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RECRYSTALLIZATION IN METALS

FLORENT LEFEVRE-SCHLICK and DAVID EMBURY

Department of Materials Science and EngineeringMcMaster University, Hamilton, ON, Canada

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OUTLINE

RecrystallizationWhat is it?How is it usually treated?Importance of local misorientation/strain gradients on “nucleation”First stages of recrystallization; how can we investigate the “nucleation”?

Rapid heat treatmentsWhat are they?What can we expect from them?Recrystallization in metals

Modeling

Conclusions-Future work

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What is it?Fe

E =Estored=~100J/mol

Deformation

Heat

Recovery (rearrangement of dislocations in sub grains)

Recrystallization (development of new strain free grains)

Recrystallization

4

Recrystallization

HOW DOES RECRYSTALLIZATION START?

“nucleation”

Strain Induced Boundary Migration

∆Θ1∆Θ2

∆Θ3

∆Θ4

∆Θ1

∆Θ3

∆Θ4

Θ1

Θ2

Θ1

Θ2

Θ2

E 1 E 2>

Coalescence and growth of subgrains

Migration of a boundary

In simple systems: small number of “nuclei” lead to recrystallized grains

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Improving the mechanical properties of materials

How does recrystallization proceed? How to control recrystallization? How to achieve an important grain refinement? Can we control more than just the scale?

0

1000

2000

3000

4000

5000

6000

7000

0 2 4 6 8 10d-1/2 (µm-1/2)

σY (

MPa

)

CuFeAl

Recrystallization

Grain refinement strengthening

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Johnson, Mehl, Avrami, Kolmogorov approach

1 exp( )nX Bt= − −

0

1

recr

ysta

llize

d fr

action

X

time

Random distribution of nucleation sites Constant rate of nucleation and growth n=4 Site saturation n=3

Recrystallization

7

Johnson, Mehl, Avrami, Kolmogorov approach

Recrystallization

Is n misleading?

<1Fe-Mn-C

1.7Aluminium+ small amount of copper, 40% cold rolled

4Fined grained Aluminium, low strain

4/3/2Constant nucleation rate 3d/2d/1d

3/2/1Site saturation 3d/2d/1d

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“NUCLEATION” OF RECRYSTALLIZATION

Recrystallization

Hu et al. (1966) Adcock et al. (1922)

Large orientation gradient(transition bands)

Strain heterogeneities(shear bands)

Fe-Si system Cu

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Particle Stimulated Nucleation

Leslie et al. (1963) Humphreys et al. (1977)Oxide inclusions in Fe Al-Si system Cluster of SiO2 in Ni

Recrystallization originates at pre-existing subgrains within the deformation zone

Nucleation is affected by particle size and particle distribution

“NUCLEATION” OF RECRYSTALLIZATION

Recrystallization

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INVESTIGATING THE “NUCLEATION” EVENT

Injecting nucleation sites to increase N:

• Local misorientation (twins)

• Local strain gradient (high deformation)

Recrystallization

o

Impeding growth of recrystallized grains

• Rapid heat treatments

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What are rapid heat treatments?

T

time

•“Slow” heat treatment (salt bath)

•“Rapid” heat treatment (spot welding machine)

•“Ultra-fast” heat treatment (pulsed laser)

T

timeT

time

seconds

mseconds

nano/pico/femtoseconds

Rapid heat treatments

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“Slow” heat treatment: Salt bathTim e/Tem perature profile during salt bath

heat treatm ent

0

100

200

300

400

500

600

700

0 5 10 15Time (sec)

Tem

pera

ture

(C)

Duration of the heat treatment: 5 seconds.

Temperature range: 500oC to 650oC.

Heating rate ~300C/sec

Cooling rate ~1000C/sec

Salt bath

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“NUCLEATION” IN IRONFe deformed by impact at 77K

50 µm B=[011]

01-1 -21-1

-200

21-1

-2-11

2-22(-2-11)

(1-11)

graintwinTwinning plane {112}

Shear direction 111

Production of deformation twins to promote a variety of potential nucleation sites for recrystallization, either at twin/grain boundary or twin/twin intersections

4 µm

Salt bath

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ZA=[011]

ZA=[113] ZA=[113]

ZA=[113]200

0-11

22-2 21-1

ZA=[133]

-110

0-31

12-1 -301

21-1

-110

0-31

12-1-301

21-1

-110

0-31

12-1 -301

21-1

Kikuchi patterns of the parent grain, a twin and a cell of dislocations. Shift of about 0.5 deg in the ZA between the grain (green circle) and the cell (red circle).

-301-310

5 seconds at 500oC

BF images of a nuclei along a deformed twin.

Salt bath

“NUCLEATION” IN IRON

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“NUCLEATION” IN COPPER

50 µm

1 µm 4 µm

25 µm

Cu 60% cold rolled Cu ~ 2% recrystallized5 seconds at 250oC

No noticeable effect of annealing twins on nucleation

Salt bath

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45% cold rolled @ 77K

100µm

Stainless steel 316L

Cooperation with X. Wang

Salt bath

“NUCLEATION” IN STAINLESS STEEL

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2 min @ 950C

25µm

Stainless steel 316L

Average grain size: 7µm

Salt bath

“NUCLEATION” IN STAINLESS STEEL

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25µm

2 min @ 900C

Stainless steel 316L

Average grain size: 5µm

Salt bath

“NUCLEATION” IN STAINLESS STEEL

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Stainless steel 316L

25µm

2 min @ 850C

Average grain size: 3µm

Salt bath

“NUCLEATION” IN STAINLESS STEEL

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1 min @ 800C

10µm

Role of annealing, deformation twins and phases on nucleation and growth?

