[METALLOGRAPHY] 1

1

METALLOGRAPHY

By

MUHAMMADAL-ASYRAFBINDINMUHAMADAMERULMUKMINBINYAHAYAMUHAMMADFADZILANBINABDLATIF

DEPARTMENT OF MATERIAL ENGINEERING, FACULTY OF MANUFACTURING

UNIVERSITY OF TECHNICAL MALAYSIA MELAKA

This report is submitted to the Faculty Of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements in the subject of BMFB 3253 METALLURGY for Bachelor of

Manufacturing Engineering (Engineering Materials). The member of the supervisory committee is as follow:

DR SHARIZA BT ISMAIL

Submitted on 2 NOVEMBER 2015

2 [METALLOGRAPHY]

2

1.0 INTRODUCTION

Metallography Metallography is the scientific discipline of examining and determining the constitution and the

underlying structure of (or spatial relationships between) the constituents in metals, alloys and

materials or it can be materialography. It is also has relation to the properties, of metals and

alloys. Moreover, the characterization of structure of alloys or metals can be seen mostly with

optic microscope. Microstructural analysis of a material's metallographic microstructure aids in

determining if the material has been processed correctly and is therefore a critical step for

determining products reliability (Quality Control) and for determining why a material failed

(Metallographic Failure analysis).

Metals Metal is an element that readily forms positive ions (cations) and has metallic bonds. Metals are

sometimes described as a lattice of positive ions surrounded by a cloud of delocalized electrons.

This material is typically hard, opaque, shiny, and has good electrical and thermal conductivity.

Alloys An alloy is a mixture of metals or a mixture of a metal and another element. Alloys are defined

by metallic bonding character.

Microstructure The structure which is in micro dimension. This structure only can be seen by using microscope.

2.0 IMPORTANCE OF METALLOGRAPHY The importance of metallography:

• The understanding of the metals behavior

• To determine the strength and other characteristics of new alloys.

• Examination of defects that appear in finished or partly finished products and studies of

parts that have failed in service.

Metallographic Applications Metallographic testing and Material Science failure analysis and quality control testing are used

in industries that require produce reliability such as:

[METALLOGRAPHY] 3

3

Industries Metallographic Applications • Aerospace • Advanced Materials • Superalloys • Ceramic Matrix Composites • Metal Matrix Composites • Polymer Matrix Composites • Biomedical Devices • Medical Implants • Materials Science Education • Metallurgical Engineering • Mechanical Engineering • Aerospace Engineering • Electronics • Solder Joint analysis • Integrated IC chip failure analysis • Printed Circuit board or PCB quality

control and failure analysis • Dielectric layer coating analysis • Automotive • Heat Treating • Metal Fabrication • Forging • Castings • Thermal Spray • Welding • Powder Metallurgy • Deep Drawing • Fastener Testing • Mining Metallurgical Testing Labs

• Grain Size Analysis ! ASTM E112

• Porosity ! ASTM A276

• Phase Analysis ! ASTM E566 ! ASTM E1245

• Inclusions ! ASTM E454 ! ASTM E1245

• Graphite Nodularity ! ASTM A247

• Coating Thickness ! ASTM B487

• Decarburization ! ASTM E1077

• Welding Analysis • HAZ Sensitization • Twin Boundaries • Cracks • Dendrites • Corrosion • Carburizing thickness • Nitriding thickness • Intergranular fracture • Weld sensitization • Flow line stress • Microhardness testing • Rockwell hardness testing • Superficial hardness testing

3.0 SAMPLE PREPARATION

The sample preparation consists of five major steps:

• Cutting • Mounting • Grinding • Lapping

4 [METALLOGRAPHY]

4

• Polishing

CUTTING

When cutting a specimen from a larger piece of material, care must be taken to ensure that it is

representative of the features found in the larger sample, or that it contains all the information

required to investigate a feature of interest. One problem is that preparation of the specimen may

change the microstructure of the material, for example through heating, chemical attack, or

mechanical damage. The amount of damage depends on the method by which the specimen is cut

and the material itself. Cutting with abrasives may cause a high amount of damage, while the use

of a low-speed diamond saw can lessen the problems. There are many different cutting methods,

although some are used only for specific specimen types.

