ISSUES TO ADDRESS...• Transforming one phase into another takes time.

Fe

(Austenite)

Eutectoid transformation

C FCC

Fe3C(cementite)

(ferrite)

+

(BCC)

Chapter 10:Phase Transformations

Phase Transformation

• Reaction rate is a result of nucleation and growth of crystals.

• Examples: Eutectoid reaction

Nucleation and Growth

% Pearlite

0

50

100

Nucleation regime

Growth regime

log (time)t0.5

Nucleation rate increases with T

Growth rate increases with T

T just below TE

Nucleation rate low

Growth rate high

pearlite colony

T moderately below TE

Nucleation rate med Growth rate med.

Nucleation rate high

T way below TE

Growth rate low

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

Fe-Fe3C phase diagram

Solidification: Nucleation Processes

• Homogeneous nucleation – nuclei form in the bulk of liquid metal– requires supercooling (typically 80-300°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 0.1-10ºC

supercooling

r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow

Homogeneous Nucleation & Energy Effects

GT = Total Free Energy = GS + GV

Surface Free Energy- destabilizes the nuclei (energy for makinmg surface)

24 rGS = surface tension

Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy)

GrGV3

3

4

volume unit

energy free volume G

TH

Tr

S

m

2* HS = latent heat of

solidification

Homogenous and Heterogeneous Nucleation

Rate of Phase Transformation

Completely growth

log t

Fra

ctio

n tr

an

sfo

rme

d, y

Fixed T: Isothermal

By convention r = 1 / t0.5

maximum rate reached –

now amount unconverted decreases so rate slowerrate increases as surface

area increases & nuclei grow

Isothermal Transformation Diagrams

• Fe-C system, Co = 0.76 wt% C• Transformation at T = 675°C.

100

50

01 102 104

T = 675°C

y,

% tr

ansf

orm

ed

time (s)

400

500

600

700

1 10 102 103 104 105

0%pearlite

100%

50%

Austenite (stable) TE (727C)Austenite (unstable)

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

675°C (T smaller)

0

50

y (%

pea

rlite

)

600°C (T larger)

650°C

100

Course pearlite formed at higher T - softer

Fine pearlite formed at lower T - harder

Rate of Phase Transformations

135C 119C 113C 102C 88C 43C

1 10 102 104

% Recrystallization of Rolled Copper

Percent recrystallization is function of time and temperature

Transformations & Undercooling

• Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it!)

• Eutectoid transformation (Fe-C System): + Fe3C0.76 wt% C

0.022 wt% C6.7 wt% C

Fe 3

C (

cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

1148°C

T(°C)

ferrite

727°C

Eutectoid:Equil. Cooling: Ttransf. = 727ºC

T

Undercooling by Ttransf. < 727C

0.7

6

0.0

22

Eutectoid Transformation Rate

• Growth of pearlite from austenite:cementite (Fe3C)

Ferrite ()

pearlite

growth

direction

Diffusive flow of C needed

• Higher T give higher diffusivity

• Eutectoid composition, Co = 0.76 wt% C• Begin at T > 727°C• Rapidly cool to 625°C and hold isothermally.

Effect of Cooling History in Fe-C System

400

500

600

700

0%pearlite

100%

50%

Austenite (stable)TE (727C)

Austenite (unstable)

Pearlite

T(°C)

1 10 102 103 104 105

time (s)

Transformations with Proeutectoid Materials

Hypereutectoid composition – proeutectoid cementite

CO = 1.13 wt% C

TE (727°C)

T(°C)

time (s)

A

A

A+

C

P

1 10 102 103 104

500

700

900

600

800

A+

P

Fe 3

C (

cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

T(°C)

727°CT

0.7

6

0.0

22

1.13

T-T-T of Eutectoid Composition

A – AusteniteP – PearliteB – BainiteM – MartensiteC – Cementite

Non-Equilibrium Transformation Products: Fe-C

• Bainite: -- lathes (strips) with long

rods of Fe3C --diffusion controlled.• Isothermal Transf. Diagram

Fe3C

(cementite)

5 m

(ferrite)

10 103 105

time (s)10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%

50%

pearlite/bainite boundary

A

A

100% bainite

100% pearlite

T-T-T DiagramTime – Temperature – Transformation

• Spheroidite: -- grains with

spherical Fe3C -- diffusion

dependent. -- heat bainite or

pearlite for long times near TE

Spheroidite: Fe-C System

60 m

(ferrite)

(cementite)

Fe3C

• Martensite: --(FCC) to Martensite (BCT)

• Isothermal Transf. Diagram

• to M transformation -- is rapid! -- % transformation

depends on T only.

