• 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)