Material.pdf

GAZİ UNIVERSITY
Engineering Faculty
Department of Industrial Engineering
MATERIALS SCIENCE
ENM223
2020-2021 Fall Semester
1

Text Book (1) and References (2-5):
1) Materials Science and Engineering, W.D. Callister, 8th ed., John Wiley,
2011.
2) The Science and Engineering of Materials, D.R. Askeland, P.P. Phule, 4th ed.,
Thomson Brooks/Cole, 2003.
3) Foundations of Materials Science and Engineering, W.F. Smith, J. Hashemi, 4th
ed. McGraw-Hill, 2006.
4) Engineering Metallurgy and Materials, Süleyman Sarıtaş, Gazi University, 1995.
5) Materials Science and Engineering, W.D. Callister, Translation to Turkish
from 8th ed., John Wiley, 2011.
2

Course Outline
Subject Weeks
1. Introduction to Engineering Materials……………………… 1
Classissification, Metals, Ceramics, Polymers, Composites
2. Interatomic Bonding……………………………………………… 1
3. The Structure of Crystalline Solids ….………...…………… 2
Atomic arrangements and crystals, Imperfections in crystals
Microstructural examination of materials
4. Diffusion………………………..…………………………….…….. 1
5. Phase Diagrams…………...……………………………………… 1
Rules of phase diagrams, Construction of phase diagrams
3

Course Outline, cont.
Subject Weeks
6. Mechanical Testing and Properties……………………… 2
Tensile test, Hardness test, Fatigue test, Creep test
Impact test, Bending and Shear (Torsion) tests
7. Ferrous and Nonferrous Alloys…………………………… 2
8. Iron-Carbon System and Heat Treatments….………… 2
Atomic arrangements and crystals, Imperfections in crystals
Microstructural examination of materials
9. Strengthening Mechanisms………………………….…….. 1
Work hardening, Solid solution hardening, ,
Grain size strengthening, Dispersion hardening
4

• Course Grading:
One Midterms : 60%
Final Exam : 40%
• Attendance:
Attendance to classes (70 %) is compulsory.
5

CHAPTER 1
INTRODUCTION TO ENGINEERING MATERIALS
(Text Book Pages 1-17)
.
6

“Because without materials, there is no engineering.”
Materials are probably more deep-seated in our culture than most of us
realize. Transportation, housing, clothing, communication and food
production-virtually every segment of our everyday lives is influenced by
materials. Then, what are materials?
Materials may be defined as substance of which something is
composed or made.
Materials are everywhere. Some of commonly encountered
materials are wood, concrete, brick, steel, plastic, glass,
rubber, aluminum, copper and paper.
7

Materials Science and Engineering?
8
Materials Science?
 Involves Investigating the relationships between structure,
processing and properties of materials.
Materials Engineering?
 On the basis of these structure-property correlations, designing or
engineering the structure of a material to produce a predetermined set
of properties. Designing the structure to achieve specific properties
of materials.
Structure means a description of the arrangements of atoms or ions in a material.
The structure of materials had a profound influence on many properties of materials,
even if the overall composition does not change!

9
Relation between
processing & properties
roll
round and coarse grains
low strength
high ductility
elongated grains
high strength
low ductility
Hot rolling
Recrystallised fine
grains
low strength
high ductility

Why the study of materials is important?
Engineers choose materials to suit design or new materials might be needed for some new
applications. For example:
 Mechanical engineers search for high temperature resistant materials so that jet engines
can operate more efficiently.
 Aerospace engineers search for materials with higher strength to weight ratios for aircraft
and space vehicles.
 Chemical engineers need more highly corrosion resistant materials.
So, engineers in all disciplines should have some basic and applied knowledge of
engineering materials.
Corrosion is the disintegration of an engineering material into its constituent atoms due to chemical reactions with
its surroundings. Formation of an oxide of iron due to oxidation of the iron atoms in solid solution is a well-known
example of corrosion (rusting).
12
Figure. A section through a jet engine. The forward compression section operates
at low to medium temperatures, and titanium parts are often used. The rear
combustion section operates at high temperatures and nickel-based superalloys
are required. The outside shell experiences low temperatures, and aluminum and
composites are satisfactory.

CLASSIFICATION OF MATERIALS
Engineering materials are divided into three main classes: metals, ceramics,
and polymers.
13
In addition, there are
the composites,
combinations of two or
more of these basic
material classes.

METALS
Metals are metallic elements such as iron, aluminum, copper, titanium, nickel and
gold. Although pure metals are occasionally (rarely) used, alloys which
combinations of two or more metals are normally designed to provide desirable
properties. Examples:
Metals and alloys are generally divided into two groups: Ferrous (steel, cast iron)
and nonferrous (Al, Cu, Zn, etc.).
Ferrous metals and alloys that contain a large percentage of iron such as the
steel and cast irons. Nonferrous metals do not contain iron.
14
Alloys: Brass: Cu-Zn, Bronze: Cu-Sn,
Steel: Fe-C, Solder: Pb-Sn, etc.
Pure metals: Fe, Au, Ag, Cu, Ni, etc.
Properties of metals
a) Atoms arranged in a regular repeating structure (crystalline structure)
b) Good strength: strong
c) Dense
d) Malleable or ductile (i.e., capable of large amounts of deformation without fracture):
high plasticity
e) Resistant to fracture and shock resistance: tough
f) Good thermal and electrical conductivity
g) Easy machinability: Thus, metals can be formed
h) They are least resistant to corrosion , such as iron-oxide (rust)
i) Some of the metals (Fe, Ni, Co) have desirable magnetic properties.
Applications:
 Buildings
 Automobiles
 Airplanes
 Machine and tools
 Gears
 Electrical wires
 Pressure vessels, etc.

Ceramics
 Ceramic materials consist of metallic and
non-metallic elements chemically bonded
together.They are most frequently oxides,
nitrides, and carbides.
For example: SiC, Al2O3, ZrO2, Si3N4,.MgO
Examples:
 Abrasive Materials (for cutting, grinding
and polished other materials of lower
hardness; aluminium oxide and silicon
carbide)
 Refractories (corrosion and high heat-
resistant; brick)
 Whitewares (e.g. porcelains)
 Electrical Ceramics (capacitors,
insulators, transducers, etc.)
 Chemically Bonded Ceramics (e.g.
cement and concrete)
 Glass (transparency,hardness at room
temperature, excellent resistance to most
environments)
15

16
Properties of ceramics
 Except for glasses they have crystalline structure
 They have lower density than most metals
 Generally they have high melting points
 They have high chemical stabilities (resistant to environment)
 They have high wear resistance
 They have high hardness and high-temperature strength (heat resistant)
but they are very brittle
 They have low ductility or malleability: low plasticity
 They have very high elastic modulus
Ceramics are usually poor electrical and thermal conductors

 Aluminium (Al) is a common metal, but aluminium oxide, a
compound of aluminium and oxygen such as Al2O3, is ceramic.
Aluminium oxide has two basic advantages over metallic
aluminium.
 First, Al2O3 is chemically stable in a wide variety of severe
environments whereas metallic aluminium would be oxidized.
 Second, the ceramic Al2O3 has a significantly higher melting
point (2020 C) than does the metallic Al (660 C).
 This makes Al2O3 a popular refractory, that is, a high-
temperature-resistant material of wide use in industrial furnace
construction.
 With its superior chemical and temperature-resistant
properties, why isn’t Al2O3 used for certain
applications,such as automotive engines, in place of metallic
aluminium?
 The answer to this question lies in the most limiting property of
ceramics-brittleness. Aluminium and other metals have the
desirable property of ductility whereas aluminium oxide and
other ceramics do not. Thus ceramics are eliminated from
many structural applications.
17
Applications
 Electrical
insulators
 Abrasives
 Thermal insulation
and coatings
 Windows,
television screens
(glass)
 Corrosion resistant
applications
 Electrical devices
 Highways and
roads (concrete)

Polymers
 Polymers are in our everyday life. An alternative name for this category is
plastics. The “mer” in a polymer is a single hydrocarbon molecule. Polymers
are long-chain molecules composed of many mers bonded together.
 Many important polymers are simply compounds of hydrogen and carbon.
 Others contain oxygen, nitrogen,and silicon.
 Examples: nylon,teflon, PVC (polyvinyl chloride), polyester.

Plastics can be divided into two classes, thermoplastics and
thermosetting plastics, depending on how they are structurally bonded. 18

 Most thermoplastics consist
of very long molecular chains and
have good ductility and formability.
 These materials can be reheated
and reformed into new shapes a
number of times.
 Polyester (PE), polystyrene
(PS) and PVC are thermoplastics.
THERMOPLASTICS & THERMOSETTINGS
 Thermosetting plastics are
stronger but more brittle because the
molecular chains are tightly linked.
 Thermosetting plastics can not be
remelted and reformed into another
shape. Thus, they cannot be
recycled.
 Epoxies, phenolics and some
polyster resins are thermosets.
19

Properties of polymers
 Most polymers are non-crystalline, but some
consist of mixtures of both crystalline and non-
crystalline regions
 They generally have low densities
 They have low elastic modulus
 Their ductility may vary considerably
 They have low strength
 Most of them are corrosion resistant
 They cannot be used at high temperature
because of their low melting points
 They have low cost
 Most are poor conductors of electricity and heat
20
 In spite of the limitations, (they have low electrical and thermal conductivities, low
strength, and are not suitable for use at high temperature) polymers are highly versatile
(can be used for many different purposes) and useful materials.
 Unlike brittle ceramics, polymers which are frequently lightweight, low-cost and resistant
to higher chemical reactivity alternatives to metals in structural design applications.
Applications
 Adhesives and glues
 Containers
 Moldable products (computer
casing, telephone handset)
 Clothing and upholstery material
(polyester, nylon)
 Biodegradable products (packing)
 Low friction materials (teflon)
 Gaskets and O-rings (rubber)

Composite Materials
 Composites are produced when two materials are joined to give a combination
of properties that cannot be obtained in the original materials.
With composites we can produce
 lightweight
 strong
 ductile
 high temperature-resistant materials that are otherwise unobtainable.
21
 Composites can be
 metal-metal
 metal-ceramic
 metal-polymer
 ceramic-polymer
 ceramic-ceramic
 polymer-polymer
 Furthermore, some naturally-occurring materials are also considered to be
composites-for example, wood and bone.

