CHAPTER 11: PHASE TRANSFORMATIONS - cribMEcribme.com/uta/data/Engineering/Material Science...

Chapter 11Phase Transformations

Time Temperature TransformationsContinuous Cooling Transformations

Precipitation Hardening

Slides contain Copyrighted material from Callister

text, 2nd

ed.

Chapter 11 – Phase Transformations•

Phase Diagram tells us….? •

Solid state transformations ‐

diffusion dependent. (T and t)•

Phase transformation study allows prediction of reasonably

attainable microstructure –

Properties depend on microstructure

Phase transformation rate ‐

Temperature dependence

fastslow

When is transformation complete?When does transformation stop?Rate vs. % transformed

T1

T2

Final Microstructure depends on T of transformation1.

Solidification

2.

Solid state precipitation

N&

rate

T

TmTΔ

Why this shape?

Nucleation Rate

By Definition:Transformation rate

5.0

1t

ln(time) at T

Frac

tion tran

sformed

Phase transformation rate ‐

Time dependence (kinetics)

Transformation rate changes over

time –

driving force diminishes

Why? ‐

Consider case of precipitation of

2nd

phase from solid solution

k and

n

are constant for given

T and transformation

)exp(1 nkty −−=Curve described by Avrami

eqn.

Transformation rate: Time and Temp effect

)exp(1 nkty −−=

5.0t

Transformation rate =

5.0

1t

Percen

t trans

form

ed

Temp(0C) Transformation rate (min‐1)135 1/988 1/210

Fe‐C TTT diagrams

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

• Eutectoid transf. (Fe-C System):

α ferrite

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6

.7

L

γ austenite

γ+L

γ+Fe3C

Fe3C

cementiteα+Fe3C

α+γ

L+Fe3C

(Fe) Co, wt% C

Eutectoid:

0.7

7

727°C

T(°C)

ΔT

0.0

22

γ ⇒ α +Fe3C0.77wt%C

0.022wt%C6.7wt%C

Undercooling by ΔT: Ttransf. < 727ºC

Equil. cooling: Ttransf. = 727ºC

Adapted from Fig. 9.21,Callister 6e. (Fig. 9.21 adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

TRANSFORMATIONS & UNDERCOOLING

•

Growth of pearlite from austenite:

Adapted from Fig. 10.3, Callister 6e.

EUTECTOID TRANSFORMATION RATE ~ ΔT

γαααα

α

α

pearlite growth direction

Austenite (γ) grain boundary

cementite (Fe3C)

ferrite (α)

γ

Diffusive flow of C needed

α

αγ γ

α

% Pearlite

0

50

100

Nucleation regime

Growth regime

log (time)t50

Nucleation rate increases w/ ΔT

Growth rate increases w/ Tat given temp, T1

•

Reaction rate is a result of nucleation and growthof crystals.

•

Examples:

Nucleation rate high

T just below TE T moderately below TE T way below TENucleation rate low

Growth rate high

γ γ γ

pearlite colony

Nucleation rate med .Growth rate med. Growth rate low

NUCLEATION AND GROWTH

675°C (ΔT smaller)

1 10 102 103time (s)

0

50

100

y (%

pe

arl

ite

) 0

50

100

600°C (ΔT larger)

650°C

% a

ust

en

ite

At 15% transformed.

•

Fe-C system, Co

= 0.77wt%C•

Transformation at T = 675C.

Adapted from Fig. 10.4,Callister 6e. (Fig. 10.4 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.)

ISOTHERMAL TRANSFORMATION DIAGRAMS

400

500

600

700

1 10 102 103 104 105

0%pearlite

100%

50%

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

Pearlite

T(°C)

100

50

01 102 104

T=675°C

y,

% t

ran

sfo

rme

d

time (s)

time (s)

isothermal transformation at 675°C

675°C (ΔT smaller)

1 10 102 103time (s)

0

50

100

y (%

pe

arl

ite

) 0

50

100

600°C (ΔT larger)

650°C

% a

ust

en

ite

•

Eutectoid composition, Co

= 0.77wt%C•

Begin at T > 727C

•

Rapidly cool to 625C and hold isothermally.

Adapted from Fig. 10.5,Callister 6e. (Fig. 10.5 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

EX: COOLING HISTORY Fe‐C SYSTEM

1 10 102 103 104 105 time (s)

500

600

700

T(°C)

γ γ

γγγγ

γ

Austenite (stable)

Pearlite

0%pearlite

100%

50%

TE (727°C)

10

μm

- Smaller ΔT: colonies are larger

- Larger ΔT: colonies are smaller

• Ttransf

just below TE

Two cases:• Ttransf

well below TE

PEARLITE MORPHOLOGY

interface energyHigherT: diffusion is fasterPearlite

is coarserLower T: diffusion is slowerPearlite

is finer.

•

Bainite:--α

lathes (strips) with longrods of Fe3C

--diffusion controlled.•

Isothermal Transf. Diagram

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.)

(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 Metals, Materials Park, OH, 1973.)

