On the interest of microgravity experimentation for studying convective effects during the directional solidification of metal alloys

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On the interest of microgravity experimentation for

studying convective effects during the directional

solidification of metal alloys

Henri Nguyen-Thi, Guillaume Reinhart, Bernard Billia

To cite this version:

Henri Nguyen-Thi, Guillaume Reinhart, Bernard Billia. On the interest of microgravity

experimen-tation for studying convective effects during the directional solidification of metal alloys. Comptes

Rendus Mécanique, Elsevier Masson, 2017, 345 (1), pp.66-77. �10.1016/j.crme.2016.10.007�.

�hal-01694469�

C. R. Mecanique 345 (2017) 66–77

Contents lists available atScienceDirect

Comptes

Rendus

Mecanique

www.sciencedirect.com

Basic and applied researches in microgravity/Recherches fondamentales et appliquées en microgravité

On

the

interest

of

microgravity

experimentation

for

studying

convective

effects

during

the

directional

solidification

of

metal

alloys

Henri Nguyen-Thi

∗

,

Guillaume Reinhart,

Bernard Billia

AixMarseilleUniversité,CNRS,IM2NPUMR7334,CampusSaint-Jérôme,case142,13397Marseille,France

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received18March2016 Accepted2September2016 Availableonline15November2016 Keywords: Directionalsolidification Microstructures Convection Microgravity Insitucharacterization Metallicalloys

Underterrestrial conditions,solidificationprocessesare oftenaffectedbygravity effects, whichcan significantly influence the final characteristics of the grown solid. The low-gravityenvironmentofspaceoffersauniqueandefficientwaytoeliminatetheseeffects, providingvaluablebenchmarkdataforthevalidationofmodelsandnumericalsimulations. Moreover,acomparative studyofsolidification experimentsonearthand inlow-gravity conditions can significantly enlighten gravity effects. The aim of this paper is to give asurvey of solidification experiments conducted in low-gravity environment on metal alloys, with advanced post-mortem analysis and eventually by in situ and real-time characterization.

©2016Académiedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Structural materialpropertiesaredirectlyrelatedtotheir solidificationmicrostructures,sothat aprecisecontrolofthe growthprocessiscrucialinengineering[1].Dependingontheappliedprocessingparametersandonthematerialphysical parameters, a wide variety of solidified microstructures are observed in casting, in welding, and in other solidification processes. The most common structure is dendrite, which can be either equiaxed, when solidification occurs in nearly isothermalconditionslikeinthecaseofasnowflake,orcolumnarinthepresenceofatemperaturegradient.Dependingon theapplication,onetypeofgrainstructureispreferredandthusfavored,e.g.,equiaxedgrainsincarenginesandcolumnar grainsinturbineblades. Thesepatternscanbe describedby suitable lengthscales suchasdendriticprimary spacingand tipradius.Inpurediffusivetransportregime,characteristiclawshavebeenproposedtorelatethoseshapecharacteristicsto processingconditions.However,formostrealsolidificationprocesses,andespeciallyatlowgrowthrate,gravityeffectssuch asnaturalconvection,sedimentation/buoyancy,mechanicaleffectsandchangeinhydrostaticpressurecannot beneglected andsignificantly affectmorphological instability[2].The couplingbetweengravity effectsandsolidification hasbeen the subjectofagreatdealofexperimental,theoreticalandnumericalworkssincethebirthofmicrogravityexperimentationin theearlyeighties.Themainconclusionofallthesestudiesisthatgravityisthemajorsourceofvariousdisturbingeffects, which can significantlymodify ormask importantphysicalmechanisms onEarth(1g).Areview of some effectsinduced by gravityonthesolidification processandinvestigatedbymeanofsynchrotronX-rayradiographyattheESRF(European

*

Correspondingauthor.

E-mailaddress:henri.nguyen-thi@im2np.fr(H. Nguyen-Thi).

http://dx.doi.org/10.1016/j.crme.2016.10.007

1631-0721/©2016Académiedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Fig. 1. Insituobservationofgravityeffectsduringdirectionsolidification.(a)Deformationofthesolid–liquidinterfaceduetoaconvectionloop form-ingduringdirectionalsolidificationofAl–4wt.%Cu(coolingrate=0.3K/min,temperaturegradient=35.5 K/cm).(b)Sequenceofradiographsrecorded duringAl–10wt.%Cuequiaxedsolidificationshowingthesedimentationofequiaxedgrains.(c)Radiographsshowingthebendingandthefragmentationof secondaryarmsduringthedevelopmentofacolumnardendriteofAl–7wt.%Si(coolingrate=0.5K/min,temperaturegradient=15K/cm).

SynchrotronRadiationFacility)inthinaluminum-basedalloysispresentedelsewhere[2],namelyconvectioneffectsonthe solid–liquidinterface during theinitial transient ofsolidification[3](Fig. 1a), thesedimentationorfloatationofequiaxed grains(Fig. 1b),anditseffectonthecolumnar-to-equiaxedtransition[4,5],andfinallythecumulativemechanicalmoments inducedbygravityonthedendriteduringsolidification[6,7],leadingtothebendingofthesecondaryarmofthedendrite (Fig. 1c).