 Fiberglass, in which small glass fibers are embedded within a polymeric material
(epoxy, polyster) is a common ceramic-polymer composite (GFRP: Glass Fiber
Reinforced Polymer). The glass fibers are relatively strong (but also brittle), whereas
the polymer is ductile (but also weak). Thus, the resulting composite material is
relatively strong and ductile. In addition, it has a low density.
 Another of these technologically important materials is the carbon fiber reinforced
polymer composite (CFRP: Carbon Fiber Reinforced Polymer). These composites
are stronger than glass fiber-reinforced composites, but they are more expensive.
They are used in some aircraft and aerospace applications, as well as high-tech
sporting equipment (e.g., bicyles, tennis rackets, and skis).
 Metal matrix-ceramic composites, for example, include silicon carbide or aluminium
oxide fiber reinforced aluminium alloy.
 WC reinforced cobalt matrix composite materials are also metal-ceramic composites.
It is used as a cutting tool for machining.
 Composites can be placed into three categories: particulate, fiber, and laminar-based
on the shapes of the material.
22
Applications
 Aircraft and aerospace applications
 Sports equipment (golf club shafts, tennis rackets, bicycle frames)
 Thermal insulation
 Concrete
 Brake materials

Fig.1.3. Bar chart of room temperature density values for various
materials
23

Fig.1.4. Bar chart of room temperature stiffness (i.e. Elastic
modulus) values for various materials
24

Fig.1.5. Bar chart of room temperature strength (i.e. Tensile
strength) values for various materials
25

Fig.1.6. Bar chart of room temperature resistance to fracture for
various materials
26

Fig.1.7. Bar chart of room temperature electrical conductivity
ranges for various materials
27

Advanced Materials
• Materials that are utilized in high-technology (or high-
tech) applications are sometimes termed advanced
materials.
• By high technology we mean a device or product that
operates or functions using relatively intricate and
sophisticated principles; examples include electronic
equipment (camcorders, CD/DVD players, etc.),
computers, fiber-optic systems, spacecraft, aircraft, and
military rocketry.
• These advanced materials are typically traditional
materials whose properties have been enhanced, and,
also newly developed, high-performance materials.
28

Advanced Materials, Cont.
Semiconductors
• Semiconductors have electrical properties that are intermediate
between the electrical conductors (viz. metals and metal alloys) and
insulators (viz. ceramics and polymers)—Figure 1.7.
• Furthermore, the electrical characteristics of these materials are
extremely sensitive to the presence of minute concentrations of
impurity atoms, for which the concentrations may be controlled over
very small spatial regions.
• Semiconductors have made possible the advent of integrated
circuitry that has totally revolutionized the electronics and computer
industries (not to mention our lives) over the past three decades. 29

Biomaterials
• Biomaterials are
employed in
components
implanted into the
human body for
replacement of
diseased or
damaged body
parts.
30

Biomaterials, cont
• These materials must not
produce toxic substances
and must be compatible
with body tissues (i.e.,
must not cause adverse
biological reactions).
• All of the above
materials—metals,
ceramics, polymers,
composites, and
semiconductors—may be
used as biomaterials.
31

Smart Materials
• Smart (or intelligent) materials are a group of new and
state-of-the-art materials now being developed that will
have a significant influence on many of our technologies.
• The adjective “smart” implies that these materials are
able to sense changes in their environments and then
respond to these changes in predetermined manners—
traits that are also found in living organisms.
• In addition, this “smart” concept is being extended to
rather sophisticated systems that consist of both smart
and traditional materials.
32

Smart Materials, cont.
• Components of a smart material (or
system) include some type of sensor (that
detects an input signal), and an actuator
(that performs a responsive and adaptive
function).
• Actuators may be called upon to change
shape, position, natural frequency, or
mechanical characteristics in response to
changes in temperature, electric fields,
and/or magnetic fields.
• Four types of materials are commonly
used for actuators: shape memory
alloys, piezoelectric ceramics,
magnetostrictive materials, and
electrorheological/magnetorheological
fluids.
33

Nanomaterials
• One new material class that has fascinating
properties and tremendous technological promise is
the nanomaterials.
• Nanomaterials may be any one of the three basic
types: metals, ceramics, or polymers.
• The ability to carefully arrange atoms provides
opportunities to develop mechanical, electrical,
magnetic, and other properties that are not otherwise
possible.
• We call this the “bottom-up” approach, and the study
of the properties of these materials is termed
“nanotechnology”;
• The “nano” prefix denotes that the dimensions of
these structural entities are on the order of a
nanometer (10-9 m) 34

Modern materials’ Needs
• In spite of the tremendous progress that has been made in the
discipline of materials science and engineering within the past few
years, there still remain technological challenges, including the
development of even more sophisticated and specialized materials,
as well as consideration of the environmental impact of materials
production. Some comment is appropriate relative to these issues so
as to round out this perspective:
– Nuclear energy
– Engine components (higher temperature capabilities)
– Hydrogen fuel cell
– Pollution control techniques
35

END of the CHAPTER 1
Don’t forget to study from the
TEXT BOOK!
pages 1-17
36

Interatomic Bonding
Learning Objectives
• Briefly describe ionic, covalent, metallic, hydrogen, and van der
Waals bonds.
Text Book: Pages: 26-36
1

Electronegativity
• Electronegativity concept helps in understanding the bonding behavior of elements.
• Electronegative elements accept electrons to produce negative ions (anion). The most
electronegative elements are in groups VIA and VIIA of the periodic table.
• Electropositive elements give up their valance electrons in chemical reactions to produce
positively charged ions (cation). The most electropositive elements are in groups IA and IIA of the
periodic table.
• Electronegativities of the elements increases from left to right in the periodic table.
Electronegativity is measured on a scale from 0.7 to 4.0. The most electronegative elements
fluorine, oxygen, and nitrogen, which have electronegativities of 4.0, 3.5, and 3, respectively.
4

ATOMIC BONDING
Chemical bonding between atoms occurs since there is a net decrease in the
potential energy of atoms in the bonded state. That is, atoms in the bonded
state are in a more stable energy condition than when they are unbonded.
What types of bonds are there?
Four important atomic bonds in engineering materials
• Ionic Bond
• Covalent Bond Primary (chemical) bonds. Strong.
• Metallic Bond
•
•Van der Waals Bond Secondary (physical) bond. Weak.
5

BOND ENERGY
• Bond energy is the energy required to create or break the bond.
• Materials having a high bond energy (It means that a greater force is required to stretch
the bond) also have
 a high strength
 a high melting temperature (Tm)
 high modulus of elasticity (E); (i.e. the slope of the stress‐strain curve in the elastic
region)
 low coefficient of thermal expansion (α)
6

Attractive, repulsive, and net potential energies as a function of interatomic separation
for two atoms are shown below. The net curve, which is again the sum of the other two,
has a potential energy trough or well around its minimum.
The bonding energy for these two atoms, Eo , corresponds to the energy at this
minimum point; it represents the energy that would be required to separate these two
atoms to an infinite separation.
The magnitude of this bonding energy and the shape of the energy-versus-interatomic
separation curve vary from material to material, and they both depend on the type of
atomic bonding.
Attractive energy
Net energy
Repulsive energy
Interatomic separation r
interatomic
spacing, 2r
7

Ionic Bonding
• Ionic bonds can form between highly electropositive (metallic) elements and highly
electronegative (non‐metallic) elements.(Dissimilar electronegativities)
• In the ionization process, electrons are transferred from atoms of electropositive elements to
atoms of electronegative elements. Atoms of a metallic element easily give up their valence
electrons to the nonmetallic atoms.
• For example, attraction between sodium (metal) and chlorine (non‐metal) ions produces
sodium chloride, or table salt, NaCl.
3s1 3p5
Electropositive element (unstable)
atomic radius of Na, rNa =0.192 nm
Electronegative element (unstable)
rCl =0.099 nm
Z=17,
1s22s22p63s23p5
Z=11,
1s22s22p63s1
Valence electron transfer
(Metal) (Non-metal)
11

• Both atoms now have filled (or emptied)
outer energy levels but both have
acquired an electrical charge and behave
as ions.
• The atom that gives up the electrons is
a cation (with positive charge), while the
atom that accepts the electrons is an
anion (with negative charge).
• The oppositely charged ions are then
attracted to each another and produce the
ionic bond.
rNa+ =0.095 nm
(stable)
rCl- =0.181 nm
(stable)
ionic bond, (NaCl)
12

The Covalent Bonding
• Covalent bonding takes place between atoms with small differences in
electronegativity and which are close to each other in the periodic table.(similar
electronegativities)
• Covalently bonded atoms share their electrons with other atoms (two or more).
13

PROPERTIES OF COVALENT BONDING
•Covalently bonded materials have poor electrical and thermal conductivity. Because the valance
electrons are locked in bonds between atoms.
•Diamond (C), silicon carbide (SiC), silicon nitride (Si3N4), and boron nitride (BN) all exhibit
covalency. Their bonding energies may be weak as in bismuth or strong as in diamond; melting
temperatures range from 270 C for bismuth to 3550 C for diamond.
• Many nonmetallic elemental molecules (H2, Cl2, F2, etc.), as well as molecules containing
dissimilar atoms, such as CH4, H2O, HNO3 and HF are covalently bonded.
• Polymeric materials (e.g. CH4) typify covalent bond. The basic molecular structure being a long
chain of carbon atoms that are covalently bonded together with two or their available four bonds
per atom.
14

Metallic Bonding
• Metallic bonding occurs in metals and their alloys.
• Metallic atoms have three or less valence electrons.
• Metallic materials give up their valence electrons to form a "sea of electrons “ (or an electron cloud)
surrounding the atoms. The valence electrons are weakly bonded to the positive‐ion cores and readily
move in the metal crystal.
• Aluminum (z=13), for example, gives up its three
valence electrons, leaving behind a core consisting of
the nucleus and inner electrons.
• Since three negatively charged electrons are missing
from this core, the core becomes an ion with a
positive charge of three.
• The valence electrons, which are no longer
associated with any particular atom, move freely
within the electron sea and become associated with
several atom cores.
• The positively charged atom cores are held together
by mutual attraction to the electron, thus producing
the strong metallic bond. These free electrons act as
a “glue” to hold the ion cores together.
valence electrons=3
positively charged
ion core
electron cloud
(free electrons)
Metallic bond
15

