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Mech1006 Materials & MechanicsLaboratory Report Cover & Feedback Sheet

Metallography & Microstructure: Lab Session M, 2013, Dr A Wojcik

Students must complete the top part of this sheet in block capitals (or typed).

Student Name:Loc Nguyen Lab Group:1

I confirm this report constitutes all my own work

SIGNED Loc Nguyen Please date stamp this sheet upon submission

Degree Course: Mechanical Engineering Personal Tutor: Dr.Shipley

Date of Lab Session 8/10/2013 Due Date 5/11/2013

Date Submitted......................................... Received/Initials...........................................(for Departments use)

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Presentation of report (structure, appearance)

Language (grammar, spelling and punctuation)

Quality of drawn microstructures

Analysis and discussion of results

Evidence of understanding of subject

Answers to questions in lab script

General comments

Overall

Grade ........................

Marked by Dr A Wojcik, 2013-2014

Department of Mechanical Engineering, University College London, WC1E 7JE

Grades A, B, C, D & E correspond broadly to First, UpperSecond, Lower Second, Third and Pass degree standardsrespectively

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Content Page

1. Introduction/page 32. Theory/page 3-9

Structure of crystal Dendrites, grains and grain boundaries: Structure of alloys Sample preparation in metallography:

3. Results/page 9 -124. Discussions/page 12-16

a. Purpose of metallography:b. Idealised dendrite and the reason of the difference in X6, X7s dendrite to

the idealized version:c. Equiaxed grains vs. columnar grains in essentially same metals X1 and

X1A

d. Three forms of porositye. Coringf. Eutectic structureg. Oil immersion can improves the resolutions of a lens

5. References/page 176. Sketches of each specimens/ page 18-19

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1.Introduction

The following report will concentrate on optical microscopy, as well as the samplepreparation required to evaluate microstructures. The optical microscopy testing on 7

different materials to evaluate the grain size, phases in specimens and composition

inside the materials. From the observations, we can be able to determine different

compositions, phases, deflects inside the materials and from that we can conclude what

affect on them. Also to do the test, microscope also is learnt and discussed how to

improve the resolution of the lens.

2. Theory

Structure of crystals:All matter is considered to be a mixture of chemical elements. These are the smallestunits that are distinguishable on the basis of their chemical activity and physical

properties. Inside the elements are atoms, which have a distinct structure characteristic

of each element. Atoms are too small to be seen without the aid of ordinary

microscopes.

The mechanical properties of metals then derive from their crystalline structure. That

is, the atoms in solid state of a metal are arranged in definite 3D geometric patterns to

form crystals or grains of the metal. The network formed by joining the centre of the

atoms in a crystal is called the lattice. The smallest volume in a lattice, which properly

represents the position of the atoms with respect to each other, is known as the unit

cell. There are 14 types of unit cells but the structures are common and can separated asfollowing:

(a) Body-centred cubic (BCC); (b) Face-centred cubic (FCC); (c) Hexagonal close-

packed (HCP)

The alignment of the atoms has an important part in mechanical properties of metals.

For example, BCC is strong structure, and in general, the metals that are hard and strong

are in this form at normal temperature; FCC is characteristic of ductile metals and the

metals with HCP structure are quite susceptible to work-hardening.

(a) (b) (c)

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Dendrites, grains and grain boundaries:When a metal is cooled from liquid state to solid state, because cooling cannot be

exactly the same for every atom, certain atoms will be attracted to each other to form a

unit cell ahead of others. This unit cell becomes the nucleus for crystal formation.

During the solidifying, the nuclei keep growing and crystallising then form dendrite. The

term dendritecomes from Greek word Dendron, which means a tree. This description

is appropriate because we can describe the form and structure of a metallic dendrite as

like a tree: With a main branch, from which grow side branches, from which grow

smaller side branches, and so on, until all the main branches and the side branches grow

into each other and there is no room for any more branches to grow then it form grains

and keep growing. This orderly growth of the grain continues in all directions until it

runs into interference from other grains that are forming simultaneously about other

nuclei.

