CH02 Nature of Materials(1)

Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

Materials for Civil and Construction Engineers

CHAPTER 2 Nature of Materials


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Basic Materials Concepts • At equilibrium atoms Ø specific atomic and molecular spacing Ø dictated by the size and arrangement of atoms

• Spacing varies changes in energy Ø  temperature Ø  mechanical (force)

• Atoms arrangement (electron configuration) Ø  bonding mechanisms Ø  molecular structure

• Bonding and structure of the atoms strongly influence strength and mechanical response


3

Basic Material Concepts-Bonding Energy


4

Basic Material Concepts - Bonds 1.  Primary Bond: forms when atoms interchange or

share electrons in order to fill the outer (valence) shells like noble gases. Types:

a)  Ionic b)  Covalent c)  Metallic

2.  Secondary Bond: forms from an imbalanced electric charge among atomic arrangements.


5

Ionic Bond Electrons transfer from one atom to another.

Na Cl + Na Cl

Na sodium atom

Cl chlorine atom

Na sodium cation atom has positive charge

Cl chlorine anion atom has negative charge

NaCl molecule


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Covalent Bond Atoms share electrons to fill outer shells

Ø Strength of the bond depends on

the number of valence electrons

needed (shared) to fill the subshell

Ø Materials with covalent bonds

have good heat and electricity

insulation properties


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Metallic Bond Ø Atoms share electrons with many neighboring atoms Ø Atoms with few valence electrons like to join with many others Ø Extremely strong and tight packing Ø Electrons

Ø  free to move between atoms

Ø  good conductor of heat and electricity

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

+ Cations fixed in lattice structure

-

- - -

- - - -

- -

- - - -

- - - -

Electrons “floating”


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Secondary Bonds • Dipolar electrostatic attraction and are much weaker than primary bonds. Ø Dipolar molecules (e.g., H2O) are asymmetric and

have one side positive while the other pole is negative. Ø van der Waals force. Ø Hydrogen bonds are a stronger type of secondary

bond because hydrogen atoms easily form dipoles and can bond this way in chains with many other atoms.


9

Materials Classification by Bond Type • Metals Ø metallic bonds between atoms with 1, 2, or 3 valence electrons Ø steel, iron, aluminum, etc.

• Inorganic Solids Ø covalent and ionic bonds between atoms with 5, 6, or 7 valence electrons Ø Ceramics – Portland cement concrete, bricks, diamond, glass, aggregates (rock)

• Organic Solids Ø long molecules of covalent hydrogen-carbon molecules with secondary bonds between chains Ø hydrocarbons Ø asphalt, plastics, wood


10

Metallic Materials • Crystal Lattice Structure

Ø Lattice repeating pattern of atoms

Ø 3-D geometric pattern

Ø Unit Cell – smallest repeating unit

• Grain Structure – collection of unit cells


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3-D Lattice Structures

• 14 possible 3-D lattice structures

• Three common ones:

Ø body center cubic (BCC)

Ø face center cubic (FCC)

Ø hexagonal close pack (HCP)

BCC

FCC

HCP


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Body Centered Cubic Face Centered Cubic Hexagonal Close Pack

center of lattice each corner each corner

center of faces each corner center top and bottom face center plane

9 atoms 14 atoms

17 atoms


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Equivalent Number of Atoms in Unit Cell Nine atoms but corner atoms are shared

Corner atoms shared with seven other cells

Each corner atom contributes 1/8 to the equivalent atom count

BCC Number of equivalent atoms Center atom 1 Corner atoms 8x(1/8) 1 Total eq. atoms 2

Number of equivalent atoms BCC 2 FCC 4 HCP 6


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• Volume of unit cell occupied by the atoms

cellunitofVolcellunitinatomsofVolAPF =

Atomic Packing Factor

FCC radius of atom: r

4r

Length of side, a

Volume of atoms in unit cell, Va 3

34 rVsphere π=

No. eq. atoms, FCC n=4

spherea VnV ×= 3

34 rnVa π×=

Volume of unit cell, Vc

( )74.0

2234

3

3

=×

×=

r

rnAPF

π

ra ×= 22 ( )322 rVc ×=


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Density

Where, Ø ρ = density Ø n = number of equivalent atoms in unit cell Ø A = atomic mass (gram/mole) Ø Vc = volume of unit cell Ø NA = Avogadro’s number (6.023 x 1023 atoms/mole)

AcNVnA

=ρ


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Imperfect World

• Perfect lattice structures only exist under ideal conditions and small quantities of material.

