Hydrogen trapping by VC precipitates and structural defects in a high strength Fe-Mn-C steel studied by small-angle neutron scattering

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O

pen

A

rchive

T

OULOUSE

A

rchive

O

uverte (

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)

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To link to this article : DOI:10.1016/j.msea.2011.12.080

URL :

http://dx.doi.org/10.1016/j.msea.2011.12.080

To cite this version : Malard, Benoit and Remy, B. and Scott, Colin

and Deschamps, Alexis and Chêne, Jacques and Dieudonné, Thomas

and Mathon, Marie-Hélène Hydrogen trapping by VC precipitates and

structural defects in a high strength Fe-Mn-C steel studied by

small-angle neutron scattering. (2012) Materials Science and Engineering A,

vol. 536. pp. 110-116. ISSN 0921-5093

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Hydrogen

trapping

by

VC

precipitates

and

structural

defects

in

a

high

strength

Fe–Mn–C

steel

studied

by

small-angle

neutron

scattering

B. Malard

a,∗

_,

_B.

_Remy

b

_,

_C.

_Scott

b

_,

_A.

_Deschamps

a

_,

_J.

_Chêne

c

_,

_T.

_Dieudonné

b,c

_,

_M.H.

_Mathon

d

a_SIMaP,_INP_Grenoble_–_CNRS_–_UJF,_BP_75,₃₈₄₀₂_St_Martin_d’Hères_Cedex,_France b_{ArcelorMittal}_(R&D_Automotive_Products),_Voie_Romaine,₅₇₂₈₀_{Maizières-les-Metz,}_France c_CEA/CNRS:_{“Hydrogène/Matériaux}_de_{structure”,}_UMR_8587,₉₁₁₉₁_Gif_sur_Yvette,_France d_Laboratoire_Léon_Brillouin,_CEA/Saclay,₉₁₁₉₁_Gif_sur_Yvette_Cedex,_France

Keywords:

Small-angleneutronscattering Hydrogenembrittlement

Hydrogentrappedbynanoprecipitates

a

b

s

t

r

a

c

t

ThetrappingofhydrogenbyVCprecipitatesandstructuraldefectsinhighstrengthFe–Mn–Csteelwas studiedbysmallangleneutronscattering.NointeractionbetweenHandVinsolidsolutionhasbeen detectedbutasignificantinteractionbetweenHandstructuraldefectsintroducedbyplasticdeformation hasbeenmeasured.ThislasteffectwasreversibleuponoutgassingoftheH.Moreoverasignificant interactionbetweenHandVCprecipitateshasbeenmeasured;5ppmwt.ofHcouldbetrappedinthe precipitates.ThisisconsistentwiththehomogeneoustrappingofHwithintheprecipitatesratherthan attheprecipitate/matrixinterface.

1. Introduction

Fe–Mn–Causteniticsteelsthat exhibitthetwinninginduced plasticity(TWIP)effectareveryattractivematerialssolutionsfor weightreductionofcrash-resistantpartsinnewpassengervehicle developments,duetotheircombinationofextremelyhighenergy absorbingcapacity coupledwithlargeresidual elongationafter forming[1–6].

The combination of very high tensile strength (typically >1000MPa)coupledwithextendedformabilityinevitablyleadsto thepresenceofregionsofhighlevelsofinternalstressesaftercold formingoperations.Iftheconcentrationofmobileatomic hydro-gen(H)inthesezonesexceedsacriticallimit,thematerialcanbe susceptibletodelayedcracking[7].Hcontentinthesteelcanbe rigorouslycontrolledattheproductionstagebutmobileHmaybe introducedatlaterstagesbywelding,surfaceorheattreatments, corrosion,etc.ItisthusadvantageoustointroduceHtrappingsites inthemicrostructurethatcanactassinksformobileHandhence reducetheriskofdelayedcracking.Itisnowappreciatedthatthe additionofsmallamountsofvanadium(V)canreducetheseverity ofdelayedcrackingcausedbythepresenceofmobileH[8]whileat thesametimeenhancingthemechanicalproperties[9].However, thephysicalinteractionsbetweenmobileHandVeitherinsolid

∗ Correspondingauthor.Tel.:+330476826669;fax:+330476826644. E-mailaddress:benoit.malard@simap.grenoble-inp.fr(B.Malard).

solutionorintheformofvanadiumcarbides(VC)precipitatesin austeniticTWIPsteelsneedtobestudiedinordertoincreaseour understandingofthetrappingprocess.

