Materials Moments:

Arthur C—Food Containers

Lewis & Ray—Al Composites

Exam I

Friday 21 FebruaryCovers 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:1. Solid-solution strengthening (Diffusion)

a) Case hardeningb) Alloying

1. Solid-solution strengthening (Diffusion)2. Alloying

Carburizing furnace

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

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

Solid-Solution Strengthening:Larger Substitutional Impurity

Fig. 7.18

Compressive strains

2. Solid-Solution Strengthening:

Interstital Impurity

Fig. 7.18

Compressive strains

Fits in interstitial sites

2. Solid-Solution Strengthening:

Interstital Impurity

Fig. 7.18

Compressive strains

Fits in interstitial sites

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

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

Grain-size reductionDislocation Pile-ups at grain boundaries

Young Modulus and Yield Strength 2:11

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)

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

Strain Hardening in Copper

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:

Strain Hardening:Example: Rolling

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Cold WorkingExample: Shot peening

Cold WorkingExample: Shot peened surface

DislocationDensities

PlasticDeformation:

Stainless Steel

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

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 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)

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

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

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?

1) Grain-size reduction

2) Solid Solution Strengthening

3) 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 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