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Influence of local mechanical loadings paths on the
oxidation assisted crack initiation of alloy 718
Benoît Ter-Ovanessian, Dominique Poquillon, Jean-Marc Cloué, Eric Andrieu
To cite this version:
Benoît Ter-Ovanessian, Dominique Poquillon, Jean-Marc Cloué, Eric Andrieu. Influence of local
mechanical loadings paths on the oxidation assisted crack initiation of alloy 718. Materials Science
and Engineering: A, Elsevier, 2011, vol. 533, pp. 43-49. �10.1016/j.msea.2011.11.030�. �hal-00837751�
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: DOI:10.1016/j.msea.2011.11.030
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http://dx.doi.org/10.1016/j.msea.2011.11.030
This is an author-deposited version published in:
http://oatao.univ-toulouse.fr/
Eprints ID: 5462
To cite this version:
Ter-Ovanessian, Benoît and Poquillon, Dominique and Cloué, Jean-Marc
and Andrieu, Eric Influence of local mechanical loadings paths on the
oxidation assisted crack initiation of alloy 718.
(2011) Materials Science
and Engineering A, vol. 533 . pp. 43-49. ISSN 0921-5093
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Influence
of
local
mechanical
loading
paths
on
the
oxidation
assisted
crack
initiation
of
alloy
718
Benoît
Ter-Ovanessian
a,b,∗,
Dominique
Poquillon
b,
Jean-Marc
Cloué
a,
Eric
Andrieu
baAREVA,AREVANP10,rueJ.Récamier,69456LyonCedex06,France
bUniversitédeToulouse,CIRIMAT,INP/UPS/CNRS,INP-ENSIACET,4,alléeEmileMonso-BP44362-31030TOULOUSECedex4,France
Keywords: Ni-basedsuperalloys Environment-assistedcracking Portevin-LeChâteliereffect Finiteelementsimulation
a
b
s
t
r
a
c
t
Athightemperatures,alloy718,likemanyothernickel-basedsuperalloys,issensitivetoan oxidation-assistedintergranularcrack(OAIC)growthmechanism.Formerstudieshavepointedoutthatevenif intergranularoxidationstilloccurred,intergranularcrackinitiationwasinhibitedduetospecific mechan-icalloadingswhichwereidentifiedasPortevin-LeChâtelier(PLC)plasticinstabilities.Inthepresentwork, keyparameterstriggeringcrackinitiationorPLCinstabilitiesinthe[550–700◦C]temperaturerangewere
determinedforthestudiedgradebymeansoftensiletestsonsmoothspecimens.Then,inordertoassess theapplicabilityofsuchafindingatascalecompatiblewiththematerialmicrostructure,adedicated ten-sileV-shapedspecimenwasdesignedtogeneratedifferentsurfaceandsub-surfacestrainandstrainrate histories.Thankstoadualapproachbasedontheobservationofcrackinitiationlocationonthisspecific experimentalspecimentogetherwithassociatedFEcalculations,thecriticalmechanicalloadingpaths inducingOAICinitiationhavebeenspecified.Thus,assumingthatthemetallurgicalstateishomogenous atthestructurescale,amappingofintergranularcrackinitiationisthenobtainable.
1. Introduction
Alloy 718is known tobe sensitiveto anoxidation assisted intergranular crack (OAIC) growth mechanism at temperatures closetothoseencountered industriallyfor turbo machinedisks (650◦C-air)[1–9].Recentwork[6–9],investigatingthedetrimental
effectofoxidationonthelifespanandthemechanicalbehaviour ofalloy718, hasdemonstratedthat duringtensile testscarried outunder syntheticairat a 400–600◦C temperaturerange and
ata5×10−7s−1to10−1s−1 strainraterange,thedisappearance oftheintergranularbrittleareaonthefracturesurface systemati-callycorrespondedtotheoccurrenceofPortevin-LeChâtelier(PLC) phenomenon.Moreover,thesame testscarried outunderinert atmospheresystematicallyexhibitedafullytransgranularductile fracture mode,whatever theplasticflow regimewas.Although theoccurrenceofplasticinstabilities(jerkyflow, orPortevin-Le Châtelierphenomenon),andmoregenerallydynamicstrainageing (DSA)duringplasticflowofalloy718forsuchrangeof temper-atureiscommonlyobserved[10–13],differenthypothesesabout theirimpactonoxidationassistedintergranularcrackgrowthwere
∗ Correspondingauthorat:UniversitédeToulouse,CIRIMAT,INP/UPS/CNRS, INP-ENSIACET,4,alléeEmileMonso-BP44362-31030TOULOUSECedex4,France. Tel.:+33534323449;fax:+33534323498.
