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