govt. polytechnic lisana (rewari)
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Govt. Polytechnic Lisana (Rewari)Mechanical Engg. Deptt.
Subject: Materials & Metallurgy
Faculty: Amit Hooda
Atomic Structure & Interatomic bonding
Atomic Structure Atoms are the smallest particle of a chemical element
that can exist.
They are made of small particles called Electrons(-),
Protons(+)&Neutrons.
Each atom consist of a very small nucleus composed of
Protons and Neutrons which is encircled by moving
Electrons.
Electrons & Protons are electrically charged with
magnitude 1.60×10−19C.
Proton & Neutron have approximately 1.67×10−27 of mass
whereas mass of Electron is 9.11×10−31 kg.
Atomic mass unit (amu) = 1/12 mass of Carbon 12 (12C)
1 mol of substance contains 6.023 x 1023 (Avogadro’s number) atoms or
molecules.
Atomic weight = 1 amu/atom (or molecule) = 1 g/mol = Wt. of 6.023 x 1023
atoms or molecules.
For example, atomic weight of copper is 63.54 amu/atom or 63.54 g/mole
❖ No. of Protons in the nucleus of any atom is its Atomic Number(Z).
❖ Sum of masses of Neutrons and Protons within the nucleus is its Atomic Mass(A)
❖ Atoms of some elements have two or more different atomic masses are called ISOTOPES.
Interatomic bonding:
❖ Ionic bonding:
It is the complete transfer of valence electron(s) between atoms. It is a type of
chemical bond that generates two oppositely charged ions. In ionic bonds, the
metal loses electrons to become a positively charged cation, whereas the non
metal accepts those electrons to become a negatively charged anion.
❖ COVALENT BONDING:
A covalent bond, also called a molecular bond, is a chemical bond that involves
the sharing of electron pairs between atoms. These electron pairs are known as
shared pairs or bonding pairs, and the stable balance of attractive and repulsive
forces between atoms, when they share electrons, is known as covalent
bonding.
❖ METALLIC BONDING:
Metallic bonding is the force of attraction between valence electrons
and the metal ions. It is the sharing of many detached electrons between
many positive ions, where the electrons act as a "glue" giving the
substance a definite structure.
Electronic Configuration:
It is the distribution of electrons in various sub shells around the nucleus.
There can be only 2𝑛2 electrons with the same total quantum number n.
eg- if n=2, no of electrons=8
In the 𝑛𝑡ℎ shell there are n subgroups.
Crystal Structure & Defects
Crystal Structure
Crystal structure is one of the most important aspects of materials science
and engineering as many properties of materials depend on their crystal
structures.
The basic principles of many materials characterization techniques such as X-
ray diffraction (XRD), Transmission electron microscopy (TEM) are based on
crystallography
Crystal Structure
Crystalline Amorphous
Crystalline:-periodic arrangement of atoms: definite repetitive pattern
Amorphous:-random arrangement of atoms.
The periodicity of atoms in crystalline solids can be
described by a network of points in space called lattice
Unit Cell & Space Lattice
The metallic crystals can be considered as consisting of tiny blocks which are repeated in 3-D pattern.
The tiny block formation
by the arrangement of
small group of atoms
is called the unit cell.
If each atom in a lattice is
Replaced by a point, then
Each point is called lattice point.
The arrangement of points is referred to as the lattice array.
An array of points in the 3-D in which every point has surroundings
identical to that every other point in the array is known as Space lattice.
The distance between the atoms points is called inter-atomic or lattice
spacing.
Bravais Lattice
The unit vectors a, b and c are called lattice parameters. Based on their length
equality or inequality and their orientation (the angles between them, α, β and
γ) a total of 7 crystal systems can be defined. With the centering (face, base and
body centering) added to these, 14 kinds of 3D lattices, known as Bravais
lattices, can be generated.
THEORETICAL DENSITY, r
Example: Copper
• crystal structure = FCC: 4 atoms/unit cell
• atomic weight = 63.55 g/mol (1 amu = 1 g/mol)
• atomic radius R = 0.128 nm (1 nm = 10 cm)
Result: theoretical rCu = 8.89 g/cm3
Compare to actual: rCu = 8.94 g/cm3
√
Iron- carbon Phase
Diagram
Iron-Iron carbide Phase Diagram
Iron-‘Iron carbide’ phase diagram
Its not a true equilibrium phase diagram because iron carbide is not a stable
phase
Iron carbide decomposes into iron and carbon (graphite)
Even at elevated temperature (like 700C), it will take several years for
decomposition
Hence for all practical purpose Iron-Iron carbide phase diagram represents
equilibrium changes
Iron-Iron carbide phase Diagram
Carbon being a very small atom gets into the interstitial of ferrite/ austenite
phases to form solid solution
Ferrous metals - based on iron, comprises about 75% of metal tonnage in the
world. Broadly three main alloys
❖ Iron = C content < 0.008 wt%
❖ Steel = Fe-C alloy (0.008 to 2.11% C)
❖ Cast iron = Fe-C alloy (2.11% to 6.7% C)
Transformation Temperatures
A1 = Temperature at which austenite begins to form during heating
A2 = Temperature at which α iron becomes nonmagnetic
A3 = Temperature at which transformation of α iron to austenite is completed
during heating
A4 = Temperature at which austenite transforms to delta ferrite
Am = Temperature at which solutionizing of cementite in austenite is
complete
Allotropes of Iron and various phases
Where Does the carbon Atom go??
