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Eutectics,Dispersion Strengthening,

and otherThree-Phase

Reactions

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Eutectic Diagrams:

• Consider a system with limitedsolubility:

T

 wt. % SnPb Sn

αβ

α + β

liquid

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• Determine cooling curves forvarious alloys.

T

time

Slope change

Isothermal hold

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•  Transfer the transition point dataand consider various alloys:

T

 wt. % SnPb Sn

αβ

α + β

liquid1 4

3

2

!1."%

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• Alloy 1

 – Appears to be like isomorphoussystem

• liquidus and solidus with a freezing range

time

Tliquidus

solidusα

α

α

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Alloy 2

T

 wt. % SnPb Sn

αβ

α + β

liquid1 4

3

2

!1."%

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• Alloy 2

 – ame as Alloy !" but crosses asolubility line #a solvus$

 –%esults in the precipitation of a hightin second phase on grain boundarysurfaces

time

T liquidussolidus

sol#us

β ppt

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Alloy

T

 wt. % SnPb Sn

αβ

α + β

liquid1 4

3

2

!1."%

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• Alloy --!1"#$ Tin

 – At !&'(C) system is all liquid of*!.+, Tin

 – At !&-(C) all solid #solid α  solid β$• α / !+, Tin

• β / +0.1, Tin

 – A signi2cant chemistry separation hasoccurred upon solidi2cation to

produce a -3phase mi4ture

time

T1$3&

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%amellar Structures:

• 9amellar structures consist ofalternating layers #plates$ of thetwo components

• ach material has its ownchemistry and structure" and theinterfaces act as barriers to

dislocation movement

• ;any eutectics and eutectoidshave lamellar structures

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%amellar (icrostructure

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Alloy )

T

 wt. % SnPb Sn

αβ

α + β

liquid1 4

3

2

!1."%

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• Alloy )

 – Cooling through “left ear” – At !&'

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Alloy *

T

 wt. % SnPb Sn

αβ

α + β

liquid1 4

3

2

!1."%

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• Alloy * – Cooling through “right ear” 

 – >rimary phase is now β #i.e. Tin3rich$

 – At !&=rimary β  utecticstructure

eutectic

Primar' β

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•  Terminology – Alloy ' is hypoeutectic #hypo / less

than$ – Alloy 1 is hypereutectic #hyper /

greater$

•   Note:  verywhere eutecticstructure is present" it always hasthe same composition and the

same properties•   Note:  4pect the eutectic

structure to be high strength 3

2ne -3phase structure contains

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Properties o+ Eutectics:

• 9owest melting point coupled withhighest strength – Attractive for soldering" brazing" and casting

• ach phase has solid solutionstrengthening plus the additional two3phase dispersion strengthening

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9ead3Tin utectic Diagram

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Additional Resources:

nline resources +or Eutectic Phase Diagrams:

 

http:..///"materials"ac"u0.elearning.matter.PhaseDiagrams.EutecticAlloy.introduction"html

 

and +or phase diagrams in general:

 http:..///"southampton"ac"u0.pasr1. inde"htm

http:??www.doitpoms.ac.uk?tlplib?phase3diagrams?inde4.php

http://www.materials.ac.uk/elearning/matter/PhaseDiagrams/EutecticAlloy/introduction.htmlhttp://www.materials.ac.uk/elearning/matter/PhaseDiagrams/EutecticAlloy/introduction.htmlhttp://www.southampton.ac.uk/~pasr1/index.htmhttp://www.doitpoms.ac.uk/tlplib/phase-diagrams/index.phphttp://www.doitpoms.ac.uk/tlplib/phase-diagrams/index.phphttp://www.southampton.ac.uk/~pasr1/index.htmhttp://www.southampton.ac.uk/~pasr1/index.htmhttp://www.materials.ac.uk/elearning/matter/PhaseDiagrams/EutecticAlloy/introduction.htmlhttp://www.materials.ac.uk/elearning/matter/PhaseDiagrams/EutecticAlloy/introduction.html

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Revie/ Strengthening(echanisms:

