EM025 CHAPTER 1 ENGINEERING MATERIAL

IONIC BOND

• Certain atoms tend to donate or receive electrons in order to become more stable by
completely occupying their outermost orbit.

• Atoms with very few electrons in their outermost shell tend to donate the electrons and
become positively charged ions, while atoms with more electrons in their outermost orbit
have a tendency to receive electrons and become positively charged ions.

• When these ions are brought together, the attraction forces are occurred due to opposite
charges of ions.

• These forces are called ionic bonds. These stable bonds are also called electrostatic bonds.
• Solids bonded with ionic bonds have crystalline structures and low electrical conductivity,

which is due to lack of free moving electrons.
• Bonds usually occur between metal and non-metal that are having a large difference in

electronegativity. Examples of ionically bonded materials include LiF, NaCl, BeO, CaF2 etc.

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IONIC BOND

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IONIC BONDING

PROPERTIES OF IONIC-BOND COMPOUND

➢ Ionic compounds tend to form giant structures
➢ Crystalline at room temperature.
➢ Have high melting points and boiling points compare to covalent bond.
➢ Hard but brittle.
➢ Conduct electricity if dissolved in molten or solution state but not in the solid state.

COVALENT BOND

• Covalent bonds are formed when two atoms share their valence electrons.
• The two atoms have a small difference in electronegativity.
• Covalent bonds occur between same atoms or different types of atoms.
• For example, fluorine needs one electron to complete its outer shell, thus, one

electron is shared by another fluorine atom by making a covalent bond resulting
F2 molecule.
• Covalently bonded materials are found in all three states; i.e., solid, liquid and gas.
Examples of covalently bonded material include hydrogen gas, nitrogen gas, water
molecules, diamond, silica etc.

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COVALENT BOND

Properties of Covalent-Bond compound

➢ Have much lower melting and boiling points than ionic compounds.
➢ Soft compared to ionic compound.
➢ Do not conduct electricity in water.
➢ Do not soluble in water.
➢ If small molecules are formed these are likely to be gases, for example : hydrogen.
➢ If giant structures are formed, these substances will be hard with high melting

points, for example : diamond.

COVALENT BOND

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METALLIC BOND

• In a metal lattice, valence electrons are loosely attached by the nuclei of metal atoms. Thus,
valence electrons require very low energy to release themselves from nuclei.

• Once these electrons detach, metal atoms become positively charged ions. These positively
charged ions are surrounded by a large number of negatively charged, free moving electrons called
an electron cloud.

• Electrostatic forces are formed due to the attractions between the electron cloud and ions. These
forces are called metallic bonds.

• In metallic bonds, almost every atom in the metal lattice shares electrons; so there is no way to
determine which atom shares which electron. Because of this reason, electrons in metallic bonds
are referred to as delocalized electrons.

• Due to the free moving electrons, metals are known for good electricity conductors. Examples of
metals with metallic bonds include iron, copper, gold, silver, nickel etc.

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METALLIC BOND

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DIFFERENTIATE BETWEEN IONIC, COVALENT AND METALLIC

CHARACTERISTICS IONIC COVALENT METALLIC

1. Definition Ionic bonds are electrostatic forces Covalent bonds are bonds that occur Metallic bonds are forces between
arising between negative and when two elements share a valence negatively charged freely moving
2. Bond Energy positive ions. electron in order to get electron electrons and positively charged
3. Formation configuration of neutral gasses. metal ions.
Bond Energy is higher than metallic
bonds. Bond Energy is higher than metallic Bond Energy is lower than other
Ionic bonds form when one atom bonds. primary bonds.
provides electrons to another atom.
Covalent bonds form when two atom Metallic bonds form when a
shares their valence electrons. variable number of atoms share a
variable number of electrons in a
4. Conductivity Ionic bonds have a low conductivity. Covalent bonds have a very low metal lattice.
conductivity.
5. Boiling & Melting Ionic bonds have higher melting and Metallic bonds have very high
Point boiling points. Covalent bonds have lower melting electrical and thermal conductivity.
Ionic bonds only exist in the solid and boiling points.
6. Physical State state. Metallic bonds have high melting
Covalent bonds exist in the form of and boiling points.
solids, liquids, and gasses.
Metallic bonds exist in the form of
solid only.

