B. Engg. Mechanical Engineering Level 2
MECH 2104 Material Science Lecture Notes 1999/2000
Hurreeram D K
1.0 Atomic Structure and Inter Atomic Bonding
1.1 Course Perspective
1.2 Atomic and Crystalline Structure of Metals
1.3 Atomic Structure
1.4 Electrons in an atom
1.5 Atomic Bonding in Solids
2.0 Crystalline Structure of Solids
2.1 Introduction
2.2 Crystal Structure
2.3 Imperfection in Solids
3.0 Physical and Mechanical Properties of Metals
3.1 Testing Techniques DT and NDT (Tension, Compression, Torsion,
Surface)
3.2 Physical Properties
3.3 Mechanical Properties
3.4 Other Properties
4.0 Deformation of crystalline Materials
4.1 Dislocations and Plastic Deformation
4.2 Slip Systems
5.0 Strengthening Mechanism in Metals
5.1 Equilibrium Phase Diagrams
5.2 Iron-Carbon System
6.0 Thermal Processing of Metals and Alloys
6.1 Cold and Hot Working
7.0 Failure Mechanism of Materials in Service
7.1 Brittle and Ductile Failure
7.2 Creep and Fatigue
7.3 Corrosion
Prepared by: Dr.D.K.Hurreeram
August 04
1.0
Course Perspective
Materials involved in all aspects of human life
• Transportation, Housing, Clothing, Communication, Food Production, Recreation
• Civilization designated by level of materials development
• Stone, wood, clay, skins, pottery, metals
• New developments in materials production and use
• ability to alter properties according to requirement
• selection of materials out of specifications
• aim to provide comfort to man
• Material Science - investigate relationship between structures and properties of materials
• Structure relate to arrangement of its internal components
• Microscopic (using microscope) and macroscopic (viewed with naked eye)
structures
• Property relate to the response of materials exposed to external stimuli
• Force, heat, environment
• Basically six properties
• mechanical, thermal, electrical, magnetic, optical, deteriorative
• Why study material science?
• design problems, material selection problems, economic or cost problems
• Classification of materials
• Metals, Ceramics, Polymers, Composites, Semiconductors
• Modern Materials needs
• Energy consumption
• Nuclear Power Usage
• Environment quality (pollution control)
• Renewable resources
1.1 Atomic and Crystalline Structure of Metals
1.2 Atomic Structure
Properties of solid materials - Function of geometrical atomic arrangement and interaction
between constituent atoms or molecules
•
The Atom - Nucleus (Neutrons, Protons), Electrons,
Cloud of electrons
Neutrons and
protons (in
nucleus)
•
•
•
Electron, Proton are electrically charged ( magnitude of 1.6E-19 C)
Neutron - electrically neutral
Neutron and Protons have approximately the same mass (1.67E-27 kg)
Prepared by: Dr.D.K.Hurreeram
August 04
•
•
•
•
•
• Electron Mass (9.11E-31 kg, negligible compared to the above)
Atomic Number (Z) - Number of Protons in the nucleus (same for all atoms of a given
element), 1 for Hydrogen to 94 for Plutonium (Periodic Table)
Atomic Mass (A)- Sum of Masses of Protons and Neutrons within nucleus. The number
of neutrons(N) may be variable (isotopes)
Atomic Weight - weighted average of the atomic masses of the atom’s naturally occurring
isotopes
Atomic Mass Unit - used for computations of atomic weight. 1 AMU = 1/12 of the atomic
mass of the most common isotope of Carbon, Carbon 12, 12C (A=12)
A=Z+N
Atomic Weight of an element or molecular weight of a compound can be specified on the
basis of AMU’s per atom or mass per mole of material. In one mole of a substance there
are 6.023E23 atoms or molecules (Avogadro’s Number)
1 AMU/atom (or molecule) = 1 g/mol
1.3 Electrons in an atom
• Quantum Mechanics (Bohr atomic model - Fig 1 above)
• electrons assumed to revolve around nucleus in discrete orbital (position well
defined in terms of its orbital)
• energy of electrons are quantised (change of energy levels or states)
• Calculations concerning Bohr’s model
• momentum equation
• energy emission or absorption
• force of attraction between electrons and nucleus
• radii of orbits
• velocity of electrons
• orbital frequency
• electron energy
• energy levels
• Wave mechanical Model (position defined by the probability of electrons being at various
locations around the nucleus - electron cloud instead of orbit, Fig 1b above)
• Quantum Numbers - each electron characterised by four parameters; the size,
shape and spacial orientation of an electron’s probability density are specified by
three of these quantum numbers.