Dendrite structure

How the dendrite forms

and ideal dendrite

structure

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The process of the formation of grains and grains boundaries

If two grains that have the same orientation meet, they will join and form a larger grain,

but if they are forming about different axes, the last atoms to solidify between the

growing grains will be attracted to each other and much assume compromise positions

in an attempt to satisfy a double desire to join with each other. These misplaced atoms

are in layers about the grains and are known as grain boundaries. They are

interruptions in the orderly arrangement of the lattices and offer resistance to

deformation of the metal. A fine-grained metal with a large number of interruptions will

be harder and stronger than a coarse-grained metal of the same composition and

condition. Polishing the metal specimen can sometimes reveal the grain structure but

chemically etching the specimen, after polishing, dramatically enhances the

appearances of the grain boundaries.

Grain boundary

in brass

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Structure of alloysGrains of a particular composition are referred to as being made from the same phase. A

pure metal will cool to give single phase grains. An alloy is a substance that has metallic

properties and is composed of two or more chemical elements, of which at least one is a

metal. Most commercially used metallic materials are not pure metals but alloys, which

consist of more than one elements. Some of them may be non-metallic elements.Fundamentally, there are modes of arrangement of atoms or phases exist in alloys.

These phases are: pure metal, solid solution and compound. For simplicity of

illustration, with 2 elements A and B, shall be considered in the following discussion:

Pure metal: If there exist no B-atoms in A-crystal grains and no A-atoms inB-grains.

Solid solution: Any solution is composed of two parts: a solute and asolvent. The solute is the minor part of the solution, which is dissolved,

while the solvent constitutes the major portion of the solution. There exist

B-atoms (solute) in A-crystal grain (solvent). Compounds: There are generally formed between chemically dissimilar

metals and are combined by the following rules of chemical valence.

Elements A and B form a compound AB but unlike solid solution, the ratio

of the number of A-atoms to B-atoms is fixed (ex. 1:2).

Phase transformation can happen by both the concentration of the alloying

elements and the temperature of the metal.

To determine the phases inside the metals, we can use optical microscope

experimentally on the metal but also can analysis theoretically from a

composition/temperature map called as an equilibrium phase diagram.

From the graph, we can identify the phases in metal at each different

temperature and condition. However, to minimise error, we should also

combine it with using microscope.

Copper and iron

phases in a cold

pressed metal.

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Sample preparation in metallography:In order to identify and evaluate the microstructure of metals, it is very important to

prepare the samples properly and with great care. The various steps in specimen

preparation include:

Selecting a representative sample of the materials to properlycharacterize the microstructure or the features of interest. This step is

critical to the success of any subsequent study. The second, equally

important step is to correctly prepare a metallographic specimen.

Sectioning the sample carefully to avoid altering or destroying thestructure of interest. The most widely used sectioning device is the

abrasive cut-off machine, ranging from units using thin diamond-rimmed

watering blades to those using wheels. To minimise burning and

deformation, a lubricant or coolant is typically used.

However no matter how carefully we sectioned the sample, a smallamount of deformation occurs on the sample surface. This is reason we

need to grind the sample later.

Mounting the section without damage to the sample in a plastic or epoxymaterial to facilitate handling and the grinding and polishing steps.

Because large specimens are generally more difficult to prepare more

than small ones, specimen size should be minimised.

Grinding to achieve a flat sample with a minimum amount of damage tothe sample surface. Grinding is generally performed by the abrasion of the

specimen surface against water-lubricated abrasive wheels. Grindingdevelops a flat surface with a minimum depth of deformed metal and

Fig 2.6: An example of

equilibrium phase diagram

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usually is accomplished by using progressively finer abrasive grits on the

grinding wheels.

Polishing the mounted and ground sample. Often automated, polishingcan be performed by hand by holding specimen against a wheel, covered

with a cloth pad impregnated with a polishing compound. Polishing

should yield a scratch-free surface

Improper polishing vs. proper polishing

Etching in the proper etchant to reveal the microstructural details such asgrains, grain boundaries and phase boundaries. Various etching

techniques are available which chemical and electrochemical attack are

mostly used.

Washing the specimen in water and examination under a microscope.