Itisnoteasytoexperimentallydetecthydrogeninsteelsbecause ofitshighmobilityandlowsolubility.Therefore,theunderstanding ofhydrogenbehaviourincludingdiffusionandtrappinghasbeen limitedduetothelackofsuitablemonitoringtools[10].Thermal desorptionanalysis(TDA)isusuallyusedtodeterminethe hydro-gen desorption characteristics, giving useful information about trappingsitesandmobilehydrogencontentovertheentire speci-men[11–14].TDAisoftenusedinconjunctionwithelectrochemical permeationtechniques(EP)whichprovidecomplementary infor-mationaboutdiffusioncoefficients[15,16].However,information aboutthestate,numberanddistributionofhydrogenatomswithin thesamplearestilluncertain.

Techniquesinvolvingneutronscatteringareamongthemost suitabletoolsforobtaininginformationonHdistributionbecause neutronshavetheadvantageofahighsensitivitytolightelements likehydrogen.Neutronscatteringtechniquesareappropriatefor measuringmicrostructuresinthescalerangefromsub-angstrom (neutrondiffraction)[17]tomillimetrelevel(neutronradiography)

[18,19].Inparticular,smallangleneutronscattering(SANS)isan appropriatetechniquefordetectingHinteractionswithnanoscale microstructuralfeatures.For example,Rossetal.[20]and Max-elonetal.[21]haveusedthistechniqueinPd alloystoreacha consistentdescriptionof theinteractionbetweenhydrogenand dislocations. In addition, Kluthe et al. reported detection of H

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segregationattheinternalinterfaceofAg/MgOusingSANS[22]. Theyshowedcleardifferencesinthescatteringprofilesbetween Hordeuterium(D)chargedandunchargedsamples.Insituations withlowerH concentrations,a differencein neutronscattering profilescanalsobeobservedbetweenH-chargedanduncharged steels.Ulbrichtetal.,usingSANS,reportedthatthebehaviourofH trappedinthemicrostructuralheterogeneitiesofapressurevessel steelwasmainlycorrelatedwithirradiationeffects[23].Recently, aJapaneseteamatNIMShasappliedSANStechniquestothe detec-tionofHtrappedbyniobiumcarbides(NbC)inferriticsteels[24]. TheneutronscatteringintensityfromnanometrescaleNbC pre-cipitatesappearstobehigherinthecaseofH-chargedsamples. Ifthesamesamplesarethen heatedtooutgasH theenhanced scatteringintensitydisappears.Theauthorsclaimthatthiseffect iscausedbythesegregationortrappingofHattheNbC/ferrite interface.

H also interacts withlattice defects such as vacancies, dis-locations and stacking faults. For example the trapping of H bydislocationsand«martensitehasbeenobserved experimen-tally by tritium autoradiography on austenitic stainless steels by Chêne et al. [25]. An indirect demonstration of the phe-nomenonof trapping is given by thedecrease in theapparent Hdiffusioncoefficientwhenthematerialisplasticallydeformed

[26]. The peculiarity of H trapping at dislocations lies in the fact that these traps are mobile and can therefore assist the mechanism of H diffusion. The important role of mobile traps for thetransport of H was firstproposed byBastien and Azou

[27]50yearsago.Thistransportmechanismwasexperimentally demonstrated by Donovan [28] by measuring the flow of tri-tiumdesorptionintroducedintensile specimens.Recently,new observationsconductedbyChêneandBrass[29–31]wereusedto quantifychangesintheamountoftritiumdesorbedduringa ten-siletestperformedonasinglecrystalofnickel-basedsuperalloy

[30].

Inthispresentwork,toshedlightontheinteractionbetween HandnanoscaleVCprecipitatesaswellasotherstructuraldefects suchasstackingfaults,twinsanddislocations,SANSandTDSwere employedtoinvestigatetwo TWIPsteelcompositions; aV-free steel,and a V-containingsteel where theVwaseither in solid solutionorintheformofprecipitates.Theeffectoflatticedefects wasinvestigatedbycomparingsampleswithdifferentdegreesof coldrolling.MeasurementswerecarriedoutwithandwithoutH charging.

