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• Transforming one phase into another takes time.
TOPIC 10:
PHASE TRANSFORMATIONS
• Transforming one phase into another takes time.
• How does the rate of transformation depend on
time and T?
• How can we control the transformation so that
we can engineering non-equilibrium structures?
• How different are the mechanical properties of • How different are the mechanical properties of
non-equilibrium structures?
IMPORTANCE OF COOLING TIME
Cu-Ni alloy
Slow coolingFast cooling
First α to solidfy:
46wt%Ni
Uniform Cα:
35wt%Ni
Last α to solidfy:
< 35wt%Ni
Slow coolingEquilibrium phases
Fast coolingNon-equilibrium phases
COOLING AUSTENITE
Austenite
Pearlite
• Mainly interested in eutectoid cooling: γγγγ ���� αααα + Fe3C (pearlite), 0.77 wt% C• Cooling rate can result in a wide variety of phases and microstructures
– Equilibrium phases: pearlite, bainite– Non-equilibrium phases: martensite
MECHANICAL PROPERTIES
• Martensite• Martensite• Tempered martensite• Bainite• Fine pearlite• Coarse pearlite
Stre
ngth
Duc
tilit
y
• Can control the formation of specific phases and
microstructure so that desired properties result
• Fraction transformed depends on time,
at constant temperature (e.g., γ � pearlite)
FRACTION OF TRANSFORMATION
nktey −−= 1
• Transformation rate , r = 1/t0.5
ktey −−= 1Avrami equation
(k, n are constants)
• Growth of pearlite from austenite:
EUTECTOID TRANSFORMATION RATE ~ ∆T
• Reaction rate increases with ∆T.
• Fe-C system, Eutectoid composition (Co = 0.77wt%C)
• Transformation at T = 675C.
TIME-TEMPERATURE TRANSFORMATION
(TTT) DIAGRAMS
Also called
isothermal
transformation
diagram
• Eutectoid composition, Co = 0.77wt%C
• Begin at T > 727C
• Rapidly cool to 625C and hold isothermally.
• Cooling to lower temperatures results in finer microstructures
EX: COOLING HISTORY Fe-C SYSTEM
• Ttransf just below TE--Larger T: diffusion is faster
Two cases:
• Ttransf well below TE--Smaller T: diffusion is slower
PEARLITE MORPHOLOGY
- Smaller ∆T:
colonies are larger
- Larger ∆T:
colonies are smaller
transf E--Larger T: diffusion is faster
--Pearlite is coarser.
transf E--Smaller T: diffusion is slower
--Pearlite is finer.
larger smaller
• Bainite:--α strips with long, fine
rods of Fe3C Fe3C
OTHER TRANSFORMATION PRODUCTS
• Isothermal Transf. Diagram
(Adapted from Fig. 10.8, Callister, 6e. (Fig.
10.8 from Metals Handbook, 8th ed.,
Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for
3
(cementite)
5 µm
α (ferrite)
Phase Diagrams, American Society for
Metals, Materials Park, OH, 1973.)
Note: reaction rate
increases with decreasing
temperature first, and then
decreases
• Reaction rate is a result of nucleation and growth
of crystals.
NUCLEATION AND GROWTH
Nucleation rate increases with ∆T
• Examples:
γ γ γ
pearlite colony
Nucleation rate increases with ∆T
Growth rate increases with T
Nucleation rate high
T just below TE T moderately below TE T way below TENucleation rate low
Growth rate high
Nucleation rate med .Growth rate med. Growth rate low
• Martensite:--rapid cooling from above eutectoid temperature to room T
--γ(FCC) to Martensite (Body Centered Tetragonal)
--involves collective motion of a lot of atoms
OTHER PRODUCTS: MARTENSITE
• Isothermal Transf. Diagram• Isothermal Transf. Diagram
• γ to M transformation..-- is rapid! At speed of sound
-- % transf. depends on T only.
• Martensite:--γ(FCC) to Martensite (BCT)
xpotential Fe atom
(involves single atom jumps)
µm
OTHER PRODUCTS: Fe-C SYSTEM (2)
(Adapted from Fig. 10.12, Callister, 6e. (Fig. 10.12 courtesy United
States Steel Corporation.)
