metallurgy e-matl., lecture note

7/21/2019 Metallurgy E-matl., Lecture Note

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0SUDHARSHAN BOLLAPU., M.TECH. (PH.D.) CENTURION UNIVERSITY OF ECHNOLOGY AND MANAGEMENT, PARALAKHEMUNDI

et llurgySubject Code: PCME2104

2013

Sudarshan Bollapu., M.Tech. (Ph.D.), Department of Mechanical Engineering

Centurion University of Technology and Management, Paralakhemundi.

CENTURION UNIVERSITY OFTECHNOLOGY AND MANAGEMENT, PARALAKHEMUNDI.|


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SUDHARSHAN BOLLAPU., M.TECH. (PH.D.) CENTURION UNIVERSITY OF ECHNOLOGY AND MANAGEMENT, PARALAKHEMUNDI

1May 30, 2013

DEPARTMENT OF MECHANICAL ENGINEERING

Metallurgy e-material

Syllabus

PCME 2104 METALLURGY (3-1-0)

MODULE-I (15 Lectures)Classification of Engineering Materials, Engineering properties of materials. Characteristic

property of metals, bonding in solids, primary bonds like ionic, covalent and metallic bond,

crystal systems, common crystal structure of metals, representations of planes and directions

in crystals, atomic packing in crystals, calculation of packing density, voids in common

crystal structures and imperfections crystals.

Concept of plastic deformation of metals, critical resolve shear stress, dislocation theory,

deformation by slip and twin, plastic deformation in polycrystalline metals, yield point

phenomenon and related effects, concept of cold working preferred orientation. Annealing;

recovery; recrystallization and grain growth; hot working.MODULE-II (15 Lectures)

Cofactor, valency factor, crystal structure factor and chemical affinity factor; order-disorder

transformation. Binary phase diagrams a) Isomorphism system, (b) Eutectic system, (c)

Peritectic system, (d) Eutectoid system and (e) Peritectoid system. Allotropic transformation.

Lever rule and its application, Interpretation of solidification behaviors and microstructure of

different alloys belonging to those systems, Effect of non-equilibrium cooling, coring andhomogenization. Iron-cementite and iron-graphite phase diagrams, microstructure and

properties of different alloys (alloy steels; stainless steel, tool steel, HSS, high strength low

alloy steel) types of cast iron, their microstructures and typical uses. Specification of steel.

T.T.T. diagram: concept of heat treatment of steels i.e. annealing, normalizing, hardening and

tempering; microstructural effects brought about by these processes and their influences onmechanical properties; factor affecting hardenability.

MODULE-III (12 Lectures)Optical properties of Materials: Scattering, Refraction, Theory of Refraction and absorption,

Atomic Theory of optical properties. Lasers, Optical fibres- Principle, structure, application

of optical fibres.

Plastic-: Thermosetting and thermoplastics.

Ceramics: Types, structure, Mechanical properties, application

Composite Materials: Agglomerated Materials: Cermets .Reinforced Materials: Reinforced

Concrete. Glass fibre reinforced plastics, Carbon fibre reinforced plastics, and fibre

reinforced plastics, laminated plastic sheets. Teflon, Properties of composites, Metal matrixcomposites, manufacturing procedure for fibre reinforced composite. Introduction to Nano-

materials

Text Books:

1. Engineering Physical Metallurgy and Heat Treatment by Y.Lakhtin, Mir Publisher,

Moscow.

Chapters (1; 6; 7; 8; 9 and 13)

2. Introduction to Physical Metallurgy by Avner, Tata McGraw Hill

Chapters ( 2; 3; 4; 5; 6; 7; 8; 9; 10; 11 and 12)


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3. Materials Science and Engineering by W.D.Callister, Wiley and Sons Inc.

Chapters ( 2; 3; 4; 5; 6; 7; 8; 12; 13; 15 and 20)

Reference Books

1. Elements of Material Science and Engineering, L.H.Van Vlack, Addison Wesley

2. Physical Metallurgy: Principles and Practice by Ragahvan, PHI

3. The Science and Engineering of Materials by Donald R. Ask eland and Pradeep P Phule,

Thomson

Learning (India Edition)

4. Materials Science and Engineering by V.Raghavan, Prentice Hall of India Pvt.Ltd.

5. Essentials of Material Science and Engineering by Donald R. Askeland and Pradeep P

Phule, Thomson Learning6. Processes and Material of manufacture by Lindberg, PHI.

7. Elements of Materials Science & Engineering by Van Vlack, Pearson

8. Mechanical Metallurgy by Dieter, Tata MacGraw Hill

9. Materials Science and Metallurgy by Daniel Yesudian, Scitech

10. Material Science and Metallurgy by C.K.Dutta, Dhanpat Rai

11. Materials Science and Metallurgy by R.B.Choudhary, Khanna Publishers

12. Principles of Engineering Metallurgy by L.Krishna Reddy, New Age International

13. Material Science and Processes by S.K.Hazra Chowdhury, Indian Book distributing Co


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Engineering materials and their properties

1.0.Introduction to Eng. Materials:

Since the earliest days of the evolution of mankind, the main distinguishing features betweenhuman begins and other mammals has been the ability to use and develop materials to satisfy

our human requirements. Nowadays we use many types of materials, fashioned in many

different ways, to satisfy our requirements for housing, heating, furniture, clothes,

transportation, entertainment, medical care, defence and all the other trappings of a modern,

civilised society.

Most materials doesn't exist in its pure shape, it is always exist as an ores. During the present

century the scope of metallurgical science has expanded enormously, so that the subject can

now be studied under the following headings:

a) Physical metallurgy

b) Extraction metallurgy

c) Process metallurgy

1.1. Classification of Engineering Materials

1.1. a. Engineering materials:

Almost every substance known to man has found its way into the engineering workshop at

some time or other. The most convenient way to study the properties and uses of engineering

materials is to classify them into families as shown in figure below:

Figure. Classification of engineering materials

1. 1.a.Metals:

1.1. b Ferrous metals

These are metals and alloys containing a high proportion of the element iron.

They are the strongest materials available and are used for applications where high

strength is required at relatively low cost and where weight is not of primary

importance.

As an example of ferrous metals such as: bridge building, the structure of large

buildings, railway lines, locomotives and rolling stock and the bodies and highly

stressed engine parts of road vehicles.

The ferrous metals themselves can also be classified into families',


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Figure. Classification of ferrous metals.

1.2. Nonferrous metals

These materials refer to the remaining metals known to mankind.

The pure metals are rarely used as structural materials as they lack mechanical

strength.

They are used where their special properties such as corrosion resistance, electrical

conductivity and thermal conductivity are required.

Copper and aluminium are used as electrical conductors and, together with sheet zinc

and sheet lead, are use as roofing materials. They are mainly used with other metals to improve their strength.

Some widely used non-ferrous metals and alloys are classified as shown in figure.


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Figure. Classification of non-ferrous metals and alloys.

1.2.0. Nonmetallic materials

1.2.1 Nonmetallic (synthetic materials)

These are non metallic materials that do not exist in nature, although they are manufactured

from natural substances such as oil, coal and clay. Some typical examples are classified as

shown in figure.

Figure. Classification of synthetic materials.

They combine good corrosion resistance with ease of manufacture by moulding to

shape and relatively low cost.

Synthetic adhesives are also being used for the joining of metallic components even inhighly stressed applications.

1.2.2 .Nonmetallic (Natural materials)

Such materials are so diverse that only a few can be listed here to give a basic introduction to

some typical applications.

Wood: This is naturally occurring fibrous composite material used for the

manufacture of casting patterns.

Rubber: This is used for hydraulic and compressed air hoses and oil seals. Naturally

occurring latex is too soft for most engineering uses but it is used widely for vehicle

tyres when it is compounded with carbon black.

Glass: This is a hardwearing, abrasion-resistant material with excellent weathering

properties. It is used for electrical insulators, laboratory equipment, and optical

components in measuring instrument set and, in the form of fibres, is used to reinforceplastics. It is made by melting together the naturally occurring materials: silica (sand),

limestone (calcium carbonate) and soda (sodium carbonate)

Emery: This is a widely used abrasive and is a naturally occurring aluminium oxide.Nowadays it is produced synthetically to maintain uniform quality and performance.