Numerousexperimentsinmicrogravity conditionshaveshownthat microgravity(μg)environmentis auniqueand ef-ficientwaytoeliminate buoyancyandconvectiontoprovide benchmarkdataforthe validationofmodelsandnumerical simulations. For these reasons, materials science and more particularly solidification of metal alloys has been a promi-nent topicofresearch inthe microgravityfield, since theearly stagesof microgravityexperimentation.In amicrogravity environment, transport phenomena are essentially diffusive and buoyancy forcesvanish, which highly simplifies the ex-periment’sanalysisandallowamoredirect andprecisecomparisonwiththeoreticalmodels [8].Letusmentionherethe outstandingexperimentIDGE (IsothermalDendritic GrowthExperiment),performedinlow-earthorbitby M.E.Glicksman andco-workers[9–13].TheIDGEwasdevelopedspecificallytotestdendriticgrowththeoriesbyperformingmeasurements with succinonitrile (SCN) and pivalicacid (PVA) under strictly diffusion-controlled conditions.The IDGE instrumentwas flown threetimes aboardthe spaceshuttle Columbia,aspart ofNASA’s missions,from1994to 1997[12].Thisseriesof experiments(hundredsofrepeatedexperimentsofsteady-statedendriticgrowth)providedthefirstsolidevidencethat den-driticgrowthisindeedgovernedbyheatdiffusionfromthesolid–liquidinterface.However, experimentsalsoshowedthat Ivantsov’ssolutiondescribingthediffusionprocessforaparaboloidcrystaltiprequiressomemodifications.Thetipvelocity andtipradiusdataweremeasuredasfunctionsoftheinitialundercoolingandGlicksman’sanalysisindicatedthat,atleast forpure SCN, the averagescaling factor

σ

∗

∼

0

.

02 isa value that isindependent ofboth undercooling andgravitational environment.Therobustness oftheinterfacial scalinglaw,namely V R2

₌

_{constant,whereV isthevelocityofthetipand}

R is thecurvatureradius atthetip, appearsto beindependent ofthegravity environment,witha slightdependenceon undercooling.

In addition, comparative studies of solidification experiments in 1g andμg can also enlighten the effects of gravity. Ademonstrative example ofsuch astudyon thistopicisthe work performedbyM.D. Dupouy,D. Camel andJ.-J.Favier inthe early 1990sby post-mortemanalysisof Al–Cusamplessolidified during theD1 – Spacelabmission [14–16]. Low-concentration(1wt%)hypo- andhyper-eutecticalloys(26wt%and40wt%)weredirectionallysolidifiedunderatemperature gradientof25 K/cm.Theinfluenceofgravityonthestructuraltransitions(dendritic/eutecticandcell-dendritetransitions), thedifferencebetweenthespaceandgroundsamplesprimaryspacingandthedendriticarraymorphologywerethoroughly analyzed,showingastrongimpactofthetransportmodeinthemelt.Inparticular,theprimaryarmspacingwasfoundfive timeslargerinspacethanonEarth[14–16].

The resultspresentedinthisarticle area selection ofmicrogravitysolidification experimentscarried out byour team onboardtwo platforms (American shuttle andsoundingrocket)on aluminum-basedalloys. Foreach experiment, we first brieflydescribethescientificobjectives,theapparatusandtheexperimentalprocedure.Thenwepresentthemostsignificant resultsobtainedintheframeworkoftheexperimentsconductedingravity-reducedconditions,andtheeffectsofgravity-free conditions willbe enlightened. We omittedin thisreview the recent experimentsperformed on theInternational Space Stationontransparentalloys[17,18]oronAl–Sialloys[19,20],whosedetailedanalysisisstillongoingandwoulddeservea longerpaper.

68 H. Nguyen-Thi et al. / C. R. Mecanique 345 (2017) 66–77

Fig. 2. (a)and(b):Longitudinalsectionsshowingthemacroscopicshapesofthequenchedsolid–liquidinterfaceofsamplesprocessedinmicrogravity(F2) andontheground(G2).(c)Schematicrepresentationofconvectiveflow(boundarylayermode).(d)Axialcompositionprofileforflight(opensymbols)and ground(fullsymbols)samples.

2. InfluenceofsolutalconvectiononthesolidificationofAl–3.5wt%Lialloy

Formostrealsolidificationprocesses,inparticularatlowgrowthrate,naturalconvectionanditsinfluenceonthegrain structure or the solid–liquid microstructure cannot be neglected [21–23]. On Earth, natural convection or fluid flow is inducedbybuilt-inconstitutionalandthermalgradientsintheliquidphase.Inpractice,solutalconvectiondominatesinthe caseofmetallicalloysowingtotheir largeLewisnumberdefinedastheratioofthethermaldiffusivitytothesolutalone (Le

=

Dth

/

D),typically oftheorderof 103 to104. Thecoupledconvective–morphological instabilitymaybe separatedin

twodistinctregions:withinthemushylayer(generallyconsideredasaporousmedium)andaboundarylayeraheadofthe mush/liquidinterface.Inthelattercase,thecharacteristiclengthscalemayvaryfromseveraldozentimesthesolutallength

D

/

V ,withV thesolidificationvelocity(convectioninabufferregion),uptothesamplesize(bulkconvection),depending on thealloys(partition coefficientk andrespective densitiesofliquid andsolid phases) andtheexperimental conditions (inparticulargrowthdirectionversusgravity).Thecomplexityofthetopicwasthemajorreasontocarryoutsolidification experiments inconvection-freeconditions,whenthermalandsolutal transportsare essentiallydiffusive [8]. Theobtained resultscanbethenusedasquantitativereference/benchmarkdatatovalidatetheoriesornumericalsimulations.