PROPERTIES OF METALLIC BONDING
•
•Their bonding energies may be weak or strong;
energies range from 68 kJ/mol for mercury to
850 kJ/mol for tungsten.
•Melting temperatures of metals range from
• -39 oC to 3410 oC.
• Metallic bond also allows metals to be good
electrical and thermal conductors. Under the
influence of an applied voltage, the valence
electrons move, causing a current to flow if the
circuit is complete. Other bonding mechanisms
require much higher voltages to free the electrons
from the bond.
Valence electrons move
16

Van der Waals Bonding
• Van der Waals bonds join molecules or groups of atoms by weak electrostatic attractions.
• Van der Waals bonding is somewhat similar to ionic bonding, that is, the attraction of opposite
charges. The key difference is that no electrons are transferred.
• Many plastics, ceramics, water, and other molecules are polarized; that is, some portions of the
molecule tend to be positively charged, while other portions are negatively charged.
• The electrostatic attraction between the positively charged regions of one molecule and the
negatively charged regions of a second molecule weakly bind the two molecules together.
In water, the resulting charge
difference permits the molecule to
be weakly bonded to other water
molecules.
Heating water to the boiling
point breaks the van der Waals
bonds and changes water to
steam, but much higher
temperatures are required to
break the covalent bonds
joining oxygen and hydrogen
atoms.
Covalent bond
(primary bond, strong)
Van der Waals bond
(secondary bond, weak)
Water molecule Water molecule
17

The Structure
of
Crystalline Solids
1
Text Book: Chapter 3 / Pages 44-89

Learning Objectives
1. Describe the difference in atomic structure between crystalline and noncrystalline
materials.
2. Draw unit cells for face-centered cubic, bodycentered cubic, and hexagonal
close-packed crystal structures.
3. Derive the relationships between unit cell edge length and atomic radius for face-
centered cubic and body-centered cubic crystal structures.
4. Compute the densities for metals having facecentered cubic and body-centered
cubic crystal structures given their unit cell dimensions.
5. Given three direction index integers, sketch the direction corresponding to these
indices within a unit cell.
6. Specify the Miller indices for a plane that has been drawn within a unit cell.
7. Describe how face-centered cubic and hexagonal close-packed crystal structures
may be generated by the stacking of close-packed planes of atoms.
8. Distinguish between single crystals and polycrystalline materials.
9. Define isotropy and anisotropy with respect to material properties.
2

Levels of atomic
arrangements in materials:
(a) Inert monoatomic gases
have no regular ordering of
atoms:
(b,c) Some materials,
including water vapor,
nitrogen gas, amorphous
silicon and silicate glass
have short-range order.
(d) Metals, alloys, many
ceramics and some
polymers have regular
ordering of atoms/ions that
extends through the
material.

Fundamental Concepts
Solid materials may be
classified according to the
regularity with which atoms or
ions are arranged with respect
to one another.
A crystalline material is one in
which the atoms are situated in
a repeating or periodic array
over large atomic distances;
that is, long‐range order exists,
such that upon solidification,
the atoms will position
themselves in a repetitive
three‐dimensional pattern, in
which each atom is bonded to
its nearest‐neighbor atoms.
4

Fundamental Concepts, cont.
• Metals, many ceramics, and some polymers have a crystalline structure in
which the atoms are arranged in a regular repeating structure.
• The atoms form a lattice in the crystalline structure.
• The unit cell is a subdivision of the lattice that still retains the
characteristics of the lattice. In other words, unit cells are small group of
atoms which form a repetitive pattern.
5
A unit cell defines the crystal structure
by virtue of its geometry and the atom
positions within. By stacking unit cells,
the entire lattice can be constructed.

Bravais Lattices
• Although there are 7
crystal system, there are
14 distinct
arrangements of
lattice points. These
unique arrangements of
lattice points are known
as the Bravais lattices.
• Lattice points are
located at the corners,
the faces, the center or
the base of the unit
cells. In the
orthorhombic system all
four types are
represented.
The length of unit cell
edges and the angles
between
cyristallographic axes
are referred to as lattice
parameters.
There are 7 unique arrangements, known as crystal systems, which fill in a 3-dimensional
space.
6

Metallic Crystal Structures
Most elemental metals (about 90%) have three densely packed crystal structures:
 Face‐centered cubic (FCC), (e.g. Cu, Al, Ag, Au, Pb, Ni, etc.)
 Body‐centered cubic (BCC), (e.g. Cr, Fe, W, V, Mo, etc.)
 Hexagonal close packed (HCP) (e.g. Zn, Mg, Co, Ti, Cd, etc.)
• Most metals crystallize in these dense‐packed structures because energy is released
as the atoms come closer together and bond more tightly with each other. Thus, the
densely packed structures are in lower and more stable energy arrangements.
• HCP is denser version of simple hexagonal crystal structure.
BCC Structure FCC Structure HCP Structure
8

• (a) a hard sphere unit cell representation, (b) a reduced‐sphere unit cell,
and (c) an aggregate of many atoms.
9
BCC
FCC
Unit cell

Close‐Packed Hexagonal
• Figure 3.3 For the hexagonal close‐packed crystal structure, (a) a reduced‐sphere unit cell (a
and c represent the short and long edge lengths, respectively), and (b) an aggregate of many
atoms.
10

Coordination Number
• Coordination number is the number of atoms touching particular atoms, or the number of
nearest neighbors. This is one indication of how tightly atoms are packed together.
• Each atom in the SC lattice has a coordination number of 6.
• Each atom in the BCC lattice has a coordination number of 8.
• Each atom in the FCC lattice has a coordination number of 12.
• Each atom in the HCP lattice has a coordination number of 12.
llustration of coordinations in (a) SC and (b) BCC unit cells. 6 atoms touch each atom in SC, while
the 8 atoms touch each atom in the BCC unit cell.
11

Models for BCC
• Body‐centered cubic (BCC)
In BCC structures, atoms are located at all eight corners and a single atom at the cube
center. Atoms touch along the body diagonal.
Unit cell edge length for
BCC structure

Models for FCC
• Face‐centered cubic(FCC)
In FCC structures, atoms are located at each of the corners and
centers of all the cube face . Atoms touch along the face
diagonal of the cube. There are four atomic radii along this
length—two radii from the face‐centered atom and one radius
from each corner.
13
Unit cell edge length for
FCC structure

Atomic packing Factor, APF
The APF is the sum of the sphere volumes of all atoms within a a unit cell
(assuming the atomic hard‐sphere model) divided by the unit cell volume ‐
that is,
1. How we can determine the volume of atoms in a unit cell?
2. How we can determine total unit cell volume?
14

• There is one‐half atom (i.e., one atom shared between two unit cells) in the center of each unit
cell face and one‐eighth atom each unit cell corner, for a total of 4 atoms In each FCC unit.
Number of atoms in BCC and FCC unit cells
• One complete atom is located at the center of the unit cell, and one-eighth of a sphere
is located at each corner of the cell. Thus there is a total of 2 atoms per BCC unit.
cell
unit
atom
2
cell
unit
center
1
center
atom
1
cell
unit
corners
8
corner
atom
8
1






























BCC
cell
unit
atom
4
cell
unit
face
6
face
atom
2
1
cell
unit
corners
8
corner
atom
8
1






























FCC
16

Number of Atoms per Unit Cell in Cubic Crystal Systems
In the SC unit cell
cell
unit
atom
1
cell
unit
corners
8
corner
atom
8
1
or
cell
unit
int
po
lattice
1
cell
unit
corners
8
corner
int
po
lattice
8
1






























A lattice point at a corner of one unit
cell is shared by seven adjacent unit
cell (thus a total of 8 cells); only one-
eighth of each corner belongs to
one particular cell.
17

Relationship between atomic radius and lattice parameter in Cubic Crystal
Systems
In the SC unit cell: Atoms touch along the edge of the cube, so:
r
2
a0 
3
r
4
a0 
BCC
SC
In the BCC: Atoms touch along the body diagonal, which
is in length. There are two atomic radii from the center
atom and one atomic radius from each of the corner atoms
on the body diagonal.
o
a
3
In the FCC: Atoms touch along the face diagonal of the
cube, which is in length. There are four atomic radii
along this length-two radii from the face-centered atom
and one radius from each corner.
o
a
2
FCC
2
r
4
a0 
18

Hexagonal‐close packed (HCP) structure
Aggregate of many
atoms for HCP
Schematic representation of HCP. a
and c represent the short and long
edge lengths, respectively.
• Metals do not crystallize into the simple hexagonal
crystal structure, because atomic packing factor is
too low. The atoms can attain a lower energy and a
more stable condition by forming the HCP structure.
So, HCP is a special form of the hexagonal
structure.
• In the hexagonal close packed structure, the top
and bottom faces in the unit cell consist of six corner
atoms and a single center atom. Another plane
(midplane) which is situated between the top and
bottom planes consist of three atoms. In other
words, the six atoms is contained in each unit
cell; one-sixth of each of the 12 top and bottom face
corner atoms, one-half of each of the 2 center face
atoms, and all 3 midplane interior atoms.
cell
unit
atom
6
cell
unit
face
center
2
face
center
atom
2
1
cell
unit
center
1
center
atom
3
cell
unit
corners
12
corner
atom
6
1















































19

20
cell
unit
atom
6
cell
unit
face
center
2
face
center
atom
2
1
cell
unit
center
1
center
atom
3
cell
unit
corners
12
corner
atom
6
1
















































The APF
 0,74 in the FCC
0,74 in the HCP
0,68 in the BCC
0,52 in the SC
No common engineering metals have the SC structure.
22

Density of BCC Iron
Example:
Determine the density of BCC iron, which has a lattice parameter of 0.2866 nm.
SOLUTION
• Atoms/cell = 2, a=0.2866 nm = 2.866  10‐8 cm
• Atomic mass = 55.847 g/mol
• Volume of unit cell = (2.866  10‐8 cm)3 = 23.54  10‐24 cm3/cell
• Avogadro’s number NA = 6.02  1023 atoms/mol
3
23
24
cm
/
g
882
.
7
)
10
02
.
6
)(
10
54
.
23
(
)
847
.
55
)(
2
(