Ceramic: These are produced by baking naturally occurring clays at high temperatures

after moulding to shape. They are used for high voltage insulators and high

temperature resistant cutting tool tips.

Diamonds: These can be used for cutting tools for operation at high speeds for metal

finishing where surface finish is greater importance. For example, internal combustion

engine pistons and bearings. They are also used for dressing grinding wheels.


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Oils: Used as bearing lubricants, cutting fluids and fuels.

Silicon: This is used as an alloying element and also for the manufacture of

semiconductor devices.

These and other natural, non-metallic materials can be classified as shown in figure.

Figure. Classification of natural materials.

1.2.3. Composite materials (composites)

These are materials made up from, or composed of, a combination of different materials to

take overall advantage of their different properties. In man-made composites, the advantagesof deliberately combining materials in order to obtain improved or modified properties was

understood by ancient civilizations. An example of this was the reinforcement of air-dried

bricks by mixing the clay with straw. This helped to reduce cracking caused by shrinkage

stresses as the clay dried out. In more recent times, horse hair was used to reinforce the

plaster used on the walls and ceiling of buildings. Again this was to reduce the onset of

drying cracks.

Nowadays, especially with the growth of the plastics industry and the development of high-

strength fibres, a vast range combinations of materials is available for use in composites.

For example, carbon fibre reinforced frames for tennis rackets and shafts for golf clubs have

revolutionized these sports.

1.2.4. Factors affecting materials properties:

The following are the more important factors which can be influence the properties and

performance of engineering materials.

1. Heat treatment

This is the controlled heating and cooling of metals to change their properties to improve

their performance or to facilitate processing.

An example of heat treatment is the hardening of a piece of high carbon steel rod. If it is

heated to dull red heat and plunged into Coldwater to cool it rapidly (quenching), it will


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become hard and brittle. If it is again heated to dull red heat but allowed to cold very slowly it

will become softer and less brittle (more tough). In this condition it is said to be annealed.

After the heat treatment happened on the material it will be in its best condition for flow

forming, during flow forming (working) the grains will be distorted and this will result in

most metals becomingwork hardened if flow formed at room temperature. To remove any

locked in stresses resulting from the forming operations and to prepare the material formachining, the material has to be normalized.

2. Processing

Hot and cold working process will be referred to understand what is meant by terms hot and

cold working as applied to metals. Figure shows examples of hot and cold working.

Figure. Examples of (a) hot-working and (b) cold-working process.

Metal is hot worked or cold worked depending upon the Temperature at which it is flow

formed to shape. These temperatures are not easy to define. for instance , lead hot works at

room temperature and can be beaten into complex shapes without cracking , but steel does

not hot work until it is red hot .When metal are examined under the microscope it can be seen

that they consist of very small grains. When most metals are bent or worked at room

temperature (cold worked) these grains become distorted and the metal becomes hard and

brittle.

When metals are hot worked the crystals are also distorted. However, they reform instantly

into normal crystals because the process temperature is above the temperature of

recrystallization for the metal being used and work hardening does not occur. This cold

working is the flow forming of metals below the temperature the recrystallization, while lasthot working is the flow forming of metals above the temperature of recrystallization.

1.2.5. Environmental reactions

The properties of materials can also be effected by reaction with environment in which they

are used.

For example:

Resting of steel

Unless steel structures are regularly maintained by rest neutralization and painting process,

resting will occur. The rest will eat into the steel, reduce its thickness and, therefore, its

strength. In extreme cases an entire structure made from steel may be eaten away.

Dezincif ication of brass


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Brass is an alloy of copper and zinc and when brass is exposed to amarine environment for a

long time, the salt in the sea water pray react with the zinc content of the brass so as remove

it and leave it behind on spongy, porous mass of copper. This obviously weakness the

material which fails under normal working conditions.

Degradation of plastic

Many plastic degrade and become weak and brittle when exposed to theultraviolet content of sunlight. Special dyestuffs have to be incorporated into the

plastic to filter out these harmful rays.

1.2.6. Engineering properties of materials:

Engineering properties of materials such as:

Electrical conductivity, strength, toughness, ease of forming by extrusion, forging and

casting, machinability and corrosion resistance.

1.3.0. Ionic solids:

A standard ionic solid consists of atoms held together by ionic bonds, that is, by the

electrostatic attraction of opposite charges (the result of transferring electrons from atoms

with lower electronegativity to atoms with higher electronegativity). Among the ionic solids

are compounds formed by alkali and alkaline earth metals in combination with halogens; a

classic example is table salt, sodium chloride.

Ionic solids are typically of intermediate strength and extremely brittle. Melting points are

typically moderately high, but some combinations of molecular cations and anions yield an

ionic liquid with a freezing point below room temperature. Vapour pressures in all instances

are extraordinarily low; this is a consequence of the large energy required to move a bare

charge (or charge pair) from an ionic medium into free space.

1.3.1. Metallic solids:

Metallic solids are held together by a high density of shared, delocalized electrons, resulting

in metallic bonding. Classic examples are metals such as copper and aluminium, but some

materials are metals in an electronic sense but have negligible metallic bonding in a

mechanical or thermodynamic sense (see intermediate forms). Metallic solids have, by

definition, no band gap at the Fermi level and hence are conducting.

Solids with purely metallic bonding are characteristically ductile and, in their pure forms,

have low strength; melting points can be very low (e.g., Mercury melts at 234 K (39C).These properties are consequences of the non-directional and non-polar nature of metallic

bonding, which allows atoms (and planes of atoms in a crystal lattice) to move past one

another without disrupting their bonding interactions. Metals can be strengthened by

introducing crystal defects (for example, by alloying) that interfere with the motion of

dislocations that mediate plastic deformation. Further, some transition metals exhibit

directional bonding in addition to metallic bonding; this increases shear strength and reduces

ductility, imparting some of the characteristics of a covalent solid (an intermediate case

below)


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1.4.0. Bonding In Solids, Primary Bonds Like Ionic, Covalent And Metallic Bond:

Bonding in Materials:

It depends on the bonding between atoms and molecules where the atoms are held together in

molecules by various types of bonds that depends on the valence electrons. By comparison,

molecules are attracted to each other by weaker bonds, which generally result from the

electron configuration in the individual molecules.

Thus, we have the following types of bonding:

1.4.1. Ionic Bond

In the ionic bond, the atoms of one element give up their outer electron(s), which an in turn

attracted to the atoms of some other element to increase their electron count in the outermostshell to eight, as shown in figure 3. This bond is naturally provides a very Strong bond

between atoms and as a properties of solid materials with the ionic bonding include low

electrical conductivity and poor ductility.

Figure. Ionic bond

As an example of this bond is the Sodium chloride (table salt) is a more common example.

Because of the transfer of electrons between the atoms, sodium and chlorine ions are formedas shown in this reaction:

Na++ Cl

Na +Cl

1.4.2. Covalent Bond

In the covalent bond, electrons are shared (as opposed to transfer) between atoms in theiroutermost shells to achieve as table set of eight. As shown in figure

Figure. Covalent bond.


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Solids with covalent bonding generally possess high hardness and low electrical conductivity.

As an example of covalent bond the molecule of the gas methane (CH4), four hydrogen

atoms are combined with one carbon atom. The carbon atom has four electrons in its outer

shell, but these are joined by four more electrons, contributed singly by each of the four

hydrogen atoms as shown in figure.

H

HCH

H

Figure. (i) Covalent Bonding in a Molecule of Methane, CH4. (ii) Chemists express the

structural formula for the methane molecule.

1.4.3. Metallic Bond

It is the atomic bonding mechanism in pure metals and metal alloys. The metallic bonding

involves the sharing of outer shell electrons by all atoms to form a general electron cloud that

permeates the entire block as shown in figure.

Figure .Diagrammatic Representation of the "Metallic Bond".

This cloud provides the attractive forces to hold the atoms together and form a strong, rigid

structure in most cases. Because of the general sharing of electrons and their freedom to

move within the metal, metallic bonding provides typical properties of materials

characterized such as good electrical conductivity, good conduction of heat and good

ductility.

1.4. 4.Van der Waals Force

They are very small forces of attraction acting between atoms in cases where the formation ofionic or covalent bonds is not possible. Basically similar forces also act between atoms which


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are already bounded in neighbouring molecules, giving rise to weak Van derWaals forces

between long-chain molecules in polymers.