The microscope we use to investigate the metals is a reflected light microscope. It

passes the visible light transmitted through of reflected from the sample through a

A layer of disturbed metal A layer of disturbed metal is

removed

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single or multiple lenses to allow a magnified view of the sample. The resulting image

can be showed up directly to our eyes through eyepieces or captured digitally and put

on a TV screen. To get a between view or to have bigger magnification, we can adjust the

focus or change lenses. Before put the specimen inside the microscope, we need to

secure it on a glass slide and avoid human contact unless fingerprint is the only thing we

want to see. After that, starting with lowest option of the magnification so we can havebetter look of the whole structure of metals and change the focus if necessary to get a

clear view. To minimise error, we also need to redo the experiment at different area of

the specimen to ensure a whole structure of the specimen is seen.

3.Results

Following specimens are used in this experiment:

X1 Pure zinc ingot. Sand cast, etched in hydrochloric acid, macro view X1A Zinc ingot as above but impure. Macro view, note central pore X2 Copper with 4% tin alloy, sand cast, etched in alcoholic ferric

chloride. Dendritic structure may be cored. Micro view

X3 Same as X2 but annealed for 2 hours at 700C, coring gone X5 Copper with 8.4% phosphorus alloy. Sand cast, etched in ferric

chloride solution. Eutectic composition. Micro view

X6 Same as X5 but 4.5% phosphorus. Hypoeutectic, with coreddendrites

X7 As X5 but 10.5% phosphorus. Hypereutectic, with un-coreddendrites.

X1

With bare eyes, we can see this specimen has relatively long, columnar grains radiating

in toward the centre. The grains also have different degrees of brightness of light

reflecting from the specimen when we move the object. It happens due to the light

orientations into the specimen at different angles so we can see a brighter and darker

part. This is a pure zinc ingot so there is only 1 phase, which is zinc.

As mentioned above, the columnar grains radiate toward the centre of the specimen; it

is because of the casting process. Since the casting process extracts heat from outside

through walls of a mould, small equiaxed grains from at this stage at the outside edge of

the ingot.As the solidification process happens, some of small grains, having favourable growth

axis into the centre, start to grow in the direction opposite to the direction of heat flow.

They extend more rapidly than the ones next to them and begin to take more places

than the slower growing grains. This is why we have different length and width

columnar grain structure toward the centre.

X1A

Different from the specimen X1, this is an impure zinc ingot that means there are two or

more elements in the structure of specimen. With bare eyes, we can see an equiaxed

grains structure and there is a little black dot in the centre of ingot. The equiaxed grainshappen because X1A is an alloy and different elements inside the specimen can be the

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starting point for the grains to growth. So they growth at random points and various

directions

The black pore we see on top of the surface is due to pouring metal defects during the

casting.

X2

Microscopic examination at 50X shows a difference in shade of the grains giving us

information that there are 2 elements in this material; and there are black lines as

boundaries appearing. Moreover, a dark skeleton pattern will be observed inside the

grains. This represents coring, or unevenness in the composition that is, in the present

sample, variations of the tin content in the copper.

X3

This specimen is similar to X2 except that there is no coring in the grains because the

specimen was annealed at 700C for 2 hours. This proves that heating and let the

material cool down for a specific time can help reduce the coring.

We can also see there are lots of lines containing tiny black dots representing the

porosity within the structure.

X5

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Microscopic examination reveals that the grains are composed of colonies of fine

eutectic structure, which contains two solid phases. These phases may have different

crystal structures or the same crystal structure but different lattice. High magnification

is required to show the structure more properly. There are evidences of cracks and

black dots showing the effect of shrinkage deflects and evidences of porosity. Copper

crystals (solid solution -phase) and crystals of copper-phosphide (Cu3P) have

intergrown in an apparently lamellar pattern, which has tended to develop radially in

each colony with often a coarsening of the structure at the boundaries. The -phase

appears dark or brown as it is preferentially attacked by the etching solution. The Cu3P

appears white. Occasionally, free pieces of either Cu3P or -phase may be seen.