2.1. MicrogravityexperimentsduringtheD2mission

Forthis purpose,solidificationexperiments wereperformedon Al–3.5wt%Liinthe BridgmanapparatusGFQ(Gradient Furnace withQuenching)inboth normalandmicrogravity conditions.Forgroundexperiments,alloyswere solidified ver-tically upwardtoobtaina solutaldestabilizingconfiguration.Indeed,duringthegrowthprocess,therejectedsolute(Li) is lighter thanthesolvent(Al),whichyields thatboth interstitialfluid andsolute boundarylayerareunstablystratifiedand thatbuoyancy-drivenconvectioninbothzonesisunavoidable.Threemicrogravityexperiments(labeledF1,F2andF3)were performedonboardtheAmericanspaceshuttleColumbia duringtheGermanspacelabmissionD2in1993,witha tempera-turegradientcloseto70 K/cmandpullingratesof6.3,7.2and12.2 μm/s[24,25].Theseexperimentswerecomparedtotwo groundsamples(labeledG1andG2)carriedoutinthesameapparatusandfornearly thesameexperimentalparameters (G

=

70 K

/

cm and V

=

5

.

6 and9.3 μm/s).

2.2. Effectsofnaturalconvectiononmacroscopicscalecharacteristics

The quenchedsolid–liquid interfaceswere revealed bymetallographic analysisoflongitudinal section ofthesolidified samples.Asexpected,thesolid–liquidinterfacesforμg samplesarenearlyflat(Fig. 2a),whereastheyaremodulatedatthe macroscopicscaleforsamplesgrownverticallyupwardontheground(Fig. 2b).Themacroscopicmodulationofthegrowth front isthefootprintofthethermo-solutalconvective rollsasexplainedbyNguyen-Thietal.[26] forthe solidificationof Pb–30wt%Tl andrepresented inFig. 2c: belowan upwelling current, there isan excessof solute thus creatinga trough,

whereasbelowadownwellingcurrentbringinglessrichfluidthereisadepletionofsolute,thuscreatingacrest.Notethat exchangeofsolutebetweentheregionaheadofthefrontandthebulkliquidmustoccurtoaccountforthemeasuredaxial macrosegregation.

Indeed, the axial composition profiles obtained by chemical analysis of thin slices along the samples show a larger deviationfromtheidealdiffusivecaseforgroundsamplesthanforμg samples(Fig. 2d).Forμg samples,thecomposition profiles continuously decreased alongthe sample, probablydueto aprogressive lossoflithium during thecourse ofthe experiments that were performed under vacuum. For ground samples, sharp variations were found at the beginning of thesolidification(from3.5wt.%to2.7wt.%forG1 and2.9wt.%forG2)andafterquenching(upto3.3wt.%forboth ground samples),whichtestifiestotheoccurrenceofaxialmacrosegregation.Inaddition,ithasbeenpossibletoestimatethevalues oftheeffectivepartitioncoefficient keff

=

CS

/

C∞,where C∞ isthebulkliquidcomposition, assumedequalto thatofthe

quenchedliquid,andCSisthesolidcompositionmeasuredduringsteady-statesolidificationandassumedtobeequaltothe

compositionofthesolidjustbeforethequench.Thevaluesofkeff,deducedfromtheaxialmacrosegregationmeasurement

inFig. 2d,wereclosetounityformicrogravityexperiments,asexpectedinessentiallydiffusivetransportconditions,while theyareabout0

.

83

±

0

.

03 forG1and0

.

88

±

0

.

03 forG2,whicharebetweentheequilibriumpartitioncoefficientk

=

0

.

55 andunity[27].Thisconfirmedapartialmixingofthebulk liquidbyconvectionduringsolidification. Itcanbealsonoted that, forthistype of convection,despitethe curvature ofthefront, the radial segregationisnot strong, duetothe fluid flowalongthesolid–liquidinterface[25,26].Inconclusion,thesolid–liquidinterfaceismacroscopicallyplanar,withneither radialnoraxialmacrosegregationonlyduringthesolidificationperformedinmicrogravityconditions.

2.3. Primaryarmspacingmeasurements

For all experiments performed, a detailed analysisof several microscopic characteristics was conducted andreported inreference[25].Inthispaper,we focusona keyparameterofthecellular/dendritic arrays,namelythe averageprimary spacing

λ

,whichcanbe easily measuredoncross-sectionsofthe solidifiedsamples(Fig. 3a).Like inourprevious exper-iments onPb–30wt.%Tl alloys[28], we found that,during boththeir dynamical formationfrommorphological instability andtheir asymptoticstate,thecell/dendritearrayscouldbedescribed byahoneycombonwhichalargeamountof disor-der(displacementofcellcentersfromtheir idealpositions)issuperimposed.Thestatisticalanalysisofsuchtypeofimage hasbeendescribedindetailselsewhere[29].Intheseexperiments,averagevaluesandstandarddeviationswerecalculated formorethan250cells. Byacomparativestudyofmeasurements performedon groundandmicrogravitysamples,it has beenconfirmedthat,eveninnearlyidealconditions,thereisnoselectionofauniqueprimaryspacingandonlyanaverage value can be determined with a large standard deviation(Fig. 3b), typically about20–30%.The large standard deviation formicrogravitysamples,identicaltoground-basedones, alsoruledout thehypothesis thatconvectionisthemain cause ofthehighdisordergenerallyobservedincellular/dendriticpatterninexperimentscarriedouton Earth.The influenceof someadditionalextrinsiceffects,namelyeffectofcrucible,grainboundary,gasporesortheexperimentalprocedure,onthe dynamicsandselectionofcellulararrayswasalsostudied[30].Duringthesolidificationofmassivetransparentsamples,it hasbeenestablished thatthe mainsourceofdisorderin cellulararraysisthefront curvature,whichinduces acollective glidingofthewholecellulararraydowntheinterfaceslope,andtheeliminationofcoarsecellsatthecentralsink[31].The mainconclusionisthatthedisorderinacellular/dendriticpatternisintrinsictothedynamicsofthesolid–liquidinterface. Meanprimary spacingsofflightsamplesaregiveninFig. 3c,asafunctionofthegrowthrate.HuntandLu[32] consid-eredthat theminimumstablespacingofacellulararrayisthespacingforwhichthecompositionalong thecellgrooveis nearesttotheliquidusone.Fromnumericalcalculations,theseauthorsproposedthefollowingexpression:

λ

min

=

8

.

18 k−0.335



Γ

mLC∞

(

k

−

1

)



0.41



D V



0.59 (1)

where

Γ

is the Gibbs–Thomson coefficient andmL is the liquidus slope. This equation is plotted in Fig. 3c with

Γ

=

2

·

10−5 K

·

cm (typicalorderof magnitudeforAl-basedalloys), mL

= −

8

.

3 K/wt%Li [33], D

=

1

.

9

·

10−4 cm2

/

s, k

=

0

.

55,

and C_∞

=

3

.

32wt.%Li (mean value forflight samples,see Fig. 2d). The results ofFig. 3cshow that in μg conditionsthe experimentalspacingdecreasesforincreasinggrowthrates,asexpectedforcells,andisonly10–20%,i.e.lessthanwhatis predictedby themodelofHuntandLu,inparticularforthelowestpullingrate, whentheinfluenceofnaturalconvection wasexpectedtobemoreimportant.

3. Convectioncausedbyradialthermalandsolutalgradients

Intheprevious case(Al–3.5wt%Li), therejectedsolute waslighter,sothatthe liquidconfigurationwas solutally desta-bilizinginupwardsolidificationonground.Inordertoavoidnaturalconvectioninthemelt,itseemsreasonabletoensure thatthedensitygradientissolutallystable,i.e.verticaleverywhereandwiththeheaviestmaterialbeingatthebottom.This situationismoreorlessapproachedwhen growthtakesplaceupwardsinan alloy systeminwhichtherejected (respec-tivelyabsorbed)solute isdenser(respectivelylighter)thanthesolvent.ThisisthecaseforAl–1.5wt%Ni,forwhichalmost allthesolute(Ni) isrejectedduringsolidification(k

=

0

.

0087).Actually,eveninthisexperimentalconfiguration,flow pat-terns adjacentto thefront are knownto develop undertheeffect ofanyhorizontaldensitygradient, asrevealedby the

70 H. Nguyen-Thi et al. / C. R. Mecanique 345 (2017) 66–77

Fig. 3. (a)CrosssectionofsampleF2inthequenchedsolid–liquidzone.(b)HistogramoftheprimaryspacingforF2(about250cells)and(c)oftheprimary spacingasafunctionofthegrowthrateforsamplesprocessedinmicrogravityandontheground;comparisonwiththeHuntandLu’smodel.

macroscopicfrontdeformationandlateralmicrostructureheterogeneityobservedatlowsolidificationrates.Afirst qualita-tive evidenceoftheseeffectswas givenin[34],wherefrontshapeswerestudiedasafunctionofthesolidificationratein severalAl–Cualloyssolidifiedabovethemorphologicalinstabilitythreshold.Sincethen,similareffectshavebeenobserved belowthisthresholdboth intransparent[22] andsemiconductor systems.Inaddition,theinfluenceofconvectionon the dendriticmicrostructurewas demonstratedin[14] bycomparingthepatternsobtainedonEarthwiththoseformedunder identicalsolidificationconditionsinamicrogravityenvironment.Itwasinparticularfoundthatinsuchaconfigurationwith

Table 1

ExperimentalcontrolparametersoftheAGHFexperiments:FM(FlightModel)forexperimentsinmicrogravityconditionsandGM(GroundModel)for experimentsonEarth.

Sample μg Thermal gradient (K/cm) Pulling rate (μm/s) Sample 1g Thermal gradient (K/cm) Pulling rate (μm/s)

FM1-1 34 3.83 GM1-1 35 3.83

FM1-2 34 1.83 GM1-2 35 1.79

FM2 34 0.83 GM2 33 1.0

FM3-1 24 2.33 GM3-1 25 2.67

FM3-2 24 1.5 GM3-2 25 1.67

ahighly permeablemush,thefluid flowwasabletostronglyreducetheprimarydendritespacingintheregionswhereit waslocallyorientedtowardsthemush.

3.1. Microgravityexperiments

Directional solidification was carried out on Al–1.5wt%Ni alloysin both terrestrialand microgravity conditionsin the Advanced Gradient Heating Facility (AGHF) of the European SpaceAgency (ESA). Microgravity experiments (labeled FM) were conductedduringthe LMSandSTS-95spacemissions intheAmerican spaceshuttleColumbia (1996) andDiscovery

(1998), respectively. Forground experiments (labeled GM),samples were solidified vertically upwards in a thermal and solutalstabilizingconfigurationwithrespecttoconvectiondrivenbytheaxialgradients(Table 1).