1. Sawing

" Using hacksaws, band saws, and wire saws

" Hand-held hacksaws or band saws generally do not generate enough frictional heat to

alter the microstructure

" Saw cut surface are tough, and coarse grinding is required to obtain a flat surface

2. Electric discharge machining (EDM)

" Electric discharge machining (EDM), or spark machining is a proses that uses sparks

in a controlled manner to remove material from a conducting workpiece in a

dielectric fluid like kerosene

" The material is remove from the sample in the form of microscopic craters.

3. Fracturing

" Breaking specimens with blow of a hammer or by steadily applying pressure

" Location of the fracture can be controlled by nicking or notching the material

" Not recommended because it rarely follows desired directions and damage from

fracturing can mask inherent features

[METALLOGRAPHY] 5

5

" Also lengthy coarse grinding may be required to obtain a flat surface.

4. Shearing

" Low-carbon sheet steel and other thin, soft materials can be cut to size by shearing

" The area affected by shearing must be removed by grinding

MOUNTING

Mounting of specimens is usually necessary to allow them to be handled easily. It also minimizes

the amount of damage likely to be caused to the specimen itself. The mounting material used

should not influence the specimen as a result of chemical reaction or mechanical stresses. It

6 [METALLOGRAPHY]

6

should adhere well to the specimen, and if the specimen is to be electropolished later in the

preparation then the mounting material should also be electrically conducting. Specimens can be

hot mounted (about 150 °C) using a mounting press either in a thermosetting plastic, example

phenolic resin, or a thermosoftening plastic example acrylic resin. If hot mounting will alter the

structure of the specimen a cold-setting resin can be used example epoxy, acrylic or polyester

resin. Porous materials must be impregnated by resin before mounting or polishing, to prevent

grit, polishing media or etchant being trapped in the pores, and to preserve the open structure of

the material. A mounted specimen usually has a thickness of about half its diameter, to prevent

rocking during grinding and polishing. The edges of the mounted specimen should also be

rounded to minimize the damage to grinding and polishing discs.

[METALLOGRAPHY] 7

7

GRINDING

Surface layers damaged by cutting must be removed by grinding. Mounted specimens are ground

with rotating discs of abrasive paper, for example wet silicon carbide paper. The coarseness of

the paper is indicated by a number: the number of grains of silicon carbide per square inch. So,

for example, 180 grit paper is coarser than 1200 grit. The grinding procedure involves several

stages, using a finer paper (higher number) each time. Each grinding stage removes the scratches

from the previous coarser paper. This can be easily achieved by orienting the specimen

perpendicular to the previous scratches. Between each grade the specimen is washed thoroughly

with soapy water to prevent contamination from coarser grit present on the specimen surface.

Typically, the finest grade of paper used is the 1200, and once the only scratches left on the

specimen are from this grade. The series of photos below shows the progression of the specimen

when ground with progressively finer paper.

Specimen ground with 180 grit paper Specimen ground with 400 grit paper

Specimen ground with 800 grit paper Specimen ground with 1200 grit paper

8 [METALLOGRAPHY]

8

[METALLOGRAPHY] 9

9

LAPPING

The lapping process is an alternative to grinding, in which the abrasive particles are not firmly

fixed to paper. Instead a paste and lubricant is applied to the surface of a disc. Surface roughness

from coarser preparation steps is removed by the micro-impact of rolling abrasive particles.

POLISHING

Polishing discs are covered with soft cloth impregnated with abrasive diamond particles and an

oily lubricant or water lubricant. Particles of two different grades are used a coarser polish -

typically with diamond particles 6 microns in diameter which should remove the scratches

produced from the finest grinding stage, and a finer polish – typically with diamond particles

1 micron in diameter, to produce a smooth surface. Before using a finer polishing wheel the

specimen should be washed thoroughly with warm soapy water followed by alcohol to prevent

contamination of the disc. The drying can be made quicker using a hot air drier.

Specimen polished to 6 micron level Specimen polished to 1 micron level

Mechanical polishing will always leave a layer of disturbed material on the surface of the

specimen. Electropolishing or chemical polishing can be used to remove this, leaving an

undisturbed surface.

