ch02 nature of materials

8/21/2019 Ch02 Nature of Materials

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THE NATURE OF MATERIALS

Manufacturing Processes, 1311

Dr Simin NasseriSouthern Polytechnic State University


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Manufacturing ProcessesProf Simin Nasseri

THE NATURE OF MATERIALS

1. Atomic Structure and the Elements

2. Bonding between Atoms and Molecules

3. Crystalline Structures

4. Noncrystalline (Amorphous) Structures
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Importance of Materials in Manufacturing

Manufacturing is a transformation process

It is the material that is transformed

And it is the behavior of the material when subjected to

the forces, temperatures, and other parameters of the

process that determines the success of the operation


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Atomic Structure and the Elements


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Atomic Structure and the Elements

The basic structural unitof matter is the atom

Each atom is composed of a positively charged nucleus,

surrounded by a sufficient number of negatively charged

electrons so the charges are balanced

More than 100 elements, and they are the chemicalbuilding blocks of all matter


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Element Groupings

The elements can begrouped into families and

relationships established

between and within the

families by means of the

Periodic Table

Metalsoccupy the left and

center portions of the table

Nonmetals are on right

Between them is a

transition zone containing

metalloidsor semi-metals

Metals Metalloids orSemimetals

NonMetals

Beryllium Be Boron B Helium He

LithiumLi SiliconSi NeonNe

Magnesium Mg ArsenicAs ArgonAr

Cadmium

Cd Antimony

Sb Krypton

Kr

Copper- Cu Polonium - Po Xenon Xe

Iron Fe Tellurium - Te Radon Rn

Zinc Zn Germanium - Ge FluorineF

Titanium Ti Chlorine Cl

GoldAu Oxygen O


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Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listedfor the 103 elements.

Periodic Table


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Question?

What are the noble metals?

Copper

Silver

Gold

Noble metals (precious metals) are metals that are resistant to corrosion or oxidation, unlike most base metals.

Platinum (Pt), Palladium (Pd)


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Bonding between Atoms and Molecules


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Atomsare held together in molecules by varioustypes of bonds

1. Primary bonds- generally associated with formation

of molecules

2. Secondary bonds- generally associated withattraction between molecules

Primary bonds are much stronger than secondary

bonds


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Primary

Bonding

Secondary

Bonding

Ionic

Covalent

Metallic

Dipole forces

London forces

Hydrogen bonding


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Primary Bonds

Characterized by strong atom-to-atom attractionsthat involve exchange of valence electrons

Following forms:

Ionic

Covalent

Metallic

The ones on the outer shell


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Ionic Bonding

Figure 2.4 First

form of primary

bonding: (a) Ionic

Atoms of one element give up their outer electron(s),which are in turn attracted to atoms of some other

elementto increase electron count in the outermost shell.

Properties:

Poor Ductility

Low Electrical Conductivity

Example: Sodium Chloride (NaCl)


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Covalent Bonding

Figure 2.4Second form of

primary bonding:

(b) covalent

Outer electrons are shared between two local atoms ofdifferent elements.

Properties:

High HardnessLow Electrical Conductivity

Examples: Diamond, Graphite


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Metallic Bonding

Figure 2.4 Thirdform of primary

bonding: (c) metallic

Outer shell electrons are shared by all atoms to forman electron cloud.

Properties:

- Good Conductor

(Heat and Electricity)- Good Ductility


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Secondary Bonds

Secondary bonds involve attraction forcesbetween molecules(whereas primary bonds involve

atom-to-atom attractive forces),

No transfer or sharing of electrons in

secondary bonding Bonds are weaker than primary bonds

Three forms:

1. Dipole forces

2. London forces

3. Hydrogen bonding


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Macroscopic Structures of Matter

Atoms and molecules are the building blocksof more macroscopic structure of matter

When materials solidify from the molten state,

they tend to close ranks and pack tightly,

arranging themselves into one of twostructures:

Crystalline

Noncrystalline


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Crystalline Structures


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Crystalline Structure

Structure in which atoms are located at regularand recurring positions in three dimensions

Unit cell- basic geometric groupingof atoms

that is repeated The pattern may be replicated millions of times within a

given crystal

Characteristic structure of virtually all

metals, as well as manyceramics and somepolymers


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Crystallinity

When the monomers are arranged in a neat orderly manner, the

polymer is crystalline.Polymers are just like socks. Sometimes

they are arranged in a neat orderly manner.

An amorphous solid is a solid in which the molecules have no order or

arrangement.Some people will just throw their socks in the drawer in one

big tangled mess. Their sock drawers look like this:


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Question?

Antique windowpanes are thicker at the bottom, because

glass has flowed to the bottom over time!

Glass has no crystalline structure, hence it is NOT a solid.

Glass is a supercooled liquid. Glass is a liquid that flows very slowly.

Glass is a highly viscous liquid!!


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Three Crystal Structures in Metals

1. Body-centered cubic (BCC)2. Face centered cubic (FCC)

3. Hexagonal close-packed (HCP)

Figure 2.8 Three types of crystal structure in metals.

# of atoms in unit cell: 9 # of atoms: 14 # of atoms: 17


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Crystal Structures for Common Metals

Room temperature crystal structures for some of thecommon metals:

Body-centered cubic (BCC)

Chromium, Iron, Molybdenum, Tungsten

Face-centered cubic (FCC) Aluminum, Copper, Gold, Lead, Silver,

Nickel, (Iron at 1670oF)

Hexagonal close-packed (HCP)

Magnesium, Titanium, Zinc


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Imperfections (Defects) in Crystals

Imperfections often arise due to inability ofsolidifying material to continue replicationof unit

cell,e.g., grain boundaries in metals

It is in fact: Deviation in the regular pattern of the

crystalline lattice structure.

