6. Pearlite reaction in Fe-C alloys
Pearlitic nodules
Create at grain boundaries,
triple points, grain corners,
and surfaces
Form alternating parallel
lamellae of two product
phases (α + Fe3C)
Grow with constant radial
velocity into adjoining
austenite grains
6
← Partially transformed eutectoid steel
7. Nucleation of pearlite
Heterogeneous nucleation
generally at γ grain boundaries
Cooling below A1,
Small undercooling
Small nucleation rate – nodules grow as spheres
hemispheres without interfering with each other
High undercooling
Higher nucleation rate – nodules quickly cover
all boundaries
20-25 % of total transformation time
7
8. Nucleation of pearlite
Eitherα or Fe3C plates nucleate and
grow which promotes the growth of the
other phase.
At the region ahead of α
C is depleting and to promote α formation
At the region ahead of Fe3C
C is increasing and to promote Fe3C formation
8
9. Nucleation of pearlite
Hypoeutectoid steel
γ’ → αproeutectoid + γ
→ αproeutectoid + α + Fe3C
Formation of proeutectoid α leads to the
rejection of C to the surrounding γ phase
When supersaturation of γ phase with
respect to both α and Fe3C is reached,
Pearlite begins to form
9
10. Nucleation of pearlite
To minimize the activation free energy
barrier to nucleation
Epitaxial relationships exist between
γ, α, and Fe3C
10
12. Orientation relationships
Fe3C - γ1 Pitch
( 100) Fe C
3
( 554 ) γ1
( 001) Fe C
3
( 225) γ1
( 010) Fe Cγ ( 110)
3 1
Fe3C - γ2 Incoherent interface
12
13. Orientation relationships
α - Fe3C Pitch/Patch (eutectoid composition)
( 001) Fe C ( 521) [ 110] Fe C 2.6 from 131α
3 α 3
[ 010] Fe Cα2.6 from [ 113]
3
or Bagaryatski (off-eutectoid)
( 001) Fe C 3
( 211) α [ 100] Fe C 011α
3
[ 010] Fe Cα [ 111]
3
13
14. Growth of pearlite
Edgewise growth occurs by the motion
of the incoherent boundary
Sidewise growth occurs by Nucleation
Repeated nucleation (Mehl)
Branching (Hillert)
Growth rate is as a function of
Time
Transformation temperature
Nucleation
Prior-austenite grain size
14
15. Cellular growth
Composition and orientation
of α’ phase changes
discontinuously from Cα’ to Cα
for the α phase colony
Solutes diffuse
to form
β phase colony from
neighboring α colonies
with a distance of d = So/2
15
16. Pearlite transformation
Fora given temperature and γ grain size,
transformation rate occurs in 3 stages.
1st,low transformation rate,
site-saturation dependent
2nd, more nodules develop,
increase transformation rate
3rd, nodules impinge, the rate
slows as microstructure
gradually approaches
complete transformation 16
17. Pearlite transformation
Volume fraction of γ transformed to pearlite
π NG 3t 4
f = 1 − exp −
3
t – a given temperature
N – nucleation rate of
pearlite colonies
G – rate at which the
colonies grow into γ
17
18. Pearlite transformation
Temperature at which the austenite is
transformed also affects the pearlite
growth rate
Lowering temperature
increases driving force
for nucleation, which
increases transformation
rate
18
19. Pearlite transformation
Maximum rate of transformation occurs
at about 550°C
Above bainite grows
faster than pearlite
and results in bainitic
transformation
19
20. Pearlite transformation
Decreasingγ grain size will increase the
number of nucleation sites
(more heterogeneous nucleation sites)
More nuclei growing into γ
Decrease transformation time
Increase transformation rate
20
21. Pearlite transformation
Interlamellar spacing is also a strong
function of transformation
temperature
Lower temperatures will
result in a finer lamellar
structure
21
23. Finer pearlite structure
α - Fe3C Pitch/Patch or Bagaryatski
Cementite ledges
stop advancing at a
boundary
Bending of lamellar
because of series of
growth steps
23
24. Bainite transformation
Decomposition of γ in
steels at temperatures
below pearlite reaction,
but above martensitic
transformation
Two types of eutectoid
transformation
Pearlitic transformation
Bainite transformation
24
25. Bainite
Influence of carbon content in Fe-C
alloys to bainitic transformation
temperature
25
26. Bainite
Ferrous bainite consists of
Non-lamellar aggregate
of lath- or plate- shaped
α grains
Carbide precipitation
within the α grains
or in the inter-laths
(between thin strip)
26
27. Bainite
In steels containing high Si content,
Carbide precipitation can be
suppressed completely
Result in carbide-free structures
Still referred to as bainitic
structures.
27
28. Bainite
Important characteristic of bainite in
ferrous and nonferrous alloys
Formation of bainitic α plates
Results in surface relief
Indication : shape change
accompanied by shear
component similar to that
found in martensite plates
28
30. Bainite transformation
Dependence of transformation temperature
Bainitic microstructural differences are
presented in the distribution of carbides
formed in
Upper portion
Lower portion
of temperature
range.
