ISSUES TO ADDRESS...• When we combine two elements... what equilibrium state do we get?• In particular, if we specify... --a composition (e.g., wt% Cu - wt% Ni), and --a temperature (T )

then... How many phases do we get? What is the composition of each phase? How much of each phase do we get?

Chapter 9: Phase Diagrams for Metallic Systems

Phase BPhase A

Nickel atomCopper atom

Phase Equilibria: Solubility LimitIntroduction

– Solutions – solid solutions, single phase– Mixtures – more than one phase

• Solubility Limit: Max concentration for which only a single phase solution occurs.

Question: What is the solubility limit at 20°C?

Answer: 65 wt% sugar.

If Co < 65 wt% sugar: syrup

If Co > 65 wt% sugar: syrup + sugar.

65

Sucrose/Water Phase Diagram

Pu

re

Su

gar

Tem

per

atu

re (

°C)

0 20 40 60 80 100Co =Composition (wt% sugar)

L (liquid solution

i.e., syrup)

Solubility Limit L

(liquid)

+ S

(solid sugar)20

40

60

80

100

Pu

re

Wat

er

• Components: The elements or compounds which are present in the mixture (e.g., Al and Cu)

• Phases: The physically and chemically distinct material regions that result (e.g., and ).

Aluminum-CopperAlloy

Components and Phases

(darker phase)

(lighter phase)

Effect of T & Composition (Co)• Changing T can change # of phases: path A to B.• Changing Co can change # of phases: path B to D.

D (100°C,90)2 phases

B (100°C,70)1 phase

A (20°C,70)2 phases

70 80 1006040200

Tem

pera

ture

(°C

)

Co =Composition (wt% sugar)

L (liquid solution

i.e., syrup)

20

100

40

60

80

0

L (liquid)

+ S

(solid sugar)

water-sugarsystem

Phase Equilibria

CrystalStructure

electro negativity

r (nm)

Ni FCC 1.9 0.1246

Cu FCC 1.8 0.1278

• Both have the same crystal structure (FCC) and have similar electronegativities and atomic radii (W. Hume – Rothery rules) suggesting high mutual solubility.

• Ni and Cu are totally miscible in all proportions.

Simple solution system (e.g., Ni-Cu solution)

Phase Diagrams• Indicate phases as function of T, Co, and P. • For this course: -binary systems: just 2 components.

-independent variables: T and Co (P = 1 atm is almost always used).

• PhaseDiagramfor Cu-Nisystem

• 2 phases: L (liquid)

(FCC solid solution)

• 3 phase fields: LL +

wt% Ni20 40 60 80 10001000

1100

1200

1300

1400

1500

1600T(°C)

L (liquid)

(FCC solid solution)

L + liquidus

solid

us

Phase Diagrams:# and types of phases

• Rule 1: If we know T and Co, then we know: --the # and types of phases present.

• Examples:A(1100°C, 60): 1 phase:

B(1250°C, 35): 2 phases: L +

wt% Ni20 40 60 80 10001000

1100

1200

1300

1400

1500

1600T(°C)

L (liquid)

(FCC solid solution)

L +

liquidus

solid

us

Cu-Niphase

diagram

B (

1250

°C,3

5)

A(1100°C,60)

Phase Diagrams:composition of phases

• Rule 2: If we know T and Co, then we know: --the composition of each phase.

• Examples:

At TA = 1320°C:

Only Liquid (L) CL = Co ( = 35 wt% Ni)

At TB = 1250°C:

Both and L CL = C liquidus ( = 32 wt% Ni here)

C = C solidus ( = 43 wt% Ni here)

At TD = 1190°C:

Only Solid ( ) C = Co ( = 35 wt% Ni)

Co = 35 wt% Ni

wt% Ni

20

1200

1300

T(°C)

L (liquid)

(solid)L +

liquidus

solidus

30 40 50

L +

Cu-Ni system

TAA

35Co

32CL

BTB

DTD

tie line

4C3

• Rule 3: If we know T and Co, then we know: --the amount of each phase (given in wt%).• Examples:

At TA: Only Liquid (L)

W L = 100 wt%, W = 0At TD: Only Solid ( )

W L = 0, W = 100 wt%

Co = 35 wt% Ni

Phase Diagrams:weight fractions of phases

wt% Ni

20

1200

1300

T(°C)

L (liquid)

(solid)L +

liquidus

solidus

30 40 50

L +

Cu-Ni system

TAA

35Co

32CL

BTB

DTD

tie line

4C3

R SAt TB: Both and L

% 733243

3543wt

= 27 wt%

WL S

R +S

W R

R +S

• Tie line – connects the phases in equilibrium with each other - essentially an isotherm

The Lever Rule

How much of each phase? Think of it as a lever

ML M

R S

RMSM L

L

L

LL

LL CC

CC

SR

RW

CC

CC

SR

S

MM

MW

00

wt% Ni

20

1200

1300

T(°C)

L (liquid)

(solid)L +

liquidus

solidus

30 40 50

L + B

TB

tie line

CoCL C

SR

• Phase diagram: Cu-Ni system.