Studying about imperfections is important:

Imperfection is bad: a perfect diamond (with no flaws) is

more valuable than one containing imperfections.Imperfection is good: the addition of an alloying

ingredient in a metal to increase its strength (this is

an imperfection which is introduced purposely).


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Types of defects or imperfections

Point defects, Line defects,

Surface defects.


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Point Defects

Imperfections in crystal structure involving eithera single atom or a few number of atoms

Figure 2.9 Point defects: (a) vacancy, (b) ion-pair vacancy (Schottky),

(c) interstitialcy, (d) displaced ion (Frenkel Defect).

Extra atom present

Dislocation of an atom


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Line Defects

Defect happens along a line( Connected group of pointdefects that forms a line in the lattice structure)

Most important line defect is a dislocation, which cantake two forms:

Edge dislocation

Screw dislocation


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Edge Dislocation

Figure 2.10 Line

defects: (a) edge

dislocation

Edge of an extra plane of atoms that exists inthe lattice


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Screw Dislocation

Figure 2.10 Line

defects: (b) screw

dislocation

Spiral within the lattice structure wrapped aroundan imperfection line, like a screw is wrapped

around its axis


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Surface Defects

Imperfections that extend in two directionstoform a boundary

Examples:

External:the surface of a crystalline object

is an interruption in the lattice structure

Internal:grain boundaries are internal

surface interruptions


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Elastic Strain


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Elastic Strain

When a crystal experiences a graduallyincreasing stress, it first deforms elastically

If force is removed lattice structure returns toits original shape

Figure 2.11 Deformation of a crystal structure: (a) original lattice: (b)

elastic deformation, with no permanent change in positions of atoms.


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Plastic Strain

If stress is higher than forces holding atoms intheir lattice positions, a permanent shape

change occurs

Figure 2.11 Deformation of a crystal structure: (c) plastic deformation(slip), in which atoms in the lattice are forced to move to new "homes.


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Effect of Dislocations on Strain

In the series of diagrams, the movement of the dislocationallows deformation to occur under a lower stress than in aperfect lattice.

Slipinvolves the relative movement of atoms on the

opposite sides of a plane in the lattice, called slip plane.

Figure 2.12 Effect of dislocations in the lattice structure under stress.
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Slip on a Macroscopic Scale

When a lattice structure with an edge dislocationis subjected to a shear stress, the material

deforms much more readilythan in a perfect

structure.

Dislocations are a good-news-bad-news situation

Good newsin manufacturingthe metal is easier to

form

Bad newsin designthe metal is not as strong as thedesigner would like


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Twinning

A second mechanism of plastic deformationin

which atoms on one side of a plane (the twinningplane) are shifted to form a mirror imageof theother side

Figure 2.13 Twinning, involving the formation of an atomic mirror image on

the opposite side of the twinning plane: (a) before, and (b) after twinning.


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Polycrystalline Nature of Metals

A block of metal may containmillions of individual crystals,

called grains

Such a structure is called

polycrystalline Each grain has its own unique

lattice orientation; but

collectively, the grains are

randomly oriented in the block


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Crystalline Structure

Growth of crystals in metals

Grain

How do polycrystallinestructures form? As a block of metal

cools from the moltenstate and begins tosolidify, individualcrystals nucleate atrandom positions and

orientations throughoutthe liquid

These crystals grow andfinally interfere witheach other, forming attheir interface a surfacedefect - a grainboundary

Grain boundaries aretransition zones, perhapsonly a few atoms thick

Grain

boundary


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Noncrystalline (Amorphous) Structures


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Noncrystalline (Amorphous) Structures

Many materials are noncrystalline Water and air have noncrystalline structures

A metal loses its crystalline structure when melted

Some important engineering materials have

noncrystalline forms in their solid state:

Glass

Many plastics

Rubber


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Features of Noncrystalline Structures

Two features differentiate noncrystalline(amorphous) from crystalline materials:

1. Absence of long-range order in

molecular structure

2. Differences in melting and thermal

expansion characteristics

What are the

differences

between

them?


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Crystalline versus Noncrystalline

Figure 2.14 Difference in structure between: (a) crystalline and (b)noncrystalline materials.

The crystal structure is regular,repeating, and denser The noncrystalline structure is randomand less tightly packed.


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Solidification

Alloy Metal Pure Metal


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Volumetric Effects

Figure 2.15 Characteristic change in volume for a pure metal (a

crystalline structure), compared to the same volumetric changes

in glass (a noncrystalline structure).

Tg=glass temperature

Tm=melting temperature


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Summary: Characteristics of Metals

Crystalline structuresin the solid state, almostwithout exception

BCC, FCC, or HCP unit cells

Atoms held together by metallic bonding

Properties: high strength and hardness, high

electrical and thermal conductivity

FCC metalsare generally ductile


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Manufacturing Processes

Prof Simin Nasseri

Summary: Characteristics of Ceramics

Most ceramics have crystalline structures,while glass(SiO2) is amorphous

Molecules characterized by ionic or covalent

bonding, or both

Properties: high hardness and stiffness,

electrically insulating, refractory, and

chemically inert

Refractorymaterials retain their strength at high temperatures. Theyare used to make crucibles and linings for furnaces, kilns and

incinerators.

?


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Manufacturing Processes

Summary: Characteristics of Polymers

Many repeating mersin molecule held togetherby covalent bonding

Polymers usually carbon plusone or more

other elements: H, N, O, and Cl

Amorphous(glassy) structure or mixture of

amorphous and crystalline

Properties: low density, high electrical

resistivity, and low thermal conductivity,

strength and stiffness vary widely

ch02 nature of materials

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