30
31. Upper bainite
Upper bainitic microstructure forms at
temperatures of 350-500 °C
Needles/laths of α with Fe3C precipitates
between the
α laths
31
32. Upper bainite
Ferritelaths grow into γ in a similar way to
Widmanstätten side-plates
Ferrite nucleates on grain
boundary with
Kurdjumov-Sachs orientation ( 011) α ( 111) γ
relationship with austenite 111 101
α γ
large undercooling, ferrite nucleus grow
rapidly into austenite and form ferrite
laths with semicoherent interfaces 32
33. Upper bainite
As ferrite
laths thicken,
Carbon content of austenite
increases till reaching a level
of cementite formation
Cementite nucleates and
grows from carbon-rich
regions in austenite
33
34. Upper bainite
Iftemperature of formation upper
bainite increases,
Upper bainitic structure is more similar
to Widmanstätten side-plates
34
35. Upper bainite
As temperature of formation increases,
It is difficult to distinguish the pearlite
colonies and the upper bainite
Both grow competitively
Pearlite cementite may form as
broken lamellae
HW 1
How to distinguish these two structures?
35
36. Upper bainite
Bainitic microstructure in
hypo-eutectoid steel
Aggregate of ferrite laths are
usually formed in parallel groups,
called sheaves.
36
38. Upper bainite
Bainitic microstructure
in hypo-eutectoid steel
Decreasing transformation
temperature or
Increasing carbon content
Decreases widths of individual ferrite laths
Increases amount of carbide precipitation
38
39. Upper bainite
Bainitic microstructure
in hypo-eutectoid steel
Orientation relationship
between Fe3C and bainitic α
Bagaryatski
( 001) Fe Cα ( 211) [ 100] Fe C 011α
3 3
Isaichev
( 001) Fe C
3
( 111) [ 103]
α Fe3Cα
[ 101]
39
40. Upper bainite
Bainitic microstructure
in hypo-eutectoid steel
Orientation relationship
between Fe3C and parent γ
Pitsch
( 010) Fe Cγ ( 110) [ 001] Fe C
3 3
225
α
40
41. Upper bainite
Bainitic microstructure
in hypo-eutectoid steel
High carbide contents can form as
stringers
Poor mechanical properties,
particularly if a crack is created on
the carbides
Crack will easily propagate through
the carbide 41
42. Lower bainite
Lower bainitic microstructure forms at
lower portion of bainitic transformation
curves
42
43. Lower bainite
Bainitic microstructure changes from
laths to plates
Carbide precipitates become much finer
Lower bainitic structure consists of
heavily dislocated
ferrite plates, rather
than laths
43
44. Lower bainite
Most characteristic metallographic
difference is the distribution of carbides
Carbide precipitates are located within the
ferrite plates rather than between plates
Carbide precipitates are
oriented at a characteristic
angle of ~60° to the long axis
of the bainitic plate
44
46. Lower bainite
C rejection is slow and
C cannot move away fast
Precipitates occur and move to the next
level with the advance of ferrite plate
Carbide will form exactly about the same
size and lattice orientation
Orientationrelationships between Fe3C
has and α plane
Bagaryatski ( 001) Fe Cα ( 211) [ 100] Fe C 011α
3 3
Isaichev ( 001) Fe C
3
( 111) [ 103]
α Fe3Cα
[ 101] 46
47. Other bainite
Inverse bainitic structure
in hyper-eutectoid steels
Carbide phase
nucleate first
Precipitates as a lath or
plate and then become
surrounded ferrite
47
48. Other bainite
Nonferrous bainite
Ti – 4 Ni
Nonlamellar α
Retained β phase
Precipitates of Ti Ni
2
Cu – 27 Sn
α laths/plates
Interlath precipitations
48
49. Effect of alloying elements
Alloying elements added to Fe-C system
can alter eutectoid transformation.
Austenite stabilizers: Zr, Cu, Ni, Mn, N, C
Expand γ field (Reduce A temperature)
1
Ferrite stabilizers: Cr, Si, Be, Al, Mo, W, Nb,
V, P, Sn, Ti
Expand α field (Increase A temperature)
H
1 He
Li Be Austenite stabilizers Ferrite stabilizers B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs
Fr
Ba
Ra
La
Ac
Hf
Rf
Ta
Db
W
Sg
Re
Bh
Os
Hs
Ir
Mt
Pt Au Hg Tl Pb Bi Po
Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo
At Rn
49
50. Effect of alloying elements
Alloying elements added to Fe-C system
can alter eutectoid transformation.
Effect on A1
H He
Li Be Austenite stabilizers Ferrite stabilizers B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs
Fr
Ba
Ra
La
Ac
Hf
Rf
Ta
Db
W
Sg
Re
Bh
Os
Hs
Ir
Mt
Pt Au Hg Tl Pb Bi Po
Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo
At Rn
50
51. Effect of alloying elements
Alloying elements added to Fe-C system
can alter eutectoid transformation.