2. Materialsandtechniques

Themeasuredspecimenswereintheformof1.2mmcoldstrips preparedwiththreedifferentmicrostructures(seeTable1foralloy compositions).

Thefirstmicrostructure(A)wasareferencealloywithnoV addi-tionsinafullyrecrystallisedstate.Thesecond(B)usesthesame basecompositionmodifiedbytheadditionof0.215wt.%V.Itwas studiedinthefullhardstate(coldrolledwith50%reduction)where allVisinsolidsolution(validatedbyselectivechemicaldissolution) andcontainingahighdensityofcrystallinedefects(dislocations, twinsandstackingfaults).Thethirdspecimen,(C)wasidenticalto (B)exceptrecrystallisationannealedwith60%oftheavailableV

Table1

Compositionoftheausteniticsteelsin%wt.

C Mn Si P N V

Sample(A) 0.585 18.4 0.195 0.032 0.020 0

Samples(B)and(C) 0.600 18.14 0.185 0.032 0.016 0.215

precipitatedassphericalshapeVC[9].1 _Different_H_charging_and outgassingcycleswereappliedtothesemicrostructures.

Theprocessingcycleforthethreemicrostructureswasthe fol-lowing.Afterreheating(1180◦_C_–₁₅_min)_the_cast_ingots_were_hot

rolledto2.4mmandwaterquenchedfromabove900◦_C._The

sam-pleswerethenchemicallystripped(HClbath),andcoldrolledwith areductionrateof50%to1.2mm.Atthisstepthereference(B)with Vinsolidsolutionfinisheditsprocessingcycle.Themicrostructures (AandC)wereannealedat800◦_C_for₁₈₀_s_to_form_{semi-coherent}

VCwithanaverageprecipitatediameterof7–8nm[9,32](C)anda fullyrecrystallisedmicrostructure(AandC).

Samples12mm×12mm in areawere mechanicallythinned andcathodicallychargedwithHin0.1MaqueousNaOHsolution. Twoseries ofcharging experimentswere carriedout, resulting invery differentH concentrationsand distribution(see Table2

fordetailsonsample thicknessand chargingconditions).Inthe moderateHchargingconditions(18◦_C,₂₄_h),_assuming_that_the

apparentdiffusioncoefficientofHatroomtemperatureisabout DRT=10−12cm2s−1[33],theHconcentrationprofileisexpectedto

extendoverabout10mmonbothsidesofthesamplethickness (500mm).Thiscorrespondstoanonhomogeneousdistributionof Hinthespecimenandalowaverageconcentration.Inthesevere Hchargingconditions(80◦_C,₆₂_h),_according_to_the

experimen-talvalueoftheapparentdiffusioncoefficientat80◦_C_in_Fe–Mn–C

TWIPsteels[34],theHconcentrationprofileextendsabout100mm onbothsidesofthesamplethickness(200mm),resultinginamore homogeneoushydrogenationandahigheraverageconcentration. Moreovershort-circuitdiffusionofHalonggrainboundariesmay slightlyenhanceHabsorption[34].ImmediatelyafterHcharging, thesampleswerestoredinliquidnitrogentopreventHdesorption beforeSANSmeasurement.

Thetotal H contentwasmeasuredwiththe meltextraction technique.Thesampleismeltedinacrucibleandthehydrogenis extractedusinganinertcarriergaspulse(argon).Thesignal gener-atedbythethermalconductivityanalyserisintegratedtocalculate theconcentrationbyweightofthesample.Thesensitivityofthis techniqueisabout0.1ppmwt.;itwasnotpossibletogivea stan-darddeviationontheconcentrationvaluesbecauselackofmaterial meantthattoofewmeasurementscouldbemadeonhydrogenated samples.

SANSexperimentswereperformedonthePAXYinstrumentat theLaboratoireLéonBrillouin(LLB)inSaclay,France.Neutronsare monochromatedbyavelocityselectortoameanwavelengthof6 ˚A and9.5 ˚A,thewavelengthspreadbeingd/=9%.Thiswavelength waschoseninordertoavoiddoublediffractionproblemsthatcan arisefromtheaustenitemicrostructure.Sample–detectordistances of1.865mand6.065mwereusedwiththewavelengthof6 ˚Aand 9.5 ˚Awithappropriatecollimationsettingstogiveaccessto scat-teringvectorsQ(Q=4psin/)rangingfrom0.06 ˚A−1 _to_{0.2 ˚A}−1_.