• Isothermal Transf. Diagram
xx x
x
x
potential C atom sites
Fe atom sites
600
800
T(°C)Austenite (stable)
P
TEA
S
Martentite needlesAustenite
60
µ
(Adapted from Fig.
10.11, Callister, 6e.
11
Adapted
from Fig.
10.13,
Callister 6e.
States Steel Corporation.)
time (s)10 103 10510-1
400
200
B
0%
100%50%
A
S
M + AM + A
M + A
0%50%90%
• γ to M transformation..-- is rapid!
-- % transf. depends on T only.
PRODUCTS OF COOLING AUSTENITE
• Slow cooling � pearlite• Cool rapidly to upto 550
C, and hold � pearlite• Cool rapidly to 550-225
C and hold � bainite• Cool rapidly to below • Cool rapidly to below
225 C � martensite
COOLING EX: Fe-C SYSTEM (1)
Rapidly cool to 350 C
Hold for 10000 seconds
Rapidly cool to room T
100% Bainite
Rapidly cool to room T
100% Austenite
100% Bainite
COOLING EX: Fe-C SYSTEM (2)
Rapidly cool to 250 C
Hold for 100 secondsHold for 100 seconds
Rapidly cool to room T
100% Austenite100% Austenite
Mostly Martensite + traces of Austenite
COOLING EX: Fe-C SYSTEM (3)
50% Austenite,
Rapidly cool to 650 C
Hold for 20 seconds
Rapidly cool to 400 C
100% Austenite50% Austenite,50% Pearlite
50% Austenite,50% Pearlite
50% Bainite, 50% Pearlite
Rapidly cool to 400 C
Hold for 1000 seconds
Rapidly cool to room T
50% Bainite, 50% Pearlite
α
(ferrite)
• Spheroidite:--α crystals with spherical Fe3C
--diffusion dependent.
--heat bainite or pearlite for long times
OTHER PRODUCTS: Fe-C SYSTEM (1)
60 µm
Fe3C
(cementite)
--heat bainite or pearlite for long times
--reduces interfacial area (driving force)
• Isothermal Transf. Diagram
(Adapted from Fig. 10.10, Callister, 6e. (Fig. 10.10 copyright United
States Steel Corporation, 1971.)
600
800
T(°C)Austenite (stable)
P
TEA
Spheroidite100% spheroidite
100% spheroidite
10
Adapted from Fig. 10.9,Callister 6e.
(Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for
Metals, 1997, p. 28.)
States Steel Corporation, 1971.)
10 103 105time (s)10-1
400
200
B
0%
10
0%
50
%
A
100% spheroidite
• reduces brittleness of martensite,
• reduces internal stress caused by quenching.
TEMPERING MARTENSITE
TS(MPa)
Adapted from
Fig. 10.24,
Callister 6e.
(Fig. 10.24
copyright by
United States
Steel
Corporation,
Adapted from
Fig. 10.25,
Callister 6e.
(Fig. 10.25
adapted from
Fig. furnished
courtesy of
Republic Steel
YS(MPa)
1000
1200
1400
1600
1800
40
50
60
%AR
TS
YS
%AR
9 µ
m
18
Corporation,
1971.)
Republic Steel
Corporation.)
• decreases TS, YS but increases %AR
80030
40
200 400 600Tempering T (°C)
• produces extremely small Fe3C particles surrounded by α.
MECHANICAL PROPERTIES
• Martensite• Martensite• Tempered martensite• Bainite• Fine pearlite• Coarse pearlite• Spheroidite
Stre
ngth
Duc
tilit
y
• Spheroidite
• Can control the formation of specific phases and
microstructure through a cooling schedule
so that desired properties result
HYPOEUTECTOID & HYPEREUTECTOID
• Eutectoid (0.77 wt% C) �pearlite (ferrite & cementite layers)
Austenite
layers)• Hypoeutectoid (< 0.77 wt%
C) � pearlite & ferrite• Hypereutectoid (> 0.77 wt%
C) � pearlite & cementite• Ferrite is soft and cementite
is hardis hard• Thus, hardness and strength
increase with carbon content
Pearlite
HYPEReutectiod Steel TTT Curve
Alloy Steel TTT Curve
Continuous Cooling Transformation (CCT)
Continuous Cooling Transformation (CCT)
Continuous Cooling Transformation (CCT)
MECHANICAL PROP: Fe-C SYSTEM (1)
• Fine Pearlite vs Martensite:
MECHANICAL PROP: Fe-C SYSTEM (2)
• Hardness: fine pearlite << martensite.
