Phase Diagrams Chapter 9 4 th Edition Chapter 10 5 th Edition.

Phase Diagrams

Chapter 9 4th EditionChapter 10 5th Edition

So far….

We have concentrated on the solidificaton of a pure metal.

We have looked at strengthening approaches that do not require that we change the composition of the metal.

In this chapter we start to examine alloying, and see why alloys are stronger than pure metals

Let’s Start with a basic question.. What is a phase? Homogeneous in crystal structure and

atomic arrangement Same physical and chemical properties A definite interface with its surroundings Mechanically separable

For example

A glass of ice water has two phases Solid water phase Liquid water phase

Homogeneous in crystal structure and atomic arrangementSame physical and chemical properties

A definite interface with its surroundingsMechanically separable

A mixture of salt and pepper has two phases Solid salt Solid pepper

Homogeneous in crystal structure and atomic arrangementSame physical and chemical properties

A definite interface with its surroundingsMechanically separable

Solid, Liquid or Gas?

We usually think of matter as having 3 phases, but..

It’s possible to have more than one solid phase.

For example at atmospheric pressure when iron first freezes it is BCC As it cools it changes to FCC Upon further cooling it changes to BCC

Phase Diagram

A visualization technique to help you see where different phases exist, based on “state variables”

Solid

Liquid

Gas

Pres

sure

Temperature

Typical phase diagram – for example for Carbon Dioxide

Phase Diagram

A visualization technique to help you see where different phases exist, based on “state variables”

Solid

Liquid

Gas

Pres

sure

Temperature

Phase diagram for pure water

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

http://www.enm.bris.ac.uk/teaching/projects/2002_03/jb8355/review.html

From the following article:Superconductivity: Iron cast in exotic role S. S. Saxena and Peter B. Littlewood

Nature 412, 290-291(19 July 2001) doi:10.1038/35085681

We are going to start with Solidification from a Melt of… Binary Isomorphous (single solid phase) Metals What we learn also applies to ceramics

In the next chapter we’ll look at phase diagrams that include multiple solid phases

Binary Isomorphous Phase Diagrams for Metal Alloys

Tem

pera

ture

X Y% Y added

Liquid Solution

Solid Solution

Two Phase

Constant Pressure!!!

Phase Diagrams NomenclatureTe

mpe

ratu

re

X Y% Y added

Liquidus

Solidus

Freezing Range

Tem

pera

ture

Time

Cooling Curve for Simple Alloys

Liquid

Two Phase

Solid

Tem

pera

t ure

X Y% Y added

Isopleth

What Constitutes a Simple Alloy? A single phase solid solution of two or

more metals Most metals are alloys But what is a solid solution?

Solutions

We usually think of liquid solutions

Pour alcohol into water and it dissolves

Alcohol and water are completely miscible

Miscible means completely soluble


Limited Solubility

Salt is a solid, but it dissolves in water too. Eventually though, you can’t add any more salt, and

you get a two phase system (What are the two phases?) Solid and Liquid But its not pure

water and solid salt – its salt water and solid salt


Insoluble Species

Oil and water don’t mix Does that mean there is absolutely no oil in

the water, or that there’s no water in the oil? Absolutely not!! It just means that not very much dissolved. This is a two phase system too – they are

both liquid phases


Solid Phases

You can dissolve one solid in another

•One way we’ve looked at dissolving one solid in another in the past is through diffusion

•Now we’ll look at forming a solid solution as the metals solidify

Solids have varying solubility – just like liquids Copper and nickel are completely soluble

(miscible) in each other Copper and Zinc display limited solubility Lead and copper are considered insoluble

Why?

Consider liquid solubility

Like dissolves like Water and Alcohol are miscible because they

are similar chemically (polar molecules) Water and Oil are immiscible because they are

different (polar vs non-polar)

Melt Single Phase Solid Solution

Two solid phases – each of which are solutions

Solubility RulesHume-Rothery Rules Size – up to a 15% difference in radius Crystal Structure – the same Valence – the same Electronegativity – Approximately the same If these conditions are met, the two metals are

usually completely soluble These rules also apply to ceramics There is a quiz on the CD what came with your