1.4.5. Crystalline Structures:

Many substances, including metals, have a crystalline structure in the solid state. Metal

crystals from when the molten metals cools and solidifies, whereas crystals of other

substances, for example copper sulphate, and sodium chloride (salt), form when a saturated

solution of compound evaporates causing the solid to crystallize out.

In crystalline structure, the atoms are located at regular and recurring positions in three

dimension. The pattern may be replicated millions of times within a given crystal. The

structure can be viewed in the form of aunit cell, which is the basic geometric grouping ofatoms that is repeated.

1.4.5.1. Levels of Material Structure:

The structure of materials can be classified as:

Crystalline: Metals

Semi-crystalline: HDPE (High Density Poly ethylene)

Non-crystalline: Plastics, Ceramics, Rubbers, LDPE (also known as amorphous)

Based on the observation made by human being or not, materials structures are broadly

classified as:

Macro Structure: observed by naked human eyes, 0.1 mm resolution

Micro-Structure: Can be observed with the help of some instrument or external aids

Micro-structure: Order of 10-4 to 10-6 m: Crystal

i. Sub-structure: order of 10-6 to 10-8 m, Observation Level - Crystalii. Crystal Structure : order of 10-8 to 10-10 m , Ob. Level - Unit Cell

iii. Electron Structure: order of 10-6 to 10-8 m, Ob. Level electron of outer shell

iv. Nuclear Structure : order of < 10-10 m , Ob. Level Proton and Neutron

Smart Materials or Intelligent Materials:

The materials, which can sense, process, stimulate and actuate a response, are known as smart

materials. Their functioning is analogous to human brain, slow and fast muscles action or

living organisms.

The intelligent materials comprises three basic components:

Sensor: for signal input: Piezoelectric polymers, optical fibres

Processors: for processing/analysing: Micro-chipsActuators: for actual functioning / output: Shape memory alloys, Polypyrrole

These materials have ability to change their inherent properties with surrounding condition.

They may change their dimensions with respect to environmental radiations, stress,

temperature, pressure, voltage etc.

Example:

Piezoelectric Ceramics: generates emf from mechanical pulse or vise-versa, Quartz

Visco-elastic (VE): Damping in space-crafts, earthquake prone structures

Shape Memory Alloys (SMA): Change in dim (plastic deformation) after transition

temperature and used in Fire alarm


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1.4.5.1. Crystallography:

The study of geometry and structure of solids is known as crystallography. The solids are of:

Crystalline Solids

Non-crystalline or amorphous solids

The solids, which possess regular and periodically repeated arrangement of atoms, the solidsare termed as crystalline solids. The solids in which there is no regular arrangement of atoms

are known as amorphous solids.

Factors for formation of non-crystalline solids:

Periodically repeated and regular arrangement of atoms get distorted owing to following

reasons:

Larger free energy

Fast rate of cooling

Absence of primary bonds in all directions

Weak secondary bonding

Open network of atomic packing

Non-parallel, entangled chain configuration

Mono-crystalline solids are those solids, which are having single crystal for its use or

smallest part, while polycrystalline solids are those, which are having more than one crystal

for its use or smallest part.

1.4.5.2 Space Lattice or Bravais Lattice:

An infinite array of points in 3 Dimensional space in which each point has identical

surroundings is known as space lattice or Bravais Lattice.

I. Unit Cell:

The smallest cell or portion of points or atoms or molecules which when repeated infinitely,

generates the space lattice. Mono-atomic unit cell contains one atom (one molecule that

comprises one atom) at its each point. Diatomic unit cell contains two atoms (one molecule

that comprises two atoms) at its each point. Poly-atomic or multi-atomic unit cell are those

which contains more than two atoms (one molecule that comprises more than two atoms) at

each point.

II. Crystal:

When many unit cells are repeated in a definite order in 3 dimensional space, a crystal isgenerated. It is the smallest ordered portion of material, which may be in use.

Crystals may be mono-atomic (that contains one atom at each lattice point), diatomic (that

contains two atom at each lattice point) and poly-atomic (that contains more than two atom at

each lattice point). Diatomic and poly-atomic crystals are known as molecular crystals.

III. Basis:

Replacing of points in space lattice by atoms or molecules is known as basis.

Space lattice + Basis (replacing all points by atoms or molecules) = Unit Cell

IV. Bravais Crystal System and Space Lattices:


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To represent a unit cell in 3 dimensional space, three linear vectors are required along all

three Cartesian coordinate axes ie x-axis, y-axis, and z-axis. These are a, b, c. along which

these tree linear vector, three angular position with respect to these axes are also required.

These are, , . Thus total required parameters are a, b, c (three linear vectors) and ,

,(three angles).

Based on the symmetry of possible geometries of unit cell and 3 dimensional space, there for14 types of Bravais Lattices or Space Lattices which are grouped under 7 crystal systems.

These are as:

C T O R H M T

3 2 4 1 1 2 1 (except in Monoclinic Simple and End Centered)

1. Simple: That contains atoms at all lattice points (corners)

2. Body Centered: That contains eight atoms at all corners (at all eight points) and one atom

at the Centre of body (cutting point of both body diagonals)

3. Face Centered: That contains eight atoms at all corners (at all eight points) and one atom at

the Centre of each face (cutting point of diagonal of each face, at all six faces)

4. End Centered: That contains eight atoms at all corners (at all eight points) and one atom atthe Centre of opposite face (cutting point of face diagonal, at only three, alternate) Other than

these, there are two types other structures as HCP and DC.

I .Hexagonal Closed Pack (HCP):

HCP is denser than the hexagonal one as HCP unit cell shares 17 number of atoms. There are

12 atoms at all angular points of two faces of hexagonal (top and bottom). One atom is at the

Centre of these top and bottom face. Three atoms are present at the Centre of alternate

vertical planes. Effective number of atoms for HCP is 6 and coordination number is 12. Its

APF is 0.74.

II. Diamond Cubic Structure (DC):

Carbon exists in two hybrid covalent bonding forms as sp2 and sp3. Diamond has sp3 hybrid

covalent bond. Each of its atom has four bonds and are directional in nature. This primary

bonding is extended to 3 Dimensional network. The bond angle is 109.50. It is the known

hardest solid. Effective numbers of atoms in the unit cell of DC are 8 and has APF of 0.34.

DC structure shares 18 atoms in its unit cell. 8 atoms are present at all eight corners of a cube.

One atoms is present at the Centre of each face. Two atoms are present at 1/4 th distance from

the base and two are present at 3/4 th distance from base along the body diagonal, all of them

are inside the unit cell.

III. Graphite Structure:

Graphite is another form of carbon that has sp2 hybrid cov1lent bonding. It has hexagonal

honey bee type of sheet structure. In a plane, unit cell comprises primary bonding along the

sheet while atomic planes are bonded by secondary bonding like Ver dar Waals type (along

its thickness). Bond angle is 1200. It has directional property and is used as solid lubricant.

Graphite fibres are used to make fibrous composites and also used as moderator in nuclear

reactors. Graphite is very useful to use at high temperature and pressure because of its low

coefficient of friction. It can be converted into synthetic diamond at 16000C by the

application of pressure of about 50,000 to 60,000 atmospheric.

Most symmetric crystal system: CubicMost un-symmetric crystal system: Triclinic


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1.4.6. Common crystal structure of metals

There are several types of pattern in which metallic atoms can arrange themselves on

solidification, but the most common is as follows:-

1. Body-Centered-Cubic [BCC]

As shown in figure (a), as an example of the materials for this type:

Chromium, Molybdenum, Niobium, Tungsten, Iron.

2. Face-Centered-Cubic [FCC]As shown in figure (b), as an example of the materials for this type:

Aluminium, Copper, Lead, Nickel, Iron, Gold, Silver.

3. Hexagon al-Closed -Packed [HCP]

As shown in figure (c), as an example of the materials for this type:Beryllium, Cadmium, Magnesium, Zinc.

Fig: a Fig: b Fig: c

There are some metals that are undergo a change of structure at different temperatures. Iron

metal for example is arranged in a body entered-cubic (BCC) at room temperature, when the

metal is heated and reaches a temperature of 910C, the atoms rearrange themselves into

Face-Centered-Cubic (FCC) crystals. If the metal is heated to the still higher temperature of

1400C the atoms again rearrange themselves, this time back into Body-Centered-Cubic

form.

