1 2 San Francisco, CA March 19, 2010 FE/EIT Review Materials Properties 3 This Review Session’s Agenda Materials Science – Quick Review – 60 minutes – You should already know these materials Practice Problems – 20 minutes to take the practice test – 10 minutes to go over problems 4 Materials Science Quick Review 5 Materials Science/Properties – 7% of total A. Properties mechanical chemical electrical physical B. Corrosion mechanisms and control C. Materials engineered materials ferrous metals nonferrous metals 6 Materials Science Review Atomic Bonding & Crystal Structures 7 Metals form crystals; 14 basic crystalline lattice structures (unit cells) based on 7 crystal systems
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San Francisco, CAMarch 19, 2010
FE/EIT Review
Materials Properties
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This Review Session’s Agenda
� Materials Science – Quick Review
– 60 minutes
– You should already know these materials
� Practice Problems
– 20 minutes to take the practice test
– 10 minutes to go over problems
4
Materials Science
Quick Review
5
Materials Science/Properties – 7% of total
A. Properties
� mechanical
� chemical
� electrical
� physical
B. Corrosion mechanisms and control
C. Materials
� engineered materials
� ferrous metals
� nonferrous metals
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Materials Science Review
Atomic Bonding & Crystal Structures
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Metals form crystals; 14 basic crystalline lattice structures (unit cells) based on 7 crystal systems
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How are the 7 crystal systems different?
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90% of metals have FCC, BCC, or HCP crystal structures
3 most common unit cells
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Some Nomenclatures
� Crystal or Space Lattice – 3D “wireframe”
– like a scaffold at a construction site
� Lattice constant
– a = 0.24nm
– b = 0.24nm
– c = 0.24nm
� 90% of metals crystallize upon solidification into 3
densely packed crystal structures:
– Body-centered cubic (BCC)
– Face-centered cubic (FCC)
– Hexagonal close-packed (HCP)
� How many unit cells in 1mm of space? (3.5 million)
ab
cLattice Constant
A Space Lattice
Examples
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Crystalline structures of common metals
� Body-Centered Cubic (BCC):
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Crystalline structures of common metals
� Face-Centered Cubic (FCC):
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Crystalline structures of common metals
� Hexagonal Close-Packed (HCP):
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Number of atoms in a unit cell
� Most atoms are only ‘partially’ in the unit cell!
� In a simple cubic unit cell, only 1 atom is in the unit cell
1/8 x 1 atom = 1/8 atom
There are 8 of these� Total number of atoms = 1/8 x 8 = 1
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Coordination Number & Atomic Packing Factor
� Coordination number: number of nearest neighbors
� Atomic Packing Factor (APF)
APF = Volume of atoms in unit cell*
Volume of unit cell
*assume hard spheres
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Miller Indices for Crystal Planes (BCC, FCC)z
x
ya b
c
4. Miller Indices (110)
example a b cz
x
ya b
c
4. Miller Indices (100)
1. Intercepts 1 1 ∞
2. Reciprocals 1/1 1/1 1/∞1 1 0
3. Reduction 1 1 0
1. Intercepts 1/2 ∞ ∞2. Reciprocals 1/½ 1/∞ 1/∞
2 0 03. Reduction 2 0 0
example a b c
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Miller Indices for Crystal Planes (HCP)
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Materials Science Review
2. Materials Testing
Engr Stress-Strain Curve (non-ferrous matls)
19Engineering Strain (εεεε)
En
gin
ee
rin
g S
tre
ss
(σσ σσ
)
0.002 Offset Strain
Permanent Set
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Engr Stress-Strain Graph (some ferrous matls)
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Stress-strain graphs for various metals & alloys
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Linear Elastic Properties
• Modulus of Elasticity, E:(also known as Young's modulus)
• Hooke's Law: σ = E ε
σ
Linear-elastic
E
ε
F
Fsimple tension test
Tension
Compression
F
FSimple
compression test
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Young’s Modulus – Stiffness of a metal
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• Energy to break a unit volume of material• Approximated by the area under the stress-strain curve.• Unit : J/m3
• Strength ≠≠≠≠ Toughness
Toughness
Brittle fracture: elastic energy
Ductile fracture: elastic + plastic energy
very small toughness (unreinforced polymers)
Engineering tensile strain, ε
Engineering tensile stress, σ
small toughness (ceramics)
large toughness (metals)
Metals are tough – they absorb a lot of energy prior
to fracture, e.g. metal bumper, auto body
Metals are tough – they absorb a lot of energy prior
to fracture, e.g. metal bumper, auto body
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Hardness
� Hardness: resistance of a material to plastic
deformation
� How to test hardness?
