PY2M20PY2M20Material Properties and Phase DiagramsL t 8Lecture 8

P. Stamenov, PhDS h l f Ph i TCDSchool of Physics, TCD

PY2M20-8

Phase transformations

1600

L

1400

1200

L

+LL+Fe C1148°C

T(°C)

tite)

1200

1000

(austenite)

F C

L+Fe3C1148°C

cem

ent

800

+ Fe3C

727°C

Fe3C

(c

600 + Fe3C F

4000 1 2 3 4 5 6 6.7

C wt% CCo, wt% C

Eutectic, Eutectoid and Peritectic R tiReactionsEutectic ReactionEutectic Reaction

)2()1( phaseSolidphaseSolidLiquid

Eutectoid reaction

)3()2()1( phaseSolidphaseSolidphaseSolid

Peritectic Reaction

)3()2()1( phaseSolidphaseSolidphaseLiquid )()()( pppq

Phase Transformations

NucleationNucleation nuclei act as seed points to grow crystals for nucleus to form for nucleus to form

rate of addition of atoms > rate of loss once nucleated, growth equilibrium, g q

Driving force to nucleate increases as we increase Tg supercooling (eutectic, eutectoid) superheating (peritectic)

Small supercooling few nuclei - large crystalsLarge supercooling rapid nucleation - many nuclei,

small crystals

Solidification: Nucleation PProcesses

H l i Homogeneous nucleation nuclei form in the bulk of liquid metal requires supercooling (typically 80-300 °C max)

Heterogeneous nucleation Heterogeneous nucleation much easier since stable “nucleus” is already present

Could be wall of mold or impurities in the liquid Cou d be a o o d o pu t es t e qu dphase

Thi k f h ’d b k d dThink 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!

Rate of Phase Transformation

All out of material –y1.0

All out of material transformation complete

orm

ed, Fixed T

maximum rate reached now amount0 5

n tr

ansf

o maximum rate reached – now amount unconverted decreases so rate slows

0.5

rate increases as surface area increases & l i

log tFrac

tion t0.5 & nuclei grow

gF

Avrami (JMAK) rate equation: y = 1- exp(-k t n) k & n are constants for specific system – n is roughlyk & n are constants for specific system n is roughly

related to the growth front dimensionality + 1

The rate is: r = 1 / t0.5 JMAK – Johnson, Mehl, Avrami, Kolmogorov

Rate of Phase Transformations

Adapted from Fig. 10 11 Callister 7e

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

10.11, Callister 7e.(Fig. 10.11 adapted

from B.F. Decker and D. Harker,

"Recrystallization in

1 10 102 1041021

Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p.

888.)

T

1 10 102 10410 102

1041

In general, rate increases as T

r = 1/t0 5 = A e -Q/RT0.5

R = gas constant T = temperature (K) T = temperature (K) A = pre-exponential factor Q = activation energy

• r often small: equilibrium not possible!

Eutectoid Transformation Rate

Diffusive flow of C needed

• Growth of pearlite from austenite:

Austenite ()grain boundary

cementite (Fe3C)Ferrite ()

pearlite growth

boundary

gdirection

• Eutectoid ) 100transformationrate increases

675°C50pe

arlit

e 600°C (T larger)

650°C

with T.675 C (T smaller)

0y(%

Course pearlite formed at higher T - softerFine pearlite formed at low T - harder

Nucleation and Growth

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

% Pearlite100

G thNucleation rate increases with T

% Pearlite50 Nucleation

regime

Growth regime

t

Growth rate increases with T

• Examples:0 log(time)t0.5

p

pearlite colony

T j st belo T T moderatel belo T T a belo TT just below TENucleation rate lowGrowth rate high

T moderately below TENucleation rate med Growth rate med.

Nucleation rate highT way below TE

Growth rate low

Transformations & Supercooling

• Can make it occur at:• Eutectoid transf. (Fe-C System): + Fe3C

0 76 wt% C 6 7 t% C• Can make it occur at: ...727ºC (cool it slowly)...below 727ºC (“supercool” it!)