25

Atomic density
Example: Niobium has an atomic radius of 0.1430 nm and a density of 8.57 g/cm3.
Determine whether it has an FCC or BCC crystal structure. Atomic weight of Nb is 92.91 g/mol.
Solution:
26

Allotropy (polymorphism)
• Many elements or compounds
exist in more than one crystalline
form under different conditions
of tempereture and pressure.
This phenomenon is termed
polymorphism or allotropy.
• Iron exist in both BCC and FCC
crystal structures over the
temperature range from room
temperature to its melting point
1538°C.
• Alpha (α) iron exist from ‐273 to
912°C and has the BCC crystal
structure.
BCC
FCC
BCC
1539ºC
1400ºC
910ºC
-Fe
-Fe
-Fe
liquid iron system
27

Crystallographic Points, Directions, and Planes
When dealing with crystalline materials, it often becomes necessary to specify a particular
point within a unit cell, crystallographic direction or plane of atoms.
Crystallographic Points (Atom Positions in Cubic Unit Cell)
To locate atom positions in cubic unit cell; x, y, and z axes are used.
 y axis is the direction to the right.
 x axis is the direction coming out of the paper.
 z axis is the direction towards top.
 Negative directions are to the opposite of positive directions.
 Atom positions are located using unit distances along the axes.
x, y, and z axes for locating atom
positions in cubic unit cell
Atom positions in BCC unit cell
29

• Directions are especially important for metals and alloys. Because the properties vary
with crystallographic orientation.
• In cubic crystals, Direction Indices are vector components of directions resolved along
each axes, resolved to smallest integers.
• To indicate a direction in a cubic unit cell, a direction vector is drawn from an origin, which
is usually a corner of the cubic cell, until it emerges from the cube surface.
• The direction indices are enclosed by square brackets with no separating commas.
• The letters u, v, w are used in a general sense for the direction indices in the x, y, and z
directions, respectively, and are written as [uvw]
• All parallel direction vectors have the same direction indices.
Directions in Cubic Unit Cell
33

Some directions in cubic unit cell
37

Example: Draw the following direction vector in cubic unit cell
     
1
2
3
)
c
10
1
)
b
112
)
a
Solution
38

Crystallographic Planes
•Miller Indices:
• The orientations of planes for a crystal structure are represented in a similar manner.
• Again, the unit cell is the basis, with the three‐axis coordinate system as represented in
Figure.
• In all but the hexagonal crystal system, crystallographic planes are specified by three
Miller indices as (hkl).
• Any two planes parallel to each other are equivalent and have identical indices.
40

The procedure employed in determination of the h, k, and l index numbers is
as follows:
41

 
0
12
5
Example: Determine the Miller
indices of the cubic crystallographic
plane shown in the figure.
Solution:
1) Transpose the plane parallel to the z axis ¼
unit to the right along the y axis as shown in
the figure so that the plane intersects the x
axis at a unit distance from the new origin
located at the lower‐right back corner of the
cube.
2) The new intercepts of the transposed plane
with the coordinate axes are now (+1, ‐5/12,
∞).
3) Take the reciprocals of these
intercepts to give (1, -12/5, 0).
4) Clear the 12/5 fraction to obtain
for the Miller indices of this plane.
43

Reversing the directions of all indices specifies another plane parallel to, on the
opposite side of and equidistant from, the origin. Several low‐index planes are below.
50

A family of planes contains all those planes that are crystallographically equivalent‐that is, having
the same atomic packing; and a family is designated indices that are enclosed in braces‐such as .
.
Example:
 
100
Family of Planes {hkl}
                 
1
1
1
,
1
1
1
,
1
11
,
1
11
,
11
1
,
11
1
,
111
,
111
:
111
Also, in the cubic system only, planes having the same indices, irrespective of
order and sign, are equivalent.
      .
123
2
1
3
3
2
1
, family
the
to
belong
and
both
example
For
51

In cubic crystals,
[111] direction is perpendicular to (111) plane
52
One interesting and unique characteristic of cubic crystals is that
planes and directions having the same indices are perpendicular to one
another;
however, for other crystal systems there are no simple geometrical
relationships between planes and directions having the same indices.

Linear and planar densities
Linear and planar densities are important considerations relative to the
process of slip-that is, the mechanism by which metals plastically deform.
Slip occurs on the most densely packed crystallographic planes and, in
those planes, along directions having the greatest atomic packing.
53

Linear and planar densities
Planar Density (PD) =
Example
55

Close Packed Crystal Structures
• Both face‐centered cubic (FCC) and hexagonal close‐
packed (HCP) crystal structures have atomic packing
factors of 0.74 which is the most efficient packing of
atoms.
A
B
C c
a
A sites
B sites
A sites
FCC HCP
56

• Coordination number= 12
• ABAB... Stacking Sequence
• APF = 0.74
• 3D Projection
• 2D Projection
Hexagonal Close‐Packed Structure (HCP)
6 atoms/unit cell
ex: Cd, Mg, Ti, Zn
c
a
A sites
B sites
A sites
Bottom layer
Middle layer
Top layer
For HCP, the stacking sequence is
termed AB. This stacking sequence,
ABABAB......, is repeated over and over.
57

Study questions
1. (a) Drive linear density expressions for FCC [100] and [111] directions in terms of the atomic radius R.
(b) Compute and compare linear density values for these same two directions for copper. RCu=0.128 nm
2. (a) Drive linear density expressions for BCC [110] and [111] directions in terms of the atomic radius R.
(b) Compute and compare linear density values for these same two directions for iron. RFe=0.124 nm
3. (a) Drive planar density expressions for FCC (100) and (111) planes in terms of the atomic radius R.
(b) Compute and compare planar density values for these same two planes for aluminum. RAl=0.143 nm
4. (a) Drive planar density expressions for BCC (100) and (110) planes in terms of the atomic radius R.
(b) Compute and compare planar density values for these same two planes for molybdenum. RMo=0.136 nm
58

Crystalline and Noncrystalline Materials
Single crystal
• The repeated arrengement of atoms
is perfect in single crystal.
• All unit cell have the same
orientation in single crystal.
• They are anisotropic; properties are
dependent of direction.
• Single crystals exist in nature, but
they also be produced artificially.
But, they are ordinarily difficult to
grow.
Polycrystalline materials
• They are composed of collection of
many small grains.
• There exist some atomic mismatch
within the region where two grains
meet; these area called grain
boundary.
• They are anisotropic or isotropic
(properties are independent of
direction). Even though each grain
may be anisotropic, a specimen
composed of the grain aggregate
behaves isotropically.
• Most of the engineering materials
are polycrystalline.
59

Solidification of a crystalline material
60

ANISOTROPY
 The physical properties of single crystals of some substances depend on the crystallographic direction in
which measurements are taken. For example, the elastic modulus, the electrical conductivity, and the index
of refraction may have different values in the [100] and [111] directions.
 This directionality of properties is termed anisotropy, and it is associated with the variance of atomic or
ionic spacing with crystallographic direction.
 Substances in which measured properties are independent of the direction of measurement are isotropic.
61

• Is any metal isotropic or not? Why?
62

Imperfections in Solids
117
self-
interstitial
distortion
of planes

IMPERFECTIONS IN CRYSTAL STRUCTURES
In reality, crystals are never perfect and contain various types of imperfections and defects
that affect many of their physical and mechanical properties, which in turn affect many
important engineering properties of materials such as the cold formability of alloys, the
corrosion of metals, the electric conductivity of semiconductors.
Crystal lattice imperfections are classified according to their dimension:
1)Zero-dimensional (0D) or point defects: atoms missing (vacancies) or in irregular
places in the lattice (interstitials, impurities).
2)One-dimensional (1D) or linear defects (dislocations): groups of atoms in irregular
positions (e.g. screw and edge dislocations).
3)Two-dimensional (2D) or planar defects: interfaces between homogeneous
regions of the material (grain boundaries, external surfaces, stacking faults, twin
boundaries).
4)Three-dimensional (3D) or volume defects: extended defects (pores, cracks).
118

POINT DEFECTS: VACANCY
• Vacancy is the simplest point defect. A vacancy is produced when an atom or
an ion, is missing from its normal atomic site in the crystal structure.
• Vacancies may be produced during solidification and growth of crystals.
Additional vacancies can be introduced in or treatment (plastic deformation,
quenching (rapid cooling), etc.)
• All crystalline solids have vacancy defects. In metals the equilibrium
concentration of vacancies rarely exceeds about 1 in 10,000 atoms.
• Vacancies can move by exchanging positions with their neighbours. Atomic
diffusion in crystalline solids is extremely difficult without the point
defects.
Vacancy
distortion
of planes
119

POINT DEFECTS: Self interstitial
self-
interstitial
distortion
of planes
• Interstitial defects are also point defects. An interstitial defect is formed
when an extra atom or ion is inserted into the crystal structure at a
normally unoccupied position. This extra atom may be same type of atom
(self interstitial) or an impurity atom (impurity interstitial).
• It exists in very small concentration which is lower than vacancies.
• A self-interstitial defect introduces large distortions in the surrounding
lattice, because the atom is larger than the interstitial position.
120

Impurities in Solids
 A pure metal consisting of only one type of atom just is not possible !!!
 Impurity or foreign atoms will always be present. It is difficult to refine metals to a purity in
excess of 99.9999%. At this level, ~1023 atoms/m3 impurity atoms will be present in a material.
 A few metals are used in the nearly pure form. For example, high-purity copper of 99.99%
purity is used for electronic wires because of its high conductivity.
 Impurity atoms in ALLOYS are added intentionally to improve properties of materials such as
mechanical strength and corrosion resistance. For example, carbon in small amounts in iron
makes steel. Steel is stronger than pure iron. Sterling silver is a 92.5% silver – 7.5% copper alloy.
Pure silver has highly corrosion resistance, but sterling silver is stronger than pure silver.
Impurity point defects are found in solid
solutions, of which there are two types:
1) Substitutional: For the substitutional
type, solute or impurity atoms replace
or substitute for the host atoms.
2) Interstitial: For interstitial solid
solutions, impurity atoms fill the voids
or interstices among the host atoms.
121