E-mailaddress:benoit.terovanessian@ensiacet.fr(B.Ter-Ovanessian).
proposed[6–9].Amongthem,Fournieretal.[6],suggestedthatthe correlationbetweenthedisappearanceoftheintergranular frac-turemodewiththeoccurrenceofPLCinstabilitieswaslinkedto thestronglocalizationofthestraininthePLCbandsgivingriseto theductilefracturemode.Garatetal.[7],establishedan experi-mentalmapincludingdeformationmodes(DSA–PLC)andfracture modesforalargerangeofstrainratesandtemperatures.Fromthese results,theauthorssuggestedthatthepropagationofatypeCPLC band,whichisinitiatedinareasofhighstrain/stresslocalization likegrainboundariesor,atanotherscale,intheplastic zoneat thetipofapropagatingdefect,couldmodifythelocalstress dis-tribution.Thus,iftheplasticzoneinfrontofadefectisextended enoughtoincludeseveralgrains,itbecomespossiblethattherate ofthestressredistributionduetotheinstabilityoccurrenceinduces eitherachangeinthecrackpathoracrackarrest,hencepreventing intergranularcrackpropagation.
Furthermore,themicrostructureofalloy718isknownto gen-erateahighpropensitytoconcentratetheshearingalongafewslip planespergrainleadingtonoticeabledeformationheterogeneity. Whendealingwithintergranularcrackinitiationatthesurfaceof suchamaterial,thelocalizationofcrackinitiationsitesremains anopenquestion.Asallofthesurface/sub-surfacegrain bound-ariesexhibitedintergranularoxidation,theanswermightbefound inthereleaseofgraintograinstrainincompatibilitiesatthefree surfaceofthepolycrystalwhereonlysomegrainboundarieskeep highstresslevelssusceptibletoinitiateanintergranularcrackor
Table1
Chemicalcompositionofas-receivedalloy718[wt.%].
Al C Co Cr Cu Fe Mn Mo Ni Nb+Ta Si Ti
0.48 0.034 0.022 18.3 0.009 Bal. 0.046 3.04 53.69 5.17 0.031 1.05
Fig.1.Geometryoftensilespecimensaftercoldstamping(dimensionsinmm).
tobereleasedbyPLCinstabilities.Thistypeofapproachassumes thatthemechanicalbehaviouratthesurfaceofthematerialisnot verydifferentfromthebulk.Inordertoassessthevalidityofthe approach,adedicatedtensileV-shapedspecimenwasdesignedto generatesurfaceandsub-surfacestrainandstrainrategradientsat ascalecompatiblewiththematerialmicrostructure.Moreover,the improvementofthemechanicalbehaviourcharacterizationofthis V-shapedtensilespecimen[8,14]highlightedthemainadvantages ofthisspecificgeometryinstudyingstrainratedependenceofcrack initiation.Indeed,thisgeometryenablestocontroltheamountof materialattheapexoftheVsubmittedtotheimposedstrainrate [14].Asaresult,therequiredloadingconditionsforcrack initia-tionorforPLCbandsgenerationareexpectedtobeonlyfulfilled locally.Previously,Deleumeetal.[14]alreadyprovedthat,ona globalscale,tensiletests’mechanicalresponseandfiniteelements modellingwereingoodagreementforasimilargeometry. How-ever,fromthispreliminarystudy,itappearednecessarytoobtaina completedescriptionandevaluationofstrainandstrainrate gradi-ents’distributionallalongthetensiletestinordertobebetterable toexplainthelocalizationofintergranularcracksobservedatthe endofthemechanicaltests.Thepurposeofthepresentstudyisto providethiskeyinformationat600◦Cinordertoaccuratelyanalyze
theroleoflocalloadingpaths(i.e.:criticalstrain,criticalstrainrate, plasticflowmode,etc.)onintergranularcrackinitiationbehaviour ofalloy718.
2. Materialandexperimentalprocedures
Thematerialsusedinthisstudywereobtainedthrougha dou-blemeltingprocess:vacuuminductionmeltingpluselectrodeslag remelting.Thenominalcompositionoftheevaluatedheatisgiven inTable1.
