RECRYSTALLIZATION IN METALS
FLORENT LEFEVRE-SCHLICK and DAVID EMBURY
Department of Materials Science and Engineering
McMaster University, Hamilton, ON, Canada
1
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
2
Recrystallization
What is it?
Fe
E =Estored=~100J/mol
Deformation
Heat
Recrystallization
(development of new strain free grains)
Recovery
(rearrangement of dislocations in sub grains)
3
Recrystallization
HOW DOES RECRYSTALLIZATION START?
 “nucleation”
Coalescence and growth of subgrains
∆Θ4
∆Θ4
∆Θ3
∆Θ3
∆Θ1
∆Θ2
 Strain Induced
Boundary Migration
Migration of a boundary
∆Θ1
Θ1
Θ1
Θ2
Θ2
E
1
> E
Θ2
2
In simple systems: small number of “nuclei” lead to recrystallized grains
4
Recrystallization
Improving the mechanical properties of materials
Grain refinement strengthening
7000
Cu
Fe
Al
6000
σ Y (MPa)
5000
4000
3000
2000
1000
0
0




2
4
6
-1/2
-1/2
d (µm )
8
How does recrystallization proceed?
How to control recrystallization?
How to achieve an important grain refinement?
Can we control more than just the scale?
10
5
Recrystallization
Johnson, Mehl, Avrami, Kolmogorov approach
X = 1 − exp( −Bt )
recrystallized fraction X
n
1
0
time
 Random distribution of nucleation sites
 Constant rate of nucleation and growth n=4
 Site saturation n=3
6
Recrystallization
Johnson, Mehl, Avrami, Kolmogorov approach
Is n misleading?
Site saturation 3d/2d/1d
3/2/1
Constant nucleation rate 3d/2d/1d
4/3/2
Fined grained Aluminium, low strain
4
Aluminium+ small amount of copper, 40% cold
rolled
1.7
Fe-Mn-C
<1
7
Recrystallization
“NUCLEATION” OF RECRYSTALLIZATION
Large orientation gradient
(transition bands)
Strain heterogeneities
(shear bands)
Fe-Si system
Cu
Hu et al. (1966)
Adcock et al. (1922)
8
Recrystallization
“NUCLEATION” OF RECRYSTALLIZATION
Particle Stimulated Nucleation
Oxide inclusions in Fe
Leslie et al. (1963)
Al-Si system
Cluster of SiO2 in Ni
Humphreys et al. (1977)
 Recrystallization originates at pre-existing subgrains within the deformation zone
 Nucleation is affected by particle size and particle distribution
9
Recrystallization
INVESTIGATING THE “NUCLEATION” EVENT
o
 Injecting nucleation sites to increase N:
• Local misorientation (twins)
• Local strain gradient (high deformation)
 Impeding growth of recrystallized grains
• Rapid heat treatments
10
Rapid heat treatments
What are rapid heat treatments?
T
seconds
•“Slow” heat treatment
(salt bath)
time
T
•“Rapid” heat treatment
(spot welding machine)
•“Ultra-fast” heat treatment
(pulsed laser)
mseconds
time
T
nano/pico/femtoseconds
time
11
Salt bath
“Slow” heat treatment: Salt bath
T im e/T em perature profile during salt bath
heat treatm ent
700
600
Temperature range: 500oC
to 650oC.
Heating rate ~300C/sec
Cooling rate ~1000C/sec
Tem perature (C )
Duration of the heat
treatment: 5 seconds.
500
400
300
200
100
0
0
5
10
15
Tim e (sec)
12
Salt bath
“NUCLEATION” IN IRON
Fe deformed by impact at 77K
Production of deformation twins to promote a variety of potential
nucleation sites for recrystallization, either at twin/grain
boundary or twin/twin intersections
(1-11)
2-22
(-2-11)
-2-11
-200
21-1 01-1
50 µm
4 µm
-21-1
B=[011]
Twinning plane {112}
Shear direction 111
grain
twin
13
Salt bath
“NUCLEATION” IN IRON
5 seconds at 500 C
o
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).
0-11
ZA=[133]
-110
-310
0-31
-301
22-2
21-1
21-1
12-1
-301
200
ZA=[011]
ZA=[113]
-110
BF images of a nuclei
along a deformed twin.
-110
0-31
21-1
21-1
12-1
0-31
-301
ZA=[113]
12-1
-301
14
ZA=[113]
Salt bath
“NUCLEATION” IN COPPER
Cu 60% cold rolled
50 µm
1 µm
5 seconds at 250oC
Cu ~ 2% recrystallized
25 µm
4 µm
No noticeable effect of annealing twins on nucleation
15
Salt bath
“NUCLEATION” IN STAINLESS STEEL
Stainless steel 316L
45% cold rolled @ 77K
100µm
Cooperation with X. Wang
16
Salt bath
“NUCLEATION” IN STAINLESS STEEL
Stainless steel 316L
2 min @ 950C
25µm
Average grain size: 7µm
17
Salt bath
“NUCLEATION” IN STAINLESS STEEL
Stainless steel 316L
2 min @ 900C
25µm
Average grain size: 5µm
18
Salt bath
“NUCLEATION” IN STAINLESS STEEL
Stainless steel 316L
2 min @ 850C
25µm
Average grain size: 3µm
19
Salt bath
“NUCLEATION” IN STAINLESS STEEL
Stainless steel 316L
1 min @ 800C
10µm
Role of annealing, deformation twins and phases on nucleation and growth?
