limit thethe resolution)

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

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

16 α

r = d/2

Aperture

Rayleigh’s criterion ::when thethe maximum ofof intensity ofof ananAiry disc coincides with thethe first minimum ofof thethe second, then thethe two points can bebe distinguished

RESOLUTION ININTHE LIGHT MICROSCOPE

••From thethe diffraction theory, thetheresolution isisgiven byby Abbe’s equationequation::

d 61.0 λ 61.0 λ r = = = 2 µ sin α NA

•• λλ isis thethe wavelength ofoflight, µµ isis thethe refractive index forfor thethemedium through which thethe light passes (air ==11,,waterwater ==11..3333,,oiloil ==11..44))

•• The refractive index isis ==ratioratio ofof thethe speed ofof light inin aavacuumvacuum toto that inin aa second medium ofof greater density

17 •• µµ sinsin αα isiscalled thethe numerical aperture (NA)

•• InInorder totoobtain higher resolution (smallest r),r), itit isispossible toto decrease λλ ororincrease µµ oror αα..

••The higher thethe NANA thethe greater thetheresolution

••The limits ofof thethe objective lens arearethat αα cannot bebegreater than 9090 oo,, and that thethe object space can only reach anan NANA ==11..44

Material NANA Air 1.0003 Water 1.33 Immersion oil 1.4 Glass 1.52 Zircon 1.92 Diamond 2.42

18 N.A. 0.13 0.51 0.95

D --separationseparation distance

19 DEPTH OFOFFIELD

••The depth ofoffiled isisdefined asas thethe distance from thethe nearest part ofof thethe specimen toto thethefarthest part ofof thethe specimen that isis ininfocus when thethe picture isisbeing taken

DEPTH OFOFFIELD

••The depth ofoffield can bebe AA estimated from thethe Figure, which Aperture shows rays converging atat thethe dd specimenspecimen..Simple geometry givesgives:: αα Plane of hh optimum 61.0 λ focus h = µ sin α tan α

20 Light Microscope vs Electron Microscope

AsAs λλ increases, rrincreasesincreases(low resolution)

Best resolution isiswhen λλ isislowest

Since electron microscopes use electron asas thethesource ofof illumination, λλ isisshorter, thus resolution isishigher

OMOM SEM

Depth of Field

CONSIDERATIONS IN MICROSCOPY

••Magnification ••Resolution ••High magnification without high resolution is “ EMPTY ” magnification

••RESOLVING LIMIT

••Eye 1010 55 nm (0.1 mm) This data is outdated!!! ••Light Microscope200 nm (0.2 µµm)m) Check the latest ••SEM 3 nm machines ••TEM 0.2 nm

21 LENS ABERRATIONS

••Image quality, in all optical instruments, is limitlimiteded by distortions which arise from optical defects called aberrations

••Lens aberrations include:

Monochromatic (spherical) aberration

Chromatic aberration

Astigmatism

Diffraction

SPHERICAL ABERRATION Ideal lens

These aberrations occur when light waves passing through thethe periphery ofof aalens arearenot brought into identical focus with those passing closer toto thethe centre

Electrons further from axis areare focused closer than thethe electrons closer toto axis

22 CHROMATIC ABERRATION Ideal lens

These aberrations areare thethe result ofof thethe fact that white light isis composed ofof numerous wavelengthswavelengths..

When white light passes through aa convex lens, thethe component wavelengths arearerefracted according tototheir frequencyfrequency..

ASTIGMATISM

The offoff--axisaxis image ofof aaspecimenspecimen point appears asas aadiscdisc oror blurred lines instead ofof aapointpoint

Depending onon thetheangle ofof thethe offoff-- axis rays entering thethe lens, thethe line image may bebeoriented either tangentially oror radially

23