Thecastingotwashotandcoldrolleddowntostripsof0.3mm ofthickness,followedbyasolutionannealingheattreatmentat 1050◦Cforonehourendedbyairquenching.Thintensile
speci-mensweremachinedfromtheannealedstrips.Then,whilestillat anannealedmetallurgicalstate,V-shapedhumpspecimenswere formedfromtensilespecimens,bycoldstampingwithaspecific stainlesssteeldye(Fig.1).
V-shapedspecimensandconventionaltensilespecimenswere thenheattreatedinanargonatmospherefollowingthe conven-tional aeronautical route: holdat 720◦C – 8h, cooling 50◦C/h
downto620◦C,holdat620◦C–8handfinalaircoolingtoroom
temperature. Such heat treatment aims at precipitating and
stabilizingthehardeningphasesg′(Ni3Al,averagesize50nm)and
g′′(Ni3Nb,averagesize15nm×50nm).
All the mechanical tensile tests were performed on a hard electro-mechanicaltensilemachinewithcontrolleddisplacement rate,outfittedwithanenvironmentalchamber,aradiationfurnace andalaserextensometer.Twodifferenttypesoftensiletestswere carriedoutonthinflatconventionalspecimensundersyntheticair orargoninthe550–700◦Ctemperaturerangefordifferentimposed
strainratesin the10−5s−1,10−1s−1 range. Thefirstoneswere
carriedoutuntilfailurewhereasthesecondonesconsistedin inter-ruptedtestsatagivenplasticstrain.Afterbeingtested,gagelength and/orfracturesurfaceswereexaminedwithaLEO435VP scan-ningelectronmicroscope(SEM).Thepurposeoftheconventional tensiletestsonthinflatspecimenswastoestablishthe mechan-icalbehaviourofthatspecificgradeatthesetemperaturesandto determinethethresholdvalueofdisappearance/appearanceofthe PLCphenomenon.Thepurposeofthetensiletestsinterruptedata givenplasticstrainwastoevaluatethecriticalcumulatedplastic strainnecessarytoobservecrackinitiation.Forsuchinterrupted tests,tensilespecimenswereheatedthenloadeduntilthewanted plasticstrainwasreached,thenunloadedandfinallycooleddown. Then,SEMobservationsalongthegagelengthallowedtodetect crackinitiation.
Ontheotherhand,tensiletestshavebeencarriedoutin sim-ilartestingconditionsonV-shapedspecimens.Sometestswere carriedout uptofailure but othertestswereinterrupted for a givenloadcorrespondingtoagivenmaximalplasticstrainatthe tipoftheV.Thepurposewastodeterminethelocalized mechan-icalbehaviourofthisspecificgeometry,withoutincreasingstress triaxiality.
Forbothtensilespecimens,themicrostructurewas character-izedbyequiaxedfullyrecrystallizedsmallgrains(ASTMgrainsize number=8–9)withrandomgrainboundariesandbythepresence ofsomeNb/Ticarbiderows.Fig.2showsthatthemicrostructureis homogeneousallacrosstheVspecimenthicknesseveninthemost deformedparts.
Becauseofthecoldstampingofthespecimen,workhardening intheV-shapedisestimatedbygeometricalconsiderationstoan averagevalueof13%.Inordertoevaluatetheroleofthiswork hard-eningonthesubsequentbehaviouroftheV-shapedspecimenfor OAICtests,tensiletestswerecarriedoutontensilespecimenwhich werepre-strainedeitherat12%or20%atroomtemperatureinthe solutionannealedstatebeforebeingagedhardened.The mechan-icalresponseobtained,showedthatthemechanicalbehaviour,at 600◦C,wasnotsignificantlymodifiedbythepreviouswork
hard-ening.Therefore,onlyoneflowrulemaybeusedtocharacterize thewholebehaviouroftheVshapedspecimen.
FiniteelementcalculationswereperformedwithCast3mfinite elementcode[http://www-cast3m.cea.fr],inordertodetermine the local stress and strain tensor fields scale and distribu-tion in the V-shaped specimen throughout the tensile tests. Becauseofthegeometry(symmetryandwidth)ofthespecimen, the simulation was carried out using plane strain hypothe-sis and a 2D mesh (8node-quadratic elements, corresponding to 9mm×9mm) of half of the specimen. For calculations, the flow rule wasdescribed byan isotropic elastoplastic mechani-calbehaviour.Indeed,inCast3mcode,thismechanicalbehaviour with non-linear hardeningenables to fit the true stress–strain curve.