Thediameteroftheneutronbeamwas8mm.Dataprocessingwas carriedoutusingtheGRASPsoftwaredevelopedbyC.Dewhurst attheInstituteLaueLangevin(ILL).Datafileswerecorrectedfor electronicnoise,detectorresponseandbackgroundnoise,and nor-malisedusingawaterstandard.TheLaueintensitywassubtracted forallthepresenteddata.

3. Results

3.1. Microstructuralobservations

SampleAconsistedoffullyrecrystallisedaustenitewithagrain size of4–5mm. No precipitateswere detectedby TEManalysis.

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Table2

ConcentrationinH(ppmwt)ofthedifferentsamplesusedforSANSexperiments.BchgorCchgmeanschargedsample.BoutmeansoutgassedsampleBat260◦Cduring24h. CoutmeansoutgassedsampleCat150◦Cduring16h.

Microstructure NoHcharging ModerateHcharging

0.5mm;24h;18◦_C;₁_mA/cm2 SevereHcharging 0.2mm;62h;80◦_C;₅₀_mA/cm2 (A)Novanadium <1.5 (B)Vinsolidsolution 1.9 Bchg:2.5 Bout:0.8 (C)VCprecipitates 0.7 Cchg:72 Cout:11.5

Fig.1.Microstructuralobservationsofsample(C).(a)BrightfieldTEMmicrographshowingthepartiallyrecrystallisedgrainstructure.(b)Opticalmicrograph(grainsize 3–4mm).(candd)TEMextractionreplicasshowingtheVCprecipitatesinblack.

Sample B consisted of heavily cold rolled austenite (grain size priortorolling8–10mm)whichcouldnotbeusefullyanalysedin theTEM.Howeverselectivechemicaldissolutionconfirmedthat noVwasprecipitatedinthisspecimen.Fig.1shows microstruc-tural observations of sample C (with VC). Precipitation of VC slows down the recrystallisation kinetics [9] so that the final austenitic microstructurecontains a few partiallyrecrystallised grains(Fig.1a),howeverthevastmajorityofthematerialisfully recrystallised.Themeangrainsizewas3–4mm(Fig.1b).The distri-butionandthemeanradiusoftheVCprecipitatesinthesampleC weredeterminedusingTEMextractionreplicas.Fig.1canddshows VCprecipitateswithradiivaryingfrom1to25nm.

Fig.2showstheVCsizedistributionasmeasuredfromTEM replicaobservations.Theaverageprecipitateradiusis3.9nmand the distribution can be adequately described by a log-normal function. Selective chemical dissolution followed by ICP-OES indicated that 1537ppm wt. V was precipitated after anneal-ing [9]. This is equivalent to a volume fraction of ∼0.23% of VC. 0 20 40 60 80 100 120 140 16 14 12 10 8 6 4 2 0

C

o

u

n

t

Precipitate radius (nm)

Histogram of precipitate sizes

506 precipitates Average radius 3.9 nm

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0.0001 0.001 0.01 0.1 1 10 100 1000 0.1 0.01 V-free alloy V in solid solution

VC precipitated alloy

Intensity

(cm

-1

)

Q (Å-1)

Fig.3. IversusQforthe3reference(withoutH)samples.WithoutV(A),Vinsolid solution(B)andVCprecipitates(C).

3.2. SANSresults

3.2.1. Comparisonbetweentheinitial(uncharged) microstructures

Fig.3showsthescatteredintensitiesinalog–logplotforthe threeinitial(uncharged)microstructures.

Thereferencesample(A)showsthelowestscatteringintensity, duetotheabsenceofanyVCprecipitatesandthefullyrecrystallised microstructure.Therecordedsignalcorrespondstothebackground fromthematrix(scatteringfromremainingimpuritiesand crys-tallinedefects).