!$. @rain size strengthening

-$. Cold ork #strain hardening$=$. olid olution strengthening

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Can now add 333

'$. Dispersion strengthening 333#strengthening from interphase

boundaries$

33 present in eutectics andeutectoids

33 smaller spacing  more interfaces 

higher strength33 faster cooling or more rapid growth gives

smaller interlamellar spacing

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%amellar structure

;icrostructure of Ti3*Al3-;o3-Cr alloysafter cooling from !B1B(C at a rate of!.-(C s3!:

 a$ 9; micrograph" b$ T; micrograph.

rom: (icrostructure and (echanical Properties o+ 3igh Strength T/o-Phase Titanium Alloys y E. ieniawski" . FiaGa" H.Hubiak and ;. ;otyka in JTitanium Alloys 3 Advances in >roperties ControlJ"

http://www.intechopen.com/books/titanium-alloys-advances-in-properties-controlhttp://www.intechopen.com/books/titanium-alloys-advances-in-properties-controlhttp://www.intechopen.com/books/titanium-alloys-advances-in-properties-control

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Dispersion Strengthening:4hand out 3ome/or0 5126

Also have dispersion strengtheningin particulate dispersions where theparticles serve as reinforcements.

5ere we want:!$ trong" hard ppt. with soft" ductilematri4

-$ mall" numerous precipitates=$ 9arge amounts of precipitate

'$ Discontinuous precipitate

1$ %ounded precipitate

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Dispersion Strengthening:

Can obtain dispersion strengtheningby:

!$. 4ceeding solubility limits

-$. olidi2cation K eutectics

=$. olid phase transformations

utectoids" Age hardening" tc.

'$. Composite materials

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Dispersionmicrostructures

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Dispersionmicrostructures

7ar8on in cast ironstop – grey cast iron, brittlebottom – ductile cast iron

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ther Three-Phase Reactions:

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 – ince separation is now occurringentirely within the solid state)wee4pect an even 2ner dispersion of the

product phases" and a stronger resultantproduct

 – 4ample: steels

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 – >eritectics start fast and then slowdown

 – ecome very sluggish

 – Tends to produce non3equilibriumsegregated structure

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• Peritectoid: – %eaction is now all solid state so

deplete atoms from adGacent solidsand with ever increasing transportdistance.

 – uper sluggishL – α  β  γ 

 – olid!  olid-  olid=

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• (onotectic: – Two liquids

 – 9!  α 9-

 – 9iquid!  olid!  9iquid-

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9ntermetallic 7ompounds:

• Combine diagrams side3by3side:

T

 wt. % () (

*

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• M generally occurs at atomicpercentages corresponding to

whole number atom ratios.20 A4B

25 A3B

33 A2B40 A3B2

50 AB

60 A2B3

6 AB2

5 AB3

!0 AB4

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 – i4ed atomic ratio is indicative of ionicor covalent bonding  hard andbrittle

• Can be reinforcing agent #positive$• Can be embrittling component #negative$

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• toichiometric intermetallic – >recise atomic ratio

 – Appears in diagrams as a vertical line

• Non3stoichometric intermetallic – Can have some variation in atomic

ratio – Appears as a single phase in the

middle of a diagram with some width

•   Note: >hase diagrams tell only theequilibrium phases and amounts )

not their form or distribution

S i hi i

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Stoichiometric9ntermetallic

S i hi i

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on-Stoichiometric9ntermetallic

3ypothetical 7omple Phase

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3ypothetical 7omple PhaseDiagram

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9nter+acial Sur+ace Energy• Consider alloy cooling across a

solvus with a second phaseprecipitating:

•  The resulting distribution isanalogous

to rain water on a wa4ed car vs.

α

αα

α

αα

Start nd

or

α

αα

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&onsider a grain boundar' between two alpha grains

α

αThe energ' to maintain the inter,ace we will call γ α−α

I, we now insert some beta phase into the boundar'

α

α

β

The energ' to maintain this inter,ace we will call   2γ α−β

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•   Note:  8f the precipitate is a brittleintermetallic and the distribution is inthe form of a grain boundary 2lm #a

thin 2lm that “wets” the grainboundaries$" then a very smallamount of brittle intermetallic can

cause the entire material to be brittle