• Defects Ø Point Ø Line Ø Area Ø Volume


17

Interstitial impurity atom

Self interstitial

Substitutional impurity atom

Vacancy

Point Defects in Crystalline Structure


18

Line Defects


19

Plastic Deformations Along a Slip Plane Shear stresses

Shear stresses


20

Grain Development

As molten metal cools atoms loose energy and form together into lattice structures. Multiple nuclei develop creating grains.

1. Perfect grain growth 2. Grain starts at a new nuclei 3. Grains grow together with perfect

alignment (coherent boundary) 4. Grains grow together with imperfect

alignment (coherent strain boundary) 5. Grains grow together with imperfect

alignment (semicoherent boundary) 6. Grains grow together with skewed

alignment (incoherent boundary)

Grain Boundaries


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• Influence of flaws and slip planes on mechanical properties Ø Flaws & defects are weak spots, reducing toughness Ø Grain boundaries act as crack inhibitors, increasing toughness

• The size and arrangement of crystal grains influence the material behavior Ø This mainly depends on the rate of cooling of the molten metal Ø Smaller grains are formed by rapid cooling and increase toughness

• Both heat treating and plastic strains during manufacturing change grain structure


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Alloys Alloys have one or more compounds dissolved in a metal Ø Steel is an alloy of iron and carbon but frequently contains chromium, copper, nickel, phosphorous, etc.

• This is only possible if the different materials have compatible crystal structures

• Interstitial atoms fit between the metal atoms Ø Must have an atomic radius less than 60% of the host metal Ø Can dissolve only about 6% into the host

• Substitutional atoms take the place of host atoms in the lattice Ø If the atoms are similar enough, the compounds can mix easily


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• To have complete miscibility, the two alloying agents must be similar enough that the crystal lattice doesn’t strain too much.

• Hume-Rothery Rules: to have complete “miscibility” (limitless solubility), the elements must have the following characteristics: 1. Less than 15% difference in atomic radius 2. Same crystal structure 3. Similar electronegatives (ability of electron attraction) 4. Same valence


24

Phase Diagrams Also known as equilibrium diagram • Phase: liquid & solid states of a material • Phase diagram displays relationship between percent of elements & transition temperatures

• Phase diagrams for soluble, insoluble, or partially soluble materials


25

Phase Diagram Soluble Materials

Solid 0 25 50 75 100

100 75 50 25 0

Percent weight of material B

Percent weight of material A

Tem

pera

ture

Liquid + Solid

Liquid

Liquidius

Solidius

0 25 50 75 100

100 75 50 25 0



Tem

pera

ture

Tie line State

point

PsA

PsA

PA

PB

PlA

PlB

State point – combination of temperature and material composition

Tie line – horizontal line drawn through the state point

Similar for tie line-solidus vertical projection.

Vertical projection of the intersection of the tie line and liquidus identifies the percent of the liquid that is material A or B.

Melting point A

Melting point B


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Phase Diagram Soluble Materials

Solid 0 25 50 75 100

100 75 50 25 0



Tem

pera

ture

Liquid + Solid

Liquid

Liquidius

Solidius

0 25 50 75 100

100 75 50 25 0



Tem

pera

ture

PsA

PsA

PA

PB

PlA

PlB

mt = 100g pB = 40% plB = 20% psB = 70%

Given:

ml ms

Determine:

Solution: ml + ms = 100g 20ml +70ms = 40x100g 1/20(20ml +70ms) = 1/20(40x100)

ml + 3.5ms = 200 -(ml + ms) = -100

ms = 40 g, m1 = 60 g 2.5ms = 100 g 40

60

20 70


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0 25 50 75 100

100 75 50 25 0



Tem

pera

ture

Solid A+B

Liquid

Solidus

Liquidus

Similar regions to phase diagram for soluble materials. The solid is composed of particles of materials A and B since these materials are insoluble. Liquid

+B Liquid +A

Projecting a tie line in the Liquid + B area shows that the solid material is composed of 100% B.