The full hard specimen with V in solid solution (B) shows substantial scattering at low scattering vectors. This scattering decreaseslinearlyinthelog–logplot,followingapowerlawthat willbediscussedmoreindetailinthenextsection.Sincethis mate-rialdoesnotcontainadditionalprecipitatesascomparedtosample (A),2_it_can_be_inferred_that_the_measured_signal_is_related_to_the_high densityofcrystallinedefectsintroducedbycoldrolling.

The recrystallised sample containing VC precipitates (C) presents a scattering signal typical of a low volume fraction precipitate-containing matrix, witha broad shoulder. The inte-gratedintensityofthissignalQo(afterbackgroundLaueintensity

subtractionandextrapolationtoinfinitescatteringvectorsusing thePorodasymptoticbehaviour)providesameasurementofthe precipitatevolumefraction:

Qo=

Z

∞ 0

I(Q )Q2_dQ₌₂2_f

v(1−fv)(p−m)2

wherefvistheprecipitatevolumefraction,pandmarethe

den-sityofneutronscatteringlengthsintheprecipitatesandmatrix: p= bV +bC 2˝p =3.49×1014m−2, m∼=4bFe+bMn 5˝m =5.77×1014m−2

wherebiisthenuclear(coherent)scatteringlengthofspeciesi(see

Table3),and˝p=8.97 ˚A3and˝m=11.8 ˚A3aretheaverageatomic

volumesoftheprecipitateandmatrix.

Themeasuredvolumefractionis0.27%,whichisingood agree-mentwiththatobtainedbyselectivedissolutionpresentedabove (∼0.23%).

2_The_absence_of_any_{strain-induced}_{precipitation}_was_confirmed_by_chemical extractionfollowedbyICP-OES.

Table3

Nuclearcoherentscatteringlength.

Fe Mn C V H b(1×E−15_m) _9.4500 −3.7300 6.6460 −0.3824 −3.739 0.0001 0.001 0.01 0.1 1 10 100 1000 0.1 0.01 V - 50% rolling V - 15% rolling V - 0% rolling V-free alloy Intensity (cm -1 ) Q (Å-1)

Fig.4.IversusQforthesampleswithVinsolidsolution(B)atdifferentcoldrolling reduction(0%,15%and50%reductions).Thereference(A)annealedsamplewithout Visincludedforcomparison(freealloy).

Fromthisscatteringcurve,wealsocalculatedthePorodradius oftheprecipitatedistributionusingtheequationbelow:

Rp=3V S =

3Qo

K

SandVarethetotalsurfaceandthetotalvolumeoftheprecipitates, Qo is theintegrated intensityasdefined above and Kis the

Y-interceptofI·q4₌_f(q4₎_[35]_._We_obtained_a_value_of_R

p=3.3±0.2nm, whichisconsistentwiththeTEMmeasurements.

3.2.2. Effectofcoldrollingreduction

Theintensityof scatteringfromsample(B)washigher than expected. In order tofurther investigatethe origin of this sig-nalwemeasuredsampleswithdifferentcoldrollingreductions.

Fig.4showstheeffectofvariationsintheamountofcoldrolling reduction(0%,15%and50%)appliedtoreferencesample(B)on thescatteringintensityintheunchargedstate(<2ppmwt.ofH). Thescatteringintensityofthefullyrecrystallisedreferencesample withoutV(sample(A))isalsoincludedforcomparison.

Whennocoldrollingisapplied,thescatteringintensityofthe V-containingmaterialissimilartothatoftherecrystallised,non-V containingmaterial,showingthatthepresenceofVinsolidsolution andalsothedifferenceingrainsizedoesnothaveasignificant effectontheneutronscatteringintensity.Thesamplewith15% coldreductionshowsascatteringintensityintermediatebetween thatofthe0%and50%rolledmaterials.Italsoexhibitsasimilar power-lawscatteringbehaviour,withanexponentofabout−3.5. Thisbehaviourisconsistentwithwhatisknownfromthescattering ofdislocations,whichhavebeenshowntoproduceaQ−3_behaviour

[36].Thedifferenceinscatteringintensitybetweenthethreecold rollingreductionscanthereforebeattributedtothedifferencein thedensityofdislocations,twinsandstackingfaultsintroducedby theplasticstrain.