LINEAR DEFECTS: DISLOCATIONS
Dislocation: Defects that cause lattice distortion centered around a line in crystalline
solids. Dislocations are line defects. Dislocations affect mechanical properties.
They are introduced typically into the crystal during soldification of the material or when the
material is deformed permanently. Plastic deformation of crystalline solids is difficult
without dislocations.
Two main types of dislocations: edge and screw dislocations. A combination of the two
gives mixed dislocations.
EDGE DISLOCATION
 Edge dislocation is created in a crystal by the
insertion of an extra half plane of atoms.
 Within the region around the dislocation line
there is some lattice distortion (atoms bend
around this extra half-plane).
 An edge dislocation represented by the
inverted “tee”, , which also indicates position of
the dislocation line (positive edge dislocation).
 An edge dislocation may also be formed by an
extra half-plane of atoms that is included in the
bottom portion of the crystal; its designation is a
T (negative edge dislocation). 124

BURGERS VECTOR
• Burgers vector (b): magnitude and direction of the lattice distortion associated with a dislocation.
• “b” shows displacement of atoms (slip) and “b” is perpendicular to dislocation line for edge dislocation.
• Magnitude of b is equal to the interatomic spacing.
• Make a circuit around dislocation: go from atom to atom counting the same number of atomic
distances in both directions.
The perfect crystal in (a) is cut and an extra plane of atoms is inserted (b). The bottom
edge of the extra plane is an edge dislocation (c). A Burgers vector b is required to close
a loop of equal atom spacings around the edge dislocation.
dislocation
line
125
Edge-Dislocations-Texas-AM-Intro-to-Materials-MSEN-201.mp4

The process by which a dislocation moves and causes a metallic material to deform is called slip.
Dislocation motion requires the successive bumping of a half plane of atoms (from left to right here).
Bonds across the slipping planes are broken and remade in succession.
Motion of Edge Dislocation
When a shear stress is applied to the dislocation in (a), the atoms are
displaced, causing the dislocation to move one Burgers vector in the slip
direction (b). Continued movement of the dislocation eventually creates a step
(c), and the crystal is deformed. (d) Motion of caterpillar is analogous (smilar
in structure) to the motion of a dislocation.
126
puntos-de-dislocacion.mp4

SCREW DISLOCATION
 Screw dislocation is created due to shear stresses applied to regions of a perfect
crystal separated by a cutting plane.
 Distortion of lattice in form of a spiral or helical ramp.
 Burgers vector is parallel to dislocation line in a screw dislocation.
Formation of a screw dislocation: A
perfect crystal is sliced by a cutting
plane, and up and down shear stresses
are applied parallel to the cutting plane
to form the screw dislocation.
Apply shear
Motion of a screw dislocation 127

MIXED DISLOCATION
Most dislocations in crystalline materials are neither pure edge nor pure screw, but
exhibit components of both types: mixed dislocation.
The lattice distortion that is produced away from the two faces is mixed.
Dislocations (dark lines) in a
transmission electron micrograph
Burgers vector is neither
perpendicular nor parallel.
128

INTERFACIAL DEFECTS (2-dimensional defects)
Interfacial defects are boundaries that have two dimensions and normally
separate regions of the materials that have different crystal structures
and/or crystallographic orientations.
INTERFACIAL DEFECTS
 External Surfaces
 Grain Boundaries
 Twin Boundaries
 Stacking Faults
129

EXTERNAL SURFACES
 Surface atoms are not bonded to the maximum number of the
nearest neighbors.
 Surface atoms are in higher energy state than the atoms at interior
positions.
 The bonds of these surface atoms that are not satisfied give rise to
(cause) a surface energy, expressed in units of energy per unit area.
Surface energy:  (J/m2).
 To reduce this energy, materials tend to minimize the total surface
area. For example, the droplets become spherical. This is not possible
with solids, which are mechanically rigid.
130

GRAIN BOUNDARIES
 Grain boundaries separate two grains or crystals
having different crystallographic orientation.
 Grain boundaries are transition region between the
neighboring crystals.
 Grain boundaries may consist of various kinds of
dislocation arrangements.
 These regions are just several atom distances wide.
High and Low Angle Grain Boundaries
 When the misorientation between two grains is
small, the grain boundary is called a small (or low)-
angle boundary. < 20°, generally <10°.
 When the misorientation between two grains is
large, the grain boundary is called a high-angle grain
boundary.
 The magnitude of grain boundary energy is larger for
high-angle grain boundaries.
 Garin boundaries are chemically more reactive
than the grains because of the grain boundary energy.
Angle of misalignment
Angle of misalignment
High-angle
grain
boundary
Small-angle
grain
boundary
131

Tilt and Twist Boundaries
 A simple small (or low)-angle grain boundary is formed when edge
dislocations are aligned. This type is called a tilt boundary.
: angle of misorientation
 When the angle of orientation is parallel to the boundary, a twist boundary
results. This boundary can be described by an array of screw dislocations.
 It is possible to produce misorientations between grains by combined tilt
and twist boundaries. In such a case, the grain boundary structure will
consist of a network of edge and screw dislocations.
 
133

TWIN BOUNDARIES
 Twin boundaries are produced from applied mechanical forces (mechanical twins)
and also during annealing heat treatment (annealing twins).
 Annealing twins are found in metals that have FCC crystal structure.
 Mechanical twins are observed in BCC and HCP metals.
 Twinning occurs on a definite plane and a specific direction.
 Similar to dislocations, twin boundaries tend to strengthen a material.
 There is a mirror lattice symmetry in a twin boundary. The atoms on one side of
the boundary are located in mirror-image positions of the atoms on the other side.
135

STACKING FAULTS
 Stacking faults are found in FCC metals when there is an interruption in the
ABCABCABC... stacking sequence.
 For example: ABCABABC (the layer C is deficient).
 Stacking faults also tend to strengthen the material.
VOLUME DEFECTS
 Volume defects include pores, cracks, foreign inclusions.
 They are normally introduced during processing and fabrication steps.
 The size of a volume defect may range from a few nanometer to centimeters or
sometimes larger.
 Such defects have a tremendous effect on the performance of the material.
136

Solid Solutions (Alloys)
 Solid solution forms when the solute atoms are added to solvent (host) material. Ability to dissolve is
called solubility.
 A solid solution forms when the crystal structure is maintained, and no new structures are formed.
 A solid solution is also homogeneous, the impurity atoms are randomly and uniformly dispersed within the solid.
 Solvent: Element that is present in the greatest amount.
 Solute: Element that is present in a minor concentration.
Substitutional solid solution Interstitial solid solution
Substitutional alloy (e.g., Cu in Ni)
The solute or impurity atoms
replace for the host atoms.
Interstitial alloy (e.g., C in Fe)
The solute or impurity atoms can enter
the interstices or holes in the solvent or
host atom lattice. Interstitial solid
solutions can form when one atom is
much larger than another.
137

Interstitial Solid Solution
 Atomic radius of interstitial impurity must be substantially smaller
than that of the solvent atoms. The atomic radius of the carbon atom
is 0.071 nm; the atomic radius of the iron atom is 0.124 nm.
 Atoms of hydrogen, carbon, nitrogen and oxygen form an
interstitial solid solution due to their small sizes.
 Maximum concentration of interstitial impurity atoms is less than
10% . Carbon forms an interstitial solid solution when added to iron;
the max. concentration of carbon is about 2%.
 Even very small impurity atoms are larger than the interstitial sites.
Thus they introduce some lattice strains on the adjacent host atoms.
138

MICROSCOPIC EXAMINATION
 In most materials, the constituent grains are of microscopic
dimensions, having diameters that may be on the order of microns,
and their details must be investigated using some type of microscope.
 Several microscopic techniques are used in microscopy: optical
microscopy (light microscope) and electron microscopy (SEM-
Scanning Electron Microscope and TEM-Transmission Electron
Microscope).
 Material scientists and engineers use this microscopes to understand
the behavior of materials based on their microstructures, existing
defects, microconstituents, and other characteristics specific to
the internal structure.
139

Optical Microscopy
Optical metallography techniques are used
to study the features and internal makeup
of materials at the micrometer level
(magnification level of around 2000x)
For optical microscopy examinations,
some surface preparations (metallographic
techniques) are necessary.
These techniques include grinding and
polishing (using abrasive paper and
powders) and etching (using an chemical
reagent).
 The specimen surface must first be
ground and polished to a smooth and
mirrorlike finish.
The chemical reactivity of the grains of
polycrystalline materials depends on
crystallographic orientation. Thus, etching
characteristics vary from grain to grain.
Three polished and etched grains reflect
the light at angle different because of
differences in crystallographic orientation.
Microscope
Polished and
etched surface
140

 Since atoms along grain boundary
regions are more chemically active,
they dissolve at a greater rate than
those within the grains.
 These grooves (boşluklar) reflect
light at an angle different.
 Thus grain boundary grooves are
visible as a dark lines.
Optical Microscopy
141

 Some structural elements are too small to permit observation using
optical microscopy. Under such circumstances the electron
microscope may be employed.
 An image of the structure under investigation is formed using beams
of electrons instead of light radiation.
 SEM (Scanning Electron Microscope): The surface of the
specimen is scanned with an electron beam, and the reflected (back-
scattered) beam of electrons are collected, then displayed at the
screen. Specimen for SEM must be electrically conductive.
 TEM (Transmission Electron Microscope): Image is formed by an
electron beam that passes through the specimen. Details of
microstructure can be observed by TEM. For example; dislocations.
Specimen for TEM is in the form of a very thin foil.
Electron Microscopes
143

GRAIN SIZE
 Affects the mechanical properties of the material.
 The smaller the grain size, the more are the grain boundaries.
 More grain boundaries means higher resistance to slip (plastic
deformation occurs due to slip).
 More grains means more uniform the mechanical properties are.
 Grain structure is usually specified by giving the average
diameter.
 Grain size can be measured by two methods: Intercept method
and ASTM method.
144

