DOC

Precipitate Formation and Evolution in the Superalloy IN738LC

By Margaret Cruz,2014-02-18 12:12
6 views 0
Precipitate Formation and Evolution in the Superalloy IN738LCin,and,the,The,AND,i n

    Precipitate Formation and Evolution in the

    Superalloy IN738LC

    38

    2009

    增刊3

    12

    稀有金属材料与工程

    RAREMETALMATERIALSANDENGINEERING

    Vo1.38,Supp1.3

    December2009

    PrecipitateFormationandEvolutioninthe

    SuperalloyIN738LC

    DincErdeniz,ErcanBalikci

    (BogaziciUniversity,Istanbul,Turkey)

    Abstract:IN738LCisapolycrystalline,nickel-basedsuperalloy,whichprovidesahighperformanceinaggressiveenvironments

    attemperaturesabove650.C.Atthishightemperature,theexpectedpropertiesaregoodcorrosionresistance,optimalthermal

    properties,andcreepandfatigueresistance.Theserequiredpropertiesareobtainedviasolidsolutionhardeningandprecipitation

    hardeningofaface

    centeredcubic(fcc)Nimatrixphase.Thesize,morphologyanddistributionofprecipitatesdeterminethe

    propertiesofthemateria1.Thus,microstructurecontrolisveryimportantforeffectiveuseof1N738LC.Inthisstudy,anumberof

    heattreatmentswereconductedtoobservetheevolutionofprecipitates.Theformationofbimodalmicrostructureat1140.Cis

explored.Thedatashowsformationofaduplexsizeprecipitatemicrostructureafter5minofth

    eaging.Formationmechanismof

    theduplexstructureisdiscussed.Coarseninganddissolutionmechanismsat1120.Carediscu

    ssed.Precipitatesarefoundtogrow

    uptoacriticalsizeincoherentandcubicalmorphology,beyondwhichdissolutionsetsin.

    Keywords:precipitation;gamma-primephase;growth;coarsening CLCnumber:TG146.1+5Documentcode:AArticleID:1002.185X(2009)$3.142.05

    IN738LC1sanickelbased,polycrystalline

    superalloywhichisutilizedespeciallyinjetenginesand land-basedgasturbines.Goodcorrosionresistance, metallurgicalstability,excellentcreepproperties,and resistancetothermalfatiguemakethismaterialsuitable forhightemperatureapplications.Propertiesofthis materialareobtainedthroughstrengtheningtheNi matrixphasebytheNi3A1('phase)precipitates.Since

    performanceofthissuperalloyisstronglyaffectedbythe sizeandthemorphologyofprecipitates,microstructural controlisextremelyimportant.

    Precipitateevolutionfollowsthestagesofnucleation, growth,coarseninganddissolution.Nucleationisa formationprocessofaveryfineprecipitateparticlewhich iscallednucleus.Onceanucleusisformed.itstartsto growbyabsorbingsoluteatomsfromthesolidsolution matrixphase;thisprocessiscalledOstwaldripening. Duringgrowth,precipitatevolumefractionincreasesas soluteisrejectedbythematrix.Precipitatesmaygrow furtherwithoutanyincreaseinvolumefraction,andthis stageiscalledcoarsening.Coarseningphenomenonwas formulatedbyLifshitzandSlyozov]andbyWagner【】

    separately,butthesetwoworkswerecombinedasLSW

    theory.Severalothercoarseningtheoriesweresuggested basedonLSWtheory[.,刖.Subsequenttoreachinga

    criticalsize,precipitatesstarttodissolvebackintothe matrix.Afullcircleofallthesestagesmaybedefinedas thelifecycleofaprecipitate.

    Thispaperisdealingwiththeevolutionof),' precipitatesinthesuperalloyIN738LC.Experimental resultsregardinggrowth,coarsening,anddissolution stagesarediscussed.