Martensite: Fe-C System

Martensite needlesAustenite

60

m

10 103 105 time (s)10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%50%

A

A

M + AM + A

M + A

0%50%90%

xx x

xx

xpotential C atom sites

Fe atom sites

(FCC) P (BCC) + Fe3C

Martensite Formation

slow cooling

tempering

quench

M (BCT)

M = martensite is body centered tetragonal (BCT)

Diffusionless transformation BCT if C > 0.15 wt%

BCT few slip planes hard, brittle

Phase Transformations of Alloys

Effect of adding other elements

Change transition temp.

Cr, Ni, Mo, Si, Mn

retard +

Fe3C

transformation

Alloy steel (type 4340)

Continuous Cooling Curve

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:

a) 42% proeutectoid ferrite and 58% coarse pearlite

b) 50% fine pearlite and 50% bainite

c) 100% martensite

A + B

A + P

A + A

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)M (50%)M (90%)

Example Problem for Co = 0.45 wt%

a) 42% proeutectoid ferrite and 58% coarse pearlite

first make ferrite

then pearlite

course pearlite

higher T

T (°C)

b) 50% fine pearlite and 50% bainite

first make pearlite

then bainite

fine pearlite

lower T

T (°C)

A + B

A + P

A + A

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)M (50%)M (90%)

Example Problem for Co = 0.45 wt%

A + B

A + P

A + A

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)M (50%)M (90%)

Example Problem for Co = 0.45 wt%

c) 100 % martensite – quench = rapid cool

c)

T (°C)

Mechanical Prop: Fe-C System (1)

• More wt% C: TS and YS increase , %EL decreases.

• Effect of wt% C

Co < 0.76 wt% C

Hypoeutectoid

Pearlite (med)ferrite (soft)

Co > 0.76 wt% C

Hypereutectoid

Pearlite (med)Cementite

(hard)

300

500

700

900

1100YS(MPa)TS(MPa)

wt% C0 0.5 1

hardness

0.7

6

Hypo Hyper

wt% C0 0.5 1

0

50

100%EL

Imp

act

en

erg

y (I

zod

, ft

-lb

)

0

40

80

0.7

6

Hypo Hyper

Mechanical Prop: Fe-C System (2)

• Fine vs coarse pearlite vs spheroidite

• Hardness:

• %RA:

fine > coarse > spheroiditefine < coarse < spheroidite

80

160

240

320

wt%C0 0.5 1

Bri

ne

ll h

ard

ne

ss

fine pearlite

coarse pearlitespheroidite

Hypo Hyper

0

30

60

90

wt%C

Du

ctili

ty (

%R

A)

fine pearlite

coarse pearlite

spheroidite

Hypo Hyper

0 0.5 1

Mechanical Prop: Fe-C System (3)

• Fine Pearlite vs Martensite:

• Hardness: Fine Pearlite << Martensite.

0

200

wt% C0 0.5 1

400

600

Bri

ne

ll h

ard

ne

ss martensite

fine pearlite

Hypo Hyper

• Hardness: Pearlite < Bainite.

Tempering Martensite• reduces brittleness of martensite,• reduces internal stress caused by quenching.

• decreases TS, YS but increases %RA• produces extremely small Fe3C particles surrounded by

9 m

YS(MPa)TS(MPa)

800

1000

1200

1400

1600

1800

30

40

50

60

200 400 600Tempering T (°C)

%RA

TS

YS

%RA

Summary: Processing Options

Austenite ()

Bainite( + Fe3C plates/needles)

Pearlite( + Fe3C layers + a proeutectoid phase)

Martensite(BCT phase diffusionless

transformation)

Tempered Martensite ( + very fine Fe3C particles)

slow cool

moderate cool

rapid quench

reheat

Str

engt

h

Duc

tility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends