Chapter 10: Phase Transformations ISSUES TO ADDRESSmatclass/101/pdffiles/Lecture_15.pdf · Chapter...

Chapter 10:Phase Transformations

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

• How does the rate of transformation depend ontime and T?

Fe

γ(Austenite)

Eutectoid transformation

C FCC

Fe3C (cementite)

α (ferrite)

+(BCC)

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

• Are the properties of non-equilibriumstructures improved?

Fraction of Transformation

• Fraction transformed depends on time.

Adapted from Fig. 10.1, Callister 6e.

y

log (t)

Fixed T

0

0.5

1

t0.5

fraction transformed time

y = 1− e−ktn

Avrami Eqn.

• Transformation rate depends on T.

1 10 102 1040

50

100 135°

C11

9°C

113°

C10

2°C

88°C

43°Cy (%)

log (t) min

Ex: recrystallization of Cu

r = 1t

0.5

= Ae−Q /RT

activation energy

• r often small: equil not possible!

Adapted from Fig. 10.2, Callister 6e.(Fig. 10.2 adapted from B.F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p. 888.)

Transformations and Undercooling

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

• Eutectoid transf. (Fe-C System):

αferrite

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6

.7

L

γaustenite

γ+L

γ+Fe3C

Fe3C

cementiteα+Fe3C

α+γ

L+Fe3C

(Fe) Co, wt% C

Eutectoid:

0.7

7

727°C

T(°C)

∆T

0.0

22

γ ⇒ α +Fe3C0.77wt%C

0.022wt%C6.7wt%C

Undercooling by ∆T: 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.)

Eutectoid Transformation Rate ~ ∆T

γαααα

α

α

pearlite growth direction

Austenite (γ) grain boundary

cementite (Fe3C)

ferrite (α)

γ

Diffusive flow of C needed

α

αγ γ

α

• Growth of pearlite from austenite:

• Reaction rateincreases with∆T.



675°C (∆T smaller)

1 10 102 103time (s)

0

50

100

y (%

pe

arl

ite

) 0

50

100

600°C (∆T larger)

650°C

% a

ust

en

ite

Nucleation and Growth

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

• Examples:

% Pearlite

0

50

100

Nucleation regime

Growth regime

log (time)t50

Nucleation rate increases w/ ∆T

Growth rate increases w/ T

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

Adapted fromFig. 10.1, Callister 6e.

Isothermal Transformation Diagrams

• 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.)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

Ex: Cooling History Fe-C System

• Eutectoid composition, Co = 0.77 wt%C• Begin at T > 727 °C• Rapidly cool to 625 °C 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.)

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)

Pearlite Morphology

10

µm

• Ttransf just below TE--Larger T: diffusion is faster--Pearlite is coarser.

Two cases:• Ttransf well below TE

--Smaller T: diffusion is slower--Pearlite is finer.

Adapted from Fig. 10.6 (a) and (b),Callister 6e. (Fig. 10.6 from R.M. Ralls et al., An Introduction to Materials Science and Engineering, p. 361, John Wiley and Sons, Inc., New York, 1976.)

- Larger ∆T: colonies are smaller

- Smaller ∆T: colonies are larger

Non-Equil. Transformation Products: Fe-C

Bainite reaction rate:

rbainite = e−Q /RT

• Bainite:--α lathes (stripes) with long

rods 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)

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

Other Products: Fe-C System (1)

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.)

10 103 105time (s)10-1

400

600

800


200

P

B

TE

0%

100%

50%

A

A

Spheroidite100% spheroidite

100% spheroidite

Other Products: Fe-C System (2)

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


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


xx x

xx

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

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.

(Adapted from Fig. 10.11, Callister, 6e.

Time-Temperature-Transformation (TTT) Diagrams

Isothermal TTT Diagram Continuous Cooling TTT Diagram

TTT Diagrams for a Eutectoid Steel

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



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




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


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

Mechanical Prop: Fe-C System (1)

Adapted from Fig. 10.20, Callister 6e. (Fig. 10.20 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.)

Adapted from Fig. 9.27,Callister6e. (Fig. 9.27 courtesy Republic Steel Corporation.)

Adapted from Fig. 9.30,Callister 6e. (Fig. 9.30 copyright 1971 by United States Steel Corporation.)