Fig.2.DetailsofthemicrostructureattheapexofaVshapedtensilespecimenaftertheformingprocessandtheageingheattreatment.
Asafirstresultfromthintensiletests,intheinvestigatedrange of strain rates, the mechanical behaviour at 600◦C in the DSA
regimedidnotsignificantlydependonthestrainrate(Fig.3); there-fore,onlyatime-independentconstitutiveequationwasrequired forfiniteelementcalculationstomodelthematerialbehaviourin thesamples.
AsimulationwascarriedoutopeningtheVsampleuntilthe appliedloadreached60N.Fromthisstepbystepsimulationup to60N, differentlocalstainrates arededucedforthedifferent displacementratesasdetailedinSection3.
3. Results
3.1. Tensiletestsonthintensilespecimens
Tensiletestswerecarriedoutunderairenvironmentat600◦C
onconventionalthintensilespecimeninordertodeterminethe mechanicalbehaviourofthisgrade,forinstancestrainrate sensi-tivityoftheflowstress.Theresultsobtained,whicharedetailed inFig.3,indicatethattheflowstresswasnotreallymodifiedby
Fig.3.Mechanicalpropertiesanddeformationmodesofstudiedmaterialsat600◦C
fromconventionaltensiletestscarriedoutfordifferentstrainratesundersynthetic air.
thestrainratewhiletheductilitywas.Thefactthattheflowstress isnot(orfew)sensitivetostrainrateisnotsurprisingintheDSA regimewhilethebehaviouroftheductilityseemstobespecificto thestudiedcase.Indeed,contrarytotypeAorBPLCphenomenon [15–19],theoccurrenceoftypeCPLCphenomenonisassociated withanincreaseoftheductility[18,19].Besides,suchmechanical propertieswereobtainedfromtensiletestscarriedoutunderairfor anarrowdomainofstrainrates.Forhigherstrainrates,adecrease oftheductilitycouldbeobserved.
Moreover,thescanningofthestrainrateevidencedthe thresh-oldvalueofdisappearance/appearanceofthePLCphenomenon, which, in the present case, is in between 2.5×10−4s−1 and
3.5×10−4s−1.Inaddition,forthegradestudied,Fig.4showsthe obtainedmapintermsofplasticflowandrupturemodesovera widerrangeoftemperature.Aspreviousworkhasalready demon-strated[6–9],thesethresholdvaluescorrespondedtoatransition inthefracturemodeunderairenvironment.
Furthermore, while PLC occurred, whatever the strain rate domainwas,theserrationsproducedloaddropsbelowthegeneral levelofthestress–straincurveandtheamplitudeoftheserrations wasratherconstant[7,9–13].BothfeaturescharacterizedthetypeC ofPLCphenomenon,accordingtoRodriguez’sdefinition[19].This
Fig.4. Mapoftheruptureandplasticflowmodesofthestudiedgradeinthe [550–700◦C]temperaturerange–initalics:thecumulatednecessarydeformation
Fig.5. Comparisonofthepredictedandtherealglobalmechanicalresponseofthe specimentestedat600◦C.
typeiscommonlyassociatedwithanaudibleemissionandwith therandomnucleationofdeformationbandsallalongthe speci-mengage[9,12,13,19].Thecriticalplasticstrainfortheonsetof PLCinstabilitieswasestimatedtobebetween0.20%and0.60%for theentirePLCdomain.Finally,intheDSAregime,thankstothe interruptedtensiletestsatvariouslevelsofplasticstrainandSEM examinationsofthegagelength,aplasticstrainthresholdvaluefor theoccurrenceofcrackinitiationwasdetermined.Forthetested strainratesbelowthetransitionstrainrate,atleast1.5%andatmost 3%ofcumulatedplasticstrainseemednecessaryforcrackinitiation (initalicsinFig.4).