Various Transformation Reactions and
development of Microstructure
Peritectic Reaction:
L + δ ⇌ γ
Eutectic Reaction: Eutectic of austenite and cementite is known as ledeburite
L ⇌ γ + Fe3C
Eutectoid Reaction: Eutectoid of ferrite and cementite is known as pearlite.
The ferrite and cementite phases occur as alternate layers
γ ⇌ α + Fe3C
TTT Diagram
TTT Diagrams
The thickness of the ferrite and
cementite layers in pearlite
is ~ 8:1. The absolute layer thickness
depends on the
temperature of the transformation. The
higher the
temperature, the thicker the layers
TTT Diagrams
❖ The family of S-shaped curves at different T are used to construct the
TTT diagrams.
❖ The TTT diagrams are for the isothermal (constant T) transformations
(material is cooled quickly to a given temperature before the
transformation occurs, and then keep it at that temperature).
❖ At low temperatures, the transformation occurs sooner (it is
controlled by the rate of nucleation) and grain growth (that is
controlled by diffusion) is reduced.
❖ Slow diffusion at low temperatures leads to fine-grained
microstructure with thin-layered structure of pearlite (fine pearlite).
❖ At higher temperatures, high diffusion rates allow for larger grain
growth and formation of thick layered structure of pearlite (coarse
pearlite).
❖ At compositions other than eutectoid, a proeutectoid phase (ferrite or
Formation of Bainite Microstructure (I)
If transformation temperature is low enough (≤540°C) bainite rather than fine pearlite forms.
Formation of Bainite Microstructure (II)
❖ For T ~ 300-540°C, upper bainite consists of needles of ferrite separated by long
cementite particles.
❖ For T ~ 200-300°C, lower bainite consists of thin plates of ferrite containing very fine
rods or blades of cementite
❖ In the bainite region, transformation rate is controlled by microstructure growth
(diffusion) rather than nucleation. Since diffusion is slow at low temperatures, this
phase has a very fine (microscopic) microstructure.
❖ Pearlite and bainite transformations are competitive; transformation between pearlite
and bainite not possible without first reheating to form austenite
Spheroidite
❖ Annealing of pearlitic or bainitic microstructures at elevated temperatures
just below eutectoid (e.g. 24 h at 700 C) leads to the formation of new
microstructure – spheroidite - spheres of cementite in a ferrite matrix.
❖ Composition or relative amounts of ferrite and cementite are not changing in
this transformation, only shape of the cementite inclusions is changing.
❖ Transformation proceeds by C diffusion – needs high T.
❖ Driving force for the transformation - reduction in total ferrite - cementite
boundary area
Thermoplastics:
Thermosetting:
CERAMICS
What is Ceramics??
The word ceramic is derived from the greek term keramos, which means
“potter’s clay” and keramikos means “clay products”.
A ceramic material is an inorganic, non-metallic material and is often
crystalline.
Till 1950s, the most important types of ceramics were the traditional clays,
made into pottery, bricks, tiles etc.
Most recently, different types of ceramics used are alumina, silicon carbide
etc.
Latest advancements are in the bio-ceramics with examples being dental
implants and synthetic bones.
Why Ceramics??
A comparative analysis of ceramics with other engineering materials is shown in table
Classification of Ceramic Materials
Ceramics can be classified
in diverse ways i.e. there
are number of ways to
classify the ceramic
materials. Most commonly,
the ceramics can be
classified on the following
basis:
Classification based on composition 1) Silicate ceramics
Silicates are materials generally having composition of silicon and oxygen. Four large
oxygen (o) atoms surround each smaller silicon (Si) atom as shown in figure.
The main types of silicate ceramics are based either on alumosilicates or on
magnesium silicates
The former include clay-based ceramics such as porcelain, earthenware, stoneware,
bricks etc
The latter consists of talc-based technical ceramics such as steatite, cordierite and
forsterite ceramics.
Silicate ceramics are traditionally categorized into coarse or fine and, according to
water absorption, into dense (< 2 % for fine and < 6 % for coarse) or porous ceramics (>
2% and > 6 %, respectively).