• Tempering martensite (holding at high temperature)
reduces brittleness and residual stresses
Austenite (γ)
slow cool
moderate cool
rapid quench
Adapted from
Fig. 10.27,
Callister 6e.
SUMMARY: PROCESSING OPTIONS
Bainite (α + Fe3C plates/needles)
Pearlite (α + Fe3C layers + a
proeutectoid phase)
Martensite (BCT phase
diffusionless transformation)
cool cool quench
reheat
Str
en
gth
Martensite T Martensite
19
Tempered Martensite (α + very fine
Fe3C particles)
Str
en
gth
Du
cti
lityT Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends
AusteniteAusteniteAusteniteAustenite
MartensiteMartensiteMartensiteMartensite
AS: Alloy SteelPCS: Plain-carbon Steel
RapidRapidRapidRapidQuenchQuenchQuenchQuench
SpheroiditeSpheroiditeSpheroiditeSpheroidite
Moderate cooling (AS)Moderate cooling (AS)Moderate cooling (AS)Moderate cooling (AS)Isothermal treatment (PCS)Isothermal treatment (PCS)Isothermal treatment (PCS)Isothermal treatment (PCS)
Tempered Tempered Tempered Tempered
Slow Slow Slow Slow CoolingCoolingCoolingCoolingReReReRe----heatheatheatheat
BainiteBainiteBainiteBainite
coarse fine
Tempered Tempered Tempered Tempered MartensiteMartensiteMartensiteMartensite
PearlitePearlitePearlitePearlite
ReReReRe----heatheatheatheat
• Particles impede dislocations.
• Ex: Al-Cu system
• Procedure:--Pt A: solution heat treat
PRECIPITATION HARDENING
600
700
Lα+Lα
θ θ+L
T(°C)
A
CuAl2
--Pt A: solution heat treat
(get α solid solution)
--Pt B: quench to room temp.
--Pt C: reheat to nucleate
small θ crystals within
α crystals.
• Other precipitation
systems:• Cu-Be
Adapted from Fig. 11.22, Callister 6e. (Fig. 11.22 adapted
300
400
500
0 10 20 30 40 50wt%Cu(Al)
α+θθ A
B
C
composition range needed for precipitation hardening
16
• Cu-Be
• Cu-Sn
• Mg-Al
Pt A (sol’n heat treat)
Pt B
Pt C (precipitate θ)
Temp.
Time
from J.L. Murray, International Metals Review 30303030, p.5, 1985.)
Adapted from Fig.
11.20, Callister 6e.
PRECIPITATION HARDENING
T0
T2
• Two stage heat treatment. Procedure:--T0: solution heat treatment
(get single phase solid solution)
--Quench to T1.
--T2: reheat to nucleate precipitates
PRECIPITATION HARDENING
PRECIPITATION HARDENING
• 2014 Al Alloy:
• TS peaks with
precipitation time.
• %EL reaches minimum
with precipitation time.
PRECIPITATE EFFECT ON TS, %EL
precipitation time.
• Increasing T accelerates
process.
with precipitation time.
ten
sil
e s
tre
ng
th (
MP
a)
400
500 no
n-e
qu
il.
solid
so
luti
on
ma
ny
sm
all
pre
cip
ita
tes
“a
ge
d”
f
ew
er
larg
e
pre
cip
ita
tes
“ove
rag
ed
”
%E
L (
2in
sa
mp
le)
20
30
17
Adapted from Fig. 11.25 (a) and (b), Callister 6e. (Fig. 11.25 adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker
(Managing Ed.), American Society for Metals, 1979. p. 41.)
precipitation heat treat time (h)
ten
sil
e s
tre
ng
th (
MP
a)
300
400
2001min 1h 1day 1mo 1yr
204°C
149°C
“o
ve
rag
ed
”
%E
L (
2in
sa
mp
le)
10
01min 1h 1day 1mo 1yr
204°C 149°C
precipitation heat treat time (h)
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