3.2. TensiletestsonV-shapedspecimens:experimentsandglobal FEmodelling
UptofailuretensiletestsonV-shapedspecimenswere prelimi-narilyperformedinordertoacquiretheentiremechanicalresponse
of the specimen for different displacement rates of 0.2mm/s, 0.02mm/sand0.002mm/sat600◦C.Indeed,asduringthetensile
testthestrainrateschangedfromplacetoplaceduetothespecimen geometry;itismoreaccuratetocharacterizethetestsbyusingthe imposeddisplacementrate.Tensiletestscarriedoutat0.2mm/s and0.02mm/sledtoatotalopeningupoftheVfollowed bya transgranularductilefracture.Conversely,tensiletestscarriedout at0.002mm/swerecharacterizedbyaprematurefailureoccurring beforetheentireopeningoftheVassociatedwithamixedmodeof fracturewithintergranularbrittleandtransgranularductileareas. Complementaryinterruptedtensile testswereperformedtill theappliedloadreached60Nwithtwoimposeddisplacementrates (0.02mm/sand 0.002mm/s)inordertoassesstheabilityofthe finiteelementcalculationswiththechosenconstitutiveequations toreproducetherealmechanicalbehaviourofthespecimen.FE simulationandrealmechanicalresponsesforbothtestsare pre-sentedinFig.5.Itisworthnotingthatthecurvesillustratingthe appliedloadasafunctionofdisplacementareverysimilar.These resultsareevenmoreconvincingastheymakesenseinanother useofthemodeltodescribethelocalaspectsofthemechanical fields,stepbystep,especiallystrainandstrainratedistributionas thetensiletestproceeds.
Meanwhile,SEMexaminationsofthegagelengthforboth dis-placementratesrevealedthatforinterruptedtestscarriedoutat 0.002mm/s,cracksinitiated at∼180mm,∼220mmand260mm fromtheapexoftheV,whileforinterruptedtestscarriedoutat 0.02mm/s,nocrackwasdetected.Fig.6aandbillustratessomeof theSEMobservationsaftersuchaninterruptedtest.
3.3. Finiteelementmodellingoflocalmechanicalstates
Duringtheloading,theVtipisprogressivelyopened.Thus,the innerpartoftheVissubmittedtotensionandisgraduallystrained. Previouswork[14]hasalreadyfocusedonthestraingradientand itsevolutionduringtensiletestsintheapexofsuchaspecimen. Duringtheopening,aslongastheanglechangesbetweenthearms
Fig.7. TotalandplasticstrainindirectionxattheintradosoftheVasafunctionof theappliedload.Thedistancexisgivenfromthesample’splaneofsymmetry.
oftheVremainsmall,anelasticapproachcanbeusedtoaccessthe strainevolutioninthetipoftheV[8,14].However,whenplastic strainingoccurs,ittendstobelocalizedatthetipoftheVandits contributiontototaldeformationbecomesrapidlylargerthanthe elasticone.Forthestudiedspecimengeometry,plasticstrain con-tributiontothetotaldeformationbecomeshigherthantheelastic strain,oncetheappliedloadreachesF=36N(seeFig.7).Increasing theloadtendstolocalizetheplasticstrainmoreandmoreintheV, asshownbytheshapeofthecurvesinFig.7.
Byconsideringanelasticapproachandforagivendisplacement rate,thestrainrateatthetipoftheVisinitiallylowerthantheone obtainedwithaconventionaltensileflatsample.Thisbehaviourcan bemodelledbysimplegeometricalconsiderations[8,14].However whenconsideringplasticstraincontribution,thepreviouselastic approachisnolongerabletodescribethelocalstrainrateevolution asthetensiletestproceeds.Togofurtherintheanalysis,simulated strainrateevolutionsasafunctionofbothlocationand displace-mentratearepresentedinFigs.8and9.Thecurvesshowthatthe strainrateincreasesprogressivelyfromtheinitialvaluetoaplateau whosevaluedependsonthepositionoftheareaofinterest.Then, whenopeningtheV,thestressdistributionintheintradossurface changessothatfinallythetipoftheVreachesastrainratewhich tendstoreducethecrackinitiationprobabilityinthisarea.
Changingtheloadingratechangesthelocalstrainratebutdue totheinsignificantstrainratesensitivityofthematerialinthese mechanical testingconditions,theshape oftherangeof curves remainsalmostunchangedwhateverthedisplacementrateis.For
Fig.8.Totalstrain–strainratepathfollowedbyseveralareasofinterestlocated atthetipoftheVforagivendisplacementrate(0.02mm/s).PLC/DSAboundaryis superimposedwithcumulatednecessarystraintoinitiateintergranularcracks.