1.4.6. 1.Non crystalline (Amorphous) Structures:

The no crystalline solids materials do not have their basic particles arranged in a geometricpatter. Their particles have a random formation, and such as a result, such substances are said

to be amorphous (without shape).

Many important materials are no crystalline: liquids and gases, for example. Water and air

have a no crystal structures. A metal loses its crystalline structure when it is melt. Such as

glass, plastics and rubber are materials that fall into this category. While many important

plastics are mixture of crystalline and non-crystalline forms.

Two closely related features differentiate noncrystalline from crystalline materials:-

1 Absence of long range order in the molecular structure of noncrystalline. It can be

visualized with reference to figure they closely packed and repeating pattern of the

crystal structure and random arrangement of atoms in the noncrystalline materials.


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Figure. Difference in structure between (a) crystalline and (b) non crystalline materials.

2

Differences in melting and thermal expansion characteristics. It could be demonstrated by

a metal when it is melts. When the metals molten an increase in volume compared to the

materials solid crystalline state. This effect is characteristic most materials when melted (a

noble exception is ice; liquid water is denser than ice).

4.7. The crystalline state:

As we mention before, that all of the metals and its alloys have crystalline structure where the

atoms are rearranged in an organized shapes which it is called as the crystal lattice. This

lattice consisted of another smallest grouping of atoms each one is called the unit cell as

shown in figure.

Figure. Representation of part of a space lattice with a unit cell outlined.

The unit cell is the smallest parallel surfaces of the crystalline structure that can be removed

or repeated in different directions. It is also differ from each other in shape or size in the

crystalline lattice from one material to another. The atoms that belongs to the unit cell are

called the basic atoms, its number is different from one shape of arrangement to another, this

number can be found from the following equation:-N = NC + NI + NF

Where N:is the number of the basic atoms in the unit cell.

NC:is the number of the atoms in the corner.

NI:is the number of the atoms inside the cube.

NF:is the number of the atoms in the centre of the face.

For the Body-Centered-cubic (BCC), it is obvious that the unit cell have just two atoms the

first one in the corner of the cube and the second in the centre of the cube (that share the unit

cell with each atom in the corner) as in the equation:

N = 8* 1/8 + 1 = 2As for the Face-Centered-cubic (FCC) it is calculated by the equation below:

N = 8 * 1/8 + 6 * 1/2 = 4Finally for the Hexagonal-closed-packed (HCP) is also calculated as follows:


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N = 12 * 1/6 + 3 + 2 * 1/2 = 6

4.7.0 Atomic packing in crystals,

4.7.1. The atomic packing factor [A.P.F]:

It can be defined as the ratio between the volumes of the basic atoms of the unit cell (whichrepresent the volume of all atoms in one unit cell) to the volume of the unit cell itself.

For cubic crystals, there is one constant to be quoted. The unit cell constant of pure metal

crystals can be directly related to the atomic diameter of the element as below:-

1. Body-Centered-cubic (BCC)

In the body Centered cubic the length of the cube diagonal = 2 D, as shown in figure 10, and

by Pythagoras:

(2D)2

= a2+ 2a

2

D =

a or r r

a

The volume of the atom can be calculated as follows:

V =

r

3 =

(

a)3

The volume of the basic atoms in the unit cell can be calculated as follows:

Vb =

(

a)3 ]

The volume of the unit cell is

Vu= a3

A P F =

=

(

a)3] / a3

Where D: is the atomic diameter

a: is the lattice constant

r: is the atomic radius


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Figure. Body Centered cubic unit cell. Relation between a and D.

2 Face-Centered-cubic (FCC)

Figure. Face Centered cubic unit cell. Relation between a and D.

3 Hexagonal-closed-packed (HCP)

There are two lattice constant, a and c as shown in figure 12, that parameter a is equal to

one atomic diameter [a = D], the parameter is the high of the hexagonal structure.From the hexagonal structure basics:

So the volume of the basic atoms is:

And the volume of the unit cell is:


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Figure. Hexagonal structure with unit cell outlined, and showing relationship between a and

c.

1.4.8. Selection of Materials:

Following are the salient features and properties owing to which selection of material for a

product is dependent:

Availability of Material

Fabrication Ease

Service ConditionOperational Needs

Process Control

Economy

Durability and Dependability

Dimensional Stability

Resistance to adverse condition like corrosion, Cavitation, Moisture, Radiation, Chemical,

Wear and Tear and Flame etc.

Elasticity and Plasticity, depending upon service requirement

Strength values and Impact Strength


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Details of crystal system:

1.4.9. Crystal Systems:

Primitive Unit Cells:

Primitive unit cells are those unit cells, which contains atoms or molecules at the corners of

lattice only. Thus, all simple types of space lattices are primitive unit cells. The unit cells

which contains atoms or molecules at all corner points and at Centre of body or face, are not

primitive cells are known as non-primitive cells.

Coordination Number:

It is the number of nearest and equidistant atoms in a unit cell. For this, it is assumed that

atoms are of spherical shape and are in contact (or touch each other, whenever possible).

Coordination number for SC is 6, BCC is 8 and for FCC, it is 12. For dense liquids, it is

nearly 10.

4.9.1. Voids:

The space, which remains empty when atoms are in contact, is known as voids. There are two

types of voids as Tetrahedral and Octahedral. When three atoms in a plane is covered by a

single atom on their top (in middle of these three atoms), then the formed void is termed as

tetrahedral void.

When 3 atoms are present in a plane and this atomic plane is covered by another plane of

three atoms such that their centres are not matching, then the formed void is known as

octahedral void. The space available in octahedral voids is more with respect to tetrahedral

voids. The max permissible size for an atom to fit in tetrahedral void without distortion is

0.0225 r where r is the radius of parent atom. The example is availability of alloying elements

in alloys.

The max permissible size for an atom to fit in octahedral void without distortion is 0.414 r

where r is the radius of parent atom. The example is Iron in which Carbon takes position in

octahedral voids in its FCC form.


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Relation between atomic radius and Lattice constant:

For SC, FCC and BCCEffective Number of Atoms:

It is different from total number of atoms in the unit cell. Total number of atoms are the

number of total atoms which are available in unit cell or are the part of unit cell whether fully

or partially.

Effective Number of atoms are those atoms which are dedicated to the unit cell or possessed

by the unit cell. These are: For SC1, For BCC2, For FCC4.

4.9.1Atomic Packing Fraction or Efficiency:

It is the ratio of total volume of atoms (occupied by atoms) to the volume of the unit cell. Itrepresents that how much volume of total unit cell is occupied by the atoms. It is also termed

as Atomic Packing Efficiency (APF or APE).

APF = v / V

For such calculations, atom is assumed to be a sphere and is in touch or contact.

APF for SC, BCC and FCC:

1.4.9.2 Calculation of packing density,

Density:

The density of a material is defined as the ratio of mass of the unit cell to the volume of the

unit cell.

Where:

:Density of material

Aw:Atomic Weight or Molecular Weight

Ne:Number of effective atoms in the unit cell

NA:Avogadros number

a3:Volume of unit cell (cubic)

1.4.9.3 Miller Indices:

Miller is the name of scientist and indices is the plural of Index. It is the system to designate

or to represent the crystal plane and direction.

Miller Indices are the smallest integer number that represents the plane and its direction in a

unit cell or crystal. It comprises three numeric values as h, k, l (name of variables). The

representation is as:

(h k l ) : to represent a plane

{h k l } : to represent the family of the plane

[h k l ] : to represent the direction

< h k l > : to represent family of directions


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When h, k, l values are of single digit, they are not separated by commas, but, if, these are of

two or more digits, then comma is used to separate the h, k, l values with in the brackets.

Procedure to find the Miller Indices of a Plane:

Choose an origin and designate x, y and z axes in the unit cell.

Find the intercepts on these three linear axes. Say these are c1, c2 and c3 along x axis, y-axis

and z-axis respectively.

Represent them in terms of axial units ie represent them with respect to linear dimensions

of the unit cell (a, b, c) as:

C1 = p.a C2 = q.b C3 = r.c

Or p = c1 / a q = c2 / b r = c3 / c

Where p, q, r the intercepts on x, y, z axes respectively.