• Effect of wt%C

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

0 0.5 10

50

100%EL

Imp

ac

t e

ne

rgy

(Izo

d, f

t-lb

)

0

40

80

300

500

700

900

1100YS(MPa)TS(MPa)

wt%C0 0.5 1

hardness

0.7

7

0.7

7

Co>0.77wt%C

Hypereutectoid

Co<0.77wt%C

Hypoeutectoid

Pearlite (med)ferrite (soft)

Pearlite (med)Cementite

Hypo HyperHypo Hyper

(hard)


Adapted from Fig. 10.21, Callister 6e. (Fig. 10.21 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.)

• Fine vs coarse pearlite vs spheroidite

• Hardness: fine > coarse > spheroidite • %AR: fine < coarse < spheroidite

80

160

240

320

wt%C0 0.5 1

Bri

ne

ll h

ard

ne

ss

fine pearlite

coarse pearlitespheroidite

0

30

60

90

wt%C0 0.5 1

Du

cti

lity

(%A

R)

fine pearlite

coarse pearlite

spheroidite

Hypo Hyper Hypo Hyper


• Fine Pearlite vs Martensite:

• Hardness: fine pearlite << martensite.

Adapted from Fig. 10.23, Callister 6e. (Fig. 10.23 adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p. 1776.)

0

200

wt%C0 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.

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.)

• 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.

Summary: Processing Options

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

lity

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends


Chapter 11: Metal AlloysApplications and Processing

ISSUES TO ADDRESS...

• How are metal alloys classified and how are they used?

• What are some of the common fabrication techniques?

• How do properties vary throughout a piece of materialthat has been quenched, for example?

• How can properties be modified by post heat treatment?

Taxonomy of Metals

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.)


Fe3C

cementite

Metal Alloys

Steels

Ferrous Nonferrous

Cast Irons Cu Al Mg Ti<1.4wt%C 3-4.5wt%C

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

γaustenite

γ+L

γ+Fe3Cα

ferriteα+Fe3C

α+γ

L+Fe3C

δ

(Fe) Co, wt% C

Eutectic:

Eutectoid:0.77

4.30

727°C

1148°C

T(°C)

Steels<1.4wt%C

Cast Irons3-4.5wt%C

microstructure: ferrite, graphite cementite

Steels

Low Alloy High Alloy

low carbon <0.25wt%C

med carbon 0.25-0.6wt%C

high carbon 0.6-1.4wt%C

Uses auto struc. sheet

bridges towers press. vessels

crank shafts bolts hammers blades

pistons gears wear applic.

wear applic.

drills saws dies

high T applic. turbines furnaces V. corros. resistant

Example 1010 4310 1040 4340 1095 4190 304

Additions noneCr,V Ni, Mo

noneCr, Ni Mo

noneCr, V, Mo, W

Cr, Ni, Mo

plain HSLA plainheat

treatableplain tool

austentitic stainless

Name

Hardenability 0 + + ++ ++ +++ 0TS - 0 + ++ + ++ 0EL + + 0 - - -- ++

increasing strength, cost, decreasing ductilityBased on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 6e.

Basic Ideas in Alloying Steels:1. Phase Partitioning at Austenite/Pearlite Interface

Carbide FormersV, Ti, Nb

Ferrite Formers (solid solution)Ni, Si, Mn

Paritioning at the Austenite/Pearlite InterfaceSlows transformationAllows Bainite or Martensite to form on cooling

Basic Ideas in Alloying Steels:2. Alloying to Control the Eutectoid Transformation

AlloyingControl ‘Nose’ in TTT DiagramControl Eutectoid Temperature and C Composition

Cr: added (~8 – 12wt %) to make steel ‘stainless’Ni: High concentrations to stabilize austenite – austenitic steels

Hardenability of Steels

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

• Hardness versus distance from the quenched end.

24°C water

specimen (heated to γ phase field)

flat ground

4”

1”

Ha

rdn

ess

, HR

C

Distance from quenched end

Adapted from Fig. 11.10, Callister 6e. (Fig. 11.10 adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1978.)


Why Hardness Changes with Position

• The cooling rate varies with position.

Adapted from Fig. 11.12, Callister 6e. (Fig. 11.12 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 376.)

distance from quenched end (in)Ha

rdn

ess

, HR

C

20

40

60

0 1 2 3

600

400

200A → M

A → P

Martensite

Martensite + Pearlite

Fine Pearlite

Pearlite

0.1 1 10 100 1000

T(°C)

M(start)

Time (s)

0

0%100%

M(finish)

Hardenability vs. Alloy Content

• Jominy end quenchresults, C = 0.4wt%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. 11.13, Callister 6e. (Fig. 11.13 adapted from figure furnished courtesy Republic Steel Corporation.)