Intercept Method
 This is very easy and may be the preferred method for measuring grain size.
 Straight lines all the same length are drawn through several photomicrographs that show
the grain structure.
 The grain intersected by each line segment are counted; the line length is then divided by
an average of the number of grains intersected, taken over all the line segments.
 The average grain diameter is found by dividing this result by the linear magnification of
the photomicrographs.
 Several lines should be drawn to obtain a statistically significant result.
m
5
.
21
)
m
3
.
14
(
2
3
D
m
3
.
14
1000
7
10
10
l
4










Examle: L=10 cm, Nl= 7, M=1000x, D=?
M
N
L
l
l


l
D


2
3 L= length of the line, Nl= number of grain-boundary intercepts
M= magnification, = mean lineal intercept, D= grain size
l

145

ASTM Method
• This method of measuring grain size is common in engineering applications.
1
n
2
N 

N: number of grains per square inch (at 100×)
n: ASTM grain size number
 n < 3 Coarse grained
 4 < n < 6 Medium grained
 7 < n < 9 Fine grained
 n > 10 Ultrafine grained
146

Example:
1
n
2
N 

Solution: a)
b) At magnifications other than 100x, use of the following modified form of
is necessary:
1
n
2
N 

1
n
2
M 2
100
M
N 







NM: number of grains per square inch
at magnification M.
147

Example: Determine the ASTM grain size number if 30 grains per square inch are
measured at a magnification of 250.
1
n
2
M 2
100
M
N 







Solution:
148

Diffusion
147

Diffusion
 Diffusion is the movement of atoms within a material.
 Diffusion takes place in gases, liquids and solids.
 For metallic materials, diffusion is a solid state process.
 Most metallic materials contain large numbers of a lattice
defects. These defects accelerate diffusion.
148

Interdiffusion In an alloy, atoms tend to migrate from regions of high
concentration to regions of low concentration.
(a) A copper–nickel diffusion couple after a high-
temperature heat treatment, showing the alloyed
diffusion zone.
(b) Schematic representations of Cu (red circles) and
Ni (blue circles) atom locations within the couple.
(c) Concentrations of copper and nickel as a function
of position across the couple.
151

A
B
C
D
A
B
C
D
Label some atoms
After some time
152
Selfdiffusion Diffusion also occurs for pure metals, but all atoms
exchanging positions are of the same type.

Diffusion Mechanisms
153
Diffusion is the migration of atoms from lattice site to lattice
site.
The ability of atoms to move is related to
an empty adjacent site
sufficient energy of atoms to break bonds

Diffusion Mechanisms
154
1- Vacancy diffusion
• The number of vacancies increases
as the temperature increases.

Diffusion Mechanisms, cont.,
155
2- Interstitial diffusion
•Because there are more interstitial sites than vacancies, interstitial
diffusion is more rapid than vacany diffusion.

Rate of Diffusion
• The rate of diffusion in a material can be measured by the diffusion flux (J).
• J is the number of atoms passing through a plane of unit area per unit time.
• The units for J are kilograms or atoms per meter squared per second (kg/m2-s
or atom/m2-s)
157

Steady-State Diffusion (Fick’s First Law)
If the diffusion flux does not change
with time, steady state diffusion exist.
FICK’S FIRST LAW- diffusion flux for steady-state diffusion (in one direction):
158

One practical example of steady-state diffusion is found in the purification of
hydrogen gas. One side of a thin sheet of palladium metal is exposed to the impure
gas composed of hydrogen and other gaseous species such as nitrogen, oxygen,
and water vapor. The hydrogen selectively diffuses through the sheet to the
opposite side, which is maintained at a constant and lower hydrogen pressure.
159
(a)Steady-state diffusion across a thin plate.
(b) A linear concentration profile for the diffusion situation in (a).

Nonsteady-State Diffusion (Fick’s Second Law)
• Most practical diffusions are nonsteady-state (or dynamic) ones.
• If the diffusion flux changes with time, nonsteady-state diffusion exist.
161

Activation Energy for Diffusion
A diffusion atom must past
the surrounding atoms to
reach its new site. In order
to move to a new location,
the atom must overcome
an energy barrier. The
energy barrier is the
activation energy (Q).
Heat supplies the atom
with the energy needed to
exceed this barrier.
Activation energies are
lower for interstitial
diffusion than for vacany
diffusion.
A low activation energy
indicates easy difusion.
169

Dependence of the diffusion coefficient on temperature
171

Problems
Q1. At 300°C the diffusion coefficient and activation energy for Cu and in Si
are 7.8×10-11 m2/s and 41.5 kJ/mol. What is the diffusion coefficient at 350°C?
Solution:
176

Problems
Q2. An FCC Fe-C alloy initially containing 0.2 wt% C is carburized at on elevated temperature
and in an atmosphere that gives a surface carbon concentration constant at 1 wt%. If after 49.5 h
the concentration of carbon is 0.35 wt% at a position 4 mm below the surface, determine the
temperature at which the treatment was carried out. (D0= 2.3×10-5 m2/s, Q=148 kJ/mol)
Solution:
z erf(z)
0.90 0.7970
z 0.8125
0.95 0.8209
177

Problems
Q3. Carbon from a gaseous phase, with 1.2% carbon, is to be diffused into a steel piece with a
uniform carbon concentration 0.1% at 950°C. The diffusion coefficient of carbon in iron at 950°C
is 1.6×10-11 m2/s. What will be the carbon concentration at 0.4 mm below the surface after 10
hours.
Solution:
z erf(z)
0.25 0.2763
0.26 erf(z)
0.30 0.3286
179

PHASE DIAGRAMS
Textbook / Chapter 9 / pp 281-341
255

Definitions and Basic Concepts
 Solubility: The amount of one material that will completely dissolve in a second
material without creating a second phase.
A solid solution consists of atoms of at least two different types; the solute
atoms occupy either substitutional or interstitial positions in the solvent lattice.
250

 For example, the sugar-water syrup solution is
one phase, and solid sugar is another. Each has
different physical (liquid and solid) and chemical
(pure and solution) properties.
 Water and ice in a container are two phases,
physically dissimilar but chemically the same.
 FCC and BCC structures are separate phases for
a polymorphic material. The three forms of water
– gas, liquid, and solid –
are each a phase.
250
 Phase: A portion of a system that has uniform physical and chemical
characteristics. Every pure material is considered to be a phase; so also is
every solid, liquid, gaseous solution.

Solubility
 Unlimited solubility: When the amount of
one material that will dissolve in a second
material without creating a second phase is
unlimited. Alloys that form from materials that
are completely soluble in each other, e.g. Cu
and Ni are mutually soluble in any amount
(unlimited solid solubility).
Limited solubility: When only a maximum
amount of a solute material can be dissolved
in a solvent material.
 Not all materials are completely soluble.
(e.g. alcohol has unlimited solubility in water,
carbon has a limited solubility in Fe, sugar
has a limited solubility, oil is insoluble).
258

Solubility limit
 Solubility limit: Maximum concentration of solute atoms
that may dissolve in the solvent.
 Addition of solute atoms beyond solubility limit results in
formation another solid solution or compound with different
composition.
Single-phase system: Sometimes its termed
homogeneous.
Two or more phases: They are termed mixtures or
heterogeneous systems. (e.g. water + ice, separated by a
phase boundary.)
259

260
 Equilibrium: A system is at equilibrium if its
characteristics do not change with time. This means the
system is stable. Equilibrium is achieved given sufficient
time, but that may be very long.
 Metastable (yarı dengeli): It is often the case,
especially in solid systems, that a state of equilibrium is
extremely slow; such a system is said to be in a
metastable state.
Phase diagram: Combination of temperature, pressure
or composition for which specific phases exist at
equilibrium. Phase diagrams help understand and predict
microstructures.
•

Figure 9.2 Pressure–temperature phase diagram for H2O.
Intersection of the dashed horizontal line at 1 atm pressure with the solid–liquid
phase boundary (point 2) corresponds to the melting point at this pressure (T 0C).
Similarly, point 3, the intersection with the liquid–vapor boundary, represents the
boiling point (T 100C). 261

• Another type of extremely common phase diagram is
one in which temperature and composition are
variable parameters, and pressure is held constant—
normally 1 atm.
• There are several different varieties; in the present
discussion, we will concern ourselves with binary
alloys—those that contain two components.
262

Binary Isomorphous System:
Completely Soluble Elements
•Isomorphous phase diagram: A phase diagram in which
components display unlimited solid solubility (complete solubility).
•
•The Cu-Ni system is isomorphous because of this complete liquid
and solid solubility of the two components.
263

264
Liquidus temperature: The temperature at which the first solid begins to form during
solidification.
Solidus temperature: The temperature below which all liquid has completely solidified.
Cu-Ni phase diagram
The ordinate is temperature and the
abscissa represents the composition of
the alloy in weight.
The composition ranges from 0 wt%
Ni (100 wt% Cu) on the left horizontal
extremity to 100 wt% Ni (0 wt% Cu) on
the right.
Three different phase regions appear
on the diagram, an alpha (α) field, a
liquid (L) field, and a two-phase (α+L)
field.
Phases exist over the range of
temperatures and compositions delimited
by the phase boundary lines.
 The liquid (L) is a homogeneous liquid
solution composed of both Cu and Ni.
 The α phase (FCC) is a
substitutional solid solution
consisting of both Cu and Ni atoms.

