PY2N20-8

PY2N20

Material Properties and

Phase Diagrams

Lecture 8

P. Stamenov, PhD

School of Physics, TCD

Phase transformations

Fe

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g

(austenite)

g +L

g + Fe3C

a + Fe3C

L+Fe3C

d

Co , wt% C

1148°C

T(°C)

727°C

Eutectic, Eutectoid and Peritectic

Reactions

Eutectic Reaction

Eutectoid reaction

)2()1( phaseSolidphaseSolidLiquid

)3()2()1( phaseSolidphaseSolidphaseSolid

Peritectic Reaction

)3()2()1( phaseSolidphaseSolidphaseLiquid

Phase Transformations

Nucleation nuclei act as seed points to grow crystals

for nucleus to form

rate of addition of atoms > rate of loss

once nucleated, growth equilibrium

Driving force to nucleate increases as we increase T supercooling (eutectic, eutectoid)

superheating (peritectic)

Small supercooling few nuclei - large crystals

Large supercooling rapid nucleation - many nuclei,

small crystals

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

Think of why you’d been asked never to put a used spoon back into a honey jar…

allows solidification with only 0.1-10 ºC supercooling

concentration drive is also possible and often used!

The rate is: r = 1 / t0.5

Avrami (JMAK) rate equation: y = 1- exp(-k t n)

k & n are constants for specific system – n is roughly

related to the growth front dimensionality + 1

Rate of Phase Transformation

All out of material –

transformation complete

log t

Fra

cti

on

tra

nsfo

rmed

, y

Fixed T

maximum rate reached – now amount unconverted decreases so rate slows

0.5

1.0

t0.5 rate increases as surface area increases

& nuclei grow

JMAK – Johnson, Mehl,

Avrami, Kolmogorov

In general, rate increases as T

r = 1/t0.5 = A e -Q/RT

R = gas constant

T = temperature (K)

A = pre-exponential factor

Q = activation energy

Rate of Phase Transformations

• r often small: equilibrium not possible!

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

1 10 102 104 10

102 104

1

Adapted from Fig.

10.11, Callister 7e.

(Fig. 10.11 adapted

from B.F. Decker and

D. Harker,

"Recrystallization in

Rolled Copper", Trans

AIME, 188, 1950, p.

888.)

Eutectoid Transformation Rate

Course pearlite formed at higher T - softer

Fine pearlite formed at low T - harder

Diffusive flow of C needed

a

a

g g

a

• Growth of pearlite from austenite:

g a a a a

a

a

pearlite growth direction

Austenite (g)

grain boundary

cementite (Fe3C)

Ferrite (a)

g

• Eutectoid

transformation

rate increases

with T. 675°C

(T smaller)

0

50

y (

% p

earlite)

600°C

(T larger) 650°C

100

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

• Examples:

Nucleation and Growth

% Pearlite

0

50

100

Nucleation

regime

Growth

regime

log (time) t 0.5

Nucleation rate increases with T

Growth rate increases with T

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

Transformations & Supercooling

• Can make it occur at: ...727ºC (cool it slowly)

...below 727ºC (“supercool” it!)

• Eutectoid transf. (Fe-C System): g a + Fe3C

0.76 wt% C 0.022 wt% C

6.7 wt% C

Fe

3C

(cem

entite

)

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g

(austenite)

g +L

g +Fe3C

a +Fe3C

L+Fe3C

d

(Fe) Co , wt%C

1148°C

T(°C)

a

ferrite 727°C

Eutectoid: Equil. Cooling: Ttransf. = 727ºC

T

Supercooling by Ttransf. < 727C

0.7

6

0.0

22

Isothermal Transformation Diagrams

• Fe-C system, Co = 0.76 wt% C

• Transformation at T = 675°C.

100

50

0 1 10 2 10 4

T = 675°C

y,

% tra

nsfo

rmed

time (s)

400

500

600

700

1 10 10 2 10 3 10 4 10 5

Austenite (stable) TE (727C)

Austenite (unstable)

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

• 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

Austenite (stable) TE (727C)

Austenite

(unstable)

Pearlite

T(°C)

1 10 10 2 10 3 10 4 10 5

time (s)

g g

g

g g

g

Transformations with

Proeutectoid Materials

Hypereutectoid composition – proeutectoid cementite

a

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

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g (austenite)

g +L

g +Fe3C

a +Fe3C

L+Fe3C

d

(Fe) Co , wt%C

T(°C)

727°C T

0.7

6

0.0

22

1.1

3

Non-Equilibrium Transformation

Products: Fe-C

• Bainite: (Davenport & Bain) - a strips with long

needles of Fe3C

- diffusion controlled.