    1Exoerimental

    Thematerialusedinthisstudyisacast,

    polycrystalline,Ni?basesuperalloyIN738LC,supplied byHowmetCorporation,Whitehall,MI,USA,inthe formofrods,15mmindiameterand110mminlength. TheasreceivedrodswereHIPed(hotisostatically pressed)at1185.Cfor2hinordertoremovemicropores, andthencooledtoroomtemperatureinaneutral atmosphere.AfterHIPing,solutiontreatmentat1120.C wasconductedfor2h;subsequentlyanargonbackfill

    coolingtoroomtemperaturewasgiven.Anaging treatmentat843.Cwascarriedoutfor24h,andcooling toroomtemperaturewasdoneagainby

    argonbackfilling.Alltheseprocesseswerecarriedout bythesupplier.Themicrostructureoftheas-received stockisshowninFig.1anditschemicalcompositionis giveninTable1.

    Receiveddate:2008.08.10

    Foundationitem:SupportedbyBogaziciUniversityScientificResearchProjects(BAP)(05

    HA601)

Biography:ErcanBalikci,BogaziciUniversity,Istanbul,Turkey,E

    mail:ercan.balikci@boun.edu.tr

    增刊

    3DincErdenizetal:PrecipitateFormationandEvolutionintheSuperalloy1N738LC?l43?

    Small,quarter.cylindricalpieceswithabout4mm thicknesswerecutfromtheasreceivedbarsforheat

    treatments,andeachofthemwassealedinaquartztube undervacuumat10Tort.Beforesealing.thequartz

    tubeswerefirstvacuumedandthenpurgedwith 99.995%purityargonthreetimes.Thesampleswere individuallywrappedwithastainlesssteelfoilinorder topreventreactionwithquartztube.Theheattreatments werecarriedoutinatiltable,tubularfumacewhichcan reachtemperaturesuDto1400?.Attheendofa

    prescribedagingtime,thefumacewastilted,lettingthe quartztubedropinawaterfilledbucketandsamplebe

    quenchedveryquickly.Theheattreatmentprocedureare giveninTable2.

    Subsequenttothecompletionofallheattreatments. sampleswerepreparedformicroscopicanalysis.First, theyweregroundusinggrindingdiscswithcoarse(12o) tofine(1200)gritsize,andthenpolishedwithalumina powderofsize1.O.3and0.05gm.Afterultrasonic cleaningtheywereetchedwithasolutionwhichcontained 33%HN0+33%aceticacid+33%H,O+1%HF.A

    PhilipsXL30ESEM.FEG/EDAXsystemwasusedto characterizethesizeandmorphologyoftheprecipitates

    Backscatteredelectron(BSE)detectorwasusedinSEM (ScanningElectronMicroscope)becauseofitshigher energyandcompositionsensitivity,givingagoodcontrast

    betweenthematrix(seenasthelighterbackgroundinthe micrographs)andtheprecipitates,whicharethedark regionsinthemicrographs.Precipitatesizeswere measuredfromdigitalmicrographsbytheImageJimage processingsoftware[9_

    Fig.1Microstructureoftheasreceived1N738LC

    Table1Chemicalcompositionoftheas-receivedIN738LC Table2Heattreatmentprocedure

    SolutiontreatmentAgingtreatment

    1200.C/4h/WQ114O.C/t/WQ

    ,=1,5,10,15,20,30min

    1200.C/4h/WQ

    ll2O.C/24h/FC

    1120.C/t/WQ

    t=0,12,72,144,216,288,480h

    2ResultsandDiscussions

    Thechemicalcompositionoftheprecipitatesinthe IN738LCwasanalyzedinanearlierstudyf10]which showedthattheywereoftheintermetallicNi3(A1,Ti),-y' phase,withtheL12orderedFCCsuperlatticestructure. Inthecurrentstudy,thevolumefractionofthe'

    precipitateswasdeterminedtobe42%intheFC (fumacecoolingafteraging)conditionwhichwas consistentwiththedatagivenintheliterature[10-12]. 2.1Agingsat1140.CSubsequenttotheSolution Treatmentat12000C/4h/WO