3.2.3. EffectofHchargingonthefullhardsample(sampleB) WehaveevaluatedtheeffectofHchargingonthescattered intensityfromfullhard samplescoldrolledto50%reduction(V insolidsolution).Fig.5showsthecomparisonatsmallscattering vectorsofthescatteringintensityfromtheuncharged(B)sample

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0.1 1 10 100 0.04 0.03 0.02 0.01 0.008 Uncharged (1.9 ppm H) Charged (2.5 ppm H) Outgassed (0.8 ppm H)

In

te

n

si

ty

(

cm

-1

)

Q (Å-1)

V in solid solution - 50% rolling

Fig.5. IversusQ.EffectofHconcentrationonscatteringfromcrystaldefectsofthe sample(B).

(measuredHcontent: 1.9ppmwt.),inthesamesampleafterH charging(measuredHcontent:2.5ppmwt.)andaftercharging fol-lowedbyoutgassingbyannealingundervacuumfor24hat260◦_C

(measuredHcontent:0.8ppmwt.).

Whenthefullhardsample(B)ishydrogenchargedthe scatter-ingintensitydecreases,whilethesamepowerlawexponentisstill observed.Outgassingreversesthischange:theoutgassedsample showsthesamescatteringintensityastheunchargedsample.This changeofintensityoccurringafterHchargingislikelyduetothe segregationofmobileHtodefectsthatresultsinadecreaseofthe associatedstrainfieldsandthereforetothescatteringintensitythat isrelatedtothemagnitudeofthisstrainfield.Duetoitslarge par-tialmolarvolume,atomicHisknowntobesensitivetolocalstress fields[37].Thecompletereversibilityoftheintensitychangewith outgassingimpliesthatthistreatmentremovesmostorallofthe Hsegregatedtocrystallinedefects.

Thefactthatthenuclearscatteringintensitydecreaseswiththe additionofHisnotexpectedfromtheexistingliterature,e.g.Ross etal.[20].Theseauthorsmeasuredthatthescatteringintensity, associatedwithH-dislocationinteractionsinpalladium,increases withHanddecreaseswithD.However,besidesthelarge differ-enceintheHsolubilityinPdandFe–Mn–CTWIPsteel,different typesofdefectsmaybepresentineachexperiment.Moreovertheir calculationsdonotincludetheinfluenceoftheeffectofstress relax-ationduetotrappedH[38].InourcasetheFe–Mn–CTWIPsteels arestronglytwinnedandhavelowstackingfaultenergy.Thislow energyinducesalargenumberofstackingfaultsduetothe disso-ciationofdislocations.AccordingtotheobservationofHtrapping on«martensite[25],itisanticipatedthatHsegregationonthese stackingfaultsalsooccurs.

Atlargescatteringvectors, noeffectofHisobserved.Thisis quitereasonablebecauseatlargeQthecrystallinedefectsdonot contributemuchtothescatteredintensity.

3.2.4. InteractionbetweenHandtheVCprecipitates(sampleC) WehaveevaluatedtheeffectofHchargingonsample(C) con-tainingVCprecipitates.Inthiscaseastrongchargingprocedure wasfollowed,resultingin72ppmwt.ofHinthechargedsample. Afteroutgassingfor16hat150◦_C_the_remaining_H_{concentration}

was11.5ppmwt.

Fig.6showsthecomparisonofthescatteringintensityofthe threesamples,namelyunchargedspecimen(C),specimen(C)after Hchargingandspecimen(C)afterHchargingandoutgassing.At lowscatteringvectors,itisobservedthatthescatteringintensity decreaseswiththeadditionofH,andincreaseswithoutgassing. Thisbehaviourissimilartothatobservedinthefullhardsample,

0.001 0.01 0.1 1 10 0.1 0.01 Uncharged (0.7 ppm H) Charged (72 ppm H) Outgassed (11.5 ppm H)

Intensity (cm

-1

)

Q (Å

-1

)

VC containing alloy

Fig.6. IversusQforsamples(C)withVCprecipitatesatdifferentconcentrationsof H.

anditcanbeassumedthatitisrepresentativeoftheinteraction betweenHandthecrystallinedefectsremaininginthesample. Actually,whenthesampleis outgassedat150◦_C _the_scattering

intensityatlowQincreasestovaluesthataresignificantlyhigher thanthereferencesample.Thisislogicalifweassumethatthereis strongoutgassingofHreversiblytrappedatcrystallinedefectsbut notfromtheresidualHtrappedbytheVCprecipitates.