3.2. Solutesegregationatthemacroscopicscale

The longitudinalsegregation profilewas determined by recordingthe grey-levelvariation ofthe radiographalong the sample axis,whichalsoallowedlocatingthepositionofthesolidificationfrontatthesuccessivestagesoftheexperiment (i.e.endofstabilization,velocity changeifany,andquench).The soluteconcentration profilewasthen calibratedboth by chemical analysisof thinslices (1 mm thick) cut along the sample, andby EDS(Energy-Dispersive Spectroscopy).These analyses were performedinthe unmeltedpart,the solidification zoneand thequenchedliquid.The relative influenceof convectionisderivedbycomparingtheresultsobtainedinmicrogravityandongroundsamples.

Forallsamples,therewasaplateauofalmostconstantconcentrationduringthesolidificationphase,equaltotheinitial concentration Al–1.5wt%Ni, measuredin theunmelted partofthesample (Fig. 4a). Beforethisplateau,a complexprofile wasdetectable,whichistheresultoftheTGZM(TemperatureGradientZoneMelting)phenomenoninvestigatedindetails in[33,35].Theconcentrationvariationmeasurednearthequenchedfront(Fig. 4b)indicatedthepresenceofanNi-enriched layer,which wasthe characteristicprofileresultingfromthesolute diffusionoccurringbothinthe liquidaheadthefront andintheinterdendriticorintercellularliquid[36].

RadialsegregationsweredeterminedinallsamplesbyEDSoncross-sectionsjustbelowthequenchedsolid–liquid inter-face.Formicrogravitysamples,theradialcomposition profileswere roughlyflat,asexpectedinsolidificationexperiments withessentially diffusivetransport conditions.Onthecontrary,large radial macrosegregationwas found forground sam-ples,despite thenegligiblelongitudinalmacrosegregation. Theexplanationofthiseffectisthat thesolute boundarylayer adjacent tothe tips isswept by the fluid flow driven by the residual radial temperaturegradient, dueto the difference inthermal conductivities betweenthesolid and liquidphasesandthe cruciblewall. Accordingly, a partofthe heat flux transportedalong thecrucible goesintothesolid, thus creatinga localinward heatflux that leads toa convexshapeof theisothermsandanarrowfrontdepressionatthecruciblewall. Thisdepressionisprogressivelyenhanced bythenickel accumulationontheperiphery,untiltheeutecticconcentrationisreached(Fig. 5).Astationarystateiseventuallyachieved becausetheradial-gradient-driven fluidflowisconfinedinabuffertransportedatthefrontvelocitywithnomixingwith thebulkofthemelt,whichisthemerereasonoftheabsenceoffluidfloweffectonlongitudinalsolutemacrosegregation.

3.3. Microstructurelocalization

To obtain the morphology at the quenched solidification front, a metallographic examination was first performedon a longitudinal section. Then, a slice containing the solid–liquid interface was cut and polished to obtain a sequence of closelyspaced transverse sectionsthrough the solid–liquid zone. Imageanalysistechniques [29] were applied on optical micrographsofselectedregions inordertodeterminethehistogramsofprimaryspacing.Fig. 6showstypicallongitudinal andcrosssectionsforsamplessolidifiedin1g (GM2)conditionsandinmicrogravity(FM2),respectively.Thesectionthrough the quenched front in Fig. 6a, flatted parallel to the growth, indicates microstructure localization, namely cell/dendrite clustering towards the center accompaniedby steepling,i.e. theenvelope ofthe tipsis strongly distortedand protrudes markedlyintothemeltaheadoftheeutecticfront[34].AsshowninFig. 6aandFig. 6c,athinliquidregionispresentat theperiphery down tothe eutectictemperature. Comparatively,nofront distortionis observedeven atlow solidification ratesin the microgravitysamples: thelongitudinal section (Fig. 6b) always showsa macroscopically planar shape ofthe front envelope, except for the steep but narrow depression at the contact with the crucible, which corresponds to the

72 H. Nguyen-Thi et al. / C. R. Mecanique 345 (2017) 66–77

Fig. 4. (a)ProfilesofNilongitudinalmacrosegregationinmicrogravity(FM2)andground(GM2)samples(chemicalanalysisofthinslicescutallalongthe samples),withthevariationintheTGZMregionsuperimposed(scanline).(b)VariationofNiconcentrationacrossthequenchedmushyzonededucedfrom theabsorptioncontrastonanX-rayradiograph,andprofilepredictedbymodellingin[36].

Fig. 5. Sketch of cell/dendrite localization due to radial solute segregation under fluid flow.

region whereheatflows fromthecrucibleintothesolid.Intransversesection(Fig. 6d), acellular/dendriticmicrostructure coverstheentiresurfaceandnoeutecticborderispresent.

Thecomparisonofgroundandmicrogravitysamplesdefinitelydemonstratedthatconvection,createdbyaninitial resid-ualtemperaturegradientandthenamplifiedbysoluterejection,wasthecauseofthestrongmacroscopicdistortionofthe solid–liquidinterface.Thegradualamplificationofconvectionduringtheinitialtransientwasanalyzedinsituandreal-time during thedirectionalsolidificationofAl–CualloysbyBognoetal.[3,37].Fig. 1aillustrates thiseffect,showingintwo di-mensionsthesteeplingandclusteringeffect.Thefinaldepthofthedeformationandresultingradialsegregationaremainly determined by the amount ofsolute that can be transportedlaterally. Moreover, it resultsfrom thegroundexperiments intheAGHFthatclustering andsteeplingeffectsofbuoyancy-drivenconvectionprogressivelydecreaseasthesolidification velocityisincreased,sothatdiffusive-likedendriticgrowthisachievedbeyondacriticalpullingrate[36].