0.76 wt% C0.022 wt% C

6.7 wt% C

1600

T(°C)

1400

1200

L

+L

men

tite)1200

1000

(austenite)

+Fe C

L+Fe3C1148°C

C (c

em

800

+Fe3C

+Fe C

ferrite

727°C

Eutectoid:Equil. Cooling: Ttransf. = 727ºCT

Fe3C600

4000 1 2 3 4 5 6 6 7

+Fe3CTSupercooling by Ttransf. < 727C

0.76

0.02

2

0 1 2 3 4 5 6 6.7(Fe) Co, wt%C

00

Isothermal Transformation Diagrams

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

100ed 100

50T = 675°C

y,

ansf

orm

01 102 104%

tra

time (s)

700Austenite (stable) TE (727C)

Austenite

T(°C)

600

(unstable)Pearlite

isothermal transformation at 675°C

400

500

1 10 102 103 104 105 time (s)

Effect of Cooling History in Fe-C System

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

700

Austenite (stable) TE (727C)Austenite

T(°C)

600

700 Austenite (unstable)

Pearlite

500

600 Pearlite

400

500

400

1 10 102 103 104 105

time (s)

Transformations with P t t id M t i lProeutectoid Materials

CO = 1.13 wt% CT(°C)900 1600

T(°C)

TE (727°C)A

A

900

800 te)

1600

1400 L

+L

TE (727 C)A

+C

P

700

+P em

enti1200

1000

(austenite)

+L

+Fe3C

L+Fe3C

P

500

600 A+

Fe3C

(c800

600

Fe3C

+Fe3C727°C

T

2

time (s)1 10 102 103 104

F

4000 1 2 3 4 5 6 6.7(Fe) Co, wt%C

0.76

0.02

2

1.13

Hypereutectoid composition proeutectoid cementite

o,

Hypereutectoid composition – proeutectoid cementite

Non-Equilibrium Transformation P d t F CProducts: Fe-C• Bainite: (Davenport & Bain)

t i ith l- strips with longneedles of Fe3C

- diffusion controlled.

Fe3C(cementite)

(f )diffusion controlled.• Isothermal Transf. Diagram (ferrite)

800 A t it ( t bl )

5 m600

800

T(°C)Austenite (stable)

PTEA

100% pearlite

400

600

B

pearlite/bainite boundary

A

100% bainite

00% pea e

400

200

A

10 103 105

ti ( )10-1

200

120 mtime (s) 120 m

cf: Pearlite

Spheroidite: Fe-C System

- + grains with sphericalFe3C (ferrite)

- diffusion dependent.Fe3C- heat bainite or pearlite

for long times (e g 18 h at 700°C)

(cementite)

Fe3C

(e.g. 18 h at 700 C)

- reduces interfacial area (driving force) 60 m

Martensite: Fe-C System

• Martensite: (Martens)(FCC) t M t it (BCT)- (FCC) to Martensite (BCT)

mx potentialFe atom

(involves single atom jumps)

60

xx x

xx

potential C atom sites

Fe atom sites

• Isothermal Transf. DiagramMartensite needlesA t it

800 Austenite (stable)T Austenite

600

T(°C)( )

PTEA

• to M transformation..is rapid!

400 BA- is rapid!- % transf. depends on

T only.200 M + A

M + AM A

0%50%90% y

- can be stress-induced10 103 105 t/s10-1

M + A

Martensite Formation

(FCC) (BCC) + Fe3Cslow cooling ( ) ( ) 3

quench

temperingM (BCT)

M = martensite is body centered tetragonal (BCT)

Diff i l f i BCT if C 0 1 %Diffusionless transformation BCT if C > 0.15 wt%BCT few slip planes hard, brittle

Phase Transformations of Alloys

Effect of adding other elementsChange transition

temp.p

Cr Ni Mo Si MnCr, Ni, Mo, Si, Mnretard + Fe3C t f titransformation

Dynamic Phase Transformations

On the isothermal transformation diagramOn the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label th ti t t th t d ththe time-temperature paths to produce the following microstructures:a) 42 % proeutectoid ferrite and 58 % coarse

pearlitepb) 50 % fine pearlite and 50 % bainitec) 100 % martensitec) 100 % martensited) 50 % martensite and 50 % austenite

Example Problem for Co = 0.45 wt%

a) 42 % proeutectoid

A + A800

proeutectoidferrite and 58 % coarse pearlite

A + P

A + A

P

p(amounts determined by phase diagram)

