PY3090 Preparation of Materials Lecture 1 Lecture 7.pdf · 4 Phase Transformations Nucleation...

28
PY3090 PY3090 7 7 PY3090 PY3090 Preparation of Materials Preparation of Materials Lecture 7 Lecture 7 Colm Stephens Colm Stephens School of Physics School of Physics

Transcript of PY3090 Preparation of Materials Lecture 1 Lecture 7.pdf · 4 Phase Transformations Nucleation...

PY3090 PY3090 –– 77

PY3090PY3090Preparation of MaterialsPreparation of MaterialsLecture 7Lecture 7

Colm StephensColm StephensSchool of PhysicsSchool of Physics

2

Phase transformationsPhase transformations

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

γ (austenite)

γ +L

γ + Fe3C

α + Fe3C

L+Fe3C

δ

Co, wt% C

1148°C

T(°C)

727°C

3

Eutectic, Eutectoid and Eutectic, Eutectoid and PeritecticPeritecticReactionsReactionsEutecticEutectic ReactionReaction

Eutectoid reactionEutectoid reaction

)2()1( phaseSolidphaseSolidLiquid +⇔

)3()2()1( phaseSolidphaseSolidphaseSolid +⇔

PeritecticPeritectic ReactionReaction

)3()2()1( phaseSolidphaseSolidphaseLiquid ⇔+

4

Phase TransformationsPhase Transformations

NucleationNucleationnuclei act as seed points to grow crystalsnuclei act as seed points to grow crystalsfor nucleus to form for nucleus to form rate of addition of atoms > rate of lossrate of addition of atoms > rate of lossonce nucleated, growth once nucleated, growth equilibriumequilibrium

Driving force to nucleate increases as we increase Driving force to nucleate increases as we increase ΔΔTTsupercoolingsupercooling (eutectic, eutectoid)(eutectic, eutectoid)superheatingsuperheating ((peritecticperitectic))

Small Small supercoolingsupercooling few nuclei few nuclei -- large crystalslarge crystalsLarge Large supercoolingsupercooling rapid nucleation rapid nucleation -- many nuclei, many nuclei,

small crystalssmall crystals

5

Solidification: Nucleation Solidification: Nucleation ProcessesProcesses

Homogeneous nucleation Homogeneous nucleation nuclei form in the bulk of liquid metalnuclei form in the bulk of liquid metalrequires requires supercoolingsupercooling (typically 80(typically 80--300300°°C max)C max)

Heterogeneous nucleationHeterogeneous nucleationmuch easier since stable much easier since stable ““nucleusnucleus”” is already presentis already present

Could be wall of mold or impurities in the liquid Could be wall of mold or impurities in the liquid phasephase

allows solidification with only 0.1allows solidification with only 0.1--1010ººC C supercoolingsupercooling

6

Rate is r = 1 / t0.5

Avrami rate equation y = 1- exp (-k t n)k & n are constants for specific system

Rate of Phase TransformationRate of Phase TransformationAll out of material –transformation complete

log tFrac

tion

tran

sfor

med

, 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

7

In general, rate increases as T ↑

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

R = gas constantT = temperature (K)A = preexponential factorQ = activation energy

Rate of Phase TransformationsRate of Phase Transformations

• r often small: equilibrium not possible!

135°C 119°C 113°C 102°C 88°C 43°C

1 10 102 104

8

Eutectoid Transformation RateEutectoid Transformation Rate

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

Diffusive flow of C needed

α

αγ γ

α

• Growth of pearlite from austenite:

γαααα

α

α

pearlite growth direction

Austenite (γ)grain boundary

cementite (Fe3C)Ferrite (α)

γ

• Eutectoid transformationrate increases with ΔT.

675°C (ΔT smaller)

0

50

y(%

pea

rlite

)600°C

(ΔT larger)650°C

100

9

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

• Examples:

Nucleation and GrowthNucleation and Growth

% Pearlite

0

50

100

Nucleation regime

Growth regime

log(time)t0.5

Nucleation rate increases with ΔT

Growth rate increases with T

T just below TENucleation rate lowGrowth rate high

γpearlite colony

T moderately below TE

γ

Nucleation rate med .Growth rate med.

Nucleation rate highT way below TE

γ

Growth rate low

10

Transformations & Transformations & SupercoolingSupercooling

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

• Eutectoid transf. (Fe-C System): γ ⇒ α + Fe3C0.76 wt% C

0.022 wt% C6.7 wt% C

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

γ (austenite)

γ+L

γ +Fe3C

α +Fe3C

L+Fe3C

δ

(Fe) Co, wt%C

1148°C

T(°C)

αferrite

727°C

Eutectoid:Equil. Cooling: Ttransf. = 727ºC

ΔTSupercooling by ΔTtransf. < 727°C

0.76

0.02

2

11

Isothermal Transformation DiagramsIsothermal 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 1050%

pearlite100%

50%

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

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

12

• 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 FeEffect of Cooling History in Fe--C SystemC System

400

500

600

700

0%pearlite

100%

50%

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

Pearlite

T(°C)

1 10 102 103 104 105

time (s)

γ γ

γγ γ

γ

13

Transformations with Transformations with Proeutectoid MaterialsProeutectoid Materials

Hypereutectoid composition – proeutectoid cementite

α

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

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

γ (austenite)

γ+L

γ +Fe3C

α+Fe3C

L+Fe3C

δ

(Fe) Co, wt%C

T(°C)

727°CΔT

0.76

0.02

2

1.13

14

NonNon--Equilibrium Transformation Equilibrium Transformation Products: FeProducts: Fe--CC• Bainite:

--α strips with longneedles of Fe3C

--diffusion controlled.• Isothermal Transf. Diagram

FeFe33CC(cementite)(cementite)

5 μm

α (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

120 μm

cf: Pearlite

15

-- α grains with sphericalFe3C

-- diffusion dependent.