Atlargerscatteringvectors,wheremostofthesignalrelatesto theVCprecipitates,theoppositebehaviourisfound,namelythe scatteringintensityincreaseswiththeadditionofHanddecreases againwithoutgassing.

TheintensityvariationscanbemoreeasilyobservedinaKratky plotofI·Q2_versus_Q_[35]_(see_Fig.₇_)._On_this_plot_the_scattering

vectorattheintensitymaximumattheshoulderisinversely pro-portionaltotheaverageprecipitateradiusandtheareabelowthe curveistheintegratedintensitythatdependsontheprecipitate volumefractionandthescatteringcontrastbetweenthe precipi-tatesandmatrix.ThisplotshowsthattheadditionofHstrongly increasesthescatteringintensity(byapproximately25%). More-over,thisincreaseofintensityisclosetoamultiplicationfactorin therangeofscatteringvectorswheretheintensityisdominated bythesignalofprecipitates,sincetheshapeofthesignalisnot muchaffected.Whenthesampleisoutgassedat150◦_C,_the

inten-sityisobservedtodecrease,butstayssignificantlyhigherthanthat ofthereferencesample,whichisconsistentwiththeremainingH concentrationmeasuredafterdegassing(11ppmvs.0.7ppminthe unchargedsample).

Fig.7.KratkyplotofIQ2_versus_Q_for_samples_(C)_with_VC_precipitates_at_different concentrationsofH.

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3.2.5. QuantitativeassessmentoftheinteractionbetweenVC precipitatesandH

ThechangeinneutronscatteringintensitywithHchargingand outgassingobservedinsample(C)demonstratesthatan interac-tionexistsbetweenH present inthealloy andVCprecipitates. TheincreaseinneutronscatteringintensitywithHcharging cor-respondstoanincreaseofthecontrastinscatteringlengthdensity betweentheprecipitatesandmatrix.Sincethecoherentnuclear cross-sectionofHisnegativeandtheprecipitatescatteringlength densityissmallerthanthatofthematrix,thiscontrast enhance-mentcorrespondstoHtrappingbytheprecipitates.Wehaveseen that theincrease of intensitymeasuredin the H-charged sam-plecorrespondednearlytoa simplemultiplicationfactorofthe reference(uncharged)samplesignal.Thisexperimentaldatais con-sistentwithahomogeneoustrappingofHinsidetheprecipitates, althoughitisnotpossibletoruleoutaheterogeneoustrappingof Hinthevicinityoftheprecipitate–matrixinterface.Htrappingof TiCparticlesinbccsteelshasbeenshowntooccurbothatthe parti-cle/matrixinterfaceandinthebulkandtodependonthecoherency oftheprecipitate/matrixinterface[39].MoreoverHtrappinginthe bulkofsemi-coherentandincoherentg′_(Ni

3Al)precipitatesinfcc

Nibasesuperalloyshasbeenevidencedbytritiumautoradiography

[40].AtomisticcalculationshaveshownthatsuchaHsegregationin Ni3AlprecipitatesistheresultofalargerHsolubilityintheNi3Al

phasethaninNibasematrix[41].SimilareffectsmayexplainH trappinginthebulkofVCprecipitates.

Within this hypothesis, we can evaluate the relationship betweenthetrappedHcontentintheprecipitatesandthechange inneutronscatteringintensity.

Foratwo-phasesystem,theneutronscatteringintensityI(Q) isproportionaltothesquareofthecontrastinnuclearscattering density[35]:

I(Q)∝(p−m)2

TheeffectofHonthenuclearscatteringlengthofthematrixcan beneglectedsincetheHconcentrationisverylowsothat: m∼= 4bFe+bMn

5˝m

TocalculatetheeffectofHintakeintheprecipitates,wewilluse theapproximationthatthelatticeparameteroftheVCprecipitates isnotchangedsignificantlybythepresenceofH.Wewilldefine thefractionXpHofHinsidetheprecipitatesastheratiobetween

thenumberofHatomsandhalfthenumberof(V+C)atoms: XpH∼= 2nH

nV+nC

Then the nuclear scattering length density of the precipitates becomes:

p∼= (1/2)bV

+_(1/2)b_C+_X_pH_b_H ˝p

ThefractionofH in theprecipitatesXpH canbetranslatedinto

theoverallconcentrationofHinthesamplethatistrappedinthe precipitates(XtotHinatomicfraction):

XtotH=fv˝m

˝pXpH

And finally the overall concentrationof H in thesample CtotH

trappedintheprecipitatescanbeexpressedinppmwt.: CtotH=

106 55.4XtotH

Withtheabove equations wecan relatethe relativechange in scatteringintensity1I/ItotheamountofHtrappedinsidethe pre-cipitates(expressedasanoverallHconcentrationinthesample).