Fe3C

(cementite)

5 μm

α (ferrite)

OTHER TRANSFORMATION PRODUCTS: Fe‐C

10 103 105

time (s)10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%

50%

100% bainite

pearlite/bainite boundary100% pearlite

A

A

Micrograph scale difference

60 μm

α (ferrite)

Fe3C

(cementite)

•

Spheroidite:--α

crystals with spherical Fe3C--diffusion dependent.--heat bainite or pearlite for long times--reduces interfacial area (driving force)

•


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.)

(Adapted from Fig. 10.10, Callister, 6e. (Fig. 10.10 copyright United States Steel Corporation, 1971.)

Fe‐C SYSTEM ‐

Spheroidite

10 103 105time (s)10-1

400

600

800


200

P

B

TE

0%

100%

50%

A

A

Spheroidite100% spheroidite

100% spheroidite

Fe‐C SYSTEM: Martensitic

transformation

•

Occurs when Austenite rapidly

cooled to room temp.•

Supersaturated solid solution–

C stays in interstitial sites–

No diffusion during transformation–

Very unstable

xx x

xx

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

•

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

Unique properties – very brittle

•

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


(Adapted from Fig. 10.12, Callister, 6e. (Fig. 10.12 courtesy United States Steel Corporation.)

•


time (s)10 103 10510-1

400

600

800


200

P

B

TE

0%

100%50%

A

A

S

M + AM + A

M + A

0%50%90%

Martentite needlesAustenite

60

μm

•

γ

to M transformation..

--

is rapid!--

% transf. depends on T only.--

remaining material is retained Austenite

Fe‐C SYSTEM ‐

Martensite


COOLING EX: Fe‐C SYSTEM (1)• Co = Ceutectoid• Three histories...

time (s)10 103 10510-1

400

600

800


200

P

B

0%

100%50%

A

S

M + AM + AM + A

0%50%90%

100% Bainite

A

100%A 100%B

Case I

Rapid cool to:

350°C

250°C

650°C

Hold for:

104s

102s

20s

Rapid cool to:

Troom

Troom

400°C

Hold for:

104s

102s

103s

Rapid cool to:

Troom

Troom

Troom

• Co = Ceutectoid• Three histories...

time (s)10 103 10510-1

400

600

800


200

P

B

0% 100%50%

A

S

M + AM + AM + A

0%50%90%

M + trace of A

A

100%A

Case II

Rapid cool to:

350°C

250°C

650°C

Hold for:

104s

102s

20s

Rapid cool to:

Troom

Troom

400°C

Hold for:

104s

102s

103s

Rapid cool to:

Troom

Troom

Troom


COOLING EX: Fe‐C SYSTEM (2)


COOLING EX: Fe‐C SYSTEM (3)

Rapid cool to:

350°C

250°C

650°C

Hold for:

104s

102s

20s

Rapid cool to:

Troom

Troom

400°C

Hold for:

104s

102s

103s

Rapid cool to:

Troom

Troom

Troom

• Co = Ceutectoid• Three histories...

time (s)10 103 10510-1

400

600

800T(°C)

Austenite (stable)

200

P

B

0%

100%50%

A

S

M + AM + AM + A

0%50%90%

50%P, 50%B

A

50%P, 50%A

50%P, 50%A

100%A

50%P, 50%B

Case III

Austenite (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

moderate cool

rapid quench

reheat

Str

en

gth

Du

cti

lityMartensite

T Martensite bainite

fine pearlite coarse pearlite

spheroidite

General Trends

SUMMARY: Fe‐C Processing

Spheroidite

?

TTT for Hypo‐eutectoid composition

Fe‐C: Continuous Cooling Transformations

•

reduces brittleness of martensite,

•

reduces internal stress caused by quenching.

Adapted from Fig. 10.24, Callister 6e. (Fig. 10.24 copyright by United States Steel Corporation, 1971.)

Adapted from Fig. 10.25, Callister 6e. (Fig. 10.25 adapted from Fig. furnished courtesy of Republic Steel Corporation.)

TEMPERING MARTENSITE

• decreases TS, YS but increases %AR

YS(MPa)TS(MPa)

800

1000

1200

1400

1600

1800

30

4050

60

200 400 600Tempering T (°C)

%AR

TS

YS

%AR

9 μ

m

• produces extremely small Fe3C particles surrounded by α.

•

Particles impede dislocations.

•

Ex: Al-Cu system

• Procedure:--Pt A: solution heat treat

(get α

solid solution)--Pt B: quench to room temp.--Pt C: reheat to nucleate

small θ

crystals withinα

crystals.•

Other precipitationsystems:• Cu-Be• Cu-Sn• Mg-Al

Pt A (sol’n heat treat)

Pt B

Pt C (precipitate θ)

Temp.

Time

Adapted from Fig. 11.22, Callister 6e. (Fig. 11.22 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)


PRECIPITATION HARDENING

300

400

500

600

700

0 10 20 30 40 50wt%Cu(Al)

Lα+Lα

α+θθ

θ+L

T(°C)

A

B

C

composition range needed for precipitation hardening

CuAl

Preview of Mechanical Properties

Precipitation hardening for temp, T‐

Longer time, larger ppts‐

Different curve for each T

Al ‐

Cu

Preview of Mechanical Properties

All pearlite

is in the form of fine pearlite.

Preview of Mechanical Propertiesha

rdne

ss

Duc

tility

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