Influence of local mechanical loadings paths on the oxidation assisted crack initiation of alloy 718

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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

b

a_AREVA,_AREVA_NP_10,_rue_J._Récamier,₆₉₄₅₆_Lyon_Cedex_06,_France

b_Université_de_Toulouse,_CIRIMAT,_{INP/UPS/CNRS,}_{INP-ENSIACET,}_4,_allée_Emile_Monso_-_BP₄₄₃₆₂_-₃₁₀₃₀_TOULOUSE_Cedex_4,_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]_temperature_range_were

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]_._Recent_work_[6–9]_,_{investigating}_the_detrimental

effectofoxidationonthelifespanandthemechanicalbehaviour ofalloy718, hasdemonstratedthat duringtensile testscarried outunder syntheticairat a 400–600◦_C _temperature_range _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

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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◦_C_in_order_to_accurately_analyze

theroleoflocalloadingpaths(i.e.:criticalstrain,criticalstrainrate, plasticflowmode,etc.)onintergranularcrackinitiationbehaviour ofalloy718.

2. Materialandexperimentalprocedures

Thematerialsusedinthisstudywereobtainedthrougha dou-blemeltingprocess:vacuuminductionmeltingpluselectrodeslag remelting.Thenominalcompositionoftheevaluatedheatisgiven inTable1.

Thecastingotwashotandcoldrolleddowntostripsof0.3mm ofthickness,followedbyasolutionannealingheattreatmentat 1050◦_C_for_one_hour_ended_by_air_quenching._Thin_tensile

speci-mensweremachinedfromtheannealedstrips.Then,whilestillat anannealedmetallurgicalstate,V-shapedhumpspecimenswere formedfromtensilespecimens,bycoldstampingwithaspecific stainlesssteeldye(Fig.1).

V-shapedspecimensandconventionaltensilespecimenswere thenheattreatedinanargonatmospherefollowingthe conven-tional aeronautical route: holdat 720◦_C _– ₈_h, _cooling ₅₀◦_C/h

downto620◦_C,_hold_at₆₂₀◦_C_–₈_h_and_final_air_cooling_to_room

temperature. Such heat treatment aims at precipitating and

stabilizingthehardeningphasesg′_(Ni₃_Al,_average_size₅₀_nm)_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◦_C_temperature_range_for_different_imposed

strainratesin the10−5_s−1_,₁₀−1_s−1 _range. _The_first_ones_were

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,_was_not_{significantly}_modified_by_the_previous_work

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.

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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 2.5×10−4_s−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]_temperature_range_–_in_italics:_the_cumulated_necessary_deformation

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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,_as_during_the_tensile

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

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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

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Fig.10.SchematicloadingpathsandresultsoftensiletestscarriedoutonV-shaped specimensat600◦_C_under_synthetic_air.

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.

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