Fig.9.Totalstrain–strainratepathfollowedbyseveralareasofinterestlocatedat thetipoftheVforagivendisplacementrate(0.002mm/s).PLC/DSAboundaryis superimposedwithcumulatednecessarystraintoinitiateintergranularcracks.
each case, differentmaps showingstrain–strain rate paths fol-lowedbythemateriallocatedintheapexoftheVcaneasilybe calculated. On thesemaps,the strainrateinterval demarcating thedisappearance/appearancetransitionofthePLCphenomenon togetherwiththecriticalplasticstrainintervalweresuperimposed (inFigs.8and9)inordertopointouttheboundaryoftheareainside whichthedamagingconditionsatthistemperaturearefulfilled.An interruptedtensiletesthasbeencarriedoutat0.02mm/suptoan appliedloadof60N.Thesemechanicaltestingconditions repro-ducethesimulationconditionspresentedinFig.8.Aspreviously mentioned,finite elementcalculationsconcludedthatthe dam-agingconditionswereneverfulfilledontheentireVsincestrain ratesquicklyreachedthePLCinstabilitiesdomainbefore cumulat-ingthecriticalplasticstraintoinitiateintergranularcracks.Thefact thatnocrackwasdetectedonthegagelengthofthetested spec-imencorroboratesthefiniteelementcalculations.Finally,italso explainswhythetestscarriedoutuptoruptureat0.02mm/sgave risetoafullyductilefracture.Conversely,foraninterruptedtensile testcarriedoutat0.002mm/s,associatedwithFig.9,thefinite ele-mentcalculationspredictthatthecrackinitiationmayoccurovera wideareacentredat250mmfromtheapexoftheV.Observations ofcracksinitiatedat∼180mm,∼220mmand260mmfromtheV apexonthecorrespondinginterruptedtensiletestspecimen(see Fig.6),seemtoprovethatexperimentalresultsandsimulationsare ingoodagreementandconsequentlythatthespecimengeometry isusefulininvestigatingcrackinitiationconditionsinalloy718. 4. Discussion
ElastoplasticisotropicmodelusedforFEcalculationsaccurately leadstoalocalmappingofplasticstrainandstrainrateoftheV thicknessorofitsextremesurface.Indeed,theareaofinterestin thisstudyisaboundarylayeratthetipoftheVwhosethicknessisof therangeofthemicrostructuresize,onwhichdamaging phenom-enaarebeforehandmodelledatamacroscopicscale.Thestriking factthattheprobabilitydomainofcrackinitiationevidencedbyFE calculationsandmacroscopicexperimentaldataexactlymatches thecracklocalizationfromSEMobservationsonthegagelength oftheinterruptedtensiletestsimpliesthatrandomphenomena generallyobservablemacroscopically,likeDSA/PLCphenomenaor ruleofdamageaccumulation,maybelocalized,inthepresentcase, atthesurfacelayeroftheVstructure.Indeed,theseexperiments confirmnotonlytherelevancyoftheuseofsuchgeometryforOAIC concerns,butalsothepossibilityofatranspositionofloading condi-tionsinducingOAIConthescaleofaconventionaltensilespecimen
Fig.10.SchematicloadingpathsandresultsoftensiletestscarriedoutonV-shaped specimensat600◦Cundersyntheticair.
tothescaleoftheinnersurfaceofthetipoftheV-shapedone. Thus,thankstothistransposition,themechanicalkeyparameters responsibleforOAICmaybedemonstratedandcharacterized.
Theeffectoftheenvironment,intermsofintergranular oxida-tionandweakeningreactions,isalreadyprovedtobeanecessary, butnot a sufficientcomponent, ofthedamagingprocess [1–9]. However,ifallof thegrainboundariesareoxidized,therole of localstresses,cumulatedstrain,andstrainratestillraises ques-tionsabouttheiractionsandinteractionsinthedamagingprocess. Thepresentresultstendtominimizethedirectroleofthestresses onOAICconcernsforalloy718andconversely,emphasizetherole ofstrainrateandplasticity.Firstly,therupturemodemap(Fig.4) correlatedwithFig.2,showedthatforsimilarflowstresses,the rupturemodechangesprincipallyasafunctionofthedeformation mode,i.e.: thestrainraterelatingtopureDSAorPLC instabili-ties.Secondly,toactivateOAICmechanisms,acriticalplasticstrain accumulationseemstoberequired.Therefore,ifthisplasticstrain isnotcumulatedthankstoaslowstrainrate-DSAflowmode-crack initiationdoesnotoccur.
