materials moments: arthur c—food containers lewis & ray—al composites
TRANSCRIPT
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Materials Moments:
Arthur C—Food Containers
Lewis & Ray—Al Composites
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Exam I
Friday 21 FebruaryCovers Chapters 1 – 7
Review Questions posted on Canvas
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Strengthening Mechanisms
Sections 7.8 – 7.13
Strengthening Metals
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Underlying Principle for Strengthening Metals
–Dislocations facilitate plastic deformation
–Inhibiting (binding, stopping, slowing) dislocation motion makes metals stronger
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Strengthening Metals:(Ways to restrict dislocation motion)
Composition change:1. Solid-solution strengthening (Diffusion)
a) Case hardeningb) Alloying
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1. Solid-solution strengthening (Diffusion)2. Alloying
Carburizing furnace
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City Steel Heat Treating Co.
Case Hardening – Hard Case w/ tough core
Low-C Steels (> 0.30% C):
Carburizing,Nitriding,Carbonitriding
Carburized depth of 0.030” to 0.050” in 4 hours @ 1700°F
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Alloying
http://tankiialloy.en.made-in-china.com/offer/AqCnWidOrYcV/Sell-Copper-Nickel-Alloy-Strip.html
Cu-Ni Alloy
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Atoms diffuse to a location that reduces strain energy
Underlying principle:
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Fig. 7.17
Tensile strains
Solid-Solution Strengthening:Smaller Substitutional Impurity
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Solid-Solution Strengthening:Larger Substitutional Impurity
Fig. 7.18
Compressive strains
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2. Solid-Solution Strengthening:
Interstital Impurity
Fig. 7.18
Compressive strains
Fits in interstitial sites
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2. Solid-Solution Strengthening:
Interstital Impurity
Fig. 7.18
Compressive strains
Fits in interstitial sites
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Strengthening metals:How are dislocations bound in:Solid-solution strengthening?
They seek sites near dislocations to reduce lattice strains.
This stabilizes the lattice and discourages plastic deformation.
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YouTube: Dislocation motion is analogous to the movement of caterpillar
How Solid-Solution strengthening
binds dislocations
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Cu-Ni alloy:Strength & Elongation Variation with Ni content
Fig. 7.16
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Strengthening Metals
No Composition change:
1.Grain-size Reduction— Polycrystalline metals
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Grain size reduction:Dislocation motion at a grain boundary
Fig. 7.14
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Grain-size reductionDislocation Pile-ups at grain boundaries
Young Modulus and Yield Strength 2:11
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How do we reduce grain size?
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Strengthening metals:How are dislocations bound in:
Grain-size reduction?
It’s difficult for dislocations to move past a grain boundary
The more grain boundaries, the more difficult for dislocations to move metal is strengthened
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Strengthening Metals:
(Ways to restrict dislocation motion)
1. Solid-solution strengthening (Diffusion)
2. Grain-size reduction
3. Strain Hardening a.k.a. Work Hardening
a.k.a. Cold Working
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3. Strain Hardening (Work Hardening) (Cold Working)
Includes (but not limited to)
Drawing
Rolling
Peening—Strain hardened on surface only
Strain hardened throughout
No composition change
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Strain Hardening in Copper
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Cold WorkingExample: Wire Drawing
YouTube: Wire Drawing“2.Combined Drawing Machine SH-1” 0:20 - 0:45
YouTube: Drawing Process in Manufacturing / Aluminium tube Production
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Cold WorkingExample: Drawing
2. Deep drawing of sheet metal, Tiefziehen von Metallblechen
1. Deep drawing of sheet metal, Tiefziehen von Metallblechen
YouTube:
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Strain Hardening:Example: Rolling
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Cold WorkingExample: Shot peening
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Cold WorkingExample: Shot peened surface
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DislocationDensities
PlasticDeformation:
Stainless Steel
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Strengthening due to Cold Work
Fig. 7.19
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Strengthening metals:
How are dislocations bound in:
Strain hardening?
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Increasing the dislocation density increases the number of dislocations which can repel each other.
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Plastic Deformation difficult
Dislocations can’t easily move
Metal is Strengthened
Strain Hardening
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Recovery, Recrystallization, &
Grain Growth
Sections 7.10 – 7.13
Reverse of Strengthening
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Annealing:Eliminates dislocations
1) Recovery
2) Recrystallization
3) Grain Growth
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Recrystallization 580ºC
Stages of Recrystallization and grain growth33% Cold-worked brass (Tm = 900-940ºC)
t = 3 sect = 0 t = 4 sec
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Grain size increases
Stages of Recrystallization and grain growthCold-worked brass
t = 8 sec (580ºC) t = 15 min (580ºC) t = 10 min (700ºC)
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Recovery followed by grain growth in polycrystalline camphor-ethanol mixture
YouTube Video:
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Plastic Deformation:Polycrystalline Cold-worked Nickel
Before deformation After deformation
Fig
. 7.1
1--1
70
x p
hoto
mic
rog
rap
h
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Controlled annealing
Strain-relaxed buffers due to annealing in Silicon
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Recovery, Recrystallization, and Grain Growth
Recovery (grains recover slightly from cold-working)
Recrystal-lization (new grains form)
Grain Growth (larger grains grow at expense of smaller)
See Fig. 7.22
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YouTube: Tensile Test on Work-Hardened Copper: necking effect
YouTube: Tensile Test on Annealed Copper
Compare these videos:Take note of the knurled knob on the RHS
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How do we
restore ductility to work hardened metals?
Eliminate Dislocations!
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Some little study aids follow
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Review on your own:When Strengthening metals:
How are dislocations bound in these cases?
1) Grain-size reduction
2) Solid Solution Strengthening
3) Strain hardening
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Element Crystal structure Atomic radius
Fe BCC 0.124 nm
Cr BCC 0.125 nm
Al FCC 0.125 nm
N HCP 0.065 nm
a) N in Fe at 700°C b) N in Fe at 900°C
c) Cr in Fe at 700°C d) Cr in Fe at 900°C
e) Al in Fe at 700°C f) Al in Fe at 900°C
1. For which combination of metals do you expect solid solution strengthening to occur?
2. For which combination of metals do you expect diffusion to be the fastest?
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Metallic xl Structures1) Face-Centered Cubic (FCC)
Cu, Al, Ag, Au, Pb, Ni, Pt
2) Body-Centered Cubic (BCC)Na, Fe, Cr, Mo, W
§ Hexagonal Close-Packed (HCP)Ti, Zn, Cd, Co, Mg
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