Effect on
eutectoid
carbon
content
H He
Li Be Austenite stabilizers Ferrite stabilizers B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs
Fr
Ba
Ra
La
Ac
Hf
Rf
Ta
Db
W
Sg
Re
Bh
Os
Hs
Ir
Mt
Pt Au Hg Tl Pb Bi Po
Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo
At Rn
51
52. Effect of alloying elements
Pearlite growth rate of Fe-C-X
X is substitutional element
If X diffuses more
slowly than C,
transformation rate
decreases
H He
Li Be Austenite stabilizers Ferrite stabilizers B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs
Fr
Ba
Ra
La
Ac
Hf
Rf
Ta
Db
W
Sg
Re
Bh
Os
Hs
Ir
Mt
Pt Au Hg Tl Pb Bi Po
Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo
At Rn
52
55. Ordering reaction
α’→α
Ordered structures, or called
superlattices, result from the ability of
atoms to arrange themselves into
specific ordered configurations.
55
56. Ordered structure
B2, CsCl prototype L12, AuCu3 prototype
Cl atomic position Au atomic position
½½½ 000
Cs atomic position Cu atomic positions
000 ½½0,½0½,
0½½
56
59. Ordering reaction
During cooling, ordering occurs independently
in various portions of crystal
Long-range order parameter L is given by
rA − X A r − XB
L= or B
1 − XA 1 − XB
rA and rB : probabilities that an A atom occupies
an A site and an B atom occupies
an B site, respectively
XA and XB : mold fractions of A and B,
respectively 59
60. Ordering reaction
If L = 1, the lowest internal energy.
Entropy becomes more important factor as
temperatures increase
L continuously decreases until above the
critical temperature Tc, which L = 0.
L = 0, it is impossible to
distinguish separate
sublattices extending
over long distance
60
61. Ordering reaction
Most ordering reaction occurs in what is
called “1st – order transformation”
At equilibrium transformation temperature,
the first derivatives of the Gibbs free energy
∂G/∂T and ∂G/∂P are discontinuous.
∂G/∂T = – S
H is also
discontinuous.
61
62. Ordering reaction
2nd – order transformation
∂G/∂T and ∂G/∂P are continuous.
∂2G/∂T2 and ∂2G/∂P2 are discontinuous.
(∂2G/∂T2) = – (∂S/∂T) = (∂H/∂T) /T = C /T
P P P P
H is continuous.
62
63. Ordering reaction
2 mechanisms for creating ordered phase
from disordered phase on cooling
1. Continuous increase in short-range order by
local arrangements occurring homogeneously
throughout the crystal → leading to long-
range order in final
Occur by 2nd – order transformation or at very high
supercoolings below Tc
Possible homogeneous nucleation by highly
coherent interface between ordered and
disordered regions
63
64. Ordering reaction
2. Energy barrier to form ordered domains for a process
of nucleation and growth
Generally more common
Atoms may have wrong kind of neighbors
creating well-defined boundaries, termed
antiphase boundaries (APBs).
64
66. Antiphase boundary
Antiphase boundaries can also be
generated by the motion of dislocations.
APB generated by edge-
dislocations in ordered
MnNi3 alloy
66
67. Antiphase boundary
Antiphase boundariescan also be
generated by deformation.
APB generated by
moving dislocations in
ordered AlFe3 alloy
67
69. Massive transformation
2 different crystal structures
must be
simple and stable/metastable at the
same composition, but at different
temperature
69
70. Massive transformation
An alloy must be cooled
fast enough to
temperature below T2
So, no time for precipitation
Massive transformation
appears to proceed primarily
by a non-cooperative
(random) transfer of atoms
across the interfaces between
the parent and product phases.
70
71. Massive transformation
Controlled by interface diffusion
Growth of the product phase at speeds
up to 10 to 20 mm/s
No known simple orientation
relationships exist between parent and
product phases
Microstructure often shows massive
patches of grains having irregular
boundaries
71
72. Massive transformation
Fe - 0.002 C alloy
Quenched in iced brine from 1000 °C
Microstructure shows ferrite grains with
irregular boundaries
HW 2
Differences between
massive transformation
and eutectoid
transformation?
72
73. Massive transformation
Cu-37.8 at.% Zn alloy
Aftera partial massive transformation
Massive α phase (dark, mottled) has
formed at the boundaries of and inside the
parent grains of β phase β
α
73
74. Massive transformation
Cu-21.5 at.% Ga alloy
Quenched from β structure (above 775°C)
Twinned feathery grains formed by massive
transformation, cross prior grain
boundaries
Arrows are α
precipitation
74
76. Polymorphic transformation
Polymorphic transformation involves
alteration of structure but not of
composition, and the transformation
occurs by a diffusional process.
76