Stainless steel 316L

Salt bath

“NUCLEATION” IN STAINLESS STEEL

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DF image (austenite)

DF image (austenite + martensite)

DF image (Twin)

BF image

Salt bath

1 min @ 800CStainless steel 316L

Fine and complex deformed microstructure Over a range of possible growing grains, only a few seem to grow

“NUCLEATION” IN STAINLESS STEEL

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Salt bath

Stainless steel 316L, 2 min @ 850C

25µm

RECRYSTALLIZATION AS A WAY TO CONTROL THE NATURE OF GRAIN BOUNDARIES?

10o 20o 30o 40o 50o 60o

0%

30%

~30% of Σ3 boundaries (rotation 60o, axis <111>)

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“RAPID” HEAT TREATMENT: SPOT WELDING MACHINE

3mm

250 µm

Fe annealed (thickness = 500 µm)Fe 60% cold rolled (thickness = 200 µm)

Electrode of Cu

Pulse discharge width: 1 msecEnergy output: 100 J to 1 JEstimated heating rate ~105K/sec

Spot welding machine

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PHASE TRANSITION IN IRON

50 µm 50 µm

40 J 20 JMelted zoneHeated zone

Refinement of the microstructure via phase transitions

Distribution in grain size from 40 µm down to less than 1 µm

Spot welding machine

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RECRYSTALLIZATION AND PHASE TRANSITION IN IRON

40 J

50 µm100 µm

Refinement of the microstructure via phase transitions and recrystallization

Distribution in grain size from 100 µm down to less than 1 µm

Spot welding machine

Fe 60% cold rolled

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20 J

50 µm

Localized event along specific grain boundaries

Spot welding machine

RECRYSTALLIZATION AND PHASE TRANSITION IN IRON

Fe 60% cold rolled

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Laser pulse: Energy (nJ to µJ) Time (fsec to nsec) Beam size (µm to mm)

Small volume on the surface Rapid heating and cooling (104 to 1012 K/sec) Increase in pressure (up to TPa) Shock wave.

“ULTRA FAST” HEAT TREATMENT: PULSE LASER IRRADIATION(nano/pico/femtosecond)

Cooperation with Preston/Haugen group

~100 nmto mm

Pulse lasers

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λ = 800 nm

The beam has a Gaussian profile with a radius ω0

E0: full energy pulse (~10 µJ)

τp: duration of the pulse (~ 10 nsec/ 100psec/ 150 fsec)

φ: fluence or energy per unit area (J/cm2)

φth: threshold fluence (J/cm2) fluence required to transform the surface

Pulse lasers

“ULTRA FAST” HEAT TREATMENT: PULSE LASER IRRADIATION(nano/pico/femtosecond)

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WHY PULSED LASERS?

Pulse lasers

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SINGLE PULSE ABLATION OF FEE = 9.2 µJ

10 µm

5 µm

E = 1.0 µJ

10 µm

E = 3.2 µJ

5 µm

E = 0.2 µJ

What is the temperature profile? How to characterise the irradiated volume?

Pulse lasers

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Si substrate

SiO2 isolant layer

Platinum2 mm

2 mm 100 µm25 nm2 µm

resistor connector

TEMPERATURE MEASUREMENT DEVICE

Summer work of B. Iqbar

Measuring the changes in resistivity of Pt estimating the temperature

Pulse lasers

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Fe annealed, 1 grainCorrected harmonic contact stiffness: 1.106 N/m

0

10

20

30

200 400 600 800 1000 1200

Load On Sample (mN)

Displacement Into Surface (nm)

12345[6]

U

HD

I

E

M HN

L

0

100

200

300

400

200 400 600 800 1000 1200

Reduced Modulus (GPa)

Displacement Into Surface (nm)

12345[6]

IM HN

0

2

4

6

8

10

12

14

16

0 200 400 600 800 1000

Hardness (GPa)

Displacement Into Surface (nm)

12345[6]

IM

HN

INSTRUMENTED INDENTATIONPulse lasers

0

10

20

30

40

200 400 600 800 1000 1200

Load On Sample (mN)

Displacement Into Surface (nm)

[2]34

U

HDI

EM HN

L

Fe annealed, 3 different grains

0

100

200

300

400

200 400 600 800 1000 1200

Reduced Modulus (GPa)

Displacement Into Surface (nm)

[2]34

IMHN

0

2

4

6

8

10

12

14

16

200 400 600 800 1000 1200

Hardness (GPa)

Displacement Into Surface (nm)

[2]34

IM HN

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1 2 3

12 11 10

-1

0

1

2

3

4

5

6

7

100 200 300 400

Load On Sample (mN)

Displacement Into Surface (nm)

12345678[9]101112S

U

HDI EM HN

L

-2

0

2

4

6

8

10

12

14

16

18

20

100 200 300 400

Hardness (GPa)

Displacement Into Surface (nm)

12345678[9]101112IM HN

INSTRUMENTED INDENTATIONPulse lasers

Softening of the deformed material? Is there local melting/solidification or local heating?

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SGGrain IGrain II nucleus

Grain IGrain II

)(2)(tr

tG γ>

Modeling

ZUROB’S MODEL FOR RECRYSTALLIZATION

Needs input on local misorientations

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CONCLUSIONS – FUTURE WORK

Investigation of the first stage of recrystallization by:

o Designing microstructures to promote No Using rapid heat treatments to allow nucleation but not G

o

o

Characterize the heat treatment in terms of time/temperature profile

Characterize the “nucleation” event in terms of local misorientation, local strain gradient (EBSD)

Introduce the data on misorientation into Zurob’s model