• Isothermal Transf. Diagram

Fe3C

(cementite)

5 mm

a (ferrite)

10 10 3

10 5

time (s) 10

-1

400

600

800

T(°C) Austenite (stable)

200

P

B

TE

pearlite/bainite boundary

A

A

100% bainite

100% pearlite

120 mm

cf: Pearlite

- a + grains with spherical

Fe3C

- diffusion dependent.

- heat bainite or pearlite

for long times

(e.g. 18 h at 700°C)

- reduces interfacial area

(driving force)

Spheroidite: Fe-C System

(ferrite)

60 mm

a

(cementite)

Fe3C

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

• Isothermal Transf. Diagram

• g to M transformation.. - is rapid!

- % transf. depends on

T only.

- can be stress-induced

Martensite: Fe-C System

Martensite needles Austenite

60

mm

10 10 3

10 5 t/s 10

-1

400

600

800

T(°C) Austenite (stable)

200

P

B

TE A

A

M + A M + A

M + A

0% 50% 90%

x

x x

x

x

x potential

C atom sites

Fe atom

sites

(involves single atom jumps)

g (FCC) a (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

g a + Fe3C

transformation

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

d) 50 % martensite and 50 % austenite

A + B

A + P

A + a A

B P

A 50%

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 (amounts determined by phase diagram)

first make ferrite

then pearlite

coarse pearlite

higher T

T (°C)

a. the amount of pearlite and proeutectoid ferrite (a)

note: amount of pearlite = amount of g just above TE

Co = 0.45wt% C

Ca = 0.022 wt% C

Cpearlite = Cg = 0.76 wt% C

%9.57100x

ag

a

ag

g

CC

CCo

pearlite = 58 %

proeutectoid a = 42 %

Fe

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g (austenite)

g +L

g + Fe3C

a + Fe3C

L+Fe3C

d

Co , wt% C

1148°C

T(°C)

727°C

CO

R S

Cg Ca

Example Problem for Co = 0.45 wt%

b) 50 % fine

pearlite and

50% bainite

first make pearlite

then bainite

fine pearlite

lower T

T (°C)

A + B

A + P

A + a A

B P

A 50%

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 A

B P

A 50%

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

d) 50 % martensite

and 50 %

austenite d)

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)

C ementite (hard)

300

500

700

900

1100

YS

TS(MPa)

wt% C 0 0.5 1

hardness

0.7

6

Hypo Hyper

wt% C 0 0.5 1

0

50

100

%EL

Imp

act e

ne

rgy (

Izo

d, ft

-lb

)

0

40

80

0.7

6

Hypo Hyper

TS

EL

Izod

Mechanical Prop: Fe-C System (2)

• Fine vs coarse pearlite vs spheroidite

• Hardness:

• %RA: fine > coarse > spheroidite

fine < coarse < spheroidite

80

160

240

320

wt%C 0 0.5 1

Bri

ne

ll h

ard

ne

ss fine

pearlite coarse pearlite

spheroidite

Hypo Hyper

0

30

60

90

wt%C D

uctilit

y (

%A

R)

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% C 0 0.5 1

400

600

Bri

ne

ll h

ard

ne

ss

martensite

fine pearlite

Hypo Hyper

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 a.

9 m

m

MPa

800

1000

1200

1400

1600

1800

30

40

50

60

200 400 600 Tempering T (°C)

%RA

TS

YS

%RA

Summary: Processing Options

Austenite (g)

Bainite (a + Fe3C plates/needles)

Pearlite (a + Fe3C layers + a

proeutectoid phase)

Martensite (BCT phase

diffusionless transformation)

Tempered Martensite (a + very fine

Fe3C particles)

slow cool

moderate cool

rapid quench

reheat

Str

en

gth

Ductilit

y

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends