1. Chapter 10: Phase Transformations
ISSUES TO ADDRESS...
• Transforming one phase into another takes time. Fe g
(Austenite)
Eutectoid
transformation C
FCC
Fe3C
(cementite) a
(ferrite) +
(BCC)

2. Phase Transformation

3. Nucleation and Growth
• Reaction rate is a result of nucleation and growth
of crystals.
% Pearlite 50
100
Nucleation
regime
Growth
log (time) t
0.5
Nucleation rate increases with T
Growth rate increases with T
• Examples: Eutectoid reaction
T just below TE
Nucleation rate low
Growth rate high g
pearlite
colony
T moderately below TE g
Nucleation rate med .
Growth rate med.
Nucleation rate high
T way below TE g
Growth rate low

4. Nucleation nuclei (seeds) act as template to grow crystals
Nucleation driving force increases as increase T
Supercooling (or undercooling)
superheating
Small supercooling  few nuclei,
large crystals
Large supercooling  rapid nucleation,
many nuclei,
small crystals
In Chapter 9 we looked at the equilibrium phase diagram . This indicated phase structure if we wait long enough. But due to slow diffusion may not reach equilibrium We need to consider time- kinetics - energy of phase boundaries may be high. – also nucleation
Transformation rate
How fast do the phase transformations occur?
First need nuclei (seeds) to form for the rest of the material to crystallize

5. Fe-Fe3C phase diagram

6. Solidification: Nucleation Processes
Homogeneous nucleation
nuclei form in the bulk of liquid metal
requires supercooling (typically °C max)
Heterogeneous nucleation
much easier since stable “nucleus” is already present
Could be wall of mold or impurities in the liquid phase
allows solidification with only ºC supercooling

7. Homogeneous Nucleation & Energy Effects
Surface Free Energy- destabilizes
the nuclei (energy for makinmg surface)
,g = surface tension
HS = latent heat of
solidification
DGT = Total Free Energy
= DGS + DGV
Volume (Bulk) Free Energy –
stabilizes the nuclei (releases energy)
r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow

8. Homogenous and Heterogeneous Nucleation

9. Rate of Phase Transformation
Fixed T: Isothermal
Completely growth
maximum rate reached –
now amount unconverted decreases so rate slower
Fraction transformed, y
rate increases as surface area increases & nuclei grow
log t
S.A. = surface area
By convention r = 1 / t0.5

10. Isothermal Transformation Diagrams
• Fe-C system, Co = 0.76 wt% C
• Transformation at T = 675°C.
675°C
(DT smaller) 50
y (% pearlite)
600°C
(DT larger)
650°C
100
100
T = 675°C y,
% transformed 50 2 4 1 10 10
time (s)
400
500
600
700 1 10 2 3 4 5
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
Course pearlite  formed at higher T - softer
Fine pearlite  formed at lower T - harder

11. Rate of Phase Transformations
135C
119C
113C
102C
88C
43C 1 10
102
104
% Recrystallization of Rolled Copper
Percent recrystallization is function of time and temperature

12. Transformations & Undercooling•
Eutectoid
transformation (Fe-C System): g Þ a +
Fe3C
0.76 wt% C
0.022 wt% C
6.7 wt% C •
Can make it occur at:
...727ºC (cool it slowly)
...below 727ºC (“undercool” it!)
Fe3C (cementite)
1600
1400
1200
1000
800
600
400 1 2 3 4 5 6
6.7 L g
(austenite) +L
+Fe3C a
L+Fe3C d
(Fe)
Co , wt%C
1148°C
T(°C)
ferrite
727°C
Eutectoid:
Equil. Cooling: Ttransf. = 727ºC DT
Undercooling by DTtransf. < 727C
0.76
0.022

13. Eutectoid Transformation Rate
• Growth of pearlite from austenite:
cementite (Fe3C)
Ferrite (a) g a
pearlite
growth
direction
Diffusive flow
of C needed a g g a a
• Higher T give higher diffusivity

14. Effect of Cooling History in Fe-C System
• Eutectoid composition, Co = 0.76 wt% C
• Begin at T > 727°C
• Rapidly cool to 625°C and hold isothermally.
400
500
600
700
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C) 1 10 2 3 4 5
time (s) g g

15. Transformations with Proeutectoid Materials
CO = 1.13 wt% C
TE (727°C)
T(°C)
time (s) A + C P 1 10
102
103
104
500
700
900
600
800
Fe3C (cementite)
1600
1400
1200
1000
800
600
400 1 2 3 4 5 6
6.7 L g
(austenite) +L
+Fe3C a
L+Fe3C d
(Fe)
Co , wt%C
T(°C)
727°C DT
0.76
0.022
1.13
Hypereutectoid composition – proeutectoid cementite