T (°C)

A + BBP

A

600phase diagram)

first make A50%400 M (start)

M (50%)

first make ferritethen pearlite

200M (90%)

coarse pearlite

00 1 10 103 105

higher T0.1 10 10 10

time (s)

Example Problem for Co = 0.45 wt%

a. the amount of pearlite and proeutectoid ferrite () t t f lit t f j t b Tnote: amount of pearlite = amount of just above TE

Co = 0.45wt% CCo 0.45wt% CC = 0.022 wt% CCpearlite = C = 0.76 wt% C 1600

%9.57100x

CCo te)

1400

1200

L

+LL+Fe3C1148°C

T(°C)%9.57100x

CC

emen

tit

1200

1000

(austenite)

+ Fe3C

L+Fe3C1148 C

pearlite = 58 % e 3C

(ce

800

600 + Fe C

727°CR S

pearlite 58 %proeutectoid = 42 %

F600

4000 1 2 3 4 5 6 6.7

+ Fe3C

C wt% CCOCC Co, wt% CCO

Example Problem for Co = 0.45 wt%

b) 50 % fine )pearlite and 50% bainite A + A

80050% bainiteT (°C)

A + P

A + A

Pfirst make pearlitethen bainite A + B

BP

A

600

fi lit

A50%400 M (start)

M (50%)fine pearlite

lower T 200M (90%)

00 1 10 103 1050.1 10 10 10

time (s)

Example Problem for Co = 0.45 wt%

c) 100 %

A + A800

c) 100 % martensite – quench =

A + P

A + A

P

quench = rapid cool T (°C)

A + BBP

A

600

A50%400 M (start)

M (50%)

d) 50 % martensiteand 50 % d)

200M (90%)austenite

d)

00 1 10 103 105

c)

0.1 10 10 10time (s)

Mechanical Prop: Fe-C System (1)

• Effect of wt% C Pearlite (med)

Co < 0.76 wt% C

Pearlite (med)ferrite (soft)

Co > 0.76 wt% C

Cementite(hard)

Hypoeutectoid Hypereutectoid

TS(MPa) Hypo Hyper%EL 80

Hypo Hyper

900

1100TS(MPa)

hardness

100%EL

od, f

t-lb)

80

TSIzod

500

700hardness

50

nerg

y (Iz

o

40

EL

300

500YS

16 0 Impa

ct e0

• More wt% C: TS and YS increase, %EL decreases.wt% C0 0.5 1

0.76 wt% C0 0.5 1

0

0.76

Mechanical Prop: Fe-C System (2)

• Fine vs coarse pearlite vs spheroidite

320

Hypo Hyper 90 Hypo Hyper

240

ness

fine pearlite

coarsepearlite

60

y (%

AR

)

spheroidite

160

inel

l har

d pearlitespheroidite

30Duc

tility

coarsepearlite

80

wt%C0 0.5 1

Br

0t%C

fine pearlite

0 0.5 1wt%C wt%C0

• Hardness:• %RA:

fine > coarse > spheroiditefine < coarse < spheroiditee coa se sp e o d te

Mechanical Prop: Fe-C System (3)

• Fine Pearlite vs Martensite:

Hypo Hyper

600

ness martensite

400

inel

l har

d

200

Br

fine pearlite

H d fi lit << t it

0wt% C0 0.5 1

• Hardness: fine pearlite << martensite.

Tempering Martensite

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

MPaMPa

1600

1800

TS

m

1400

1600

YS

9

1000

1200

405060

%RA%RA

800 3040

200 400 600Tempering T (°C)

• produces extremely small Fe3C particles surrounded by

Tempering T (°C)

• decreases TS, YS but increases %RA

Summary: Processing Options

Austenite ()()

slow cool

moderatecool

rapid quench

BainitePearlite Martensite

cool cool quench

Bainite( + Fe3C plates/needles)

Pearlite( + Fe3C layers + a proeutectoid phase)

Martensite(BCT phase diffusionless

t f ti )transformation)

reheatMartensite T M t it

Tempered Martensiteen

gth

ctili

tyT Martensite

bainite fine pearlite Martensite

( + very fine Fe3C particles)

Stre Ducfine pearlite

coarse pearlitespheroidite

General Trends