For a binary system of known composition and temperature
that is at equilibrium, at least three kinds of information
are available:
1. The phases that are present,
2. the compositions of these phases,
3. the percentages or fractions of the phases.
265

2)
In all two-phase regions (and in two-phase regions only), one may
imagine a series of horizontal lines, one at every temperature; each
of these is known as a tie line, or sometimes as an isotherm.
These tie lines extend across the two-phase region and terminate
at the phase boundary lines on either side. To compute the
equilibrium concentrations of the two phases, the following
procedure is used:
1. A tie line is constructed across the two-phase region at the
temperature of the alloy.
2. The intersections of the tie line and the phase boundaries on either
side are noted.
3. Perpendiculars are dropped from these intersections to the
horizontal composition axis, from which the composition of each of
the respective phases is read.
267

3)
If the composition and temperature position is located within a two-phase
region, things are more complex. The tie line must be utilized in conjunction
with a procedure that is often called the lever rule (or the inverse lever
rule), which is applied as follows:
1. The tie line is constructed across the two-phase region at the temperature of
the alloy.
2. The overall alloy composition is located on the tie line.
3. The fraction of one phase is computed by taking the length of tie line from
the overall alloy composition to the phase boundary for the other phase, and
dividing by the total tie line length.
4. The fraction of the other phase is determined in the same manner.
5. If phase percentages are desired, each phase fraction is multiplied by 100.
When the composition axis is scaled in weight percent, the phase fractions
computed using the lever rule are mass fractions—the mass (or weight) of a
specific phase divided by the total alloy mass (or weight). The mass of each
phase is computed from the product of each phase fraction and the total
alloy mass.
269

Equilibrium Cooling
At this point it is instructive to
examine the development of
microstructure that occurs
for isomorphous alloys
during solidification.We first
treat the situation in which
the cooling occurs very
slowly, in that phase
equilibrium is continuously
maintained.
274

Nonequilibrium Cooling
Conditions of equilibrium solidification and the development of microstructures
are realized only for extremely slow cooling rates. The reason for this is that
with changes in temperature, there must be readjustments in the
compositions of the liquid and solid phases in accordance with the phase
diagram (i.e., with the liquidus and solidus lines), as discussed.
These readjustments are accomplished by diffusional processes—Diffusion is
a time-dependent phenomenon (Section 5.3), to maintain equilibrium during
cooling, sufficient time must be allowed at each temperature for the
appropriate compositional readjustments.
Diffusion rates are especially low for the solid phase and, for both phases,
decrease with diminishing (azalan) temperature.
In virtually all practical solidification situations, cooling rates are much too rapid
to allow these compositional readjustments and maintenance of equilibrium;
consequently, microstructures other than those previously described
develop.
275

Another type of common and relatively simple phase diagram found for binary
alloys is shown in Figure 9.7 for the copper–silver system; this is known as
a binary eutectic phase diagram.
278

Binary Phase Diagram for
partially soluble elements or compounds
279
It is much more common to find that solids are partly soluble in one another rather than be
either completely soluble or completely insoluble. Cu–Ag, Pb-Sn systems display this behavior.
Three single-phase regions: α, β, and liquid. The α-phase is a solid solution rich in A
atom; it has B atom as the solute component. Otherwise in β-phase solid solution, A is the
solute.
There are also three two-phase regions found for the system: The α+L, β+L and α+β.
Solvus line: The solid
solubility limit line
separating the α (or β)
and α+β phase regions.
Eutectic point: The
lowest melting point of
any composition for the
alloy.

Eutectic Reaction
 Eutectic means easily fused. Upon cooling, liquid phase is transformed into the solid  and
solid  phases at the temperature TE (eutectic temperature). This is called a eutectic reaction..
 The phase diagram for lead–tin
(Pb-Sn) is an example of eutectic
system. In this diagram solid
solutions phases are designated
by  and .
  represents a solid solution of
tin in lead and  represents a solid
solution of lead in tin.
 The eutectic point is located at
61.9 wt% Sn + 38.1 wt% Pb
(eutectic composition) and 183C
(eutectic temperature).
 Eutectic composition freezes
at a lower temperature than all
other composition!!!
327°C
232°C
Pb-Sn phase diagram
2
On this diagram, at 0ºC, only 2% Sn can dissolve in α. As the temperature increases, more
Sn dissolves into the Pb until, at 183ºC, the solubility of Sn in Pb has increased to 19% Sn.
This is the maximum solubility of Sn in Pb.
Eutectic alloy

Pb-Sn Phase Diagram (Eutectic Phase Diagram)
289
Solidification and microstructure of a
Pb-2% Sn alloy. The alloy is a single-
phase solid solution.
 Alloys that contain 0 to 2% Sn
behave exactly like the Cu-Ni alloys; a
single-phase solid solution α forms
during solidification.
Solidification, precipitation, and microstructure
of a Pb-10% Sn alloy. Some dispersion
strengthening occurs as the β solid precipitates.
 Alloys containing between 2% and 19% Sn
also solidify to produce a single solid solution
α. However, as the alloy continues to cool, a
solid state reaction occurs, permitting a second
solid phase (β) to precipitate from the original α
phase.

Hypoeutectic and Hypereutectic Alloys
293
A hypoeutectic alloy is an alloy
whose composition will be between
that of the left-hand-side end of the
tie line defining the eutectic
reaction and eutectic composition.
In the Pb-Sn system, any
composition between 19% and
61.9% Sn is hypoeutectic.
A hypereutectic alloy is an alloy
whose composition will be between
that of the right-hand-side end of
the tie line defining the eutectic
reaction and eutectic composition.
In the Pb-Sn system, any
composition between 61.9% and
97.5% Sn is hypereutectic.

Hypoeutectic and hypereutectic microstructures
294
A hypoeutectic Pb-Sn alloy A hypereutectic Pb-Sn alloy
The dark constituent is the lead-rich solid α, the light constituent
is the Sn-rich solid β, and the fine plate structure is the eutectic.
eutectic
eutectic
β
α

IRON-CARBON
PHASE DIAGRAMS
Textbook / Chapter 9 / pp
250

251
 Eutectoid reaction is important in heat treatment of steels.
 Eutectic reaction is important for cast irons.
The Iron-Carbon Phase Diagram
100%
100%

252
Austenite (): The interstitial solid solution of carbon in  iron is called austenite. Austenite
has an FCC crystal structure and a much higher solid solubility for carbon than α-ferrite. The
solid solubility of carbon in austenite is a max. of 2.14% at 1147 C and decreases to 0.76%
at 727C. FCC has larger interstitial sites.
The solubility is about 100 times greater in austenite than in ferrite because of the larger
hole in FCC unit cells.
Solid Phases in Fe-C
Phase Diagram
α-ferrite: This phase is
an interstitial solid
solution of carbon in the
BCC iron crystal lattice.
Carbon is only slightly
soluble in a α-ferrite,
reaching a max. solid
solubility of 0.022% at
727C. The solubility of
carbon in α-ferrite
decreases to 0.005% at
0C. BCC has relatively
small interstitial sites.

253
  ferrite: The interstitial solid solution of carbon in  iron is called  ferrite. It has a BCC
crystal structure like α-ferrite but with a greater lattice constant. The max. solid solubility of
carbon in  ferrite is 0.09% at 1493C.
Cementite: The intermetallic compound Fe3C (iron carbide) is called cementite. Cementite
has a composition of 6.7% C and 93.3% Fe. Cementite is a hard and brittle compound.
Solid Phases in Fe-C Phase Diagram

254
Reactions in the Fe- C Phase Diagrams
 Eutectic reaction- At the eutectic reaction point, liquid of 4.3% C forms 
austenite of 2.14% C and the intermetallic compound Fe3C (cementite) which
contains 6. 7% C. This reaction which occurs at 1147 C.
 Eutectoid reaction- At the eutectoid reaction point, solid austenite of 0.76% C
produces α-ferrite with 0.022% C and Fe3C (cementite) which contains 6.7% C.
This reaction which occurs at 727C.
 Steel which contains 0.76% C is called a eutectoid steel..I
 If a steel contains less than 0.76% C, it is termed a hypoeutectoid steel.
 If a steel contains more than 0.76% C, it is termed a hypereutectoid steel.
 
C
%
wt
3
.
4
L    
C
%
wt
70
.
6
C
Fe
C
%
wt
14
.
2 3



255
Slow Cooling of Eutectoid Steels
 If a sample of a 0.76%
(eutectoid) steel is heated
to about 750C (at point a)
and held for a sufficient
time, its structure will
become homogeneous
austenite. This process is
called austenitizing.
 If this steel is then
cooled very slowly to just
below the eutectoid
temperature (at point b),
austenite transform to α-
ferrite and cementite
(Fe3C). This lamellar
structure is called pearlite.

256
Microstructure of Eutectoid Steel
Carbon atoms diffuse away from 0.022
wt% ferrite regions and to the 6.7 wt%
cementite layers, as the pearlite extends
from the grain boundary into the unreacted
austenite grain.
The layered pearlite forms because
carbon atoms need diffuse only minimal
distances with the formation of this structure.
Growth and structure of pearlite:
redistribution of carbon and iron
Pearlite microstructure: In the
micrograph, the dark areas are Fe3C
layers, the light phase is α-ferrite

257
Microstructure of Hypoeutectoid Steel
If a sample which has Co% C is heated to
point C, its structure will become austenite.
Then, if this steel is slowly cooled to
temperature d, proeutectoid ferrite will
nucleate and and grow at the austenitic grain
boundaries.
If this alloy is slowly cooled from temperature
d to e, the amount of proeutectoid ferrite formed
will continue. While the steel is cooling from d to
e, the carbon content of the remaining austenite
will be increased from 0.4% to 0.76%. At 727C,
the remaining austenite will transform into
pearlite by the eutectoid reaction:
   + Fe3C (Pearlite)
The  ferrite in the pearlite is called eutectoid
ferrite to distinguish it from the preeutectoid
ferrite which forms first above 727C.
 + Fe3C
α + Fe3C

α
α
Hypoeutectoid steel contains proeutectoid
ferrite formed above eutectoid temperature
and pearlite that contains eutectoid ferrite
and cementite.
proeutectoid α
pearlite

258
Microstructure of Hypereutectoid Steel
If a sample which contains C1% C is heated
to point g, its structure will become all
austenite.
Then, if this steel is cooled very slowly to
temperature h, proeutectoid cementite will
begin to nucleate and grow at the austenit
grain boundaries.
With further slow cooling to just above 727
C, the carbon content of the austenite
remaining in the alloy will change from 1.2 to
0.76%.
With still further slow cooling to temperature,
the remaining austenite will transform to
pearlite by the eutectoid reaction.
The cementite formed by the eutectoid
reaction is called eutectoid cementite to
distinguish it from the proeutectoid cementite
formed at temperature above 727C
 + Fe3C
α + Fe3C
α + 

Hypereutectoid steel contains proeutectoid cementite
(formed above eutectoid temperature) plus pearlite that
contains eutectoid ferrite and cementite.
proeutectoid Fe3C
pearlite

259
Calculation of the Relative Amounts of
Proeutectoid Phase (α) and Pearlite
 
 
 