Fig. 6. Comparisonofthemicrostructuresobtainedduringupwardsolidificationonground(aand c)andinmicrogravity(band d)for Al–1.5wt%Ni, G=24 K/cm andV=2.3 μm/s.(a)and(b):Longitudinalsectionsacrossthequenchedfront,(c)and(d)transversesectionsbelowthequenchedfront.

3.4. Solidificationmicrostructureinthediffusiveregime

Forthemicrogravitysamples,primaryspacingscanbemeasuredonthewholetransversesectionsforalargenumberof cells.Despitethediffusiveconditions,thestandarddeviationoftheprimaryspacingremainslarge,about10–20%.Thisresult issimilartothosebeforehanddescribedinAl–Lialloyssolidifiedinmicrogravityconditions[25].Thus,thepresent experi-mentsagainconfirmthat,astheoreticallyemphasizedbyBenAmarandMoussalam[38],thereisnoselectionofaunique wavelengthduring directional solidification. Yet,theaverage spacingcan bedetermined together withtheminimum and maximumspacings(FM3-1:

λ

min

=

190 μm,

λ

mean

=

378 μm,

λ

max

=

510 μm andFM3-2:

λ

min

=

150 μm,

λ

mean

=

394 μm,

λ

max

=

600 μm). The comparison withthe numerical predictions of Lu andHunt’s model reveals that the experimental

primaryspacingisabouthalfofthetheoreticalvalue.Uptonow,wehavenoexplanationforthislargediscrepancy. Inrelationwiththeclusteringandsteeplingphenomenacausedbyconvectionongroundasdiscussedbefore,theground samplesshow aradially non-uniformmicrostructure(Fig. 6a), cellularinthecoreofthesample andbecoming moreand moredendriticwhenapproaching theeutecticborderattheperiphery.Moreover,astrongtendencyforthesedendritesto growlaterallytowardsthiseutecticregionisobservedintheregionwherethelocalslopeofthefrontofdendritetipsishigh (Fig. 6c).Thislateralgrowthisconsistentwiththefactthattheliquidinthedepressedliquidregionisslightlyundercooled duetothehorizontalsolutegradient.Duetothisradialnon-uniformity,itisdifficulttoperformmeaningfulmeasurements ofprimaryspacing.Nevertheless,meanprimaryspacingwasestimatedfortheseexperimentsinthecentralcellularregion wherethe fluid flow U// is locallydirected towardsthe mush.Thus, theaverage primary spacingswere calculated fora limitednumberofcells (about 40–50)butthe standarddeviation

λ

of themeasurements was approximatelythesame asinthe microgravitysamples.Foreach velocity, the averageprimary spacingmeasured inground-based experimentsis lower than the one measured in space experiments, similarly to the solidification of Al–Li alloyswhere the convective configurationwas solutallydestabilizing[25].Inthepresentcase,thisreductionwas correlatedwiththeinwarddirection oftheflowusingtherelationshipderivedby[14]:

λ

=



λ

0 1

+

U//

V

(2)

where

λ

0istheprimaryspacingmeasuredinspaceunderdiffusiontransport,U// thevelocityoftheinwardfluidflowand

V thesolidificationvelocity.Theweakerreductionofprimaryspacingcomparedtofullydendriticsolidificationcanthenbe attributedtothelowerflowpenetrationinamushoflimitedpermeability,whichisreflectedbysmallvaluesoftherelative fluid-flowdownwardvelocityU//

/

V [36].

74 H. Nguyen-Thi et al. / C. R. Mecanique 345 (2017) 66–77

4. Insitucharacterizationduringmetalalloysolidificationonboardmicrogravityplatforms

As mostof the phenomena involved during solidification are dynamical, insitu andreal-time observation is manda-tory to analyzein details theexperiments. Up to now, insituobservation during microgravity solidification experiments were limitedtotheinvestigationsontransparentorganicalloys,usingopticalmethods[39].However,theirbehaviorisnot completely identicaltothatofmetallic systems,inparticularduetotheir low thermalconductivity.Recent developments ofmore powerfulmicrofocuslaboratoryX-raysources,aswell asmodern X-raydetectors,haveledtovast improvements in theperformanceofX-ray-based techniques.Accordingly,intheframe ofESA-MAPXRMON(in-situX-raymonitoringof advanced metallurgicalprocesses undermicrogravity andterrestrialconditions) project,a novelfacilitydedicated tothe studyof thedirectional solidification ofAl-based alloy withinsituX-rayradiography,hasbeen developed.Indeed,X-ray radiographyisparticularlywellsuitedforinsitustudiesandisnon-destructivetomostmetallicmaterials[40].Inthis tech-nique,thecontrastintherecordedimageisduetolocalchangesintheamplitudeoftheX-raybeamtransmittedthrough thesample.ThemainobjectiveofthisprojectistodemonstratethenewopportunitiesX-rayradiographyoffersinthefield ofmaterialsscienceinmicrogravity[41].

4.1. BriefdescriptionoftheMASER-12experiment

The experimentalfacilityconsistsofafurnace andan X-rayradiographydevicespecificallydevotedto thestudyof Al-basedalloys[42].ThegradientfurnaceisofBridgmantypeandspecialcareinthechoiceoffurnaceandcruciblematerials hasbeentakentoprovidegoodX-raytransmissionwithoutcompromisingthethermalproperties.Theexperimentsample is made ofAl–20wt.%Cu andis 5 mm inwidth, 50 mm in length and150 μm inthickness. The highamount ofsolute was chosen mainly to ensure a high contrast between the grown solid and the surrounding liquid phase. The imaging system comprisestwo parts: theX-raytube andthecamera. AmicrofocusX-raytube with3 μmfocalspotwas used in ordertomeetaspatial resolutionofroughly5 μm. TheX-raytubewas equippedwithathinlayerofmolybdenumasthe transmissiontargetonanaluminumsubstrate.

