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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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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