text - use it to practice

Consider Copper and Nickel

Size – up to a 15% difference in radius

Crystal Structure Valence

Electronegativity

Radius Cu = 1.278 ARadius Ni = 1.243 A

FCC vs FCC Cu -- +1 or +2

Ni -- +2 1.9 vs 1.9

1.278 1.243 *100 2.8%1.243

Consider Copper and Zinc



Electronegativity

Radius Cu = 1.278 ARadius Zn = 1.332 A

FCC vs HCP Cu -- +1 or +2

Zn -- +2 1.9 vs 1.6

1.332 1.278 *100 4.2%1.278

Therefore not TOTALLY miscible

Consider Copper and Lead



Electronegativity

Radius Cu = 1.278 ARadius Pb = 1.75 A

FCC vs FCC Cu -- +1 or +2

Pb -- +4 1.9 vs 1.9

1.75 1.278 *100 37%1.278

Copper and Lead are essentially insoluble

Solid Solution Strengthening

Purposely add an element into the crystal lattice

It distorts the lattice, which causes strengthening

The bigger the distortion, the bigger the improvement in strength

Remember, really large atoms or really small atoms will not be completely soluble

Effect of adding an impurity

Effect of Alloying Elements on Copper

0

5000

10000

15000

20000

25000

30000

35000

40000

0 5 10 15 20

Percent Alloying Element

Yiel

d St

reng

th (p

si) Be

Si

Sn

Al

Ni

Zn

Be

Si Sn

AlNi

Zn

Metal Radius (A) (r-rCu)/rCu *100

Cu 1.278

Zn 1.332 +4.2%

Al 1.432 +12.1%

Sn 1.509 +18.1%

Ni 1.243 -2.7%

Si 1.176 -8.0%

Be 1.143 -10.6%

Adding a small substitutional atom

Distorts the Matrix

Adding a large substitutional atom

Distorts the Matrix

Effect of adding an impurity

Effect of Alloying Elements on Copper

0

5000

10000

15000

20000

25000

30000

35000

40000

0 5 10 15 20

Percent Alloying Element

Yiel

d St

reng

th (p

si) Be

Si

Sn

Al

Ni

Zn

Be

Si Sn

AlNi

Zn

Metal Radius (A) (r-rCu)/rCu *100

Cu 1.278

Zn 1.332 +4.2%

Al 1.432 +12.1%

Sn 1.509 +18.1%

Ni 1.243 -2.7%

Si 1.176 -8.0%

Be 1.143 -10.6%

Other Property Effects

Property Tensile strength Yield Strength % Elongation Electrical Conductivity

Direction Up Up Down Down

Try the quiz on the CD that came with your text

Stre

ngth

( ps i

)

Cu NiWt % Nickel

60% Ni

Look at the Effect on Yield Strength

Monel

Inexpensive valve showing

corrosion

?

Use a phase diagram to examine the solidification process in more detail

Tem

pera

ture

X Y% Y added

Tie Line

Lever Law

Isopleth

A

B

C

Tem

pera

ture

X Y% Y added

30 %

70%

20%

( )*100% SolidConcentration OverallLiquidSolidConcentration LiquidConcentration

( )*100% LiquidConcentration OverallSolid

SolidConcentration LiquidConcentration

70 30% *10070 20

80%

Liquid

30 20% *10070 20

20%

Solid

Sample Calculation

Overall Composition

Composition of the solid

Composition of the liquid

All Liquid52% Ni45% Ni

Solidification Process of Cu - 40% Ni Melt

Almost 40% Ni

40 % Ni

Can only happen if the melt is cooled slowly

Diffusion keeps the solid a constant composition and must have time to occur

Microsegregation occurs if solidification happens more quickly

Can lead to “hot shortness”

The melting point is different at each point in the solid because the composition is different

Homogenization

If after the solid has formed it is reheated, what happens?

Diffusion! The microsegragation

can be eliminated

Microsegregation can be eliminated with heating

Macrosegregation

In a casting, the sides solidify first Because of this, the sides may have a

different composition that the middle of the casting

Can not be eliminated by an homogenization treatment

Hot working will take care of it

Phase Diagrams

You’ll need some phase diagrams to practice with, and for your homework

Use the ones in the textbook However…here are some examples for us

to play with now

Equilibrium Phase Diagram - Mg0-Fe0

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

3000

0 20 40 60 80 100

Wt% FeO

Tem

pera

ture

(C)

Equilibrium Phase Diagram - Nb-W

2400

2500

2600

2700

2800

2900

3000

3100

3200

3300

3400

0 20 40 60 80 100

Weight % W

Tem

pera

ture

(C)

Equilibrium Phase Diagram - Cu-Ni

1000

1050

1100

1150

1200

1250

1300

1350

1400

1450

1500

0 20 40 60 80 100

Wt% Ni

Tem

pera

ture

(C)

Phase Diagrams Chapter 9 4 th Edition Chapter 10 5 th Edition.

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