The firstsolidification experimentinmicrogravity usingthisfacilitywas successfullycarriedoutduring theMASER 12 sounding rocketmission on13February2012. Inaccordancewiththelimitedmicrogravity period,awell-adapted exper-imental timeline hadto be precisely defined. After a short stabilizationperiod in microgravity environment,the sample solidification was triggeredby applyingacoolingrate R1

=

0

.

15 K

/

s, tobothheaters, att

=

187 s. Att

=

400 s asecond

coolingrateR2

=

0

.

7 K

/

s wasappliedautomaticallyonbothheaters.Atlast,afewsecondsbeforetheendofthe

micrograv-ityperiod,thetwoheaterswereswitchedoffandthusthesamplenaturallycooleddownatR3

=

5 K

/

s,deducedfromthe

thermocouple measurements. Radiographswere successfullyrecorded duringthe wholeexperimentincludingthemelting andsolidificationphasesofthesample,withafieldofviewofabout5 mm

×

5 mm,aspatialresolutionofabout4 μmand afrequencyoftwoframespersecond.

The day after theflight, two ground-reference testswere carriedout ona fresh sample with thesame experimental profile, fortwo differentsample positions.Inthefirstreferencetest, named1g-upwardexperiment, thegrowthdirection was anti-parallel to the gravity vector. In the second reference test, named 1g-horizontal experiment, the sample was in horizontal position with the observation plane perpendicular to the gravity direction. A comparative study was then performed to enable direct comparisonbetween the experiments andtherefore to enlighten the effects of gravity upon microstructureformation.

4.2. Insituobservationofmicrogravityexperiments

Fig. 7displaysasequenceofimagesrecordedduringthesolidificationofanAl–20wt%Cualloyinmicrogravityconditions. Thissequenceofimagesshowsthetimeevolutionoftheinterface patternduringthesuccessivecoolingrates(0.15K/s

→

0.7 K/s

→

natural cooldown) andillustrates the great advantage of usingX-ray radiography for insitu investigationof directional solidification ofmetallic alloys. Inthe experiment, thesample was initially fully meltedin the Fieldof View (FoV), so thatnucleation ofthefirst solidsoccurredbelowtheFoV. Afterawhile,dendritetipsappeared intheFOV but only a disordereddendritic patterncould beobtained, dueto thenucleation ofseveralgrainswithdifferent orientations (Fig. 7a). Inaddition,onecanseeinFig. 7bthenucleationofanequiaxedgrainaheadofthecolumnarfront(blackcircle), most likely ona small heterogeneity of thesample oxidelayer. This grain slightly rotated,which clearly showedthat it was notstuckonthesample walls.However,duetothemicrogravityenvironmentandtheabsenceofbuoyancy,thisgrain remained at the samealtitude and was engulfed by the columnar front (Fig. 7c),and then completely merged into the columnar microstructure (Fig. 7d). At t

=

400 s,a faster cooling ratewas applied on both heaters(0.7 K/s)and a much finer microstructureachieved,whichinduced aweaker contrastintheradiographs,withamoreorienteddirectionforall dendrites (Fig. 7d). Very close to the end of the solidification, the eutecticfront was detected (dashed line in Fig. 7g), allowingustohaveanestimateofthelengthofthemushyzone,typically5 mm.Fig. 7hshowsthefinalgrainstructureat theendoftheexperiment.Thegrainstructureismainlycomposedofcolumnargrainsallalongthefieldofviewandafew elongated grainsfromnucleationonthewall,orfragmentsstuckinthethicknessforupwardsolidification experiments.It isworthnotingthat,basedonlyonthelastfigure(Fig. 7h),itwouldnotbepossibletoknowiftherewas grainnucleation ahead ofthe columnarfront ornot. Thisremark showsclearly thegreat benefitof usingin situcharacterizationfor the

Fig. 7. ColumnarsolidificationofAl–20wt%Cuinmicrogravityconditions,withatemperaturegradientofabout150 K/cmbetweenthetwoheatersandthe threedifferentcoolingrates.Coolingrateof0.15 K/sonbothheaters,(a)t=317 s,(b)t=360 s,(c)t=382 s.Changeofcoolingrateto0.7◦C/sonboth heaters,(d)t=418 s.Naturalcoolingafterswitchingoffthepoweronbothheaters,(e)t=422 s,(f)t=426 s,(g)t=433 s afterlift-off.(h)Finalgrain structuredeterminedbydrawingmanuallytheboundariesofthedifferentdendriticgrains.

studyoftime-dependentphenomena.Forthesakeofcompleteness,itisalsoworthmentioningthatadarklayerjustahead ofthesolidificationfrontisvisible,whichisduetothesolute(copper)rejectionduringtheliquid-to-solidtransformation.