T(°C)

10-1 10 103 1050

200

400

600

800

Time (s)

M(start)M(90%)

TE

A Bshift from A to B due to alloying

Cooling rate (°C/s)

Ha

rdn

ess

, HR

C

20

40

60

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

210100 3

4140

8640

5140

1040

50

80

100

%M4340

Quenching Medium and Geometry

• Effect of quenching medium:

Mediumairoil

water

Hardnesssmall

moderatelarge

Severity of Quenchsmall

moderatelarge

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

--cooling rate increases--hardness increases

Positioncenter

surface

Cooling ratesmalllarge

Hardnesssmalllarge

Nonferrous Alloys

NonFerrous Alloys

• Cu AlloysBrass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant)Bronze: Sn, Al, Si, Ni are subst. impurity (bushings, landing gear)Cu-Be: precip. hardened for strength

• Al Alloys-lower ρ: 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct.

aircraft parts & packaging)

• Mg Alloys-very low ρ: 1.7g/cm3 -ignites easily -aircraft, missles

• Refractory metals-high melting T -Nb, Mo, W, Ta• Noble metals

-Ag, Au, Pt -oxid./corr. resistant

• Ti Alloys-lower ρ: 4.5g/cm3

vs 7.9 for steel -reactive at high T -space applic.

Based on discussion and data provided in Section 11.3, Callister 6e.

Metal Fabrication Methods (1)

CASTING JOININGFORMING

Ao Ad

force

dieblank

force

• Forging(wrenches, crankshafts)

• Rolling(I-beams, rails)

• Extrusion(rods, tubing)


ram billet

container

containerforce

die holder

die

Ao

Adextrusion

roll

AoAd

roll

• Drawing(rods, wire, tubing)

often atelev. T

tensile force

AoAddie

die

Forming Temperature

• Hot working--recrystallization--less energy to deform--oxidation: poor finish--lower strength

• Cold working--recrystallization--less energy to deform--oxidation: poor finish--lower strength

• Cold worked microstructures--generally are very anisotropic!--Forged --Fracture resistant!

Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.), John Wiley and Sons, Inc., 1996. (a) Fig. 10.5, p. 410 (micrograph courtesy of G. Vander Voort, Car Tech Corp.); (b) Fig. 10.6(b), p. 411 (Orig. source: J.F. Peck and D.A. Thomas, Trans. Metall. Soc. AIME, 1961, p. 1240); (c) Fig. 10.10, p. 415 (Orig. source: A.J. McEvily, Jr.and R.H. Bush, Trans. ASM 55, 1962, p. 654.)

(a) (b) (c)

--Swaged


plasterdie formedaround waxprototype

FORMING JOININGCASTING• Sand Casting

(large parts, e.g.,auto engine blocks)

Sand Sand

molten metal

• Investment Casting(low volume, complex shapese.g., jewelry, turbine blades)

wax

• Die Casting(high volume, low T alloys)

• Continuous Casting(simple slab shapes)

molten

solidified


CASTINGFORMING JOINING• Powder Processing

(materials w/low ductility)

pressure

heat

point contact at low T

densification by diffusion at higher T

area contact

densify

• Welding(when one large part isimpractical)

• Heat affected zone:(region in which themicrostructure has beenchanged).

Adapted from Fig. 11.8, Callister 6e.(Fig. 11.8 from Iron Castings Handbook, C.F. Walton and T.J. Opar (Ed.), 1981.)

piece 1 piece 2

fused base metal

filler metal (melted)base metal (melted)

unaffectedunaffectedheat affected zone

Thermal Processing of Metals

Annealing: Heat to Tanneal, then cool slowly.

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 γ, then cool in furnace to get coarse P.

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

15-25h.

Based on discussion in Section 11.7, Callister 6e.

Precipitation Hardening

• 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.)


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

CuAl2

Precipitate Effect on TS, %El

• 2014 Al Alloy:

• TS peaks withprecipitation time.

• Increasing T acceleratesprocess.

• %EL reaches minimumwith precipitation time.

precipitation heat treat time (h)

ten

sile

str

en

gth

(M

Pa

)

300

400

500

2001min 1h 1day 1mo 1yr

204°C

149°C

non-

equi

l. so

lid s

olut

ion

man

y sm

all

prec

ipita

tes

“ag

ed”

fe

wer

larg

e

pre

cipi

tate

s

“ove

rage

d”%

EL

(2

in s

am

ple

)10

20

30

0 1min 1h 1day 1mo 1yr

204°C 149°C

precipitation heat treat time (h)

Adapted from Fig. 11.25 (a) and (b), Callister 6e. (Fig. 11.25 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.)

SUMMARY

• 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.

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