Take the reciprocal of these intercepts as

h = 1 / p k = 1 / q l = 1 / r

Represent them in order within the brackets as ( h k l ) and convert them into smallestinteger part by taking common outside the bracket (or multiplying by LCM)

Neglect the common factor, which is outside the bracket and represent these smallest

integer values that are inside the bracket as ( h k l )

Important Points:

For negative planes, bar is used as 1 or ( h k l ), in which h value is negative.

Parallel planes intercepts at infinity. Thus, value of intercept will be infinity and reciprocal

of infinity is zero. So, Miller Indices for parallel plane will be zero.

Family of Planes:

It is represented by {h k l}. It comprises the members as (h k l) in all possible orders, both

negative and positive. For example {hkl} will have planes as (hkl), (klh), (khl) and other

three negative planes.

To draw a plane whosMiller Indices are given?

The given values are (h k l). These are the miller indices with respect to x, y, z axesrespectively.

Take reciprocal of these Miller Indices and the value will be of Intercepts along x, y and z

axes respectively. As:

p = 1 / h q = 1 / k r = 1 / l Convert them with respect to linear vectors or linear dimensions of unit cell i.e. with

respect to a, b, c along x, y and z axes respectively.

Choosing origin, mark the intercepts and join them to draw the plane.

1.4.9.2. CrystalDirections:

Miller Indices are also used to represent the directions with in a unit cell or crystal. These aredesignated by [h k l]. The family of directions contains all possible orders or h k l values


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including negative and positive directions. The family of directions is represented by < h k l

>.

For Parallel: Miller Indices = 0

For Negative directions: Use bar Salient Points of Miller Indices:

Miller Indices of parallel planes are same.

for parallel planes, Miller Indices value will be zero as they intercepts at infinity andreciprocal of infinity is zero.

The plane passing throughorigin will have non-zero miller indices.

Any two planes will be perpendicular if: h1 . h2 + k1 . k2 + l1 . l2 = 0

When Miller Indices contains more than single digits, they can be separated by commas or

spaces.

All members of a family of planes may not be parallel to each other.

Inter-planer Distance:

The distance between a plane (h k l) and the other parallel plane passing through origin is

called inter-planer spacing or inter-planer distance.

Linear Density (L):

The ratio of number of effective atoms (NeL) per unit cell length on certain length (L) along

any direction to the length of unit cell in that direction (ie L) in a unit cell or crystal is called

Linear Density (L). Mathematically,L = NeL / L

For example, in FCC, Plane [110] direction:

NeL = (1/2) + 1 + (1/2) = 2 and L = (a2

+ a2)1/2

Thus, L = 2 / {(a). 21/2}

= 21/2

/ a L = 21/2

/ a

Planer Density (P):

The ratio of atoms per unit area of crystal plane is called planer density. It express the

packing of atoms on a plane. Mathematically,

L = Ne / A where : Ne is the number of effective number of atoms on a plane : A is the area of

that plane

1.5.0 Voids in common crystal structures and imperfections crystals.

1.5.1. Crystal Structure Determination Techniques:

Braggs Law

Powder Method

Crystal Imperfections:


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The crystal which are having periodically repeated arrangement of atoms (regular) in the

space lattice is known as ideal or perfect crystal. But, due to some natural processes or owing

to intentional mixing, the regular arrangement of atoms gets deviated from its ideal nature.

Such crystals do not possess regular arrangement of atoms, which are periodically repeated in

the space lattice, are known as real crystals or imperfect crystal.

Intentional: Doping to make materials for transistors, ICs in electronics applications, AlloyFormation like mixing of Carbon in the matrix of Iron to form steels, cast irons etc.

Natural: presence of any foreign material (based on dimensions) during cooling,

Solidification, manufacturing processes

5.2.1. Imperfections in Crystalline Solids:

The deviation of real crystal from its ideal one is termed at defect or imperfection.

Imperfections can be classified according to the dimension occupied at atomic level by the

impurity material or alloying material. These are:

Point Imperfections: Zero Dimensional

Line Imperfections: One DimensionalSurface Imperfections: Two Dimensional

Volume Imperfections: Three Dimensional

Based on the dimension of defect, these may be:

Nano Level Imperfections: order of 10-9 m

Angstrom Level Imperfections: order of 10-10 m

Micro Level Imperfections: order of 10-6 m

1.5.2.2. Point Imperfections:

These are zero dimensional defects and imperfect or defected regions are like a point in the

crystal. These defects are of one or two atomic diameters only. The various types of point

imperfections are:

Vacancy Defects

Interstitial Defects

Substitutional defects

Frenkels Defect

Schottkys Defect

Vacancy Imperfections:


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In such defect, atom from its regular position is missing and creates a vacant site. Atomic

bonding forces at this location are not continuous. Vacant places are of one two atomic

diameters and does not obey any rule or regular arrangement.

Interstitial Defect:

The small sized foreign atoms (size difference < 15%) occupies a void space in the parent

crystal. Such defect is called interstitial defect.

Substitutional Defect:

When foreign atom is of larger size i.e. (size difference > 15%) is present in the matrix ofparent atom, it may acquire the place of regular atom by replacing it. Such defect is known as

Substitutional defect.

Frenkels Defect:


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The defect in which an ion displaces from its regular position to interstitial location in an

ionic solid is called Frenkels Defect.

Cations are of smaller size with respect to Anions, Thus cations may move from its regular

position to any interstitial location, causing Frenkels defect. Overall electrical neutrality is

being maintained. It occurs only in Ionic solids.

Schottkys Defect:

The defect, in which one pair of anion and cation is absent from its regular arrangement in an

ionic crystal, is called Schottkys Defect. Overall electrical neutrality is being maintained and

such defect occurs in Ionic solids.

1.5.2.3. Effect of Point Imperfections:

In case of vacancy, there is absence of bonding forces with neighbouring atoms.

In case of Substitutional impurity, an elastic strain is being developed in surrounding

regions due to size difference of foreign atom. If foreign atom is of larger size with respect to

parent atom, then compressive shear stress and strains will be developed. If foreign atom is of

smaller size with respect to parent atom, then tensile shear stress and strains will be

developed.

an interstitial atom creates strain around its surroundings.

Presence of point imperfection causes lowering of total energy of crystal that affect the

stability of crystal.

Point imperfections are thermodynamically stable.


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Causes of Point Imperfections or its Origin:

Mechanical Deformations: Casting, Rolling, Forging etc.

Thermal Shocks: Quenching

Thermal Fluctuations: Heating and cooling during operations

High Energy Particles Bombardment: displacement of atomic electrons with bombardmentparticles of X-ray

To create a point imperfection, some work is required to be done. This work is known as

Enthalpy or Potential Energy of Formation and is abbreviated as Hf.

The equilibrium concentration of vacancies in a crystal will be:

n = NA . e-Hf / RT where, n: No. of vacancies per mole of crystal

NA : Avogadro No. R: Gas Constant

T: Absolute Temperature

At absolute zero temperature, number of vacancies formation will be zero.

1.5.2.4. Line Imperfections:

These are known as Dislocations. There are two types of dislocations as:Edge Dislocation

Screw Dislocation

Edge Dislocation:

The distortion caused because of any incomplete atomic plane, in the regular arrangement of

atoms, is known as Edge Dislocation In perfect crystal, atoms are in equilibrium Position.

Just above the incomplete plane, atoms are squeezed together and are in state of compression.

The bond length is smaller than the equilibrium value. But. Below this edge, atoms are pulled

apart and are in tensile state. The bond length is more than the normal value. The potential

energy increase for both condition, either increase in bond length or decrease in bond length.

Thus, there is extra stain energy owing to incomplete plane.

The direction and magnitude is represented by Burgers Vector.

To know the direction and magnitude of Burgers Vector, take a closed path by moving in +Y

direction from any atom to some atoms (say b), then move in +X direction by some no. of

atoms (say a). Then move inY direction by respective same no. of atoms as b and then in

X direction by respective same no. of atoms i.e. a. if path becomes complete and comes at

starting point, then there is no incomplete plane. If, end point is apart from starting point, then

there is incomplete plane.


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Magnitude of Burgers Vector will be equal to the magnitude of atomic distance, which is

required to come from end point to starting point. Also, the same will be the direction of

vector as of closing vector.

