1. 1
RECRYSTALLIZATION IN METALS
FLORENT LEFEVRE-SCHLICK and DAVID EMBURY
Department of Materials Science and Engineering
McMaster University, Hamilton, ON, Canada

2. 2
OUTLINE
 Recrystallization
What 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 treatments
What are they?
What can we expect from them?
Recrystallization in metals
 Modeling
 Conclusions-Future work

3. 3
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. 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

5. 5
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 10
d
-1/2
(µm
-1/2
)
σY(MPa)
Cu
Fe
Al
Recrystallization
Grain refinement strengthening

6. 6
Johnson, Mehl, Avrami, Kolmogorov approach
1 exp( )n
X Bt= − −
0
1
recrystallizedfractionX
time
 Random distribution of nucleation sites
 Constant rate of nucleation and growth n=4
 Site saturation n=3
Recrystallization

7. 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

8. 8
“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

9. 9
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

10. 10
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

11. 11
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
time
T
time
seconds
mseconds
nano/pico/femtoseconds
Rapid heat treatments

12. 12
“Slow” heat treatment: Salt bath
Time/Temperature profile during salt bath
heat treatment
0
100
200
300
400
500
600
700
0 5 10 15
Time (sec)
Temperature(C)
Duration of the heat
treatment: 5 seconds.
Temperature range: 500o
C
to 650o
C.
Heating rate ~300C/sec
Cooling rate ~1000C/sec
Salt bath

13. 13
“NUCLEATION” IN IRON
Fe deformed by impact at 77K
50 µm B=[011]
01-1 -21-1
-200
21-1
-2-11
2-22
(-2-11)
(1-11)
grain
twin
Twinning 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

14. 14
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 500o
C
BF images of a nuclei
along a deformed twin.
Salt bath
“NUCLEATION” IN IRON

15. 15
“NUCLEATION” IN COPPER
50 µm
1 µm 4 µm
25 µm
Cu 60% cold rolled Cu ~ 2% recrystallized
5 seconds at 250o
C
No noticeable effect of annealing twins on nucleation
Salt bath

16. 16
45% cold rolled @ 77K
100µm
Stainless steel 316L
Cooperation with X. Wang
Salt bath
“NUCLEATION” IN STAINLESS STEEL

17. 17
2 min @ 950C
25µm
Stainless steel 316L
Average grain size: 7µm
Salt bath
“NUCLEATION” IN STAINLESS STEEL

18. 18
25µm
2 min @ 900C
Stainless steel 316L
Average grain size: 5µm
Salt bath
“NUCLEATION” IN STAINLESS STEEL

19. 19
Stainless steel 316L
25µm
2 min @ 850C
Average grain size: 3µm
Salt bath
“NUCLEATION” IN STAINLESS STEEL

20. 20
1 min @ 800C
10µm
Role of annealing, deformation twins and phases on nucleation and growth?
Stainless steel 316L
Salt bath
“NUCLEATION” IN STAINLESS STEEL

21. 21
DF image (austenite)
DF image
(austenite + martensite)
DF image (Twin)
BF image
Salt bath
1 min @ 800C
Stainless steel 316L
 Fine and complex deformed microstructure
 Over a range of possible growing grains, only a few seem to grow
“NUCLEATION” IN STAINLESS STEEL

22. 22
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>)

23. 23
“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 msec
Energy output: 100 J to 1 J
Estimated heating rate ~105
K/sec
Spot welding machine

24. 24
PHASE TRANSITION IN IRON
50 µm 50 µm
40 J 20 J
Melted zone
Heated zone
 Refinement of the microstructure via phase transitions
 Distribution in grain size from 40 µm down to less than 1 µm
Spot welding machine

25. 25
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

26. 26
20 J
50 µm
 Localized event along specific grain boundaries
Spot welding machine
RECRYSTALLIZATION AND PHASE TRANSITION IN IRON
Fe 60% cold rolled

27. 27
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 nm
to mm
Pulse lasers

28. 28
 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)

29. 29
WHY PULSED LASERS?
Pulse lasers

30. 30
SINGLE PULSE ABLATION OF FE
E = 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

31. 31
Si substrate
SiO2 isolant layer
Platinum
2 mm
2 mm 100 µm
25 nm2 µm
resistor
connector
TEMPERATURE MEASUREMENT DEVICE
Summer work of B. Iqbar
Measuring the changes in resistivity of Pt estimating the temperature
Pulse lasers

32. 32
Fe annealed, 1 grain
Corrected 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)
1
2
3
4
5
[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)
IM
H
N
0
2
4
6
8
10
12
14
16
0 200 400 600 800 1000
Hardness (GPa)
Displacement Into Surface (nm)
1
2
3
4
5
[6]
I
M
HN
INSTRUMENTED INDENTATION
Pulse lasers
0
10
20
30
40
200 400 600 800 1000 1200
Load On Sample (mN)
Displacement Into Surface (nm)
[2]
3
4
U
HD
I
E
M 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)
I
MH
N
0
2
4
6
8
10
12
14
16
200 400 600 800 1000 1200
Hardness (GPa)
Displacement Into Surface (nm)
[2]
3
4
IM HN

33. 33
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)
1
2
3
4
5
6
7
8
[9]
10
11
12S
U
HDI EM
H
N
L
-2
0
2
4
6
8
10
12
14
16
18
20
100 200 300 400
Hardness (GPa)
Displacement Into Surface (nm)
1
2
3
4
5
6
7
8
[9]
10
11
12
IM HN
INSTRUMENTED INDENTATION
Pulse lasers
 Softening of the deformed material?
 Is there local melting/solidification or local heating?

34. 34
SGGrain I
Grain II
nucleus
Grain I
Grain II
)(
2
)(
tr
tG
γ
>
Modeling
ZUROB’S MODEL FOR RECRYSTALLIZATION
 Needs input on local misorientations

35. 35
CONCLUSIONS – FUTURE WORK
 Investigation of the first stage of recrystallization by:
o Designing microstructures to promote N
o 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