• System is: --binary i.e., 2 components: Cu and Ni. --isomorphous i.e., complete solubility of one component in another; phase field extends from 0 to 100 wt% Ni.

• Consider Co = 35 wt%Ni.

Cooling in a Cu-Ni Binary

wt% Ni20

1200

1300

30 40 501100

L (liquid)

(solid)

L +

L +

T(°C)

A

35Co

L: 35wt%Ni

Cu-Nisystem

4635

4332

: 43 wt% Ni

L: 32 wt% Ni

L: 24 wt% Ni

: 36 wt% Ni

B: 46 wt% NiL: 35 wt% Ni

C

D

E

24 36

• C changes as we solidify.• Cu-Ni case:

• Fast rate of cooling: Cored structure

• Slow rate of cooling: Equilibrium structure

First to solidify has C = 46 wt% Ni.

Last to solidify has C = 35 wt% Ni.

Cored vs Equilibrium Phases

First to solidify: 46 wt% Ni

Uniform C:

35 wt% Ni

Last to solidify: < 35 wt% Ni

Mechanical Properties: Cu-Ni System

• Effect of solid solution strengthening on:

--Tensile strength (TS) --Ductility (%EL,%AR)

--Peak as a function of Co --Min. as a function of Co

Te

nsile

Str

en

gth

(M

Pa

)

Composition, wt% NiCu Ni0 20 40 60 80 100

200

300

400

TS for pure Ni

TS for pure Cu

Elo

ng

atio

n (

%E

L)

Composition, wt% NiCu Ni0 20 40 60 80 10020

30

40

50

60

%EL for pure Ni

%EL for pure Cu

: Min. melting TE

Binary-Eutectic Systems

• Eutectic transition L(CE) (CE) + (CE)

• 3 single phase regions (L, ) • Limited solubility: : mostly Cu : mostly Ag • TE : No liquid below TE

• CE

composition

Ex.: Cu-Ag system

Co , wt% Ag

Cu-Agsystem

L (liquid)

L + L+

20 40 60 80 1000200

1200T(°C)

400

600

800

1000

CE

TE 8.0 71.9 91.2779°C

• For a 40 wt% Sn-60 wt% Pb alloy at 150°C, find... --the phases present:

EX: Pb-Sn Eutectic System (1)

+ --compositions of phases:

CO = 40 wt% Sn

--the relative amount of each phase:

L+L+

+

200

T(°C)

18.3

C, wt% Sn20 60 80 1000

300

100

L (liquid)

183°C 61.9 97.8

Pb-Snsystem

150

40Co

11C

99C

SR

C = 11 wt% SnC = 99 wt% Sn

W=C - CO

C - C

=99 - 4099 - 11

= 5988

= 67 wt%

SR+S

=

W =CO - C

C - C=R

R+S

=2988

= 33 wt%=40 - 1199 - 11

• For a 40 wt% Sn-60 wt% Pb alloy at 220°C, find... --the phases present:

EX: Pb-Sn Eutectic System (2)

+ L--compositions of phases:

CO = 40 wt% Sn

--the relative amount of each phase:

W =CL - CO

CL - C=

46 - 40

46 - 17

= 6

29= 21 wt%

WL =CO - C

CL - C=

23

29= 79 wt%

L+

+

200

T(°C)

C, wt% Sn20 60 80 1000

300

100

L (liquid)

L+

183°C

Pb-Snsystem

40Co

46CL

17C

220SR

C = 17 wt% SnCL = 46 wt% Sn

• Co < 2 wt% Sn• Result: --at extreme ends --polycrystal of grains i.e., only one solid phase.

Microstructures in Eutectic Systems: I

0

L + 200

T(°C)

Co , wt% Sn10

2

20Co

300

100

L

30

+

400

(room T solubility limit)

TE

(Pb-SnSystem)

L

L: Co wt% Sn

: Co wt% Sn

• 2 wt% Sn < Co < 18.3 wt% Sn• Result:

Initially liquid + then alonefinally two phases

polycrystal fine -phase inclusions

Microstructures in Eutectic Systems: II

Pb-Snsystem

L +

200

T(°C)

Co , wt% Sn10

18.3

200Co

300

100

L

30

+

400

(sol. limit at TE)

TE

2(sol. limit at Troom)

L

L: Co wt% Sn

: Co wt% Sn

• Co = CE • Result: Eutectic microstructure (lamellar structure) --alternating layers (lamellae) of and crystals.