Here, a = b = 4 atomic distances, Berger Vector is Perpendicular to edge dislocation.

+ Edge Dislocation: Negative Edge Dislocation

Screw Dislocation:

Screw dislocation is formed when a part of crystal displaces angularly over the remaining

part. Thus, the plane of atoms converted into helical surfaces or a screw.

Symbolically they are represented by

BurgersVector is parallel to screw dislocation

Mixed Dislocation:

When a crystal has edge dislocation as well as screw dislocation, then the imperfection is said

to be mixed dislocation. In such case an extra or incomplete plane of atoms accompanies with

angular shift of a part of real crystal. These are generally emerged with curved boundaries.

1.5.2.5. Dislocation Theory,

Characteristics of Dislocations:


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A crystal incorporates large number of dislocations and thus there exist numerous Burgers

Vectors. They meet at a point and this point is called nodal point. Sum of all Burgers vector

inside a crystal remain zero.

Dislocations vanishes at nodal points only (not ends abruptly)

Owing to dislocations, strain energy is developed inside the crystal and is the part of crystalinstability.

Dislocations have inherent tendency to keep smallest possible Burgers Vector.

Edge dislocations travel much faster (about 50 times) than screw dislocation.

Two edge dislocations of opposite nature (sigh) of equal Burgers vector and on the same

slip plane cancel-out.

Other terms related to Dislocations:

Glide Motion of Dislocation:

A edge dislocation can move along slip plane from its intermediate position to the surface

under the influence of externally applied shear stress. Such movement is called Glide Motion

and the plane of motion is called Glide Plane. A screw dislocation cannot have glide motion.

Dislocation Climb:

The plane perpendicular to glide plane is called climb plane. When edge dislocation moves

above or down to the slip plane in perpendicular (+Y or Y Direction), then the motion of

edge dislocation is called Climb Motion. The Climb motion in +Y direction is called Climb

Up and in Y direction, the climb motion is called Climb Down. Dislocation Climb is

a diffusion-controlled process. Screw Dislocation cannot climb up or climb down. It creates

vacancy in crystals. Climb motion is slower process than the glide motion.

5.2.7. Deformation by Slip

Cross Slip:

Under the influence of Shear Stress, Screw dislocation can change its slip plane while in

motion owing to any obstruction. Such motion is known as Cross-Slip. An edge or mixed

dislocation cannot have cross-slip motion. The cross slip in FCC crystals are close packed

{111}.

Jogs in Dislocation:


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A dislocation of Burgers Vector b lying in a plane other than the slip plane or glide plane

is called Jogs. The Burger Vector should be equal to the Burger Vector of a dislocation lying

in a slip plane. Thus, dislocation can jump from one plane to another and Jogs are treated as

Shorter Dislocation. (Here slip plane get changed along with motion of edge dislocation,

means both, together jumps from one location to another location of crystal)

Salient Features of Dislocations:

Dislocations are not thermodynamically stable.

Presence of dislocations, lowers the energy of the crystal.

Vacancy diffusion helps in dislocation climb.

Interstitial atoms may fir into larger spaced regions of edge dislocation. (C in BCC crystal

of Iron).

Sources of Dislocations:

mishandling during grain growth (crystals are formed by the process of crystallization)

Mechanical Deformation

Effects of Dislocation:

Lowers the mechanical strength.

Uneconomical machine structures more material is required for same strength)

Reduces electrical conduction

adversely affect surface-sensitive properties

Remedies:

Controlled process of Solidification (Nucleus formation during solidification, Grain Growth

and Recovery) and Re-crystallization etc)

Use of thermal energies

Prevention of undesirable mechanical deformations

1.5.2.8. Surface Imperfections:

These are observed on the surface of crystals and are two dimensional in nature. Due to finite

size of crystal, bonds are broken on the surface as no neighbouring atoms are present for

bonding. During solidification, solidifications starts from more than one locations and all

such crystals may have different atomic arrangement. Interaction of such crystals may

provide imperfection on mating areas or surface. Various types of surface imperfections are

as

Grain Boundaries

Twining or Twin Boundaries

Low angle Tilt Boundaries

High Angle Boundaries

Twist BoundariesStacking Fault


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1.5.8. Grain Boundaries:

During the solidifications and re-crystallization process, initially there is formation of tiny

particles as nucleus and then solidification grows from these points which is known as gain

growth. In poly-crystalline materials, atoms solidification starts from more than one point andNeighbouring atoms acquire the structure of this crystals and in total there are more than one

nucleus. Their orientation is somewhat different from each other. The boundary atoms

acquired compromising position. Finally, there is boundary formation at the surface crystals.

Such boundary is termed as Grain Boundary. The angle between the boundaries is termed as

Grain Boundary Angle.

Fig:Grain Boundaries

1.5.2.9. Twin Boundaries or Twining:

Twin boundaries occur in pair. The arrangement of atoms is such that atomic arrangement on

one side of boundary is mirror image of second side atomic arrangement. Such defect isknown as Twin Boundary or Twining or Twin. The zone ABCD is known as Twinned Zone.

Twins can be visualized by optical microscope. Annealing twins are those, which are formed

by annealing process. Deformation Twins are those twins, which are formed by the process of

mechanical deformation.

Fig:Twining:

I. Low Angle Tilt Boundaries:

Poly-crystalline materials possess the angular orientation at their boundaries. If this angle of

orientation or of grain boundaries is less than 100, then the formed grain boundaries are

termed as low angle Tilt boundaries.

II. High Angle Tilt Boundaries:


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Poly-crystalline materials possess the angular orientation at their boundaries. If this angle of

orientation or of grain boundaries is more than 100, then the formed grain boundaries are

termed as low angle Tilt boundaries.

Stacking Fault:

Say, there are three layers of atomic planes in regular order to form a crystal. The sequence is

as ABC ABC ABC ABC .

Due to any reason, if any intermediate atomic plane is missing from its regular sequence, and

is as:

ABC AC ABC.. Here, B atomic plane is missing in its regular arrangement.

Thus, the formed defect is known as Stacking Fault.

5.2.9.1. Volume Imperfections:

These are three dimensional defects. They may be formed by any of the following reasons:

Foreign Particle inclusions

Regions of Non-crystalline

Pores or Holes

Dissimilar Natured Regions

The dimensions of such defects are the order of tens of A0. The above said imperfections are

randomly located inside the material from one or more than locations.

Whisker:

Whiskers are obtained by elongating single crystal into fibrous form. With this process,

imperfections like grain boundaries are fully eliminated and other are reduced. The diameter

of whiskers varies between 2 to 20 microns. Greater strength is usually observed in wickers

of smallest diameters. Strength nearly 2.7 GPa and 13.2 GPA have been achieved in Copper

and Iron whiskers respectively.

For Mild Steel, Strength in Bulk form is about 0.48 GPA and with whisker, it is about 12.8

Gpa. Youngs Modulus of bulk form of Mild Steel is about 200 GPa and with whisker,

youngs modulus of Mild Steel is about 1000 G Pa. Elastic deformation of MS in bulk form

are about 0.2% and with whisker, these are about 5%.

Whiskers are available in metallic as well as in non-metallic, in organic and inorganicmaterials. In 1970, first synthetic whiskers were produced. The natural whiskers are Spiders

web, Bamboo Flakes, Human Bone Flakes, some kind of grass etc.

These are necessary to obtain the theoretical strength as Ultimate strength = Modulus of

Elasticity or Youngs Modulus.

1.6. Single Crystals and Polycrystalline Materials

Single crystal: atoms are in a repeating or periodic array over the entire extent of the material

Polycrystalline material: comprised of many small crystals or grains. The grains have

different crystallographic orientation. There exist atomic mismatch within the regions where

grains meet. These regions are called grain boundaries.


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Polycrystalline Materials:

Atomistic model of a Nano crystalline solid by Mo Li

Phase equilibrium diagram is a graphic relationship between temperature and weight ratios of

elements and alloys contribute to the built of the diagram. Where Phase is a uniform part of

an alloy, having a certain chemical composition and structure, and which is separated from

other alloy constituents by a phase boundary.

