9 Recovery Recrystallization Grain Growth and 3rd Universal Principle Nucleation and Growthnotes
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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
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