16. T-T-T of Eutectoid Composition
A – Austenite
P – Pearlite
B – Bainite
M – Martensite
C – Cementite

17. Non-Equilibrium Transformation Products: Fe-C
• Bainite:
--a lathes (strips) with long
rods of Fe3C
--diffusion controlled.
• Isothermal Transf. Diagram
Fe3C
(cementite) 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50%
pearlite/bainite boundary A a
(ferrite)
100% pearlite
100% bainite
5 mm
T-T-T Diagram
Time –
Temperature –
Transformation

18. Spheroidite: Fe-C System
-- a grains with spherical Fe3C
-- diffusion dependent.
-- heat bainite or pearlite for long times near TE
60 m a
(ferrite)
(cementite)
Fe3C

19. Martensite: Fe-C System
--g(FCC) to Martensite (BCT)
Martensite needles
Austenite
60 m x
potential
C atom sites
Fe atom
sites
• Isothermal Transf. Diagram 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50% A
M + A
90%
• g to M transformation
-- is rapid!
-- % transformation depends on T only.

20. Martensite Formation  (FCC) P  (BCC) + Fe3C slow cooling quench
M (BCT)
tempering
M = martensite is body centered tetragonal (BCT)
Diffusionless transformation BCT if C > 0.15 wt%
BCT  few slip planes  hard, brittle

21. Phase Transformations of Alloys
Effect of adding other elements
Change transition temp.
Cr, Ni, Mo, Si, Mn
retard    +
Fe3C
transformation
Alloy steel (type 4340)

22. Continuous Cooling Curve
Actual processes involves cooling – not isothermal
Can’t cool at infinite speed

23. Dynamic Phase Transformations
On the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures:
42% proeutectoid ferrite and 58% coarse pearlite
50% fine pearlite and 50% bainite
100% martensite

24. Example Problem for Co = 0.45 wt%
42% proeutectoid ferrite and 58% coarse pearlite
first make ferrite
then pearlite
course pearlite
 higher T
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%)
T (°C)

25. Example Problem for Co = 0.45 wt%
50% fine pearlite and 50% bainite
first make pearlite
then bainite
fine pearlite
 lower T
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%)
T (°C)

26. Example Problem for Co = 0.45 wt%
100 % martensite – quench = rapid cool
A + B
A + P
A + a A B P
50%
200
400
600
800
0.1 10
103
105
time (s)
M (start)
M (50%)
M (90%) c)
T (°C)

27. Mechanical Prop: Fe-C System (1)
• Effect of wt% C
Co < 0.76 wt% C
Hypoeutectoid
Pearlite (med)
ferrite (soft)
Co > 0.76 wt% C
Hypereutectoid
Pearlite (med) C
ementite
(hard)
300
500
700
900
1100
YS(MPa)
TS(MPa)
wt% C
0.5 1
hardness
0.76
Hypo
Hyper
wt% C
0.5 1 50
100
%EL
Impact energy (Izod, ft-lb) 40 80
0.76
Hypo
Hyper
• More wt% C: TS and YS increase
, %EL decreases.

28. Mechanical Prop: Fe-C System (2)
• Fine vs coarse pearlite vs spheroidite 80
160
240
320
wt%C
0.5 1
Brinell hardness
fine
pearlite
coarse
spheroidite
Hypo
Hyper 30 60 90
wt%C
Ductility (%RA)
fine
pearlite
coarse
spheroidite
Hypo
Hyper
0.5 1
• Hardness:
fine > coarse > spheroidite
fine < coarse < spheroidite
• %RA:

29. Mechanical Prop: Fe-C System (3)
• Fine Pearlite vs Martensite:
200
wt% C
0.5 1
400
600
Brinell hardness
martensite
fine pearlite
Hypo
Hyper
• Hardness: Fine Pearlite << Martensite.
• Hardness: Pearlite < Bainite.

30. Tempering Martensite • • • reduces brittleness of martensite,
• reduces internal stress caused by quenching.
YS(MPa)
TS(MPa)
800
1000
1200
1400
1600
1800 30 40 50 60
200
400
600
Tempering T (°C)
%RA TS YS
9 mm
produces extremely small Fe3C particles surrounded by a. • •
decreases TS, YS but increases %RA

31. Summary: Processing Options
Austenite (g)
Bainite
(a + Fe3C plates/needles)
Pearlite
(a + Fe3C layers + a
proeutectoid phase)
Martensite
(BCT phase
diffusionless
transformation)
Tempered
(a + very fine
Fe3C particles)
slow
cool
moderate
rapid
quench
reheat
Strength
Ductility
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends