Eutectic, eutectoid, peritectoid, peritectic

phase transition (or phase change) is most commonly used to describe transitions between solid, liquid and gaseous states of matter, and, in rare cases, plasma.

1. Differences between Eutectic, Peritectic,
Eutectoid and Peritectoid
transformationstransformations
8/5/2018Prem Kumar Soni1

2. 1. Eutectic Transformation
Inaneutecticreaction,whenaliquidsolutionoffixed
composition,solidifiesataconstanttemperature,formsa
mixtureoftwoormoresolidphaseswithoutanintermediate
pasty(लेई ut तरह ut लेई ut तरह ut)stage.Thisprocessलेई ut तरह ut लेई ut तरह ut
reversesonheating.
8/5/20182 Prem Kumar Soni

3. 8/5/2018Prem Kumar Soni3

4. Aeutecticreactionisathree-phasereaction,by
which,oncooling,aliquidtransformsintotwo
solidphasesatthesametime.Itisa
phasereaction,butaspecialone.Forexample:
liquidalloybecomesasolidmixtureofalphaand
8/5/2018Prem Kumar Soni4
liquidalloybecomesasolidmixtureofalphaand
betaataspecifictemperature(ratherthanovera
temperaturerange).

5. Ineutecticsystem,thereisalwaysaspecificalloy,knownas
eutecticcomposition,thatfreezesatalowertemp.thanall
othercompositions.
Attheeutectictemp.twosolidsformsimultaneouslyforma
singleliquidphase.
Theeutectictemp.&compositiondetermineapointontheTheeutectictemp.&compositiondetermineapointonthe
phasediagram,calledtheeutecticpoint.
8/5/20185 Prem Kumar Soni

6. Eutecticorinvariantpoint.Liquidandtwosolidphasesexistin
equilibriumattheeutecticcompositionandtheeutectic
temperature.
Note:
themeltingpointoftheeutecticalloyislowerthanthatofthethemeltingpointoftheeutecticalloyislowerthanthatofthe
components(eutectic=easytomeltinGreek).
Atmosttwophasescanbeinequilibriumwithinaphasefield.
Single-phaseregionsareseparatedby2-phaseregions.
8/5/2018Prem Kumar Soni6

7. Binaryalloyeutecticsystemcanbeclassedas:
1.Oneinwhich,twometalsarecompletelysolubleinthe
liquidstatebutareinsolubleineachotherinthesolid
state.
2.twometalsarecompletelysolubleintheliquidstate2.twometalsarecompletelysolubleintheliquidstate
butarepartlysolubleineachotherinthesolidstate.
8/5/20187 Prem Kumar Soni

8. 8/5/2018Prem Kumar Soni8

9. 1.Twometalscompletelysolubleintheliquidstatebutcompletelyinsoluble
inthesolidstate.
Technically,notwometalsarecompletelyinsolubleineachother.However,insome
casesthesolubilityissorestrictedthatforpracticalpurposestheymaybeconsidered
insoluble.
8/5/20189 Prem Kumar Soni

10. Alloy-1:20%Cdand80%Bi
Contrarytoalloy3,inthiscasecrystalofpureBiformfirst,enrichingthemelt
withCd.
Thecompositionofthemelt(orliquid)movestorightuntilUltimatelythepointE
isreachedandtheremainingliquidsolidi-fiesaseutectic(40%Cdand60%Bi).
Alloy-2:40%Cdand60%Bi(eutecticalloy)
Nosolidificationoccursuntilthemeltreachestheeutectictemperature(140°)
Attheeutectictemperature,thetwopuremetalscrystallizetogethertogivea
characteristicallylineaggregateknownaseutectic.
EutecticconsistsofalternatelayersofCdandBiwhichformattheeutectic
temperature(140°Cinthiscase).
8/5/201810 Prem Kumar Soni

11. Alloy-3: 80% Cd and 20% Bismuth.
AsthetemperaturefallstoT1,crystalnucleiofpureCdbegintoform.Since
pureCdisdeposited,itfollowsthattheliquidbecomesricherinBi;the
compositionofliquidmovestoleft3’andasindicatedbythediagram,no
furtherCddepositsuntiltemperaturefallstoT2.
AtT2moreCdisdepositedanddendritesbegintodevelopfromthealreadyformedAtT2moreCdisdepositedanddendritesbegintodevelopfromthealreadyformed
nuclei.
ThegrowthoftheCddendrites,ontheonehand,andtheconsequentenrichmentof
theremainingliquidinBi,ontheother,continuesuntilthetemperaturehasfallento
140°C,theeutectictemperatureinthiscase.
Theremainingliquidthencontains40%Cdand60%Bi,theeutecticcomposition.
8/5/201811 Prem Kumar Soni

12. 2. Two metals completely soluble in the liquid state, but only partly soluble in
the solid state
8/5/201812 Prem Kumar Soni

13. Sincemostmetalsshowsomesolubilityforeachotherinthesolidstate,thistypeis
themostcommonand,therefore,themostcommonalloysystem.
MetalssuchasPb-SnandPb-Sbarepartlysolubleineachotherinthesolidstate.
Fig.showstheTin-Leadequilibriumdiagramwithmicro-structures(ofcourse)
obtainedundernon-equilibriumconditionofsolidification.
I.Tinwilldissolveuptomaximumof2.6%Pbatthetemperature,formingthesolid
solutionα.solutionα.
II.Leadwilldissolveuptoamaximumof(100-80.5)i.e.19.5%tinattheeutectic
temperature,givingthesolidsolutionβ.
III.SlopeofBAandCDindicatethatthesolubilityofPbinSn(α)andthatofSninPb(β)
decreaseastemperaturefalls
ConsideranalloyofcompositionZ(70%Pb-30%Sn).Asthemelttemperature
fallstoT1,dendritesofcompositionYwilldeposit.
8/5/201813 Prem Kumar Soni

14. Thealloysolidifiesasasolidsolutionuntilat183°C,thelastlayerofsolidtoformis
ofcompositionC(80.5%Pb-19.5%Sn).
Theremainingliquidwhichhastheeutecticcomposition(38%Pb-62%Sn)then
solidifiesbydepositing,intheformofaeutectic,i.e.,alternatelayersofαandβ,of
compositionsBandCrespectively.
Ifcooledslowlytoroomtemperaturethecompositionsofthesolidsolutionsαandβ
willfollowthelineBAandCD,i.e.,αwillbecomeprogressivelypoorerinleadand
βintin.
TakeanotheralloyofcompositionZ'(95%Pb-5%Sn).Whencooledslowly,
solidificationstartsatRandiscompleteatP,theresultantsolidbeingahomogeneous
singlephase,theβsolidsolution.
Asthealloycools,thesolvuslineisreachedatpointQ.Theβsolutionisnow
saturatedintin.Belowthistemperature,underconditionsofslowcooling,theexcess
tinmustcomeoutofsolution.Sincetinissolubleinlead,theprecipitatedoesnot
comeoutasthepuremetaltin,butrathertheαsolidsolution.
8/5/201814 Prem Kumar Soni

15. 2. Eutectoid Transformation
Eutectoidreactionisanisothermalreversiblereactioninwhichasolidphase(usually
solidsolution)isconvertedintotwoormoreintimatelymixedsolidsoncooling,the
numberofsolidsformedbeingthesameasthenumberofcomponentinthesystem.
8/5/201815 Prem Kumar Soni

16. Definition:Aeutectoidreactionisathree-phasereactionbywhich,
oncooling,asolidtransformsintotwoothersolidphasesatthe
sametime.Ifthebottomofasingle-phasesolidfieldcloses(and
providedtheadjacenttwo-phasefieldsaresolidalso),itdoesso
withaeutectoidpoint.
Theeutectoid(eutectic-like)reactionissimilartotheeutectic
reactionbutoccursfromonesolidphasetotwonewsolidphases.It
8/5/2018Prem Kumar Soni16
reactionbutoccursfromonesolidphasetotwonewsolidphases.It
alsoshowsasVontopofahorizontallineinthephasediagram.
Thereareassociatedeutectoidtemperature(ortemperature),
eutectoidphase,eutectoidandproeutectoidmicrostructures.
SolidPhase1àSolidPhase2+SolidPhase3