 
022
.
0
7
.
6
C
7
.
6
X
V
U
T
X
V
U
W 0













 

 W
W
W pearlite
in
Hypoeutectoid alloy of composition C'0
Fraction of pearlite:
Fraction of proeutectoid ferrite:
Fraction of total ferrite:
Fraction of Ferrite in pearlite (eutectoid α):

260
Calculation of the Relative Amounts of
Proeutectoid Phase (Fe3C) and Pearlite
 
 
 
 
022
.
0
7
.
6
022
.
0
C
X
V
U
T
V
U
T
W 1











C
Fe
C
Fe
pearlite
in
C
Fe 3
3
3 W
W
W 


Hypereutectoid alloy of composition C'1
Fraction of pearlite:
Fraction of proeutectoid cementite:
Fraction of total cementite:
Fraction of cementite in pearlite (eutectoid Fe3C):

261
Example: Calculate the amounts of ferrite and cementite present in pearlite.
Solution: Since pearlite must contain 0.76% C, using the lever rule:
%
1
.
11
100
022
.
0
7
.
6
022
.
0
76
.
0
%
C
Fe
%
9
.
88
100
022
.
0
7
.
6
76
.
0
7
.
6
%
3 











262
Example: Solution:
(b)
39
.
0
56
.
0
95
.
0
W
W
W pearlite
in





 


(c)

The Influence of Other Alloying Elements
 Additions of other alloying elements alter iron-
carbon phase diagram.
 The extent of these alterations of the positions
of phase boundaries and the shapes of the
phase fields depends on the particular alloying
element and its concentration.
 One of the important changes is the shift in
position of the eutectoid with respect to
temperature and to carbon concentration.
263

PROBLEMS (Callister, pp. 304-310)
264
Solution:
a) The proeutectoid phase is Fe3C since 0.95 wt% C >
0.76 wt% C (the eutectoid composition).
b) Total ferrite:
Total cementite:
c)
d)

265
Solution:

266
Solution:
a)
b)
c)

PHASE TRANSFORMATION IN METALS
267
 Phase transformations are important in the processing of materials, and
usually they involve some alteration of the microstructure.
 With phase transformations, normally, at least one new phase is formed
that has different physical/chemical characteristics and/or a different structure than
the parent phase.
TTT (Time-Temperature-Transformation) Diagrams
 TTT diagrams describe the time required at any temperature for a phase
transformation to begin and end.
 Isothermal transformation diagrams
 Continuous cooling transformation diagrams
 Isothermal transformation: The amount of a transformation at a particular
temperature depends on the time (temperature is hold constant during
transformation).

268
Isothermal Transformation Diagrams: Pearlite Transformation
 Both the time and temperature
dependence of the austenite-to-pearlite
transformation is in the bottom of the Figure.
 The vertical and horizontal axes are,
respectively, temperature and logarithm of
time.
 Two solid curves are plotted.
 One represents the time required
at each temperature for the start of
the transformation.
The other is for the transformation
conclusion.
 The dashed curve corresponds to
50% of transformation completion.
 These curves were generated from a
series of plots of the percentage
transformation versus the logarithm of
time taken over a range of temperatures.
 The S-shaped curve (for 675ºC), in the
upper portion of the Figure, illustrates how
the data transfer is made.

269
 Very rapid cooling
of austenite to a
temperature is
indicated by the
near-vertical line AB.
 Isothermal
treatment at this
temperature is BCD.
 Transformation of
austenite to pearlite
begins at the
intersection, point C
after approximately
3.5 second.
 Transformation of
austenite to pearlite
finishes at point D,
after about 15 s.
Isothermal Transformation Diagrams: Pearlite Transformation

270
Microstructure of Pearlite
coarse pearlite fine pearlite
 Ferrite and cementite layer thickness in the pearlite depends on the
temperature which isothermal transformation occurs.
 Transformation at high temperatures: thick layers of ferrite and cementite
are produced. This is called coarse pearlite. At these temperatures diffusion
rates is high.
 Transformation at low temperatures: the carbon diffusion rate decreases
and the layers become thinner. This is called fine pearlite. Fine pearlite is
produced at 540 C.

271
 In addition to pearlite, other
microconstituents that are products of the
austenitic transformation exist, one of
these is called bainite.
 The microstructure of bainite
consists of ferrite and cementite
phases and is diffusional processes.
 Bainite forms as needles or plates,
depending on the temperature of the
transformation.
 All three curves have nose at point N,
where the rate of transformation is a
maximum.
 Pearlite forms above the nose (over
the temperature range of 540 to 727C.
 Bainite forms at temperatures
between 215 to 540C.
Isothermal Transformation Diagrams: Bainite Transformation
Isothermal transformation diagram for an
iron-carbon alloy of eutoctoid composition,
including austenite-to-pearlite (A-P) and
austenite-to-bainite (A-B) transformations.

272
Bainitic Microstructure
 T ~ 350-540°C, upper bainite consists of needles of ferrite separated by long
cementite particles.
 T ~ 215-350°C, lower bainite has thin plates of ferrite and fine rods of cementite.
Upper bainite Lower bainite

Martensitic transformation
 If a sample of a steel in the austenitic condition is rapidly cooled (or quenched) to
room temperature its structure will be changed from austenite to martensite.
 Austenite-to-martensite is diffusionless and fast. Since martensite is a metastable
phase, it does not appear in phase Fe-C phase diagram. However, it is represented on
the isothermal transformation diagram.
 The martensitic transformation occurs when the quenching rate is rapid enough to
prevent carbon diffusion. Since the transformation takes place so rapidly that the atoms
do not have time to intermix. Any diffusion whatsoever will result in the formation of
ferrite and cementite phases.
273
 Martensite has body-centered tetragonal (BCT: (like BCC, but
one unit cell axis longer than other two)) structure.
 Of the various microstructures in steel alloys martensite is the
hardest, strongest but most brittle.
 Martensite needs to be tempered to get better ductility.
 Tempering is the process of heating a martensitic steel at a
temperature below the eutectoid transformation temperature to
make is softer and more ductile.

 The beginning of martensitic
transformation is represented by a
horizontal line labeled M (start).
 Two other horizontal and dashed
lines, labeled M(50%) and M(90%),
indicate percentages of the austenite
to martensite transformation.
 The horizontal and linear character
of these lines indicates that the
martensitic transformation is
independent of time.
 Consider an alloy of eutectoid
composition that is very rapidly cooled
from a temperature above 727°C to,
say, 165°C. 50% of the austenite will
transform to martensite; and as long
as this temperature is maintained,
there will be no further transformation.
 100% martensite transformation lies
below the ambient for an Fe-C alloy of
eutectoid composition.
274
A: Austenite P: Pearlite
B: Bainite M: Martensite
Martensitic transformation

275
Martensitic Microstructure
Martentite needles
Austenite
60
m
 Martensite grains take o a
plate-like or needle-like
appearance.
 The austenite in the photo is
retained austenite that did not
transform during the rapid
quench.
 Martensite as well as other
microconstituents (e.g., pearlite)
can coexist.

276
Example
Using the isothermal transformation diagram for an Fe-C alloy of eutectoid composition, specify the nature of
the final microstructure (in terms of microconstituents present and approximate percentages) of a small
specimen that has been subjected to the following time-temperature treatments. In each case assume that
the specimen begins at 760 °C and that it has been held at this temperature long enough to have achieved a
complete and homogeneous austenitic structure.
Solution

278
CCT (Continuous Cooling Transformation) DIAGRAMS
 Isothermal heat treatments are not the
most practical because an alloy must be
rapidly cooled to and maintained at an
elevated temperature from a higher
temperature above the eutectoid.
 Most heat treatments for steels involve
the continuous cooling of a specimen to
room temperature.
 For continous cooling, the time
required for a reaction to begin and end is
delayed. Thus the isothermal curves
are shifted to longer times and lower
temperatures.
 The transformation starts after a time
period corresponding to the intersection
of the cooling curve with the beginning
reaction curve and concludes upon
crossing the completion transformation
curve.

279
 Normally, bainite will not form when an alloy
of eutectoid composition or any plain carbon
steel is continuously cooled to room
temperature.
 This is because all the austenite will have
transformed to pearlite by the time the bainite
transformation has become possible.
 Thus, the region representing the austenite-
pearlite transformation terminates just below
the nose (see Fig.) as indicated by the curve
AB.
 For any cooling curve passing through AB in
the Fig., the transformation ceases (stops) at
the point of intersection; with continued
cooling, the unreacted austenite begins
transforming to martensite upon crossing the M
(start) line.
 M (start), M (50%), and M (90%) lines occur
at identical temperatures for both isothermal
and continuous cooling transformation
diagrams.

280
 The critical quenching rate is the
minimum rate of quenching.
 Only martensite will exist for
quenching rates greater than the
critical.
 This critical cooling rate will just miss
the nose at which pearlitic transformation
begins.
 There will be a range of rates over
which both pearlite and martensite are
produced.
 Totally pearlitic structure developes for
low cooling rates.

281
 From the Fig., a horizontal line at 675°C
intersects the 50% and reaction completion
curves at 80 and 300 seconds, respectively.
 These are the times asked for in the
problem statement.
Solution:

282
Solution:

283
Solution:

MECHANICAL PROPERTIES
OF METALS
FAILURE
174

MECHANICAL TESTING AND PROPERTIES
TYPES OF LOADING
 Deformation occurs when forces are applied to a material.
 There are three principal ways in which a load may be applied: tension,
compression and shear. However, many loads are torsional rather than pure
shear.
Tension Load: As the ends of material are pulled apart to make the
material longer, the load is called a tension load.
Compression Load: As the ends of material are pushed in to make the
material shorter, the load is called a compression load.
Shear Load: The load imposed parallel to the upper and lower faces.
Torsion is a variation of pure shear, wherein a structural member is twisted.
Torsional forces produce a rotational motion about the longitudinal axis of
one end of the member relative to the other end.
175

MECHANICAL TESTING AND PROPERTIES
Compression
Tension
Dashed lines represent the
shape before deformation,
solid lines after deformation.
How a tensile load
produces an elongation
How a compressive load
produces contraction
F: axial force
A0: cross-sectional area
A0 perpendicular to F.
lo: original length
l : final length.
176

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