0 20 40 60 80 100 12 10 8 6 4 2 0 0.2% Volume fraction 0.3% Volume fraction 0.4% Volume fraction ∆ I / I (% )

H concentration trapped in the precipitates (ppm wt) Fig.8.1I/IversusconcentrationofHtrappedintheVCprecipitates.

Fig.8showsaplotofthisrelativevariationofintensityfordifferent valuesoftheprecipitatevolumefraction.

Experimentally,itisdifficulttoobtainaprecisioninintensity calibration that is betterthan 10–15% in SANS data, sothat it isunlikelythat,fortheprecipitatevolumefractionsinvestigated inthis work,H trappingquantitiescorrespondingtoan overall concentrationofless than 2ppm wt.canbe detectedwiththis technique.

Therelativeincreaseinneutronscatteringintensityobserved afterstrongHcharginginalloy(C)(seeFig.7)isabout25%. There-forethereshouldbeabout∼5ppmwt. ofHtrapped insidethe precipitates(assuming0.3%volumefractionofVC).Thevast major-ity (67ppm wt. H) must therefore be segregated tocrystalline defectsandgrainboundariesorhomogeneouslydistributedinthe matrix.3₅_ppm_wt._of_H_trapped_in_a_volume_fraction_of_about_0.3% correspondstoanatomicfractionofHintheprecipitatesofabout 10%.Aftervacuumoutgassing at150◦_C _the_scattering_intensity

decreases.1I/Iisabout12%morethanthereferencesamplewhich suggeststhatthereremains∼3ppmwt.ofHtrappedinthe pre-cipitatesafteroutgassing.Thus,itappearsthattheVCprecipitates arereasonablyeffectiveirreversibleHtrappingsitesfor moder-ateHconcentrations,uptoafewppmwt.Fromthedataanalysis carriedoutinthepresentwork,theHtrappingappearstooccur homogeneouslywithintheprecipitates.

4. Conclusion

InthisworkwehavestudiedtheinteractionbetweenHand an austenitic Fe–Mn–C TWIP steel containing V with different microstructuresusingTEMandSANS.Themainconclusionsthat canbedrawnfromthisexperimentalstudyarethefollowing: -No interaction between H and V in solid solution has been

detected.

-AsignificantinteractionbetweenHandstructuraldefects intro-ducedbyplasticdeformation(coldrolling)hasbeenmeasured. Namely,theneutronscatteringintensityrelatedtothese struc-turaldefectswasloweredbytheintroductionofHinthesamples. Thisispossiblyduetoareductioninthestrainfieldofthepartial dislocationsduetothesegregationofH.Thiseffectwasreversible uponoutgassing.

-AsignificantinteractionbetweenHandVCprecipitateshasbeen measured.Namely,anincreaseoftheneutronscattering inten-sityisobservedwhenthesamplesarechargedwithH.Fromthe

3_No_signs_of_damage_or_macroscopic_H

(8)

intensity increase this is consistent with the homogeneous trappingofHwithintheprecipitatesratherthanatthe precip-itate/matrixinterface.Inthestudiedconditions,wehaveshown thatapproximately5ppmwt.ofHcouldbetrappedinthisway. Thiseffectwasnotcompletelyreversibleuponsampleoutgassing at150◦_C,_showing_that_the_VC_precipitates_are_moderately_stable

Htrappingsites. Acknowledgements

TheauthorswouldliketothankC.DewhurstandD.Bowyeron D11atILL,A.RadulescuonKWS-2attheJulichNeutronCentre fortheirinstrumentalhelpsandG.PetitgandandP.Bargesfrom ArcelorMittalfortheirexperimentalworks.Wealsoacknowledge VANITECfortheirfinancialcontribution.

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