10 [METALLOGRAPHY]

10

ETCHING

Etching is used to reveal the microstructure of the metal through selective chemical attack. In

alloys with more than one phase etching creates contrast between different regions through

differences in topography or the reflectivity of the different phases. The rate of etching is

affected by crystallographic orientation, so contrast is formed between grains, for example in

pure metals. The reagent will also preferentially etch high energy sites such as grain boundaries.

This results in a surface relief that enables different crystal orientations, grain boundaries, phases

and precipitates to be easily distinguished. The specimen is etched using a reagent. For example,

for etching stainless steel or copper and its alloys, a saturated aqueous solution of ferric chloride,

containing a few drops of hydrochloric acid is used. This is applied using a cotton bud wiped

over the surface a few of times The specimen should then immediately be washed in alcohol and

dried. Following the etching process there may be numerous small pits present on the surface.

These are etch pits caused by localized chemical attack, and in most cases they do not represent

features of the microstructure. They may occur preferentially in regions of high local disorder,

for example where there is a high concentration of dislocations. If the specimen is over etched or

etched for too long, these pits tend to grow, and obscure the main features to be observed as seen

in the images below:

Etched specimen Over etched specimen

[METALLOGRAPHY] 11

11

Cleaning specimens in an ultrasonic bath can also be helpful, but is not essential. Ideally the

surface to be examined optically should be perfectly flat and level. If not, then as the viewing

area is moved across the surface it will pass in and out of focus. In addition, it will make it

difficult to have the whole of the field of view in focus - while the centre is focused, the sides

will be out of focus. By using a specimen leveling press (shown below) this problem can be

avoided, as it presses the mounted specimen into plasticene on a microscope slide, making it

level. A small piece of paper or cloth covers the surface of the specimen to avoid scratching.

4.0 MICROSCOPIC ANALYSIS

During microstructure analysis, the structure of a material is studied under magnification. The

properties of a material determine how it will perform under a given application and these

properties are dependent on the material’s structure. Metallographic analysis can be used as a

tool to help identify a metal or alloy, to determine whether an alloy was processed correctly, to

examine multiple phases within a material, to locate and characterize imperfections such as voids

or impurities, or to observe damaged or degraded areas in failure analysis investigations. The

method most often used in such evaluations is microscopy. Both optical and scanning electron

microscopy with energy-dispersive x-ray analysis can be useful in metallographic analysis. X-

12 [METALLOGRAPHY]

12

ray photoelectron spectroscopy is also useful in the measurement and identification of carbides

and graphitic inclusions and intermetallic formation.

Metallographic analysis may determine such issues as:

• Grain size and growth

• Grain structure resulting from processing

• Intermetallic phase microstructures

• Carbide formation at surfaces and grain boundaries

• Equiaxed or columnar grains

• Chemical microsegregation

• Microshrinkage and porosity

• Inclusions

• Planar, cellular, and dendritic interfaces

• Microstuctures as a function of cooling rate

• Grain size as a function of work hardening

• Graphite structures in cast irons

• Graphite replacing bulk carbides near surfaces

• Silicon structures in Al-Si alloys

5.0 PRINCIPLE AND PRACTICE

The mechanical properties of the materials are strongly affected by their microstructures. Metallography is one of the most used methods in metallurgy in order to characterize the microstructures of the materials. This method allows the study and characterization of metals, ceramics and polymers. Optical and electronic microscopy are the major techniques used to obtain images of the characteristic microstructures of the materials.

Metallography Method

The process is done according the following:

[METALLOGRAPHY] 13

13

Acquisi

tionofmaterialstructures–OpticalExamination

Opticalmicroscopy is still oneof themost important techniques toanalyze themicrostructureof thematerials. Their major components, Illumination system (light source with variable intensity),Condenserlenses(adjustablelensesforfocusingthelightbeam)andobjectivelens(Itformstheimagebeingthemostimportantcomponentofthemicroscope)

MetallographyMehthod

SampleCuDng

SampleCleaning

SampleMounEng

GrindingPolishing

AcidEtching

AquisiEionofMicrosctructure's

image

14 [METALLOGRAPHY]

14

Therearetwoexaminationmodes,whichare:

BrightField DarkField• Verticalillumination• Mostusedmethodtotheobservationof

thesurfacestructures• Light passes through the objective lens

andimpingesonthesamplesurface• Structures perpendicular to the beam

reflects the light back to the lens andappearasbrightregions

• Structures not perpendicular to thebeam reflects less lightback to the lensandappearasdarkregions

• The reflected light from regions notperpendiculartothebeamiscollected

• The reflected light from regionsperpendiculartothebeamisblocked

• The bright intensity is the opposite ofthebrightfield

• Thismodeallowsthestudyofthegrains.

