Recovery recrystallization and grain growth

Recrystallization is the process in which deformed grains of the crystal structure are replaced by a new set of stress-free grains that nucleate and grow until all the original grains have been consumed. The process is accomplished by heating the material to temperatures above that of crystallization.

1. Notes
On
“Recovery, Recrystallisation and
Grain Growth”Grain Growth”
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2. Introduction
Wenowintroducea‘technicalterm’calledCold
Work.Wewillarriveataformaldefinitionofthe
termattheendofthistopic.
Fornowweuseaworkingdefinitionofcoldwork
as:as:
Plasticdeformationinthetemperaturerange
(0.3–0.5)Tm→COLDWORK
Duringcoldworkthepointdefectdensity
(vacancies,selfinterstitials…)anddislocation
densityincrease.
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3. Typicalcoldworkingtechniquesarerolling,forging,extrusionetc.
Coldworkingistypicallydoneonductilemetals(e.g.Al,Cu,Ni)
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Coldworkingistypicallydoneonductilemetals(e.g.Al,Cu,Ni)

4. Thecoldworkedmaterialisstronger(harder),but
isbrittle(asnotedbefore).
Heatingthematerial(typicallybelow0.5Tm)is
andholdingforsufficienttimeisaheattreatment
processcalledannealing.
Dependingonthetemperatureofannealing
processeslikeRecovery(atlowertemperatures)
orRecrystallisation(athighertemperatures)mayorRecrystallisation(athighertemperatures)may
takeplace.Duringtheseprocessesthematerial
tendstogofromamicrostructurallymetastable
statetoalowerenergystate(towardsastable
state).
Further‘annealing’oftherecrystallizedmaterial
canleadtograingrowth.
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7. Recoveryisaprocessbywhich
deformedgrainscanreducetheirstored
energybytheremovalorrearrangementof
defectsintheircrystalstructure.
Thesedefects,primarilydislocations,are
introducedbyplasticdeformationoftheintroducedbyplasticdeformationofthe
materialandacttoincreasetheyieldstrength
ofamaterial.
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8. recovery
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9. Recoveryisaprocessthatkicksinatlow
temperatures;inthisstage,theexcessdefects
areannealedout,dislocationsofoppositesigns
annihilateeachother,anddislocationsalignto
formlowenergyconfigurations,namelytiltformlowenergyconfigurations,namelytilt
andtwistboundaries
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10. Basically we can say,
Basically we can say,
In Cold working
•Strength, Electrical Resistance and hardness
Increases.
•Ductility Decreases
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11. Summary
Recoverytakesplaceatlowtemperaturesofannealing
“Apparentlynochangeinmicrostructure”
ExcesspointdefectscreatedduringColdworkareabsorbed:
atsurfaceorgrainboundaries
bydislocationclimb
Randomdislocationsofoppositesigncometogetherandannihilate
eachothereachother
Dislocationsofsamesignarrangeintolowenergyconfigurations:
Edge→Tiltboundaries
Screw→Twistboundaries
POLYGONIZATION
Overallreductionindislocationdensityissmall
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12. Recrystallisation
Recrystallisationistheprocessinwhich
deformedgrainsarereplacedbystrain-free
grains.
ItisgenerallyobservedthathighercoldworkItisgenerallyobservedthathighercoldwork
andsmallerinitialgrainsizeleadtofiner
recrystallisedgrains.
Thedrivingforceforrecrystallisationisthe
storedstrainenergyinthematerial.
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13. Recrystallisationisdefinedastheprocessin
whichgrainsofacrystalstructurecomeina
newstructureornewcrystalshape.
AprecisedefinitionofrecrystallisationisAprecisedefinitionofrecrystallisationis
difficulttostateastheprocessisstrongly
relatedtoseveralotherprocesses,most
notablyrecoveryandgraingrowth.Insome
casesitisdifficulttopreciselydefinethepoint
atwhichoneprocessbeginsandanotherends.
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14. Process of Recrystallisation
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Recrystallisationofametallicmaterial(a→b)
andcrystalgrainsgrowth(b→c→d).

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17. ForRecrystallisationtotakeplace,aminimum
ofcoldworkingisneeded;ifthedeformation
isverylow,recrystallisationdoesnotoccur.
Duringrecrystallisation,thestress-freegrains
nucleateandgrow.
Inarecrystallisedsystem(aftermorethanInarecrystallisedsystem(aftermorethan
about90%deformation)theresultant
microstructureistextured;thatis,mostofthe
grainshavesimilarorientation.
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Changesinpropertieswith%coldworkandannealingtemperature;increasing
annealingtemperaturesleadstorecoveryandrecrystallisation.Notethatstillhigher
annealingtemperaturesleadstograingrowth.

19. Thetemperatureatwhichagivenmaterial
completelyrecrystallisesinanhourisdefinedas
therecrystallisationtemperature.
Therecrystallisationtemperatureislowifthe
degreeofdeformationishighand/orifthe
temperatureofdeformationislowand/orifthe
initialgrainsizeissmall.initialgrainsizeissmall.
Therecrystallisationtemperatureisalsovery
sensitivetothepurityofthematerial;anaddition
of0.01at.%ofTetopurecopper,forexample,
canincreasetherecrystallisationtemperatureby
about240◦C.
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21. Grain Growth
•Graingrowthistheincreaseinsizeofgrains
(crystallites)inamaterialathightemperature.
•Thisoccurswhenrecoveryanrecrystallisation
arecompleteandfurtherreductioninthe
internalenergycanonlybeachievedbyinternalenergycanonlybeachievedby
reducingthetotalareaofgrainboundary.
•Thetermiscommonlyusedinmetallurgybut
isalsousedinreferencetoceramicsand
minerals.
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22. Grain Growth
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23. Grain Growth
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24. a)Graingrowthoccursbythemovementofgrain
boundariesandnotbycoalescence(i.e.likewater
droplets)
b)Boundarymovementisdiscontinuousandthe
directionofmotionmaychangesuddenly.
c)Onegrainmaygrowintoanothergrainwhilstbeing
consumedfromtheotherside
d)Therateofconsumptionoftenincreaseswhenthe
grainisnearlyconsumedgrainisnearlyconsumed
e)Acurvedboundarytypicallymigratestowardsits
centreofcurvature
f)Whengrainboundariesinasinglephasemeetat
anglesotherthan120degrees,thegrainincludedby
themoreacuteanglewillbeconsumedsothatthe
anglesapproach120degrees.
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27. ThanksThanks
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