    Agingtreatmentsat1140.Cfor1t030minwere

    carriedoutsubsequenttosolutionizingatl200.Cfor4h Theaimwastodeterminetheexactformationtimefora

    duplexsizeprecipitatemicrostructure,whichconsistsof fineandrelativelycoarse),'precipitates.Previously,it wasreportedthatduplex.sizemicrostructurecouldbe observedafteranagingtreatmentfor30min[,at

    1140?.Butnotreatmentwasconductedbetween5and 30min.Thus,itwasnotclearthatduplex.size microstructurewasobtainedrightatthe30"minor before.Incurrentstudy.agingtreatmentswere conductedfor1,5,l0,15,20and30min.Theduplex microstructurewasobservedafteranagingfor5minand afterl0min.theduplexsizemicrostructurecouldbe

    seenclearly.ThesemicrostructuresareshowninFig.2. PrecipitatesizevariationwithtimeisshowninFig.3. 1200.C/4h/WQsolutiontreatmentgavea

    well-dispersed,veryfinemicrostructurewith50nm

    sizedsphericalprecipitates,buttheagingtreatmentsat 1140.Cresultedinaduplexsizemicrostructure.as

    statedabove.Thismicrostructurewasformedviagrowth ofsomefineprecipitatesthroughsoluteabsorptionfrom ?144?稀有金属材料与工程第38

    thematrix,whileotherparticlesmaintainedtheirsize.In fact,mostprobablythegrowthofsomeprecipitates startedinthefirstminuteoftheaging,butadistinct duplexsizebecameclearafter5min.Thecurrent observationsweredonewithfieldemissiongunSEM withahigherresolutionthanthepreviouslyused[,

    tungstenfilamentgunSEM,bywhichtheduplexsize microstructurewasobservedafter30min.Hence,the observationoftheduplexsizeinashorteragingtime maybeattributedtothehigherresolutionusedinthis

    currentwork.Perhaps,aTEM(TransmissionElectron Microscope)mayclearlydemonstratetheexacttimefor theformationoftheduplexmicrostructure.Itwasalso observedthatprecipitateschangedtheirinitialspherical morphologytocubicalformin10min,seeninFig.2.

    

    Fig.2Microstructuresobtainedafter1140.C/t/WQaging treatmentssubsequentto1200.C/4h/WQsolutiontreatment: (a)1rain,(b)5min,(c)l0min,(d)15min,(e)20min,and(D 30min

    Apreviousstudyreportedagingtreatmentswith longertimeatthistemperature,andtheauthorsstatedthat precipitatescontinuedtheirgrowthincubicmorphology upto24h.Then,particlesstartedtochangeintospherical shapebytheadditionofadjacentprecipitatesintoflat surfacesofcubicalprecipitatesandbythedissolutionof corners.Thisprocessresultedininitialbulgingofthe cubicalprecipitateswhichledthewaytomostlyspherical orirregularlyshapedlargerprecipitates.Itisplausibleto statethatthemicrostructuretriestoreducethetota

    internalenergybyminimizingthesurfacearea.The authorsalsoreportedthatagingtreatmentsat1150.C gaveasimilarmicrostructure,butanytreatmentsabove 1150.Cresultedinasinglesizemicrostructure.

    AgingTime/min

    Fig.3PrecipitatesizevariationwithagingtimeatI140.C 2.2.Agingtreatmentsat1120.Csubsequentto1200 .C,4h,WQ+11200C/24h/FC

    Inordertostudydissolutionofcoarseprecipitates

    andunderstandfurtherevolutionoftheseparticles,a twostepagingprocedurewasemployed.Firstagingwas conductedat1120.Cfor24handendedwithfumace cooling.Thisstepresultedinanalmostunimodal cuboidalmicrostructurewith42%volumefractionwhere precipitateswereinthesizeofapproximately1000nm. ThismicrostructureisseeninFig.4.Subsequently, sampleswereheattreatedatthesametemperaturefor differentholdingtime,suchas12,72,144,216,288,and 480h,andthenwaterquenched.Microstructuresare showninFig.5.Resultsshowedthatprecipitatesize decreasedduetothedissolutionwhichoccu~edupto12 h,whereformationoffineprecipitateswasalready started.TheprecipitatesizedataisplottedinFig.6. Balikcieta1.statedthatcoarseprecipitateswere dissolvedalongthecubecornerswhichcausedachange inshapefromcubestospheres.Thissituationwas observedafter5minofagingatl12O.Cwith-1000nm startingprecipitatesize.Inthecurrentstudy,after12h precipitateswereintheshapeofspherewhichlikely happenedbythesamemechanism.