X6

As this alloy contains excess copper with respect to the eutectic composition, there areseparate copper-rich crystals (-phase) together with the eutectic, which is distinctly

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coarser than that in X5. Further, the eutectic regions have a fringe of Cu3P around them.

The -phase crystals contain a relatively small amount of phosphorous in solid solution.

There are cored dendrite, and ranged in shade from dark to light brown. These crystals

grew first in the melt and they have developed in a characteristic dendritic shape. In

fact, the dendritic form is not well developed, the crystals being rather rounded and

stubby. Some of the apparently isolated, round shapes probably represent regionswhere the micro-section has passed through a dendrite arm.

As the illustration above, there is a huge black dot believed to be a void in the material.

It is an evidence of porosity caused by the gas formation during casting.

X7

In effect, the reverse of X6 in that rounded dendrites of Cu3P are set in the background

of the eutectic. The dendrites (white in appearance) seem to be better developed than in

X6. However the present dendrites are not cored. At higher magnification, we can see a

large gas pore presenting deflects in casting.

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4. Discussion

a) Purpose of metallography:

Metallography is the study of the microstructure of all types of metallic alloys by use of

low-magnification, optical microscopy and scanning electron microscopy. It can be more

precisely defined as the scientific discipline of observing and determining the chemicaland atomic structure and spatial distribution of the constituents, inclusions or phases in

metallic alloys. By extension, these same principles can be applied to the

characterization of any material.

Many important macroscopic properties of materials are highly sensitive to the

microstructure. Critical mechanical properties, such as tensile strength or elongation, as

well as other thermal or electrical properties, are directly related to the microstructure.

The understanding of the relationship between the microstructure and macroscopic

properties plays a key role in the development and manufacture of materials and is the

ultimate aim of metallography.

Metallography, as we know it today, owes much to the contributions of the 19th scientist

Henry Clifton Sorby. His pioneering work with modern manufactured iron and steel in

Sheffield (UK) highlighted this intimate bond between the microstructure and

macroscopic properties. As he stated towards the end of his life: In those early days, ifa railway accident had occurred and I had suggested that the company should take up a

rail and have it examined with the microscope, I would have been looked upon as a fit

man to send to an asylum. But that is what is now being done

b) Idealised dendrite and the reason of the difference in X6 , X7sdendrite to the

idealized version:

Compared to idealised dendrite, the dendrites in X6 and X7 are rounded and stubby. It

represents there may be micro-sections passing through the dendrite branches andmaking the arms not connect to each other.

c) Equiaxed grains vs. columnar grains in essentially same metals X1 and X1A

As mentioned above, X1 is a pure metal and X1A is impure so the equiaxed grainspresence is because of different elements inside X1A alloy. During the solidification

process, they can be starting points for the dendrites to growth with various directions

at random points.

So we can conclude that solid solutions have different grain structure compared to pure

metal and phases in crystal have big impact on how the dendrite grow.

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d) Three forms of porosity

From the illustrations, we can see there are three types of porosity, which are gas

porosity, cracks and misruns:

Gas porosity: Gas holes are spherical holes of varying size, usually fairly evenlydistributed and formed by gas in the metal. The larger holes tend to be found in the

heavier section. If the metal is correct prior to casting, the pinhole type of porosity is

probably due to absorption of hydrogen from steam in the mould. As example, we can

see lots of various sizes of black hole in the illustration of X5, X6 and X7 that are the

results of gas porosity.

Cracks:They are formed after the metal has become completely solid. Bad casting

design that make contraction by the mould to the solid can create cracks. Also, X5 is an

example for this.

Misruns: It is a pouring metal defect occurring when the liquid of metal does not

entirely fill up the mould, that leaving an unfilled volume. It is caused by lacking of

fluidity in the molten metal. An example of it is a pore in X1A.

e) Coring

Coring happens when a heated alloy cools in non-equilibrium conditions. As

solidification proceeds, the compositions of the solid and melt must continuously adjustthemselves in order to maintain thermodynamic equilibrium. This requires long-range

diffusion of atoms within the solid and melt and exchange of atoms across the

solid/melt interface, which will only occur if the system is cooled very slowly. If cooling

is too fast, then there is insufficient time for diffusion to take place, resulting in the

chemical unbalance in the growing crystals. This causes the exterior of the material to

harden faster then the interior. Coring cause the centres of the grains to retrain more of

the higher melting temperature element. In this case, the dendrite arms formed from

the exterior have a different composition than the alloy in the inner regions, resulting in

a local compositional difference.