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CRYSTAL STRUCTURE

INTRODUCTION

• Crystal, any solid material in which the component atoms are arranged in a definite pattern
and whose surface regularity reflects its internal symmetry.

• Metals, ceramics and certain polymers acquire crystalline form when solidify.
• In solid state atoms self-organize to form crystals.
• When the solid is not crystalline, it is called amorphous, such as glass and plastics.

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CRYSTAL STRUCTURE

• Crystal structure is a description of the ordered arrangement of atoms, ions or molecules in
a crystalline material.

• Ordered structures occur from the intrinsic nature of the constituent particles to form
symmetric patterns that repeat along the principal directions of 3D space.

• Some of the properties of crystalline solids depend on the crystal structure of the material.

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CRYSTAL STRUCTURE

• The smallest group of particles in the material that constitutes this repeating pattern is the
unit cell of the structure.

• Crystalline structure atoms can be thought as solid sphere having well defined diameters.
Model is called atomic hard sphere .

• Atomic arrangement which has identical atom is called lattice .

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CRYSTAL STRUCTURE CONFIGURATION

Two crystal structure found for mist common metals are:
➢ Face-centred cubic (FCC)
➢ Body centred cubic (BCC)

Each material has its own atomic radii that define each crystal structure radius in a unit cell. Example:

Face Centred Cubic (FCC)

➢ Atoms located at each corner and the centres of all the cube

faces.
➢ Examples of common materials: copper,aluminium, silver,

gold.
➢ These spheres or ion cores touch one another across a face

diagonal.
➢ Number of atoms per unit cell, n = 4.

FCC unit cell:
→ 6 face atoms shared by two cells: 6 x 1/2 = 3
→ 8 corner atoms shared by eight cells: 8 x 1/8 = 1

Body Centred Cubic (BCC)

➢ Crystalline structure has a cubic unit cell with atoms located
at all eight corners and single atom at the cube centre.

➢ Materials example of BCC structure: chromium, iron,
tungsten, molybdenum and tantalum.

➢ Number of atoms per unit cell, n = 2.
BCC unit cell:

→ 8 corner atoms shared by eight cells : 8 x 1/8 = 1
→ Center atom (not shared) : 1 x 1 = 1

1.3 MECHANICAL AND PHYSICAL PROPERTIES

LEARNING OUTCOMES

a) Describe mechanical and physical properties of metal.
b) Explain the concept of stress-strain.
c) Determine the stress-strain behaviour of ferrous metal:

i. stress;
ii. strain;
iii. ultimate tensile strength;
iv. yield strength; and
v. modulus of elasticity.

INTRODUCTION

➢ Material properties are physical, chemical, or mechanical components of a specific
product that would determine its functionality and manufacturability.

➢ This would mean that a product’s material properties would specifically define the
capabilities and performance of the products in all aspects.

➢ A product’s intended functionality should also set specific property requirements
that need to be met in order for the final product to be considered useful.

➢ Physical properties are things that are measurable. Those are things like density,
melting point, conductivity, coefficient of expansion, etc.

➢ Mechanical properties are how the metal performs when different forces are
applied to them. That includes things like strength, ductility, wear resistance, etc.

MECHANICAL AND PHYSICAL
PROPERTIES

MELTING
POINT

THERMAL ELECTRICAL
CONDUCTIVITY CONDUCTIVITY

METAL

➢ It can be measured without changing the material.

➢ Have important influences on materials selection, manufacturing and service life of
components.

➢ It should be considered because of theirs effects on design, services requirements and
compatibility.