    Thecomparisonofthecurrentobservationswith datareportedearlierindicatesthatprecipitatedissolution startsmuchfasterathigheragingtemperatures.For example,whiledissolutionwasreportedtostartafter 200hat1100.CJ.itstartedin5minat1140.Candin

    onesecondat116O.Catwhich700nmsizecubical precipitateswereshrunkintoabout70nmsize precipitates,stillwithaveryfewlargeones[161.

I_IIN一?0_?口00.I

    增刊3DineErdenizetal:PrecipitateFormationandEvolutioninth——

    e

    

    Superalloy

    

    IN738LC?145?

    Fig.4Microstructureobtainedafter1200.C/4h/wQ+ ll20.C/24h/FC

    Fig.5Microstructuresobtainedafter1120.C/t/WQsubsequent to1200.C/4h/WQ+I120.C/24h/FCtreatments:(a)12h, (b)72h.(c)144h,(d)216h,(e)288h,and(f)480h Agingtreatmentsfor72,144,216,288,and480h resultedinparticlecoarsening.Sizeanalysis(Fig.6) showedthatprecipitatesgrewrelativelyfastupto144h. andthengrowthratedroppeddistinctly.Thisvisible dropprobablyhappenedduetoadecreaseinlocalsolute concentrationandalsoperhapsduetoasharpreduction intotalinternalenergyoftheprecipitatemicrostructure. Botheffectsdonotfavorcoarsening.Thetotalenergyof suchamicrostructurehascontributionsfromthe interracialenergyandthemismatchenergydueto coherency.Theprecipitatescangrowincoherentform withahighcoherencyenergycontributiontothetotal energy.Whenthecoherencyislost,thecontributing factorisonlytheinterfacialenergy.Thus,the microstructuremayreduceitsenergybyminimizingthe particlesurfacearea.

    ThedatainFig.6suggeststhattheprecipitates cannotgrowbeyond1000nmsizeincubicalfoITn.

    Hence.theystartdissolvingwhichresultsinareduction inmismatch(coherency)strainenergy.Thisprocess transformstheparticlemorphologytosphericalone afterwhichmostprobablythe1atticematchislost. Moreover,thespheroidizationoftheparticlesalso reducestheinterfacialenergycontributiontothetotal energy.Thecoalescenceofthesesphericalparticles 1cadstocoarseningandafurtherdecreaseintheenergy. Precipitatecoarseningisgenerallyexplainedby Ostwaldripeningmechanismwhichwasalsoobservedin theseagingtreatments,butanothermechanismwas observedatthesametime,whichiscalledprecipitate agglomerationmechanism(PAM)anditwasfirst proposedbyBalikcieta1.Thistheorysuggeststhat secondphaseparticlesmoveasawholeandcoalesce. ThistypeofcoarseningwasalsoreportedbyPrikhodko andArdell[17.

    Agglomerationofequal-sizedparticles

    wasobservedindifferentsamplesandsomeexamples aremarkedinFig.5bd,e.Ontheotherhand,someof theprecipitateswereseenlyingveryclosetoeachother, butseparatedbyathinlayerofthematrixphase,as showninFig.5fThissituationcanbeexplainedby antiphaseboundary(APB)effect.Whentwo

    independentlynucleateddomainsmeet,theycanbeeither in.phaseoroutof-phase.Iftheyareoutof-phase,ahigh

    energyregionwouldforlTlwhichiscalledAPB.Therefore, itcanbeconcludedthatparticleswithdifferentdomains cannotmergeortheyneedhigherenergytomergeLlSJ. 1300

Report this document

For any questions or suggestions please email
cust-service@docsford.com