[METALLOGRAPHY] 15

15

Polarized Light

The light emitted by the source is not polarized (vibrations in many directions). By passing

through a polarizer the light starts to vibrate in only one direction or at a same plane towards its

propagation direction. This mode is widely used to analyze anisotropic materials (Zr, αTi, among

others). When light impinges on the anisotropic surface reflections in two different polarization

planes might occur. The directions, polarization intensity and phase difference depend on the

crystallographic structure of the material. The difference among the reflected waves is

responsible for the image contrast.

16 [METALLOGRAPHY]

16

6.0 DEVELOPMENT IN METALLOGRAPHY

Background

The name metallography was initially used in the early 1700’s to signify the description of

metals and their properties. It was first used in its modern sense in 1892. Moreover, these

techniques are widely used, or at least readily adapted to the study of engineering materials,

although full use of this commonality has only been achieved comparatively recently with the

replacement of courses in metallurgy by courses in materials science and engineering .

Major Innovations of the 1930’s

The major innovations of the early-to-mid 1930’s were the use of X-rays and electron beams to

derive crystallographic structures; this was made possible by the enunciation of Bragg’s Law of

diffraction. Also in 1938, a pioneering systematic study of the polishing and etching of steels for

optical examination was published in book. This book was to be a factor in one of the major

metallographic developments, that is Len Samuels’ work on mechanical polishing-which we will

come to later.

[METALLOGRAPHY] 17

17

Mechanical Polishing

Mechanical polishing in 1937 and, indeed for the next 20 years was to remain very much an art,

neither studied nor understood. This statement requires some qualification. A layer of anomalous

structure was found on polished (unetched) surfaces and identified by Beilby as a smeared layer

of amorphous structure, the so-called Beilby layer. The presence of amorphous metal was widely

accepted in the 1930’s, mainly as a result of work by Rosenhain, and his powerful support of it

using his great skill in debate. This did not lead to any significant advances in polishing

techniques and was eventually fully discredited.

Alternatives to Mechanical Polishing

Alternative methods of polishing by purely chemical treatment with or without an electrical

driving force emerged in the late 1930’s and were developed further in the 1940-1950’s.

Electropolishing proved a valuable method for preparing test specimens for the study of

mechanisms of deformation such as slip bands, creep and fatigue. It was less successful with

complex alloys, despite special equipment designed by Jaquet to polish small areas by dabbing them

with electrolyte using a sheathed cathode .Post 1940

Mechanical Polishing

Improved equipment and polishing material emerged gradually in the 1940’s and 1950’s which

also saw the first and most comprehensive research into mechanical polishing. The unique nature

of this work was that it placed emphasis on measurement to evaluate stages of mechanical

polishing, rather than using subject viewing. It also included elegant experiments to elucidate

details of the basic microscopic processes involved.

Steps to Evaluate Mechanical Polishing

The critical experiments in this wide-reaching series and which might well qualify to be the

turning point are the following. Firstly the depths of scratches made by the various stages of

mechanical polishing were measured, employing taper sectioning where the scratches became

small. Secondly the depth of damage beneath these scratches was determined using polished

18 [METALLOGRAPHY]

18

sections normal to the surface and etchants which revealed the regions of cold work; these could

be matched with etched patterns of specimens worked to known degrees of compression. This

then yielded the depth of material that had to be removed at each stage of polishing to leave no

distortion. The third set of experiments was to elucidate how these various stages were affected

by the abrasive particles. The scanning electron microscope was an important tool here.