According to the coring definition, it suggests that coring creates the instability inside

structure of material, which can lead to decreasing in some mechanical properties. As

we know there is no coring happens in X3 because it is annealed for 2 hours, so we can

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conclude to eliminate coring we need to maintain the thermodynamic equilibrium

inside the specimen by annealing.

According to the equilibrium phase diagram above, we can determine how different

compositions in metals affects the cooling or heating processes and solidification point

of X6, X7 can also be found out by using diagram above. With those two specimens, they

are both examples of hypoeutectic alloy. On slow cooling process, the liquid in X6 first

solidifies at about 940oC as copper rich dendrites. The composition of the remaining

liquid becomes increasing rich in phosphorus until it solidifies as a eutectic at the

eutectic temperature of 714oC. However, with the X7 specimen, on the same slow

cooling process, the liquid first solidifies as copper phosphide dendrites at

approximately 850o

C. The remaining liquid also solidifies as a eutectic at 714o

C. Thedendrites in X6 are cored, cause by non-equilibrium solidification: the cooling rate was

too fast for equilibrium to be maintained between solid and liquid phases. However,

there is no coring in X7s dendrites because of its narrow composition range.This result

suggests that the difference in percentage composition can have a big impact on the

pattern of dendrites.

f) Eutectic alloys

A eutectic-composition is an alloy of two or more metals, which will completely change

from molten state to the solid at the same temperature. During heat-up, the eutecticcomposition will be the first to melt so it takes less energy during the process.

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The silver-copper phase diagram

h) Oil immersion can improves the resolutions of a lens

In light microscopy, oil immersion is a technique used to increase the resolution of a

microscope. This is achieved by immersing both the objective lens and the specimen in a

transparent oil of high refractive index, thereby increasing the numerical aperture of

the objective lens.

The resolution of a microscope is defined as the minimum separation needed between

two objects under examination in order for the microscope to discern them as separate

objects. This minimum distance is label . If two objects are separated by a distance

shorter than , they will appear as a single object in the microscope.

A measure of the resolving power of a lens is given by its numerical aperture, NA:

Where is the wavelength of light, from this, It is clear that a good resolution (small )

is connected with a high numerical aperture.

The numerical aperture of a lens is defined as:

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Where is the angle spanned by the objective lens seen from the sample, and n is the

refractive index of the medium between the lens and specimen (=1 for air). Because

sinis always less than or equal to unity, the NA can never be greater than unity for an

objective lens in air. If the space between the objective lens and the specimen is filled

with oil however, the NA can obtain values greater than unity. This is because oil has a

refractive index greater than 1.

5. Reference

Liquid Penetrant and Magnetic particle Testing at level 2 by The International Atomic

Energy Agency

http://www.ami.ac.uk/courses/topics/0131_mb/

ME582 Laboratory Assignment and Individual Project Spring, Fall 2013 by Bill Warnes

http://www.substech.com/dokuwiki/doku.php?id=solidification

Materials and Minerals Science Course C: Microstructure

http://www.facealuminium.com/ENG/Hot_topics/Techical_papers/Defects_of_diecast_

products2.asp

Eutectics the good and the bad by Dan Kay

http://pwatlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures/mor

e-metals/copper/phosphorus/10.5wt.html

http://www.frankshospitalworkshop.com/equipment/documents/microscopes/wikip

edia/Oil%20immersion.pdf

Introduction to Metallography:www.metallographic.com

http://www.labtesting.com/resources/testing-methods/metallography-and-

microstructural-evaluation/

http://www.leica-microsystems.com/science-lab/metallography-an-introduction/

Optical Metallography by Mr Louthan, Jr., Department of Materials Engineering, Virginia

Polytechnic Institute and State University

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