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
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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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
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Basic Material Concepts-Bonding Energy
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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.
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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.
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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
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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
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Interstitial impurity atom
Self interstitial
Substitutional impurity atom
Vacancy
Point Defects in Crystalline Structure
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Line Defects
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Plastic Deformations Along a Slip Plane Shear stresses
Shear stresses
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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
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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
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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
Percent weight of material B
Percent weight of material A
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
Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.
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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
Percent weight of material B
Percent weight of material A
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
Percent weight of material B
Percent weight of material A
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
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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
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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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