Silicate
Ceramics
Structure of Silicate
Ceramics
2) Oxide ceramics
Oxide ceramics include alumina, zirconia, silica, aluminium silicate, magnesia and
other metal oxide based materials. These are non-metallic and inorganic compounds
by nature that include oxygen, carbon, or nitrogen.
Oxide ceramics possess the following properties:
(a) High melting points (b) Low wear resistance (c) An extensive collection of electrical
properties.
Oxide ceramics are used in a wide range of applications, which include materials and
chemical processing, radio frequency and microwave applications, electrical and high
voltage power applications and foundry and metal processing.
Aluminium oxide (Al2O3) is the most important technical oxide ceramic material. This
synthetically manufactured material consists of Aluminium oxide ranging from 80 % to
more than 99 %.
Aluminium Oxide Aluminium Oxide
Structure
3) Non-Oxide ceramics
The use of non-oxide ceramics has enabled extreme wear and corrosion problems to be overcome, even at high temperature and severe thermal shock conditions.
These types of ceramics find its application in different spheres such as pharmaceuticals, oil and gas industry, valves, seals, rotating parts, wear plates, location pins for projection welding, cutting tool tips, abrasive powder blast nozzles, metal forming tooling etc.
4) Glass ceramics
These are basically polycrystalline material manufactured through the controlled crystallization of base glass.
Glass-ceramics possess an amorphous phase and more than one crystalline phases
Glass-ceramics yield an array of materials with interesting properties like zero porosity, fluorescence, high strength, toughness, low or even negative thermal expansion, opacity, pigmentation, high temperature stability, low dielectric constant, machinability, high chemical durability, biocompatibility, superconductivity, isolation capabilities and high resistivity
Whitewares
Crockery
Floor and wall tiles
Sanitary-ware
Electrical porcelain
Decorative ceramics
Refractories
Firebricks for furnaces and ovens. Have high Silicon or Aluminium oxide
content.
Brick products are used in the manufacturing plant for iron and steel, non-
ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and
chemical industries.
Refractories
Used to provide thermal protection of other materials in very high temperature applications, such as steel making (Tm=1500°C), metal foundry operations, etc.
They are usually composed of alumina (Tm=2050°C) and silica along with other oxides: MgO (Tm=2850°C), Fe2O3, TiO2, etc., and have intrinsic porosity typically greater than 10% by volume.
Specialized refractories, (those already mentioned) and BeO, ZrO2, mullite, SiC, and graphite with low porosity are also used.
Amorphous Ceramics
(Glasses)
Main ingredient is Silica (SiO2)
If cooled very slowly will form crystalline structure.
If cooled more quickly will form amorphous structure consisting of disordered and linked chains of Silicon and Oxygen atoms.
This accounts for its transparency as it is the crystal boundaries that scatter the light, causing reflection.
Glass can be tempered to increase its toughness and resistance to cracking.
Glass Types
Three common types of glass:
Soda-lime glass - 95% of all glass, windows containers etc.
Lead glass - contains lead oxide to improve refractive index
Borosilicate - contains Boron oxide, known as Pyrex.
Glasses
Flat glass (windows)
Container glass (bottles)
Pressed and blown glass (dinnerware)
Glass fibres (home insulation)
Advanced/specialty glass (optical fibres)
Tempered Glass
The strength of glass can be enhanced by inducing compressive residual stresses at the surface.
The surface stays in compression - closing small scratches and cracks.
Small Scratches
Hardening Processes
Tempering:
Glass heated above Tg but below the softening point
Cooled to room temp in air or oil
Surface cools to below Tg before interior
when interior cools and contracts it draws the exterior into compression.
Chemical Hardening:
Cations with large ionic radius are diffused into the surface
This strains the “lattice” inducing compressive strains and stresses.
Crystalline Ceramics
Good electrical insulators and refractories.
Magnesium Oxide is used as insulation material in heating elements and cables.
Aluminium Oxide
Beryllium Oxides
Boron Carbide
Tungsten Carbide.
Used as abrasives and cutting tool tips.
Abrasives
Natural (garnet, diamond, etc.)
Synthetic abrasives (silicon carbide, diamond, fused alumina, etc.) are used
for grinding, cutting, polishing, lapping, or pressure blasting of materials
Advanced Ceramics
Advanced ceramic materials have been developed over the past half century
Applied as thermal barrier coatings to protect metal structures, wearing surfaces, or as integral components by themselves.
Engine applications are very common for this class of material which includes silicon nitride (Si3N4), silicon carbide (SiC), Zirconia (ZrO2) and Alumina (Al2O3)
Heat resistance and other desirable properties have lead to the development of methods to toughen the material by reinforcement with fibers and whiskers opening up more applications for ceramics
Advanced Ceramics
Structural: Wear parts, bioceramics, cutting tools, engine components, armour.
