Chapter 10 - 1

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

• How does the rate of transformation depend on time and T ?

• How can we slow down the transformation so that we can engineer non-equilibrium structures?

• Are the mechanical properties of non-equilibrium structures better?

Fe

(Austenite)

Eutectoid transformation

C FCC

Fe3C(cementite)

(ferrite)

+

(BCC)

Chapter 10:Phase Transformations

Chapter 10 - 2

Phase Transformation and Structure

Processing

Structure Properties

Phase Transformation Altering StructureChanging Temperature (on phase diagram)

Chapter 10 - 3

Phase Transformations

Nucleation – nuclei (seeds) act as template to grow crystals– for nucleus to form: rate of addition of atoms to

nucleus must be faster than rate of loss– once nucleated, grow until reach equilibrium

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

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

small crystals

Chapter 10 - 4

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

Chapter 10 - 5

Rate of Phase Transformations

Kinetics - measure approach to equilibrium vs. time

• Hold temperature constant

& measure conversion vs. time

sound waves – one sample

electrical conductivity – follow one sampleX-ray diffraction – have to do many samples

How is conversion measured?

Chapter 10 - 6

Rate of Phase Transformation

Avrami rate equation => y = 1- exp (-ktn)

– k & n fit for specific sample (from experiments)

All out of material - done

log tFra

ctio

n tr

an

sfo

rme

d, y

Fixed T

fraction transformed

time

0.5

By convention r = 1 / t0.5

maximum rate reached – now amount unconverted decreases so rate slows

t0.5

rate increases as surface area increases & nuclei grow

Chapter 10 - 7

Rate of Phase Transformations

• In general, rate increases as T

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

– R = gas constant– T = temperature (K)– A = preexponential factor– Q = activation energy

Arrhenius expression

• r often small: equilibrium not possible!

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

1 10 102 104

T

supercooling

Chapter 10 - 8

Eutectoid Transformation Rate

Course pearlite formed at higher T - softer

Fine pearlite formed at low T - harder

Diffusive flow of C needed

• Growth of pearlite from austenite:

pearlite growth direction

Austenite ()grain boundary

cementite (Fe3C)

Ferrite ()

• Recrystallization rate increases with T.

675°C (T smaller)

0

50

y (%

pea

rlite

)600°C

(T larger)650°C

100

Chapter 10 - 9

• 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)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 mediumGrowth rate medium.

Nucleation rate high

T way below TE

Growth rate low

Chapter 10 -10

Transformations & Undercooling

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

• Eutectoid transf. (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. = 727ºCT

Undercooling by Ttransf. < 727C

0.7

6

0.0

22

Chapter 10 -11

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

Chapter 10 -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 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)

Chapter 10 -13

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

Adapted from Fig. 10.16, Callister 7e.

Adapted from Fig. 9.24, Callister 7e.

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

Chapter 10 -14

Non-Equilibrium Structure of Fe-C

• Bainite: lathes (strips) with long rods of Fe3C

- Needles of Fe3C in - Very fine structure

--diffusion controlled.

Obtained from moderate cooling i.e. no enough time for diffusion

Fe3C

(cementite)

5 m

(ferrite)

Chapter 10 -15

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

Spheroidite: Fe-C System

60 m

(ferrite)

(cementite)

Fe3C

Chapter 10 -16

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

NON-Equilibrium single phase

Hard and Brittle

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

Martensite: Fe-C System

Martensite needlesAustenite

60

m

xx x

xx

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

Chapter 10 -17

(FCC) (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

Chapter 10 -18

Mechanical Prop: Effect Of Carbon Content

Increasing 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

Chapter 10 -19

Mechanical Prop: Effect Of Size

• 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%CD

uc

tilit

y (

%R

A)

fine pearlite

coarse pearlite

spheroidite

Hypo Hyper

0 0.5 1

Chapter 10 -20

Mechanical Prop: Effect Of Structure Type

• 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

Chapter 10 -21

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

Chapter 10 -22

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

reheatreheat

SpheroiditeThe most ductile

Chapter 10 -23

Summary: Structures versus Mechanical Properties

Str

engt

h

Duc

tility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends

Chapter 10 -24

Annealing: Heat to Tanneal, then cool slowly.

Based on discussion in Section 11.7, Callister 7e.

Thermal Processing of Metals

Types of Annealing

• Process Anneal: Negate effect of cold working by (recovery/ recrystallization)

• Stress Relief: Reduce stress caused by:

-plastic deformation -nonuniform cooling -phase transform.

• Normalize (steels): Deform steel with large grains, then normalize to make grains small.

• Full Anneal (steels): Make soft steels for good forming by heating to get , then cool in furnace to get coarse P.

• Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for

15-25 h.

Chapter 10 -25

a) Annealing

b) Quenching

Heat Treatments

c)

c) Tempered Martensite

Adapted from Fig. 10.22, Callister 7e.

time (s)10 10 3 10 510 -1

400

600

800

T(°C)

Austenite (stable)

200

P

B

TE

0%

100%50%

A

A

M + A

M + A

0%

50%

90%

a)b)

Chapter 10 -26

Hardenability--Steels• Ability to form martensite• Jominy end quench test to measure hardenability.