Eutectic – Sudden transition from liquid to solid without a two phase region.

•  point •  composition •  isotherm

Insoluble materials


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Partially Soluble

Liquid

Liquid+β Liquid+α

α+β

β α

Liquidus

α and β are solid solutions of the A and B materials. The materials are partially soluble.

Solidus Solubility

limit

α is solid, predominately A material with some B material.

Composition

Tem

pera

ture

State point, tie line, and lever rules for determining composition still apply.


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Eutectoid Reaction

Liquid

Liquid+β Liquid+γ

γ+β β

Liquidus Solidus

Composition

Tem

pera

ture

α+β

α

γ

α+γ

Solid region

Solid state transformation of material, αóγ depending on temperature

Eutectoid point Eutectoid composition

Eutectoid temperature


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Inorganic Solids • Ceramics – very well defined unit cell producing Ø High strength Ø High durability Ø brittle materials like diamond

Silicate tetrahedron


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Classes of Inorganic Solids 1. Glasses Ø based on silica and have a random or amorphous but very stable crystalline structure

2. Vitreous Ceramics Ø clay products like pottery, bricks, etc.

3. High-Performance Ceramics Ø expensive, highly refined materials specially developed to have very specific properties

4. Cement & Concrete 5. Rocks & Minerals


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2.4 Organic Solids • Most polymers are long molecular chains of carbon and hydrogen

• Mechanical properties depend on

Ø polymer chain length

Ø the extent of cross-linking

Ø type of radical compounds linked to the H-C


33

Classes of Organic Compounds Thermoplastics Transition from elastic to

viscous plastic behavior when heated as the cross-link bonds between chains melt

Asphalt PVC, polyethylene, polypropylene, polystyrene, Teflon (PTFE) used for pipes, tubing, bottles, electrical insulation

Thermosets Chemical reaction to harden stable cross-links that don’t soften when heated

Epoxy, polyesters, and phenol-formaldehyde used as glues, reinforcing fibers, and Formica

Elastomers or Rubbers

Limited cross-linking flexible structure

Polyisoprene (natural rubber), polybutadiene (synthetic rubber), polychloroprene (Neoprene

Natural Polymers

wood


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Polymer Basics • Mer – The repeating unit in a polymer chain

• Monomer – A single mer-unit (n=1)

• Polymer – Many mer-units along a chain (n=103 or more)

• Degree of Polymerization –average number of mer-units in a chain.


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Polymer Structures

Activated polymer

Polymer

Isotactic one side

Sindiotactic alternating

Atactic random

Terminator

Radical or side chain


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Polymer Chain Structure

Ordered structure linear polymer

Cross linked structure linear polymer


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Melting Point and Glass Transition Temperature

Tm Temperature

Volu

me

Tm Temperature

Volu

me

Well defined melting point (crystalline material)

liquid

crystalline state

Glass transition temperature

Poorly defined melting point (amorphous material)

Tm Temperature

Volu

me

Tg

Free Volume

Liquid

Rubbery

Glassy


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Covalent Bond Effect on Stiffness

Fraction of covalent bonds

Elas

tic m

odul

us, G

Pa

0.0 0.2 0.4 0.6 0.8 1.0 1

10

100

1000

Simple hydrocarbons

Noncross-linked polymers (plexiglass)

Cross-linked polymers (epoxies, polyesters)

Drawn fibers and film (drawn PE, nylon, kevlar)

100% Covalent


Advanced Construction Materials • High strength, light alloys

• High performance concrete

• Fiber reinforced polymers

• Structural laminate systems

• Fiber optics

• Nano-technology

• Utilization of waste materials

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Date post:	24-Oct-2015
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CH02 Nature of Materials(1)

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