Electrical: Capacitors, insulators, integrated circuit packages, piezoelectrics, magnets and superconductors
Coatings: Engine components, cutting tools, and industrial wear parts
Chemical and environmental: Filters, membranes, catalysts, and catalyst supports
Silicon Carbide
Automotive
Components in
Silicon Carbide
Chosen for its heat
and wear
resistance
Ceramic Armour
Ceramic armour systems are used to protect military personnel and equipment.
Advantage: low density of the material can lead to weight-efficient armour systems.
Typical ceramic materials used in armour systems include alumina, boron carbide, silicon carbide, and titanium diboride.
The ceramic material is discontinuous and is sandwiched between a more ductile outer and inner skin.
The outer skin must be hard enough to shatter the projectile.
Most of the impact energy is absorbed by the fracturing of the ceramic and any remaining kinetic energy is absorbed by the inner skin, that also serves to contain the fragments of the ceramic and the projectile preventing severe impact with the personnel/equipment being protected.
Alumina ceramic/Kevlar composite system in sheets about 20mm thick are used to protect key areas of Hercules aircraft (cockpit crew/instruments and loadmaster station).
This lightweight solution provided an efficient and removable/replaceable armour system. Similar systems used on Armoured Personnel Carrier’s.
Silicon Carbide
Body armour and
other components
chosen for their
ballistic properties.
COMPOSITES
Examples:
Cemented carbides (WC with Co binder)
Plastic molding compounds containing fillers
Rubber mixed with carbon black
Wood (a natural composite as distinguished from a
synthesized composite)
Why Composites are Important??
Composites can be very strong and stiff, yet very light inweight, so ratios of strength-to-weight andstiffness-to-weight are several times greater than steel oraluminum
Fatigue properties are generally better than for commonengineering metals
Toughness is often greater too
Composites can be designed that do not corrode like steel
Possible to achieve combinations of properties not attainablewith metals, ceramics, or polymers alone
Disadvantages and Limitations of Composite
Materials
Properties of many important composites are anisotropic - the properties differ depending on
the direction in which they are measured – this may be an advantage or a disadvantage
Many of the polymer-based composites are subject to attack by chemicals or solvents, just as
the polymers themselves are susceptible to attack
Composite materials are generally expensive
Manufacturing methods for shaping composite materials are often slow and costly
One Possible Classification of
Composite Materials
Traditional composites – composite materials that occur in nature or have been produced by
civilizations for many years
Examples: wood, concrete, asphalt
Synthetic composites - modern material systems normally associated with the manufacturing
industries, in which the components are first produced separately and then combined in a
controlled way to achieve the desired structure, properties, and part geometry
Classification
Components in a Composite Material
Nearly all composite materials consist of two phases:
Primary phase - forms the matrix within which the
secondary phase is imbedded
Secondary phase - imbedded phase sometimes referred to
as a reinforcing agent, because it usually serves to
strengthen the composite
The reinforcing phase may be in the form of fibers,
particles, or various other geometries
Primary Phase, Matrix
Secondary Phase, Reinforcement
Functions of the Matrix Material
(Primary Phase)
Protect phases from environment
Transfer Stresses to phases
Holds the imbedded phase in place, usually enclosing and
often concealing it
When a load is applied, the matrix shares the load with the
secondary phase, in some cases deforming so that the stress
is essentially born by the reinforcing agent
Reinforcing Phase (Secondary)
Metal Matrix Composites (MMCs)
A metal matrix reinforced by a second phase
Reinforcing phases:
Particles of ceramic (these MMCs are commonly called
cermets)
Fibers of various materials: other metals, ceramics,
carbon, and boron
Cermets
MMC with ceramic contained in a metallic matrix
The ceramic often dominates the mixture, sometimes up to
96% by volume
Bonding can be enhanced by slight solubility between phases
at elevated temperatures used in processing
Cermets can be subdivided into
Cemented carbides – most common
Oxide-based cermets – less common
Ceramic Matrix Composites (CMCs)
A ceramic primary phase imbedded with a secondary phase,
which usually consists of fibers
Attractive properties of ceramics: high stiffness, hardness,
hot hardness, and compressive strength; and relatively low
density
Weaknesses of ceramics: low toughness and bulk tensile
strength, susceptibility to thermal cracking
CMCs represent an attempt to retain the desirable properties
of ceramics while compensating for their weaknesses
Polymer Matrix Composites (PMCs)
A polymer primary phase in which a secondary phase is
imbedded as fibers, particles, or flakes
Commercially, PMCs are more important than MMCs or CMCs
Examples: most plastic molding compounds, rubber
reinforced with carbon black, and fiber-reinforced polymers
(FRPs)
FRPs are most closely identified with the term composite
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