Inordertounderstandandtrytodissociatetheeffectsoftime andplasticityfromthestrainrateroleontheOAICprocess, comple-mentarytestswerecarriedout.Aschematicrepresentationofthese newexperimentsispresentedinFig.10.V-shapedtensile speci-menswereloadedat10N(elasticload)or20N,whichisclosed totheelastoplasticthresholdoftheVstructure,witha displace-mentrateof0.02mm/sandmaintainedrespectivelyatthisloadfor 20min.Then,aftereachloaddwell,specimensweretestedupto ruptureatdisplacementratesof0.02mm/sor0.002mm/s.Fracture surfaceswereobservedbySEM.
Thefactthatforeachtensiletestperformedat0.02mm/safter theloaddwell,thefracturesurfaceis totallytransgranular duc-tilewhilefortensiletestsperformedat0.002mm/saftertheload dwell,thefracture surfaceexhibitsintergranular areas, demon-stratesthattheroleofthestrainrateisnotentirelyatemporalor diffusionalroleandthattheplasticitycomponentmustbetaken into account. Therefore, from a mechanistic point of view,the intergranularweakeningof thematerial isnot due tothe sim-plediffusionofreactivespeciesfromthecoreofthegraintothe grainboundariesorfromtheaggressiveenvironmenttothegrain boundaries,butseemstoberelatedtothehighlevelofgrainto grainstrainincompatibilitiesand/ortointeractionsbetween plas-ticityandmobilespecies.Previouswork[9]demonstratedthatfor tensiletestscarriedout,insimilartestingconditionsthanthose presentedin thispaper,onalow interstitialcontentalloywith similarmechanicalproperties,thetransitionDSAregimetoPLC instabilitieswasnolongerrelatedtoamodificationofthe frac-turemodewhichremainedtransgranular.Theseresultsstrongly
suggestthatinterstitialsspeciesplayafirstorderroleintheOAIC process.
Suchinteractionsbetweendislocationsandsolutespeciesare knowntooccurduringDSAandPLCregimes[10,15–24]in intersti-tialorsubstitutionalalloys.However,inthecaseofage-hardenable alloys,anotherparametermustbetakenintoaccounttorealize DSA/PLCregimes:thehardeningprecipitates.Indeed,precipitates haveasignificanteffectonthedislocationmobilityandonthe char-acteristicsofthepinning/pinningoffeffect(characteristicdistance, characteristictime,etc.)[23,25].Besides,someauthorsproposed thatduringthepiningofdislocationsonhardeningprecipitates, someinteractions in terms of migration orattraction/repulsion mayoccurbetweenprecipitatesandinterstitialsspeciesdragged bydislocationsthusimpactingplasticflowandinterstitialsspecies distributionrespectively[10,11,24].Knowing thespecificitiesof theDSAregimeandofthetypeCPLCphenomenoninsuch age-hardenablealloys,anintergranularembrittlementprocessbased ona strain-inducedsegregationmechanism mightbeproposed toexplain ourresults. Mostlikely,in thepureDSAregime (no serrations),dislocationsareassumedtodriftthroughgrain bound-ariesinterstitialspecieswhichmightreactwithoxygenproduced andtransportedbyintergranularoxidationprocess.Suchreactions betweensegregatedcarbonandoxygenatgrainboundarieshave alreadybeensuggestedbyBricknellandWoodford[26,27]during hightemperatureoxidationofnickel.Conversely,theoccurrence ofPLCbandsisassumedtotrapinterstitialspeciesinthegrainsby pinning/pinningoffeffectonthehardeningphases.Consequently, duringtheOAICdamagingprocess,twofactorsareidentifiedto berequired:asufficientcumulatedplasticstrainforcrack initia-tionandacriticalcontentofdraggedreactivespecies.Therefore, ifpreviousconsiderationsweretakenintoaccount,thisdamaging processestablishedonconventionaltensiletestsmaybeinvoked tooccurnearbythesurfaceofspecimenandbringbackapossible explanationforthelocalmechanicalloadingpathdependenceof OAIC.