• Hardness versus distance from the quenched end.

Adapted from Fig. 11.11, Callister 7e. (Fig. 11.11 adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1978.)

Adapted from Fig. 11.12, Callister 7e.

24°C water

specimen (heated to phase field)

flat ground

Rockwell Chardness tests

Har

dnes

s, H

RC

Distance from quenched end

Chapter 10 -27

• The cooling rate varies with position.

Adapted from Fig. 11.13, Callister 7e. (Fig. 11.13 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 376.)

Why Hardness Changes W/Position

distance from quenched end (in)Ha

rdn

ess

, H

RC

20

40

60

0 1 2 3

600

400

200A M

A

P

0.1 1 10 100 1000

T(°C)

M(start)

Time (s)

0

0%100%

M(finish) Martensite

Martensite + Pearlite

Fine Pearlite

Pearlite

Chapter 10 -28

Hardenability vs Alloy Composition• Jominy end quench results, C = 0.4 wt% C

• "Alloy Steels" (4140, 4340, 5140, 8640) --contain Ni, Cr, Mo (0.2 to 2wt%) --these elements shift the "nose". --martensite is easier to form.

Adapted from Fig. 11.14, Callister 7e. (Fig. 11.14 adapted from figure furnished courtesy Republic Steel Corporation.)

Cooling rate (°C/s)

Har

dne

ss, H

RC

20

40

60

100 20 30 40 50Distance from quenched end (mm)

210100 3

4140

8640

5140

1040

50

80

100

%M4340

T(°C)

10-1 10 103 1050

200

400

600

800

Time (s)

M(start)M(90%)

shift from A to B due to alloying

BA

TE

Chapter 10 -29

• Effect of quenching medium:

Mediumairoil

water

Severity of Quenchlow

moderatehigh

Hardnesslow

moderatehigh

• Effect of geometry: When surface-to-volume ratio increases: --cooling rate increases --hardness increases

Positioncentersurface

Cooling ratelowhigh

Hardnesslowhigh

Quenching Medium & Geometry

Chapter 10 -30

0 10 20 30 40 50wt% Cu

L+L

+L

300

400

500

600

700

(Al)

T(°C)

composition range needed for precipitation hardening

CuAl2

A

Adapted from Fig. 11.24, Callister 7e. (Fig. 11.24 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)

Precipitation Hardening• Particles impede dislocations.• Ex: Al-Cu system• Procedure:

Adapted from Fig. 11.22, Callister 7e.

--Pt B: quench to room temp.--Pt C: reheat to nucleate small crystals within crystals.

• Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al

Temp.

Time

--Pt A: solution heat treat (get solid solution)

Pt A (sol’n heat treat)

B

Pt B

C

Pt C (precipitate

Chapter 10 -31

• 2014 Al Alloy:

• TS peaks with precipitation time.• Increasing T accelerates process.

Adapted from Fig. 11.27 (a) and (b), Callister 7e. (Fig. 11.27 adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979. p. 41.)

Precipitate Effect on TS, %EL

precipitation heat treat time

tens

ile s

tren

gth

(MP

a)

200

300

400

1001min 1h 1day 1mo 1yr

204°C

non-

equi

l. so

lid s

olut

ion

man

y sm

all

prec

ipita

tes

“age

d”

few

er la

rge

prec

ipita

tes

“ove

rage

d”149°C

• %EL reaches minimum with precipitation time.

%E

L (2

in s

ampl

e)10

20

30

0 1min 1h 1day 1mo 1yr

204°C 149°C

precipitation heat treat time

Chapter 10 -32

Metal Alloy Crystal Stucture

Alloys• substitutional alloys

– can be ordered or disordered– disordered solid solution– ordered - periodic substitution

example: CuAu FCC

Cu

Au

Chapter 10 -33

• Interstitial alloys (compounds) – one metal much larger than the other – smaller metal goes in ordered way into

interstitial “holes” in the structure of larger metal

– Ex: Cementite – Fe3C

Metal Alloy Crystal Stucture

Chapter 10 -34

Metal Alloy Crystal Stucture

• Consider FCC structure --- what types of holes are there?

Octahedron - octahedral site = OH Tetrahedron - tetrahedral site = TD

Chapter 10 -35

Metal Alloy Crystal Stucture

• Interstitials such as H, N, B, C• FCC has 4 atoms per unit cell

metal atoms

21

21

21

21

TD sites

43 4

1,4

3 41

,

43 4

1,4

3 41

,

8 TD sites

OH sites

21

21

21

21

21

4 OH sites

Chapter 10 -36

• Steels: increase TS, Hardness (and cost) by adding --C (low alloy steels) --Cr, V, Ni, Mo, W (high alloy steels) --ductility usually decreases w/additions.• Non-ferrous: --Cu, Al, Ti, Mg, Refractory, and noble metals.• Fabrication techniques: --forming, casting, joining.• Hardenability --increases with alloy content.• Precipitation hardening --effective means to increase strength in Al, Cu, and Mg alloys.

Summary