➢ Physical Properties:
• THERMAL CONDUCTIVITY – rate at which heat can be transported through a material.
• MELTING POINT – temperature at which it changes from solid to liquid.
• ELECTRICAL CONDUCTIVITY – the movement of electrically charged particles.

MALLEABILITY ELASTICITY

DUCTILITY PLASTICITY

TENACITY TOUGHNESS

HARDNESS BRITTLENESS

METAL

It should be considered because to withstand applied load and in-service uses.

• HARDNESS – resist scratching, wear and tear and indentation
• TENACITY – grip something firmly or persistency.
• DUCTILITY – change shape usually by stretching along its length.
• MALLEABILITY – reshaped in all directions without cracking
• ELASTICITY – absorb force and returns to original shape when the load removed.
• PLASTICITY – changed in shape permanently.
• TOUGHNESS – not break or shatter while receiving a blow or a sudden shock.
• BRITTLE – fractures when applied stress.

STRESS-STRAIN

INTRODUCTION TO STRESS AND STRAIN

➢ Static or fluctuating load applied on materials or structure member may
affect its mechanical properties

➢ This can be ascertained by a simple strain-stress test under controlled room
temperature condition

➢ Two principal ways in which a load may be applied: Tension and compression

TENSILE TEST - STRESS AND STRAIN GRAPH

➢ During a tensile test, the sample is slowly pulled while the resulting change in
length and the applied force are recorded.

➢ Using original length and surface area a stress-strain diagram can generated.

STRESS AND STRAIN GRAPH

STRESS-STRAIN CONCEPT USING GRAPH

A tensile force is applied to As seen in the graph, from It curves from point C
a steel bar, it will point B, the correlation (lower yield point), to D
elongation. between the stress and (maximum ultimate stress),
strain is no longer on a ending at E (fracture stress).
straight trajectory.

If the force small, ratio of This starts the yield point
stress and strain will remain which is point B.
proportional. As a straight
line between zero and point Beyond the elastic limit is
A (limit of proportionality). plastic deformation.

If the force greater, will
experience elastic

deformation, but the ratio
of stress and strain will not
be proportional. Between

points A and B (elastic).

STRESS • An applied force causes a change in the dimension
• F (force) and A (area of force).

STRAIN • Change in the dimension
ELASTICITY •

• Material to return to its original form after the external
force is removed.

PLASTICITY • Remain deformed without fracture even after the force
is removed.

ULTIMATE LIMIT STRENGHT • Maximum stress that a material can withstand while
being stretched or pulled before breaking.

YIELD STRENGTH • Limit of elastic behavior and the beginning of plastic
behavior.

MODULUS OF ELASTCITY • Measure of stiffness, higher-modulus materials
exhibiting less deformation under load compared to 8lo1 w-

Example:

Table shows data during a tensile test of a 14.0 mm diameter mild steel rod. The gage length

was 50.0 mm. From the table:
• Create a Stress – Strain table and then plot the stress-strain diagram.
• Locate proportional limit, Yield Strength, Ultimate Tensile Strength and Fracture.
• Calculate Modulus of Elasticity

Load (N) Elongation (mm) Load (N) Elongation (mm)
0 0 46200 1.05
52400 2.12
6310 0.010 58500 3.50
12600 0.020 65400 6.10
18800 0.028 69000 9.50
25100 0.036 67800 12.50
31300 0.044 65000 15.00
37900 0.052 61500
40100 0.151 Fracture
41600 0.407

SOLUTION:

Create a Stress – Strain table by using formula :

Stress : STRESS, s (N/m²) STRAIN, e

Strain : ε = ∆L 0 0
L0 40989996 0.20
81850071 0.40
122125503 0.56
163050539 0.72
203325971 0.88
246199818 1.04
260491100 3.02
270235157 8.14
300116929 21.00
340392361 42.40
380018189 70.00
424840847 122.00
448226582 190.00
440431337 250.00
422242432 300.00
399506301 Fracture

SOLUTION:

Sketch and plot the stress-strain diagram.