• Bohr’s energy levels separate into electron shells designated by Principal
quantum number (n=1,2,3,4,5…or K,L,M,N,O….)
• Second quantum number signifies subshells (l=s,p,d,f)
• Third quantum number define energy states in each subshells
• Fourth quantum number define associated spin moment of electron +1/2 or
-1/2 orientation (up or down)
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Table 1 No. of Available Electron States in some of the Electron shells and sub-shells
Principal
Quantum No.
Shell
Designation
1
2
K
L
3
M
4
N
•
Sub-shells
Number of
States
s
s
p
s
p
d
s
p
d
f
Number of Electrons
Per Subshell
2
2
6
2
6
10
2
6
10
14
1
1
3
1
3
5
1
3
5
7
Per Shell
2
8
18
32
Electron Configuration
• Ground State and Exited state (Fig 2)
Fig 2 Relative Energies of electrons for the various shells and sub-shells
f
f
d
d
Energy
p
s
p
s
s
1
2
3
4
5
6
7
Table 2 Electron Configuration of some elements
Element
Hydrogen
Helium
Carbon
Fluorine
Neon
Aluminium
Iron
Copper
Symbol
H
He
C
F
Ne
Al
Fe
Cu
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Atomic Number
1
2
6
9
10
13
26
29
Electron Configuration
1s1
1s2
1s2 2s2 2p2
1s2 2s2 2p5
1s2 2s2 2p6
1s2 2s2 2p6 3s2 3p1
1s2 2s22p6 3s2 3p6 3d6 4s2
1s2 2s22p6 3s2 3p6 3d10 4s1
•
•
•
Valence Electrons - electrons occupying outermost shells; most important
as they participate in bonding between atoms and molecules. Many
physical and chemical properties are based on the valence electrons
Stable electron configuration (rare gases, Ne, Ar, Kr, He)
The periodic table
1.4 Atomic Bonding in Solids
• knowledge of physical properties of materials
• Interaction between two atoms; the ideal case (forces and potential energy models
Fig 3)
FA
Force of
attraction
Net force
Interatomic separation
ro
FR
Repulsive
force
Figure 3 Repulsive, attractive and net forces (potential energy) as a function of interatomic
spacing for two isolated atoms
•
•
•
Bonding energy (minimum potential energy)
In reality, three types of primary bonding + Secondary bonding
Ionic, Covalent and Metallic bonding, Van der Waals, Hydrogen bonding
• Ionic bonding - transfer of valence electrons (formation of ions)
• Covalent bonding - sharing of electrons
• Metallic bonding - ion ores and electron cloud formation
• Van der Waals bonding - from dipoles
• Hydrogen bonding - special case of Van der Waals bonding
Prepared by: Dr.D.K.Hurreeram
August 04
Table 3 Bonding energies and Melting temperatures for various substances
Bonding Type
Substance
Bonding Energy
kJ/mole
Ionic
Covalent
Metallic
Van der Waals
Hydrogen
NaCl
MgO
Si
C (diamond)
Al
Fe
W
Ar
Cl
NH3
H2O
640
1000
450
513
324
406
849
7.7
31
35
51
eV/atom, ion,
molecule
3.3
5.2
4.7
7.4
3.4
4.2
8.8
.08
.32
.36
.52
Melting
Temperature
o
C
801
2800
1410
>3550
660
1538
3410
-189
-101
-78
0
2.0 CRYSTAL STRUCTURE OF MATERIALS
2.1 Introduction
Why metals have different properties? Soft, hard, brittle, ductile
Atomic Structure - electron structure of individual atoms; bonding mechanisms
Other factors that predict properties:
• crystal structure (solid state)
• composition of metal
• impurities and vacancies
• grain size, boundaries, environment, surface condition
• method of manufacture
2.2 Crystal Structure
• Solid materials classified according to regularity with which atoms or ions are
arranged with respect to one another - over large atomic distances
• atoms bonded to nearest neighboring atom
• all metals, many ceramic materials, certain polymers form crystalline
structures on solidification
• Unit cell (small repeating entities) - basic structural unit or building block of the
crystal structure and defines the crystal structure by virtue of its geometry and the
atom positions within.