– Brinell Hardness (BHN or HB)
– Rockwell Hardness (HRB & HRC most commonly)
– Meyer, Vickers, Knoop etc.
� Hardness & Tensile Strength (TS) for steel
– TS (psi) ~ 500 BHN
– TS (MPa) ~ 3.5 BHN
e.g., 10 mm sphere
apply known force measure size of indent after removing load
dDSmaller indents mean larger hardness.
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26increasing hardness
most plastics
brasses Al alloys
easy to machine steels file hard
cutting tools
nitrided steels diamond
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Fracture : temperature-, environment-, loading rate- & loading history - dependent process
� Fracture is the separation of a body into 2 or more parts
in response to an externally applied stress
� Fracture involves 2 steps:
– Crack formation – presence of voids, hairline crack
– Crack propagation – crack continues to enlarge
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• Ductile failure:--one piece
--large deformation
Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures(2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission.
Example: Failure of a Pipe at different temperatures
• Brittle failure:--many pieces
--small deformation
Warm
Cold
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• Evolution to failure (5 steps):
• Resultingfracturesurfaces(steel)
50 mm
particlesserve as void
nucleationsites.
50 mm
From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp. 347-56.)
100 mm
Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.
Moderately Ductile Failure (cup-n-cone fracture)
necking
σ
void
nucleation
void growth
and linkage
Crack propagation fracture
Fibrous structure (“dimples”, ½ of void)
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Crack front is usually the “leading edge” of fracture due to “stress amplification”
σσσσ0σσσσ0
Crack Forms & Propagates
σσσσm σσσσm
σσσσm σσσσm
Crack front
Crack front
σ
Crack formation & evolution
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Cracks amplify stress and yield materials “prematurely” ∴∴∴∴ Flaws are Stress Concentrators
where
ρt = radius of curvature
σo = applied stress
σm = stress at crack tip
Kt = stress conc factor = 2 (a/ρt)1/2
ρt
Internal Crack
Surface Crack
Sharper crack has smaller radius of curvature and larger stress
amplification
ot
/
t
om Ka
σ=
ρσ=σ
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2
Max stress occurs at crack tip
Applied Stress
Stress conc. factor
Crack Length
(1)
Note: In this course, all cracks are treated as “elliptical cracks”.
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There are 3 methods to propagate a crack and cause failure – “Failure Modes”
To simplify things, you only need to focus on wt%C < 2.14%, i.e. austenite region
Hypereutectoid
0.76
Hypo-eutectoid
Eutectic Composition
Eutectoid Composition
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Austenite can transform into diff. microstructures at different temperatures, wt% & cooling rates
Austenite (γ)
Bainite(α + Fe3C plates/needles)
Pearlite(α + Fe3C layers + a
proeutectoid phase)
Martensite(BCT phase diffusionless
transformation)
Tempered Martensite (α + very fine
Fe3C particles)
slow cool
moderatecool
rapid quench
reheat
Str
ength
Ductilit
y
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends
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Microstructures of plain carbon steel
Spheroidite Coarse Pearlite Fine Pearlite
α-ferrite (light) Cementite (dark)73
Microstructures of plain carbon steel (continued)
Bainite Martensite
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Microstructures of plain carbon steel (continued)
MartensiteBCT, Single Phase
Tempered Martensiteα + Fe3C phases
Heating(250-650°C)
α-ferrite (dark)Cementite (light)
Note: Tempered martensite is similar to spheroidite in terms of microstructures, except the cementite particles in tempered martensite is much smaller. Hence tempered
martensite is much stronger than spheroidite
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Overview of microstructures during Austenite (γγγγ) transformation at various Temperature & wt%C
Microstructures of steel at different wt% C and temperatures