17. Theeutectoidstructureinironhasaspecialname:itis
calledpearlite(becauseithasapearlylook).Theschematic
andmicrographbelowshowpearlite.Itisimportanttonotethat
pearliteisnotaphase,butamixtureoftwophases:ferriteand
cementite.
Duringslowcoolingofaniron-carbonalloy,pearliteformsbyDuringslowcoolingofaniron-carbonalloy,pearliteformsby
aeutectoidreactionasaustenitecoolsbelow727°C(1,341°F)
(theeutectoidtemperature).Pearliteisamicrostructure
occurringinmanycommongradesofsteels.
8/5/2018Prem Kumar Soni17

18. Eutectoiddecompositionofironasanexample,austenitecontaining
0.8%Cchangesintoferrite(ironcontainingalmostnocarbon)and
cementite(Fe
3C,containing25at%carbon).Hencecarbonatoms
mustdiffusetogethertoformFe
3C,leavingferrite.
NucleiofsmallplatesofferriteandcementiteformatthegrainNucleiofsmallplatesofferriteandcementiteformatthegrain
boundariesoftheaustenite,andcarbondiffusiontakesplaceona
verylocalscalejustaheadoftheinterface(schematicbelow).Thus
theplatesgrow,consumingtheausteniteastheygo,toformpearlite.
8/5/2018Prem Kumar Soni18

19. 8/5/2018Prem Kumar Soni19

20. Thedifferenceisof%ofCarbonpresentinthesteel.
Eutectoidsteelhas0.8%C.
Hypereutectoidsteelhasgreaterthan0.8%C
whereashypoeutectoidsteelhaslessthan0.8%C....
Thus,ifthesteelishypoeutectoiditwillproduce
8/5/2018Prem Kumar Soni20
Thus,ifthesteelishypoeutectoiditwillproduce
proeutectoidferriteandifitishypereutectoiditwill
produceproeutectoidcementite.

21. 3. Peritectoid Transformation
Theperitectoidreactionisthetransformationoftwosolidintoathird
solid.
8/5/201821 Prem Kumar Soni

22. It is important to note that:
Ferriteissoftandductile
Cementiteishardandbrittle
Thus,combiningthesetwophasesinsolutionanalloycan
beobtainedwithintermediateproperties.(Mechanicalbeobtainedwithintermediateproperties.(Mechanical
propertiesalsodependonthemicrostructure,thatis,how
ferriteandcementitearemixed.)
8/5/2018Prem Kumar Soni22

23. 4. Peritectic reaction
Theperitecticreactionalsoinvolvesthreesolidin
equilibrium,thetransitionisfromasolid+liquidphaseto
adifferentsolidphasewhencooling.Theinversereaction
occurswhenheating.
8/5/201823 Prem Kumar Soni
occurswhenheating.
SolidPhase1+liquidàSolidPhase2

24. 8/5/2018Prem Kumar Soni24

25. 8/5/2018Prem Kumar Soni25

26. 8/5/201826 Prem Kumar Soni

27. Itisthereactionthatoccursduringthesolidificationofsomealloyswheretheliquid
phasereactswithasolidphasetogiveasolidphaseofdifferentstructure.
Assumingveryslowratesofcooling,theperitecticreactionwilloccuronlyinthosePt-
AgalloysthatContainbetween12and69%silver(Ag).
Consideraliquid(melt)ofcompositionZ,i.e.,containing25%Ag.Solidification
commencesatT1anddendritesofα,initiallyofcompositionW,beginforming.
SelectivecrystallizationofαcontinuesdowntoTp,theperitectictemperature;whenthe
alloyreaches.thistemperature,itiscomposedofsolidα-dendritesofcompositionBandalloyreaches.thistemperature,itiscomposedofsolidα-dendritesofcompositionBand
liquidofcompositionDintheproportionα:liquid=RD:RB.
8/5/201827 Prem Kumar Soni

28. Thank youThank you
8/5/201828 Prem Kumar Soni