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Phase Diagrams
Chapter 9 4th EditionChapter 10 5th Edition
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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
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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
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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
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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
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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
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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
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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.
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http://www.enm.bris.ac.uk/teaching/projects/2002_03/jb8355/review.html
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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
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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
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Binary Isomorphous Phase Diagrams for Metal Alloys
Tem
pera
ture
X Y% Y added
Liquid Solution
Solid Solution
Two Phase
Constant Pressure!!!
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Phase Diagrams NomenclatureTe
mpe
ratu
re
X Y% Y added
Liquidus
Solidus
Freezing Range
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Tem
pera
ture
Time
Cooling Curve for Simple Alloys
Liquid
Two Phase
Solid
Tem
pera
t ure
X Y% Y added
Isopleth
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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?
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Solutions
We usually think of liquid solutions
Pour alcohol into water and it dissolves
Alcohol and water are completely miscible
Miscible means completely soluble
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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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
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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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
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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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
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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?
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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)
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Melt Single Phase Solid Solution
Two solid phases – each of which are solutions
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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
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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
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Consider Copper and Zinc
Size – up to a 15% difference in radius
Crystal Structure Valence
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
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Consider Copper and Lead
Size – up to a 15% difference in radius
Crystal Structure Valence
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
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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
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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%
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Adding a small substitutional atom
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Distorts the Matrix
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Adding a large substitutional atom
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Distorts the Matrix
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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%
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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
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Stre
ngth
( ps i
)
Cu NiWt % Nickel
60% Ni
Look at the Effect on Yield Strength
Monel
Inexpensive valve showing
corrosion
?
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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
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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
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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
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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
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Homogenization
If after the solid has formed it is reheated, what happens?
Diffusion! The microsegragation
can be eliminated
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Microsegregation can be eliminated with heating
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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
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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
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Equilibrium Phase Diagram - Mg0-Fe0
1000
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2000
2200
2400
2600
2800
3000
0 20 40 60 80 100
Wt% FeO
Tem
pera
ture
(C)
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Equilibrium Phase Diagram - Nb-W
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0 20 40 60 80 100
Weight % W
Tem
pera
ture
(C)
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Equilibrium Phase Diagram - Cu-Ni
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0 20 40 60 80 100
Wt% Ni
Tem
pera
ture
(C)