-- heat bainite or pearlite for long times (e.g. 18 h at 700°C)

-- reduces interfacial area (driving force)

SpheroiditeSpheroidite: Fe: Fe--C SystemC System

(ferrite)

60 μm

α

(cementite)

Fe3C

16

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

• Isothermal Transf. Diagram

• γ to M transformation..-- is rapid!-- % transf. depends on

T only.

Martensite: FeMartensite: Fe--C SystemC System

Martensite needlesAustenite

60 μ

m

10 103 105 t/s10-1

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

x potential C atom sites

Fe atom sites

(involves single atom jumps)

17

γ (FCC) αα (BCC) + Fe(BCC) + Fe33CCslow cooling

Martensite FormationMartensite Formation

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

18

Phase Transformations of AlloysPhase Transformations of Alloys

Effect of adding Effect of adding other elementsother elementsChange transition Change transition

temp.temp.

Cr, Ni, Mo, Si, Cr, Ni, Mo, Si, MnMnretard retard γγ αα + + FeFe33C C transformationtransformation

19

Dynamic Phase TransformationsDynamic Phase Transformations

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

pearlitepearliteb)b) 50% fine pearlite and 50% bainite50% fine pearlite and 50% bainitec)c) 100% martensite100% martensited)d) 50% martensite and 50% austenite50% martensite and 50% austenite

20

A + B

A + P

A + αA

BP

A50%

0

200

400

600

800

0.1 10 103 105time (s)

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

Example Problem for Example Problem for CCoo = 0.45 wt%= 0.45 wt%a)a) 42% 42%

proeutectoid proeutectoid ferrite and 58% ferrite and 58% coarse coarse pearlitepearlite(amounts (amounts determined by determined by phase diagram)phase diagram)

first make first make ferriteferritethen pearlitethen pearlite

course pearlitecourse pearlite∴∴ higher higher TT

T (°C)

21

a. the amount of pearlite and proeutectoid ferrite (α) note: amount of pearlite = amount of γ just above TE

Co = 0.45wt% CCα = 0.022 wt% CCpearlite = Cγ = 0.76 wt% C

%9.57100x =−−

=+ αγ

α

αγγ

CCCCo

pearlite = 58%proeutectoid α = 42%

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

γ (austenite)

γ+L

γ + Fe3C

α + Fe3C

L+Fe3C

δ

Co, wt% C

1148°C

T(°C)

727°C

CO

R S

CγCα

Example Problem for Example Problem for CoCo = 0.45 wt%= 0.45 wt%

22

b)b) 50% fine 50% fine pearlite and pearlite and 50% bainite50% bainite

first make pearlitefirst make pearlitethen bainitethen bainite

fine pearlitefine pearlite∴∴ lower lower TT

T (°C)

A + B

A + P

A + αA

BP

A50%

0

200

400

600

800

0.1 10 103 105time (s)

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

Example Problem for Example Problem for CCoo = 0.45 wt%= 0.45 wt%

23

A + B

A + P

A + αA

BP

A50%

0

200

400

600

800

0.1 10 103 105time (s)

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

Example Problem for Example Problem for CCoo = 0.45 wt%= 0.45 wt%

c)c) 100 % 100 % martensite martensite –– quench = quench = rapid coolrapid cool

d) 50 % martensite and 50 % austenite d)

c)

T (°C)

24

Mechanical Prop: Mechanical Prop: FeFe--C System (1)C System (1)

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

• Effect of wt% C

Co < 0.76 wt% CHypoeutectoid

Pearlite (med)ferrite (soft)

Co > 0.76 wt% CHypereutectoid

Pearlite (med)Cementite

(hard)

300

500

700

900

1100

YS

TS(MPa)

wt% C0 0.5 1

hardness

0.76

Hypo Hyper

wt% C0 0.5 10

50

100%EL

Impa

ct e

nerg

y (Iz

od, f

t-lb)

0

40

80

0.76

Hypo Hyper

TS

EL

Izod

25

Mechanical Prop: Mechanical Prop: FeFe--C System (2)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

Brin

ellh

ardn

ess

fine pearlite

coarsepearlitespheroidite

Hypo Hyper

0

30

60

90

wt%CD

uctil

ity (%

AR

)

fine pearlite

coarsepearlite

spheroidite

Hypo Hyper

0 0.5 1

26

Mechanical Prop: Mechanical Prop: FeFe--C System (3)C System (3)

• Fine Pearlite vs Martensite:

• Hardness: fine pearlite << martensite.

0

200

wt% C0 0.5 1

400

600

Brin

ellh

ardn

ess martensite

fine pearlite

Hypo Hyper

27

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

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

9 μm

MPa

800

1000

1200

1400

1600

1800

30405060

200 400 600Tempering T (°C)

%RA

TS

YS

%RA

28

Summary: Processing OptionsSummary: Processing OptionsAustenite (γ)

Bainite(α + Fe3C plates/needles)

Pearlite(α + Fe3C layers + a proeutectoid phase)

Martensite(BCT phase diffusionless

transformation)

Tempered Martensite (α + very fine

Fe3C particles)

slow cool

moderatecool

rapid quench

reheat

Stre

ngth

Duc

tility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends