Objectives

1. Identify the three major classes of strengthening processes for polycrystalline metals and explain why they work.

2. Explain recovery, recrystallization and grain growth.

3. Explain the third Universal Principle: Nucleation and Growth processes.

Elastic DeformationElastic DeformationStretching of molecular bondsStretching of molecular bonds

Will be almost the same in Will be almost the same in tension as in compression.tension as in compression.

Plastic Deformation: SlipPlastic Deformation: Slip

Slip Happens on “Slip Slip Happens on “Slip Systems”Systems”

Close Packed PlanesClose Packed Planes

Close Packed DirectionsClose Packed Directions

Actual YieldActual Yield

Most Metals are Most Metals are

Polycrystalline Polycrystalline

Neighbors Interfere!Neighbors Interfere!

SSyy Polycrystals > S Polycrystals > Sy y MonocrystalsMonocrystals

Dislocations and Plasticity

1. Deformation of pure crystals is greater than expected based on number of pre-existing dislocations.

2. Dislocations density increases during plastic deformation.

3. Pre-existing dislocations do not move – they are pinned by “atmospheres” of impurities.

4. Conclusion: Dislocations must be created!

Nucleation of DislocationsNucleation of Dislocations1. Homogeneous1. Homogeneous

Uniform conditionsUniform conditions

No “Assistance” No “Assistance”

just shear stress just shear stress

Nucleation of Dislocations2. Heterogeneous

Non uniform conditions

Assistance by impurities or defects

Heterogeneous NucleationHeterogeneous NucleationFrank-Reed SourceFrank-Reed Source

Dislocation Loop: Frank ReedDislocation Loop: Frank Reed

Slip: The Highway AnalogySlip: The Highway Analogy

Slip systems are freeways Slip systems are freeways for Dislocations.for Dislocations.

Slip: Highway AnalogySlip: Highway Analogy

Three Things Slow TrafficThree Things Slow Traffic

1. Blocked Freeways 1. Blocked Freeways (construction)(construction)

2. Too Many Cars2. Too Many Cars

3. Pedestrians, Stopped 3. Pedestrians, Stopped CarsCars

TrafficTraffic

1.1. Blocked Freeways- Blocked Freeways- ConstructionConstruction

Slip

Grain Boundaries

Grain BoundariesGrain Boundaries3 3 DislocationsDislocations

8 8 DislocationsDislocations

Which is more Which is more difficult?difficult?

Yield Strength vs. Grain SizeYield Strength vs. Grain Size

TrafficTraffic

Too Many CarsToo Many CarsSlip

Dislocation Density

Called Work Hardening or Strain Hardening

Yield Strength vs. Dislocation Yield Strength vs. Dislocation DensityDensity

TrafficTraffic

Pedestrians, Pedestrians, Stopped CarsStopped Cars

Slip

Impurities

Alloying Elements

Precipitates

Hardness of Steel vs. Interstitial Hardness of Steel vs. Interstitial Carbon ContentCarbon Content

Ex: Solid SolutionEx: Solid SolutionStrengthening in CopperStrengthening in Copper

Tensile strength & yield strength increase with wt% Ni.

Adapted from Fig. 7.16 (a) and (b), Callister 7e.

Ten

sile

str

engt

h (M

Pa)

wt.% Ni, (Concentration C)

200

300

400

0 10 20 30 40 50 Yie

ld s

tren

gth

(MP

a)wt.%Ni, (Concentration C)

60

120

180

0 10 20 30 40 50

Reheating Plastically Deformed Metal

1. Recovery – elimination of dislocations

2. Recrystallization – formation of equiaxed new grains between old grains.

3. Grain Growth – equiaxed grains grow, absorbing old distorted grains.

• Effects of cold work are reversed!

Adapted from Fig. 7.22, Callister 7e. (Fig.7.22 is adapted from G. Sachs and K.R. van Horn, Practical Metallurgy, Applied Metallurgy, and the Industrial Processing of Ferrous and Nonferrous Metals and Alloys, American Society for Metals, 1940, p. 139.)

Effect of Heating After %Effect of Heating After %CWCW te

nsi

le s

tre

ngth

(M

Pa)

duc

tility

(%

EL

)tensile strength

ductility

Recovery

Recrystallization

Grain Growth

600

300

400

500

60

50

40

30

20

annealing temperature (ºC)200100 300 400 500 600 700

Annihilation reduces dislocation density.

RecoveryRecovery

• Scenario 1Results from diffusion

Dislocations annihilate and form a perfect atomic plane.

extra half-plane of atoms

extra half-plane of atoms

atoms diffuse to regions of tension

• New grains are formed that: -- have a small dislocation density -- are small -- consume cold-worked grains.

Adapted from Fig. 7.21 (a),(b), Callister 7e. (Fig. 7.21 (a),(b) are courtesy of J.E. Burke, General Electric Company.)

33% coldworkedbrass

New crystalsnucleate after3 sec. at 580C.

0.6 mm 0.6 mm

RecrystallizationRecrystallization

• All cold-worked grains are consumed.

Adapted from Fig. 7.21 (c),(d), Callister 7e. (Fig. 7.21 (c),(d) are courtesy of J.E. Burke, General Electric Company.)

After 4seconds

After 8seconds

0.6 mm0.6 mm

Further RecrystallizationFurther Recrystallization

• At longer times, larger grains consume smaller ones. • Why?

After 8 s,580ºC

After 15 min,580ºC

0.6 mm 0.6 mm

Adapted from Fig. 7.21 (d),(e), Callister 7e. (Fig. 7.21 (d),(e) are courtesy of J.E. Burke, General Electric Company.)

Grain GrowthGrain Growth

Characteristic Strength ValuesCharacteristic Strength Values

MaterialMaterial Yield Strength Yield Strength

SteelsSteels 50-200 ksi50-200 ksi

AluminumAluminum 10-70 ksi10-70 ksi

PolycarbonatePolycarbonate 6.5-10 ksi6.5-10 ksi

Nucleation and GrowthNucleation and Growth

Nucleation ProcessesNucleation Processes

Homogeneous (no Homogeneous (no assistance, high energy)assistance, high energy)

Heterogeneous (assistance, Heterogeneous (assistance, low energy)low energy)

Nucleation and Growth

Will follow standard incubation/growth laws:

Growth Processes

Often diffusion or heat flow limited

Competes with further nucleation

Nucleation and Growth