SOLUTION:

Sketch and plot the stress-strain diagram.

Ultimate Tensile Strength

Proportional limit Fracture

Yield Strength

SOLUTION:

Calculate Modulus of Elasticity
Ultimate Tensile Strength

Proportional limit Fracture

Yield Strength

By taking the value of the graph (proportional limit):

1.4 IRON CARBON PHASE DIAGRAM

LEARNING OUTCOMES

a) Sketch microstructure phase diagram of steel limited from 0 to 2% carbon content.
b) Describe microstructure transformation by given carbon percentage and temperature of
steel limited from 0 to 2% carbon content.

INTRODUCTION

• The iron-carbon phase diagram is widely used to understand the different phases of steel
and cast iron.

• Both steel and cast iron are a mix of iron and carbon. Also, both alloys contain a small
amount of trace elements.

• The amount of carbon present in an iron-carbon alloy, in weight percent, is plotted on the x-
axis and temperature is plotted on the y-axis.

• Each region, or phase field, within a phase diagram indicates the phase or phases present for
a particular alloy composition and temperature.

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MICROSTRUCTURE

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Iron- Carbon Phase Diagram

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Phase Composition For Iron-Carbon

Ferrite
◦ Known as α-iron
◦ Body-centered cubic structure
◦ Soft and ductile

Austenite
◦ Known as γ-iron
◦ Face-centered cubic
◦ Ductile and non-magnetic

Pearlite
◦ Most common constituent of steel
◦ Gives steel most of its strength
◦ Hard than ferrite

Cementite
◦ Forms in austenite when temperature reaches

between 723⁰ and 1147⁰
◦ Hard and brittle

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Points For Iron-Carbon

➢ Eutectoid
• carbon content of 0.83%.
• consists of pearlite.

➢ Hypoeutectic
• carbon content from 0 to 0.83%.
• consist of primary ferrite and pearlite.

➢ Hypereutectoid
• carbon content from 0.83 to 2.06%.
• consist of primary cementite and pearlite.

➢ Eutectic
• carbon contain with 4.3% carbon.
• consists of liquid.

➢ Steel
• Between 0 – 2.06 % of carbon

➢ Cast iron
• Between 2.06% - 6.67 % of carbon

Microstructure Transformation In Iron- Carbon Phase Diagram

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The Austenite to ferrite/cementite transformation in Fe-C diagram

• In order to understand the transformation processes, consider a steel of the eutectoid
composition 0.8% carbon, being slow cooled.

• At the upper temperatures, only austenite is present, with the 0.8% carbon being
dissolved in solid solution within the FCC.

• When the steel cools through 723°C, several changes occur simultaneously.

• The iron wants to change crystal structure from the FCC austenite to the BCC ferrite,
but the ferrite can only contain 0.02% carbon in solid solution.

• The excess carbon is rejected and forms the carbon rich intermetallic known as
cementite.

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Example:
Iron-carbon phase diagram contains ferrite (α), austenite (γ), cementite (Fe3 C) and
δ ferrite as solid phases.

i. Explain the distinction between hypoeutectic and hypereutectoid steel ?

ii. Describe the microstructural development that will occur upon slow cooling
from 1000°C to 600°C at X, Y and Z.

Solution:

i. Hypoeutectic Hypereutectoid

carbon content from 0 to 0.83%. carbon content from 0.83 to 2.06%.

consist of ferrite and pearlite. consist of cementite and pearlite.

i. X 1000°C 600°C

Phase Composition Austenite Ferrite + Cementite
Explanation Ductile and non-magnetic Hard and brittle

Microstructure
Sketch

Solution:

Y 1000°C 700°C 600°C
Phase Austenite Ferrite + Austenite Ferrite + Cementite
Composition
Ductile and non-magnetic Soft and ductile Hard and brittle
Explanation

Microstructure
Sketch