Optical Microscopy

Optical microscopy also underwent a steady development in design and quality of lenses, in

mechanical robustness and stability and probably most importantly, in the brightness and

stability of illuminating sources. This was particularly the case for those used for photography of

microstructures where unreliable carbon arcs depending on carbon rods moved by clockwork

mechanisms were replaced by much brighter and stable xenon arc lamps. Given a satisfactory

polish, resolution of micro detail was not usually a problem, even in 1937, provided one was

content to be limited by the wavelength of light.

Electron Microscopy

By the time these various techniques were understood and in many cases incorporated into

standard microscopes, they were becoming obsolete because of the outstanding performance of

electron microscopes and the multitude of instruments of the electronic kind. Electron

microscopes were first used in metallography in the mid 1940’s when their increased resolution

was seen to be their advantage. The fact that electrons only penetrate metals to very little depth

meant that polished specimens had to have their surfaces replicated by softened films of cellulose

acetate, or later by coating with a carbon layer. Contrast then often had to be enhanced by

oblique coatings of a metal. Early microscopes also suffered from problems of sustaining an

adequate degree of evacuation and sufficient stability of the power supply.

[METALLOGRAPHY] 19

19

Quantitative Metallography

It was recognised that it was important to preserve the optical expertise, not only as a reference

base but also in day-to-day characterisation and control of microstructure in industry. Another

line of development in metallography also had a notable growth and reinforced the importance of

the optical microscope. This was the technique of quantitative metallography or ‘quantitative

stereometry’.

Grain Size

The starting point in quantitative metallography was the apparently simple one of measuring

grain size. The usual method for many years was to compare (by eye) the sample microstructure

with a series of idealised charts and thus pick out a particular numbered chart as the grain size

number. Grains, even in a fully annealed single-phase alloy take up a complex shape of which

the two-dimensional polygon on a polished surface is not obviously related.

Grain size became increasingly measured by taking the average of the intercepts made by the

grain-boundary traces along specified lengths traversed. In the 1950’s, several investigations led

the way to understanding the connections between various estimates of grain size and became, in

fact, part of a broad attack on measurements of structural features in general. Serial sectioning

was used in some cases to build up an accurate three-dimensional picture. Typical features given

accurate parameters to characterise them were volume fractions of second phases, separation

distances between phases and the dimensional parameters of various shapes of particles such as

spheres, discs or rods.

Electronic Advances

This was also a period in which automatic instruments were made possible by the advances in

electronics and the introduction of computers. Thus data were collected with specified

sensitivity, collated and eventually plotted graphically if desired. This all meant that it was

possible to make experiments which could be completed in durations of time reckoned in

minutes in contrast to previous manual operation of many hours or even days of work. This was

20 [METALLOGRAPHY]

20

also an area which metallography shared with the study of other engineering materials such as

ceramics and polymers, and even with biological and botanical materials.

Concluding Remarks

The foregoing has shown, it is hoped, that metallography has developed by the accumulation of

advances, often small ones, in techniques, instrumentation and basic understanding of metals and

alloys. These have usually been driven by industrial innovation and needs. This has resulted in

gradual ‘turning points’, not sudden revolutions.

Looking back over the 20th century, it is evident that the first third of it was relatively static, the

middle third one of significant development of existing procedures followed in the final third by

the explosive proliferation of instruments utilising all aspects of the wave-like properties of

matter and the power of electronic computing.

Several, indeed many, important areas of metallography have been neglected in this article.

Examples would be measurement of distributions of crystallographic orientation of grains,

analyses of chemical composition of alloys, inclusions and precipitates, non-destructive testing

and fractography.

As to the future, this very wide ambit of metallography assures its continued importance and it is

at least certain that existing techniques will be increasingly automated and data ever more

speedily collected and collated. Beyond that it is prudent to remember that it has been said.

“Prophecy is difficult, especially about the future”.

[METALLOGRAPHY] 21

21

REFERENCES

Anderson Materials Evaluation, Inc. (2015). Retrieved November 4, 2015. Metal specimen preparation. (n.d.).

Gifkins, R. (2002, July 2).Metallography - History & Development. RetrievedNovember 4, 2015,from http://www.azom.com/article.aspx?ArticleID=1511, Institute of Materials EngineeringAustralia.

Metallography – an Introduction. (n.d.). Retrieved November 1, 2015, from http://www.leica-microsystems.com/science-lab/metallography-an-introduction/