20
Salt bath
“NUCLEATION” IN STAINLESS STEEL
Stainless steel 316L
1 min @ 800C
DF image
(austenite + martensite)
BF image
DF image (austenite)
DF image (Twin)
 Fine and complex deformed microstructure
 Over a range of possible growing grains, only a few seem to grow
21
Salt bath
RECRYSTALLIZATION AS A WAY TO CONTROL THE NATURE
OF GRAIN BOUNDARIES?
Stainless steel 316L, 2 min @ 850C
30%
25µm
0%
10o
20o
30o
40o
50o
60o
~30% of Σ3 boundaries
(rotation 60o, axis <111>)
22
Spot welding machine
“RAPID” HEAT TREATMENT: SPOT WELDING MACHINE
250 µm
Electrode of Cu
3mm
Fe annealed (thickness = 500 µm)
Fe 60% cold rolled (thickness = 200 µm)
Pulse discharge width: 1 msec
Energy output: 100 J to 1 J
Estimated heating rate ~105K/sec
23
Spot welding machine
PHASE TRANSITION IN IRON
40 J
Melted zone
Heated zone
20 J
50 µm
50 µm
 Refinement of the microstructure via phase transitions
 Distribution in grain size from 40 µm down to less than 1 µm
24
Spot welding machine
RECRYSTALLIZATION AND PHASE TRANSITION IN IRON
Fe 60% cold rolled
40 J
100 µm
50 µm
 Refinement of the microstructure via phase transitions and recrystallization
 Distribution in grain size from 100 µm down to less than 1 µm
25
Spot welding machine
RECRYSTALLIZATION AND PHASE TRANSITION IN IRON
Fe 60% cold rolled
20 J
50 µm
 Localized event along specific grain boundaries
26
Pulse lasers
“ULTRA FAST” HEAT TREATMENT: PULSE LASER IRRADIATION
(nano/pico/femtosecond)
Laser pulse:
 Energy (nJ to µJ)
 Time (fsec to nsec)
 Beam size (µm to mm)
~100 nm
to mm
Cooperation with Preston/Haugen group
Small volume on the surface
 Rapid heating and cooling
(104 to 1012 K/sec)
 Increase in pressure (up to TPa)
Shock wave.
27
Pulse lasers
“ULTRA FAST” HEAT TREATMENT: PULSE LASER IRRADIATION
(nano/pico/femtosecond)
 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
28
Pulse lasers
WHY PULSED LASERS?
29
Pulse lasers
SINGLE PULSE ABLATION OF FE
E = 9.2 µJ
E = 3.2 µJ
10 µm
10 µm
E = 1.0 µJ
5 µm
E = 0.2 µJ
5 µm
 What is the temperature profile?
 How to characterise the irradiated volume?
30
Pulse lasers
TEMPERATURE MEASUREMENT DEVICE
2 mm
2 mm
100 µm
2 µm
25 nm
Platinum
SiO2 isolant layer
resistor
connector
Si substrate
Measuring the changes in resistivity of Pt
Summer work of B. Iqbar
estimating the temperature
31
Pulse lasers
INSTRUMENTED INDENTATION
Fe annealed, 1 grain
Corrected harmonic contact stiffness: 1.106 N/m
Reduced Modulus (GPa)
Hardness (GPa)
Load On Sample (mN)
30
400
L U
16
14
300
12
20
1
2
3
4
5
[6]
10
8
6
10
1
2
200
4
4
N
5
[6]
100
M
N H
I
2
H
D
E
0
M I
3 H
0
I
NH
M
200
400
600
800
1000
1200
0
Displacement Into Surf ace (nm)
200
400
600
800
0
1000
200
Displacement Into Surf ace (nm)
400
600
800
1000
1200
Displacement Into Surf ace (nm)
Fe annealed, 3 different grains
Reduced Modulus (GPa)
Hardness (GPa)
Load On Sample (mN)
400
16
40
14
L U
300
12
30
10
20
8
I
[2]
3
4
200
H
M[2]
N
3
4
6
10
4
2
100
M
I
NH
D
E
H
0
I
M
NH
0
200
400
600
800
Displacement Into Surf ace (nm)
1000
1200
0
200
400
600
800
Displacement Into Surf ace (nm)
1000
1200
200
400
600
800
Displacement Into Surf ace (nm)
1000
1200
32
Pulse lasers
INSTRUMENTED INDENTATION
12 11 10
1 2 3
Load On Sample (mN)
Hardness (GPa)
7
20
18
6
1
2
3
4
5
6
7
8
[9]
10
11
12
5
LU
4
3
2
1
0
S
M
N
I
H
D
E
H
14
12
10
8
6
4
2
M
N
HI
0
-2
100
-1
100
200
300
400
1
2
3
4
5
6
7
8
[9]
10
11
12
16
200
300
400
Displacement Into Surface (nm)
Displacement Into Surface (nm)
 Softening of the deformed material?
 Is there local melting/solidification or local heating?
33
Modeling
ZUROB’S MODEL FOR RECRYSTALLIZATION
Grain II
Grain I
Grain II
Grain I
SG
nucleus
2γ
G (t ) >
r (t )
 Needs input on local misorientations
34
CONCLUSIONS – FUTURE WORK
 Investigation of the first stage of recrystallization by:
o
o Designing microstructures to promote N
o
o Using rapid heat treatments to allow nucleation but not G
 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
35