For example the saltwater solution have a four possible phases:

- Water vapour (steam)

- Liquid salt solution (sodium chloride in water)

- Crystals of water (ice)

- Crystals of salt (sodium chloride)


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Module-2

2.0. Alloying systems:

Alloy is a metal composing of a mixture of elements. Most of alloys are composed of a base

metal with small amounts of additives or alloying elements. The typical examples of alloysare steel/cast iron (iron base alloys), bronze/brass (copper base alloys), aluminium alloys,

nickel base alloys, magnesium base alloys, titanium alloys.

There are many types of alloying systems which they are:

Binary system

It means that alloying have two metal

- Ternary system

It means that alloying have three metals only.

Multi system

It means that alloying have three and more than that metals.

The constituent components of most commercially available binary alloys are completely

soluble in each other in the liquid (molten) state and, in general, do not form intermetalliccompounds. (The exceptions being some bearing metals.) However, upon cooling into the

solid state, binary alloys can be classified into the following types.

Simple eutectic type the two components are soluble in each other in the liquid state

but are completely insoluble in each other in the solid state.

Soli d solu tion type the two components are completely soluble in each other both in

the liquid state and in the solid state.

Combination type the two components are completely soluble in the liquid state, butare only partially soluble in each other in the solid state. Thus this type of alloy

combines some of the characteristics of both the previous types, hence the name

combination type phase equilibrium diagram.

Let's now consider these three types of binary alloy systems and their phase equilibrium

diagrams in greater detail.

1) Phase equi li bri um diagrams (Eu tectic type):

In general case, consider for studying a two components presents which are referred to as

metal A and metal B, with the phase diagram as shown in figure.


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Figure. Phase equilibrium diagram (eutectic type).

Although they are mutually soluble in the liquid state, both components retain their individual

identities of crystals of A and crystals of B in the solid state, If you refer to figure 1, you can

see that the line joining the points where solidification begins is referred to as the liquidus

and that the line joining the points where solidification is complete is referred to as the

solidus.This type of equilibrium diagram gets its name from the fact that at one particular

composition (E), the temperature at which solidification commences is a minimum for the

alloying elements present. With this composition the liquidus and the solidus coincide at the

same temperature, thus the liquid changes into a solid with both A crystals and B crystals

forming instantaneously at the same temperature. This point on the diagram is called the

eutectic, the temperature at which it occurs is theeutectic temperature, and the composition is

the eutectic composition. In practice, few metal alloys from simple eutectic type phase

diagrams. It is identical with this type of phase diagram is produced for a salt (sodium

chloride) and water solution, it is total solubility of the salt in water in the liquid state and

total insolubility (crystals of ice and separate crystals of salt) in the solid state. As an example

of eutectic are carbon steels

2) Phase equi li bri um diagrams (Soli d solu tion type):

Solid soluti on is a phase, where two or more elements are completely soluble in each other.

Depending on the ratio of the solvent (matrix) metal atom size and solute element atom size,

two types of solid solutions maybe formed: substitution or interstitial.

Substitution solid solution

If the atoms of the solvent metal and solute element are of similar sizes (not more, than 15%difference), they form substitution solid solution, where part of the solvent atoms arc

substituted by atoms of the alloying element as shown in figure 2.


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Figure. Substitution solid solution.

Interstitial solid solution

If the atoms of the alloying elements are considerably smaller, than the atoms of the matrix

metal, interstitial solid solution forms, where the matrix solute atoms are located in the

spaces between large solvent atoms as shown in figure.

Figure. Interstitial solid solution.

When the solubility of a solute element in interstitial solution is exceeded, a phase of

intermediate compound forms. These compounds (WC, Fe3C etc.) play important role in

strengthening steels and otheralloys.Some substitution solid solutions may form ordered

phase where ratio between concentration of matrix atoms and concentration of alloying atoms

is close to simple numbers like AuCu3 and AuCu.Solid solution formation usually causes

increase of electrical resistance and mechanical strength and decrease of plasticity of the

alloy. In this type as shown in figure below the line marked liquids joins the points where

solidification commences, whilst the line marked solidus joins the points where solidification

is complete. This time there is no eutectic composition. It has already been stated that copper


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and nickel are not only mutually soluble in the liquid (molten) state, they are a/so mutually

soluble in the solid state and they form a Substitutional solid solution. The phase equilibrium

diagram for copper-nickel alloys is shown in figure.

Figure. Copper-nickel phase equilibrium diagram.

3) Phase equi li bri um diagrams (Combination type):

Many metals and non-metals are neither completely soluble in each other in the solid state

nor are they completely insoluble. Therefore they form a phase equilibrium diagram of the

type shown in figure. In this system there are two solid solutions labelled and . The use ofthe Greek letters , , , etc., in phase equilibrium diagrams may be defined, in general, as

follows:

A solid solution of one component A in an excess of another component B, such that

A is the solute and B is the solvent, is referred to as solid solution .

A solid solution of the component B in an excess of the component A, so that B now

becomes the solute and A becomes the solvent, is referred to as solid solution .

In a more complex alloy, any further solid solutions or intermetallic compounds

which may be formed would be referred to by the subsequent letters of the Greek

alphabet. That is, , , etc.


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Figure. Combination type phase equilibrium diagram.

We will steady the iron-carbon phase diagram as an example on the phase equilibrium

diagram which it describes the iron-carbon system of alloys, containing up to 6.67 % carbon,

discloses thephasescompositions and their transformations occurring with the alloys during

their cooling or heating, as shown in figure.

Carbon content 6.67 % corresponds to the fixed composition of the iron carbide Fe3C.

The following phases are involved in the transformation, occurring with Iron-carbon alloys:L Liquid solution of carbon in iron.

ferrite solid solution of carbon in iron. Maximum concentration of carbon in -ferrite is

0.09 % (1493C) temperature of the Peritectic transformation. The crystal structure of -

ferrite is BCC (cubic body Centered).

Austenite interstitial solid solution of carbon in -iron. It has FCC (cubic face Centered)

crystal structure, permitting high solubility of carbon up to 2.06% at (1147C). It does not

exist below (723C) and maximum carbon concentration at this temperature is 0.83 % .

ferrite solid solution of carbon in -iron. It has BCC crystal structure and low solubility of

carbon up to 0.25 % at (723C). It is exists at room temperature.

Cementite iron carbide, intermetallic compound, having fixed compositionFe3C. It is a hard

and brittle substance, influencing on the properties of steel and cast irons.

The following phase transformations occur with iron-carbon alloys:

Alloys, containing up to 0.51% of carbon, start solidification with formation of crystals of -

ferrite. Carbon content in -ferrite increases up to 0.09% in course solidification, and at

(1493C) remaining liquid phase and -ferrite perform Peritectic transformation, resulting in

formation of austenite. Alloys, containing carbon more than 0.51%, but less than 2.06%, form

Primary austenite crystals in the beginning of solidification and when the temperature reaches

the curve ACM primary cernentite stars to form.


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Iron-carbon alloys, containing up to 2.06% of carbon, arc called Steels. Alloys, containing

from 2.06 to 6.67% of carbon, experience eutectic transformation at (1147 C). The eutectic

concentration of carbon is 4.3%.

In practice only hypoeutectic alloys are used. These alloys (carbon content from 2.06% to

4.3%) are called cast irons. When temperature of an alloy from this range reaches 2097 F

(1147 C), it contains primary austenite crystals and some amount of the liquid phase. Thelatter decomposes by eutectic mechanism to a fine mixture of austenite and cementite, called

Ledeburite.

All iron-carbon alloys (steels and cast irons) experience eutectoid transformation at (723C),

The eutectoid concentration of carbon is0.83%When the temperature of an alloy reaches

(723C), austenite transforms to pearlite (fine ferrite-cementite structure, forming as a result

of decomposition of austenite at slow cooling conditions).

2.1. Critical temperatures

Upper critical temperature (point) A3 is the temperature, below which ferrite starts to formas a result of ejection from austenite in the hypo eutectoid alloys.

Upper critical temperature (point) ACM is the temperature, below which cementite starts

to form as a result of ejection from austenite in the hypereutectoid alloys.

Lower critical temperature (point) A1 is the temperature of the austenite-to-pearlite

eutectoid transformation. Below this temperature austenite does not exist.

Magnetic transformation temperature A2 is the temperature below which a-ferrite is

ferromagnetic.

