Exam review
al_awa
361_13_strengthening_mech_c.pptMaterials Moments:
Arthur C—Food Containers
Lewis & Ray—Al Composites
Background image: http://images.iop.org/objects/ntw/news/10/4/9/image1.jpg
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Exam I
Friday 21 February
Covers Chapters 1 – 7
Review Questions posted on Canvas
Strengthening Mechanisms
Sections 7.8 – 7.13
Strengthening Metals
Underlying Principle for Strengthening Metals
- Dislocations facilitate plastic deformation
- Inhibiting (binding, stopping, slowing) dislocation motion makes metals stronger
Strengthening Metals:
(Ways to restrict dislocation motion)
Composition change:
- Solid-solution strengthening (Diffusion)
- Case hardening
- Alloying
- Solid-solution strengthening (Diffusion)
- Alloying
Carburizing furnace
Image: http://heatingitup.files.wordpress.com/2011/12/carburizing-furnace.jpg
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City Steel Heat Treating Co.
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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
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
f16_07_pg190.jpg
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.
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:
- 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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Strengthening metals:
How do we reduce grain size?
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
Strengthening Metals:
(Ways to restrict dislocation motion)
- Solid-solution strengthening (Diffusion)
- Grain-size reduction
- 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 Working
Example: 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 Working
Example: 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 Working
Example: Shot peening
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Cold Working
Example: Shot peened surface
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Dislocation
Densities
Plastic
Deformation:
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
Annealing:
Eliminates dislocations
1) Recovery
2) Recrystallization
3) Grain Growth
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Recrystallization 580ºC
Stages of Recrystallization and grain growth
33% Cold-worked brass
(Tm = 900-940ºC)
t = 3 sec
t = 0
t = 4 sec
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Grain size increases
Stages of Recrystallization and grain growth
Cold-worked brass
t = 8 sec (580ºC)
t = 15 min (580ºC)
t = 10 min (700ºC)
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Plastic Deformation:
Polycrystalline
Cold-worked Nickel
Before deformation After deformation
Fig. 7.11--170x photomicrograph
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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
How do we
restore ductility
to work hardened metals?
Eliminate Dislocations!
Some little study aids follow
Review on your own:
When Strengthening metals:
How are dislocations bound in these cases?
Grain-size reduction
Solid Solution Strengthening
Strain hardening
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?
Metallic xl Structures
Face-Centered Cubic (FCC)
Cu, Al, Ag, Au, Pb, Ni, Pt
Body-Centered Cubic (BCC)
Na, Fe, Cr, Mo, W
Hexagonal Close-Packed (HCP)
Ti, Zn, Cd, Co, Mg
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