5. Conclusions
Thankstoa dualapproach basedona specificexperimental specimenandonassociatedFEcalculations,thepresentstudyhas shownthatmechanicalloadingconditionsinducingOAIC initia-tionrandomlyobservedonconventionaltensiletestsanddefined intermsofstrainrateandthresholdvalueofplasticstrainmay belocalizedandfulfilledinadelimitedarea–tipoftheV-shaped specimen.DuetothecloseagreementbetweenFEcalculationsand mechanicaltestinginthisstudy,macroscopickeyparametersmay henceforthbediscussedonapolycrystalscale,liketheeffectof localstressorlocalstrainaccumulation.
Moreover,thegeometricalandmechanicalspecificitiesofthe V-shapedspecimen,inrelationtoFEresults,makethatdesigna usefulandrelevanttoolinunderstandingtheinfluenceofthe local-izationofgradientsofstrainandstrainrateintheOAICprocess. Theadaptationofthespecificgeometricalparameters ofthe V-shapedspecimen,associatedwithFEcalculations,makespossible tocoveralargerrangeofstrainrateandtoaccesssmootherstrain gradientsandstrainrateandthentomaketheunderstandingof thefactorsimpactingOAICsusceptibilityconcrete.Finally,allthis workdoneonconventional tensiletestsandonV-shaped spec-imens,associatedwithcalculationtools,maybeusedtofurther developstructurelifespanassessment.
Acknowledgement
TheauthorswouldliketoacknowledgetheAREVA-NPCompany foritsfinancialsupport.
References
[1] P.Valerio,M.Gao,R.P.Wei,ScriptaMetall.Mater.30(1994)1269–1274. [2]R.Molins,G.Hochstetter,J.C.Chassaigne,E.Andrieu,ActaMater.45(1997)
663–674.
[3]E.Andrieu,A.Pineau,J.Phys.IV9(1999)3–11. [4]J.Pedron,A.Pineau,Mater.Sci.Eng.56(1982)143–156.
[5]R.P.Wei,C.Miller,Z.Huang,G.W.Simmons,D.G.Harlow,Eng.Fract.Mech.76 (2009)715–727.
[6]L.Fournier,D.Delafosse,T.Magnin,Mater.Sci.Eng.A316(2001)166–173. [7] V.Garat,J.-M.Cloué,D. Poquillon,E.Andrieu,J.Nucl. Mater.375(2008)
p.95–p.101.
[8] J.Deleume,PhDthesis,INPToulouse,France,2007.
[9]B.Ter-Ovanessian,J.Deleume,J.M.Cloué,E.Andrieu,Mater.Sci.Forum595–598 (2008)951–958.
[10]W.Chen,M.C.Chaturvedi,Mater.Sci.Eng.A229(1997)163–168.
[11] M.L.Weaver,C.S.Hale,Superalloys718,625,706andVariousDerivatives,TMS, 2001,pp.421–432.
[12] S.Nalawade,M.Sundararaman,J.B.Singh,R.Kihore,V.Amit,Superalloy718 andDerivatives,TMS,2010,pp.809–823.
[13]J. Kumar, A. Kumar, V. Kumar, Mater. Sci. Eng. A 528 (2011) 4009–4013.
[14]J.Deleume,D.Poquillon,V.Garat,J.M.Cloué,E.Andrieu,Corros.Sci.50(2008) 737–743.
[15]D.Wagner,J.C.Moreno,C.Prioul,J.M.Frund,B.Houssin,J.Nucl.Mater.300 (2002)178–191.
[16]J.Belotteau,C.Berdin,S.Forest,A.Parrot,C.Prioul,Mater.Sci.Eng.A526(2009) 156–165.
[17]L.P.Kubin,Y.Estrin,J.Phys.III1(1991)929–943. [18] S.L.Mannan,Bull.Mater.Sci.16(1993)561–582. [19]P.Rodriguez,Bull.Mater.Sci.6(1984)653–663. [20] A.H.Cottrell,B.A.Bilby,Proc.Phys.Soc.A62(1949)49–62. [21]A.VanDenBeukel,ActaMetall.28(1980)965–969. [22] R.Mulford,U.Kocks,ActaMetall.27(1979)1125–1134. [23]D.M.Riley,P.G.McCormick,ActaMetall.25(1977)181–185. [24] R.Hayes,W.Hayes,ActaMetall.30(1982)1295–1301.
[25]D.Thevenet,M.Mliha-Touati,A.Zeghloul, Mater.Sci. Eng. A266 (1999) 175–182.
[26]R.Bricknell,D.Woodford,ActaMetall.30(1982)257–264. [27] R.Bricknell,D.Woodford,ScriptaMater.23(1989)599–601.