Phase compositions of the iron-carbon alloys at room temperature:

Hypo eutectoid steels (carbon content from 0 to 0.83%) consist of primary

(proeutectoid) ferrite (according to the curve A3) and pearlite.

Eutectoid steel (carbon content 0.83%) entirely consists of pearlite.

Hyper elltectoid steels (carbon content from 0,83 to 2.06%) consist of primary(proeutectoid) cementite (according to the curve ACM) and pearlite.

Cast irons (carbon content from 2.06% to 4.3%) consist of proeutectoid cementite CTejected from austenite according to the curve ACM, pearlite and transformed

ledeburite (ledeburite in which austenite transformed 10 pearlite).

2.2. Heat treatment of carbon steel

Plain carbon steels and alloy steels are among the relatively few engineering materials which

can be usefully heat treated in order to vary their mechanical properties. The other main

group is the heat-treatable aluminium alloys. Steels can be heat treated because of the

structural changes that can take place within solid iron-carbon alloys. The various heat-

treatment processes appropriate to plain carbon steels are:

Annealing.

Normalising.

Hardening.

Tempering.


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In all the above processes the steel is heated slowly to the appropriate temperature for its

carbon content and then cooled. It is the rate of cooling which determines the ultimate

structure and properties that the steel will have, providing that the initial heating has been

slow enough for the steel to have reached phase equilibrium at its process temperature. Figure

shows the types of the ranges of carbon steels.

Figure. Heat-treatment temperature Ranges of Classes of Carbon Steels in Relation to theEquilibrium Diagram.

2.2.1. Annealing

All annealing processes are concerned with rendering steel soft and ductile so that it can be

cold worked and/or machined. There are three basic annealing processes, as shown in figure,

and these are:

Stress-relief annealing at subcritical temperatures.

Spheroidised annealing at subcritical temperatures.

Full annealing for forgings and castings.

The process chosen depends upon the carbon content of the steel, its retreatment processing,and its subsequent processing and use.

Figure. Annealing temperature for plain carbon steel.

a) Stress-relief annealing

It is also called 'process annealing' , 'interstate annealing' and subcritical annealing, it is often

used for softening cold worked low carbon(0.4 % carbon content) steel or mild steel . To

fully anneal such a steel would involve heating to a temperature of more than 900C, with

consequent high cost. In a mild steel ferrite makes up about 90 % of the structure, and the

recrystallization temperature of cold worked ferrite is only about 500C. Annealing a cold


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worked mild steel in the temperature range 550 600 C will result in complete

recrystallization of ferrite,

Although the cold worked pearlite will be largely unaffected. Frequently, however, we must

apply a considerable amount of cold-work to mild steels, as, for example, in the drawing of

wire. Stress-relief annealing then becomes necessary to often the metal so that further

drawing operations can be carried out. Such annealing is carried out at about 650 C. Sincethis temperature is well above there crystallisation temperature of 500 C, recrystallization

will be

Accelerated so that it will be complete in a matter of minutes on attaining the maximum

temperature.

It should be noted that process annealing is a sub-critical operation, that is, it takes place

below the lower critical temperature (Ai). For this reason, although recrystallization is

promoted, there is no phase change and the constituentsferrite and cementite remain present

in the structure throughout the process.

Process annealing is generally carried out in either batch-type or continuous furnaces, usually

with an inert atmosphere of burnt coal gas, though cast-iron annealing "pots" are still used,their lids being luted on with clay.

b) Spheroidised annealing

The Spheroidised condition is produced by annealing the steel at a temperature between 650

and 700 C, just below the lower critical temperature. During this treatment cementite forms

as spheroidal partislesin a ferrite matrix, putting the steel into a soft, but very tough,

condition. Since the temperature involved are sub critical no phase changes take place and

spheroidisation of the cementite is purely a surface tension effects. This is referred to as the

spheroidisation of pearlitic cementite and the process is shown diagrammatically in figure.

Figure. The Spheroidisation of Pearlitic Cementite.

c) Full annealing

It is the treatment given to produce the softest possible condition in a hypo eutectoid steel. It

involves heating the steel to a temperature within the range 3050 C above the upper critical

temperatures and then allowing the steel to cool slowly within the furnace. This produces a

structure containing coarse pearlite.

This results in the formation of fine grains of austenite that transform into relatively fine

grains of ferrite and pearlite or pearlite and cementite (depending upon the carbon content) as

the steel is slowly cooled to room temperature, usually in the furnace.

Full annealing is an expensive treatment and when it is not absolutely essential for the steel to

be in a very soft condition, but a reasonably soft and ductile material is required, the process

known as normalizing is used instead.


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Ferrite, which then begins to precipitate in accordance with the equilibrium diagram, deposits

first at the grain boundaries of the austenite, thus revealing, in the final structure, the size of

the original austenite grains. The remainder of the ferrite is then precipitated along certain

crystallographic planes within the lattice of the austenite. This gives rise to a directional

precipitation of the ferrite, as shown in figure. And Plate, representing typically what is

known as Widmanstatten structure. This type of structure was first encountered byWidmanstatten in meteorites, which may be expected to exhibit a coarse structure in view of

the extent to which they are overheated during their passage through the upper atmosphere.

.The mesh-like arrangement of ferrite in the Widmanstatten structure tends to isolate the

stronger pearlite into separate patches, so. That strength, and more particularly toughness, are

impaired. The main characteristics of such a structure are, therefore, weakness and brittle-

ness, and steps must be

Taken to remove it either by heat-treatment or by mechanical working. Hot-working will

effectively break up this coarse as-cast structure and replace it by a fine-grained material, but

in this instance we are concerned with retaining the actual shape of the casting. Heat-

treatment

Must therefore be used to effect the necessary refinement of grain.

Figure. The structural effects of heating a steel casting to a temperature just above its upper

critical, followed by cooling to room temperature.

This operation need a very specific controlling on the heat temperature of annealing because

if any fault is occurs, it will make some un desired phases in the steel such as:-

1) Over heating


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Overheating during annealing, or heating for too long a period in the austenitic range, will

obviously cause grain growth of the newly formed austenite crystals, leading to a structure

almost as bad as the original Widmanstatten structure. For this reason the requisite annealing

temperature should not be exceeded, and the casting should remain in the austenitic range

only for as long as is necessary to make it completely austenitic. In fact, castings are

sometimes air-cooled to about 650 C and then cooled more slowly to room temperature, byreturning to a furnace to prevent stresses due to rapid cooling from being set up.

2) Burning (Excessive over heating)

Excessive overheating will probably cause oxidation, or burning", of the surface, and the

penetration by oxide films of the crystal boundaries following decarburization of the surface.

Such damage cannot be repaired by heat-treatment, and the castings can only be scrapped. To

prevent "burning", castings are often annealed in cast-iron boxes into which they are packed

With lime, sand, cast-iron turnings or carbonaceous material, according to the carbon content

of the castings.

3) Under-qunealing

As the lower critical temperature (723 C) is reached on heating, the patches of pearlite change

to austenite, but these crystals of austenite are very small, since each grain of pearlite gives

rise to a number of new austenite crystals. As the temperature rises, the Widmanstatten-type

plates of ferrite are dissolved by the austenite until, when the upper critical temperature is

reached, the structure consists entirely of fine-grained austenite. Cooling causes re-

precipitation of the ferrite, but, since the new austenite crystals are small, the precipitated

ferrite will also be distributed as small particles. Finally, as the lower critical temperature is

reached, the remaining small patches of austenite will transform to pearlite.

2.2. Normalising

The process resembles full annealing except that, whilst in annealing the cooling rate is

deliberately retarded, in normalising the cooling rate is accelerated by taking the work from

the furnace and allowing it to cool in free air. Provision must be made for the free circulation

of cool air, but draughts must be avoided.

In the normalising process, as applied to hyper-eutectoid steels, it can be seen that the steel is

heated to approximately 50 C above the upper critical temperature line. This ensures that the

transformation to fine grain austenite corrects any grain growth or grain distortion that may

have occurred previously. Again, the steel is cooled in free air and the austenite transformsinto fine grain pearlite and cementite. The fine grain structure resulting from the more rapid

cooling associated with normalising gives improved strength and toughness to the steel but

reduces its ductility and malleability. The increased h

metallurgy e-matl., lecture note

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