Microstructures in Eutectic Systems: III

160 m

Micrograph of Pb-Sn eutectic microstructurePb-Sn

systemL

200

T(°C)

C, wt% Sn

20 60 80 1000

300

100

L

L+ 183°C

40

TE

18.3

: 18.3 wt%Sn

97.8

: 97.8 wt% Sn

CE61.9

L: Co wt% Sn

Lamellar Eutectic Structure

• 18.3 wt% Sn < Co < 61.9 wt% Sn• Result: crystals and a eutectic microstructure

Microstructures in Eutectic Systems: IV

WL = (1-W) = 50 wt%

C = 18.3 wt% Sn

CL = 61.9 wt% SnS

R + SW = = 50 wt%

• Just above TE :

• Just below TE :C = 18.3 wt% Sn

C = 97.8 wt% SnS

R + SW = = 73 wt%

W = 27 wt%18.3 61.9

SR

97.8

SR

primary eutectic

eutectic

Pb-Snsystem

L+200

T(°C)

Co, wt% Sn

20 60 80 1000

300

100

L

L+

40

+

TE

L: Co wt% Sn LL

L+L+

+

200

Co, wt% Sn20 60 80 1000

300

100

L

TE

40

(Pb-Sn System)

Hypoeutectic & Hypereutectic

hypereutectic: (illustration only)

175 m

hypoeutectic: Co = 50 wt% Sn

T(°C)

61.9eutectic

160 m

eutectic micro-constituent

eutectic: Co = 61.9 wt% Sn

Intermetallic Compounds

Mg2Pb

Note: intermetallic compound forms a line - not an area - because stoichiometry (i.e. composition) is exact.

Eutectoid & Peritectic

• Eutectic - liquid in equilibrium with two solidsL + cool

heat

intermetallic compound - cementite

cool

heat

• Eutectoid - solid phase in equation with two solid phases

S2 S1+S3

+ Fe3C (727ºC)

cool

heat

• Peritectic - liquid + solid 1 solid 2

S1 + L S2

+ L (1493ºC)

Eutectoid & Peritectic

Cu-Zn Phase diagram

Eutectoid transition +

Peritectic transition + L

Iron-Carbon (Fe-C) Phase Diagram

• 2 important points

-Eutectoid (B):

+Fe3C

-Eutectic (A): L + Fe3C

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)

727°C = Teutectoid

A

SR

4.30

Result: Pearlite = alternating layers of and Fe3C phases

120 m

R S

0.76

Ceu

tect

oid

B

Fe3C (cementite-hard) (ferrite-soft)

Chapter 9 - 27

Ferrite and Austenite

Ferrite Austenite

Hypoeutectoid Steel

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)

727°C

(Fe-C System)

C0

0.76

proeutectoid ferritepearlite

100 m Hypoeutectoidsteel

R S

w =S/(R+S)wFe3C =(1-w)

wpearlite = wpearlite

r s

w =s/(r+s)w =(1- w)

Hypereutectoid Steel

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)

(Fe-C System)

0.7

6 Co

proeutectoid Fe3C

60 mHypereutectoid steel

pearlite

R S

w =S/(R+S)wFe3C =(1-w)

wpearlite = wpearlite

sr

wFe3C =r/(r+s)w =(1-w Fe3C )

Fe3C

Example: Phase Equilibria

For a 99.6 wt% Fe-0.40 wt% C at a temperature just below the eutectoid, determine the following

a) composition of Fe3C and ferrite ()

b) the amount of carbide (cementite) in grams that forms per 100 g of steel

c) the amount of pearlite and proeutectoid ferrite ()

Chapter 9 – Phase EquilibriaSolution:

g 3.94

g 5.7 CFe

g7.5100 022.07.6

022.04.0

100xCFe

CFe

3

CFe3

3

3

x

CC

CCo

b) the amount of carbide (cementite) in grams that forms per 100 g of steel

a) composition of Fe3C and ferrite ()

CO = 0.40 wt% C

C = 0.022 wt% C

CFe C = 6.70 wt% C3

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

Co , wt% C

1148°C

T(°C)

727°C

CO

R S

CFe C3C

Chapter 9 – Phase Equilibriac. the amount of pearlite and proeutectoid ferrite ()

note: amount of pearlite = amount of just above TE

Co = 0.40 wt% C

C = 0.022 wt% C

Cpearlite = C = 0.76 wt% C

Co CC C

x 100 51.2 g

pearlite = 51.2 gproeutectoid = 48.8 g

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

Co , wt% C

1148°C

T(°C)

727°C

CO

R S

CC

Alloying Steel with More Elements

• Teutectoid changes: • Ceutectoid changes:

TE

ute

cto

id (

°C)

wt. % of alloying elements

Ti

Ni

Mo SiW

Cr

Mn

wt. % of alloying elementsC

eut

ect

oid

(w

t%C

)

Ni

Ti

Cr

SiMn

WMo

• Phase diagrams are useful tools to determine:--the number and types of phases,--the wt% of each phase,--and the composition of each phase

for a given T and composition of the system.

• Alloying to produce a solid solution usually--increases the tensile strength (TS)--decreases the ductility.

• Binary eutectics and binary eutectoids allow for a range of microstructures.

Summary

Chapter 9 - 35

Fe-Fe3C System

Chapter 9 - 36

Cu-Ag System

Chapter 9 - 37

Pb-Sn System

Chapter 9 - 38

Sn-Bi System

Chapter 9 - 39

Cu-Zn System

Chapter 9 - 40

Mg-Pb System

Chapter 9 - 41

Ni-Ti System

Chapter 9 - 42

Sn-Au System

Chapter 9 - 43

Cu-Al System

Chapter 9 - 44

Hf-V System