• Solidification process
• Amorphous material (non-crystalline) - atomic structure resembles that of a liquid
(supercooled liquid); rapid cooling e.g. SiO2 , inorganic gases, polymers
• Atomic hard sphere model ; Lattice Structure, Space lattice
• Lattice parameters of a unit cell (Fig 2.1)
Z
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c
ß
Y
b
a
X
Fig 2.1
Form of the unit cell is a function of the interfacial angles and dimensions
Seven different crystal systems available
cubic, monoclinic, triclinic, tetragonal, orthorhombic, rhombohedral,
hexagonal, Examples of materials Table 3.1
14 possible types of space lattices in these seven systems of crystals (Bravais Lattice
structure)
•
Metallic crystal structures
• most common in metals BCC, FCC, HCP (simplest atomic structures using hard
sphere model): Fig 3.2
• Co-ordination Number
• defined as number of nearest atoms which are directly surrounding a given
atom
• Simple Cubic Structure = 6, BCC = 8, FCC = 12, HCP = 12
• Atomic Radius
defined as half the distance between nearest neighbours in a crystal of a pure
element (assume spherical model and calculate radius for each structure)
• Number of atoms per unit cell (calculate)
• Atomic Packing Factor
• defined as ratio of volume of atoms per unit cell to total volume occupied
by the unit cell (calculate)
Type of cubic cell
Simple cubic
Body centred
cubic
Face Centred
Cubic
Atomic Radius
a/2
3a/4
Number of atoms
per unit cell
1
2
a/2 2
4
Atomic packing
factor
0.53
.68
•
Worked examples
•
Crystal Directions, Planes and Miller Indices
Z
Prepared by: Dr.D.K.Hurreeram
August 04
.74
Coordination
number
6
8
12
Y
X
3.0 Imperfection in Solids
•
•
•
Ideal crystal does not exit; all contain large no. of defects or imperfections - based on
theoretical and experimental results (e.g. Modulus of Elasticity)
Properties of material affected (tensile strength, deformation mechanism etc..)
Imperfections classified according to geometry of the defect
• Point defect
• Vacancy, self interstitial
• Number of vacancies Nv=N exp (-QvkT)
Qv= Activation Energy, k=Boltzman Constant (example Nv for Cu at 1000oC. Qv= .9 eV/atom, atomic weight=
63.5 g/mol, density =8./4 g/cm3. )
•
Impurities in solid solutions (alloys)
• substitutional and interstitial solid solutions
• Cu/Ni alloy, Fe alloys with Carbon.
Vacancy
Self interstitial atom
Grain boundary
Substitutional
impurity atom
Edge dislocation
Interstitial impurity atom
•
•
•
Line defect (Dislocations-linear defects)
• edge dislocations
• screw dislocations
• mixed dislocations
Surface defect (Interfacial defects)
• External surfaces (liquids)
• Grains and Grain boundaries
• Twin boundaries
Bulk or Volume defects
• Pores, cracks, foreign inclusions, other phases
Prepared by: Dr.D.K.Hurreeram
August 04
3.1 Deformation of crystal structures
•
•
Change in dimension or forms of matter under the action of applied forces (permanent or
temporary)
Types of metal deformation
• Elastic deformation
Force or
Stress
Plastic deformation
Elastic deformation
Elastometric deformation
Extension or Strain
•
•
Elastometric deformation (some polymers, rubber, elastomers)
Plastic deformation
• Slip mechanism
4.0 Strengthening Mechanism in Metals
•
•
•
•
•
•
Plastic deformation - ability of dislocations to move
Restricting or hindering dislocation motion renders metal harder and stronger
Strengthening by grain size reduction
Solid solution hardening
Strain hardening
Recovery, Recrystallisation and Grain Growth
5.0 Physical and Mechanical Properties of Metals
•
•
•
Physical properties
Types of forces (Tensile, compressive, shear, torsion)
Concepts of Stress and Strain
• Relationship between applied load and deformation
• Tensile, Compressive, Shear stress and strain
• Tension, compressive, shear and torsional tests
• Stress/Strain behaviour
• Mechanical properties (Modulus of Elasticity, Hooke’s Law, Modulus of
Rigidity or Shear Modulus, Limit of Proportionality, Elastic and Plastic
deformation, Tensile Strength, Proof Stress, Permanent set, Necking,
Ductility, Brittleness, Toughness, Poisson Ratio, Yielding and Yield
Strength, Resilience, Stiffness, True Stress and True Strain
Prepared by: Dr.D.K.Hurreeram
August 04
6.0 Hardness
• Rockwell Hardness Tests
• Brinell Hardness Tests
• Knoop and Vickers Micro Hardness Tests
• Impact Tests
• Charpy and Izod Tests
• Hardness and Tensile Strength
• Safety Factor
• Fatigue
• Creep
• Failure Modes
Prepared by: Dr.D.K.Hurreeram
August 04