4.3. Influenceofconvectiononthesolidificationvelocity

Based onthe radiographs, it ispossible to preciselymeasure the instantaneous growthrateasa function oftime for each experiment(Fig. 8). Ineach case, the three successivegrowthrates are clearly visibleandan average value can be deduced.The sourceoftheuncertainties forallthe experimentscomesmainly fromthe limitedspatial resolutionduring themeasurementofthedendritetippositions.Theground-basedexperimentswerecarriedoutaftertheflightwithanew sample.Accordingtothemeasurements,itwas foundthatthevaluesofthegrowthratesforthemicrogravityexperiment (Fig. 8a)are closerto thevalues ofthe 1g-upwardsolidification (Fig. 8b) than tothe valuesof the1g-horizontal solidi-fication (Fig. 8c).The same trendwas observed during the breadboardtestscarried out before theflight, withdendrites growingfasterwiththesampleinhorizontalposition(20.5 μm/s)thaninverticalposition(18.6 μm/s).

The remarkablesamevalues obtainedformicrogravityand 1g-upwardexperiments canbe explained by thefact that naturalconvectioninthelattercaseisdramaticallyminimizedforseveralreasons.Firstly,solidificationintheupwardcase isperformedinbothstablethermalandsolutalconditions(hotzoneplacedabovethecoldzoneandsolutedenserthanthe solvent),whichminimizetheoccurrenceofboundarylayerconvectionsimilartoAl–Libeforehanddescribed,inparticular forhigh axial temperaturegradient andhighsolidification velocities like inthis experiment.Secondly,the highvalue of growthratespreventstheconvectiveflowinducedbyresidualtransversetemperaturegradientinthisconfigurationsimilar toAl–Ni[36]andfinallythehighconfinementinthissample(200 μminthickness)stronglydampedthefluidflow,ifany.

Thediscrepancybetweenthegrowthratesofthoseexperimentsandthe1g-horizontalexperimentisslight, but signif-icant. Actually, forthe sample solidification in 1g-horizontal position,the convective roll induced by thermalconvection and/or therejectionof heavy solutewithin the thicknessofthesample maybe strongenough tomodify thethermalor thesolutalfields,thenincreasingthegrowthrates.Indeed,inthecaseofhorizontalBridgmangrowth,thermalconvection occursassoonasthereisatemperaturegradientalongthesample(i.e.nothreshold).Fortunately, ifconvectionisnottoo strong andby neglecting solute concentration, it ispossible to estimate the magnitudeofconvection effectby an Order ofMagnitudeAnalysis(OMA) [43].Forthermalconvection,J.-J.Favierdefineda thermalconvecto-diffusiveparameter



th

astheratioofthelengthofthesolute boundarylayerinconvectiveregime tothelengthofthesolute boundarylayerin thepure diffusive regime.



th canbe expressedasa function ofthePécletnumber P e

=

eV

/

D, oftheGrashoff number

Gr

= β

TgGe4

/

υ

2 andtheSchmidtnumberSc

=

υ

/

D:



th

=

4

.

5P e

(

Gr Sc

)

−1/3 (3)

Usinge

=

150 μm,D

=

3

·

10−9m2

/

s,

ν

=

4

·

10−7m2

/

s,

β

T

=

1

.

17

·

10−4K−1,g

=

9

.

8 m

/

s2,G

=

15 K

/

mm,V1

=

18

.

6 μm

/

s,

and V2

=

69 μm

/

s, it is found that



th is always larger than unity, which means that thermalconvection is negligible

76 H. Nguyen-Thi et al. / C. R. Mecanique 345 (2017) 66–77

Fig. 8. Evolutionofthesolidificationfrontvelocityasafunctionoftimeduring(a)themicrogravityexperiment,(b)the1g-upwardreferencetestand (c) the1g-horizontalreferencetest.

Ontheotherhand,thesameOMAcanbedone forsolutalconvection[44].Inthatcasetheexpressionofthe convecto-diffusiveparameterbecomes:



sol

=

1

.

4P e4/5

(

1

−

k

)

−1/5

(

GrSSc

)

−1/5 (4)

withGrS thesolutalGrashoffnumber.Using

β

S

=

0

.

73

/

wt.%,C0

=

20wt.%,k

=

0

.

14,itwasfoundthat inourexperiments,

forbothgrowthrates,



solisalwayslowerthanunity,whichmeansthatthesolutalconvectionisnotnegligibleandcould

interactwiththecolumnarfrontandthusmodifythegrowthrates.

5. Conclusions

Itisnowobviousthattheearlierenthusiasticdreamsregardingthepotentialofspacemanufacturingofinnovative mate-rialshavegivenwaytomorerealisticexpectations.However,microgravityexperimentationstilloffersauniqueandefficient wayforin-depthanalysisofthepatternformationduringdirectionalsolidification,inthelimitofdiffusivetransport. Specif-ically, themodels ofsolidificationornumericalsimulations (inparticularthe phase-fieldapproach) needbenchmark data to be validated, firstlyin model situation,withdiffusive transport conditions andthen withconvecto-diffusive transport to be able to describe and explain experimental resultsobtained in laboratories. It is obvious that the investigations of theinfluenceofgravity,andmoreparticularlynaturalconvection,onsolidificationhavetobefurtherdevelopedtoachieve quantitativedescriptionofphenomena.

In thispaper, aselection of ourmicrogravity experimentswas described. We left aside the recentoutstanding exper-iments performed onthe International SpaceStation ontransparent alloys [17,18] oron Al–Si alloys[19,20].Our intent was to show the slow butsignificant progressin our understanding of the influence of convectionon the solidification microstructure. Thesestudiesenlightenedkey issueson thetopicbutalsoreportedopen questions,whichgaveus direc-tions for future works.From the experimental point of view, the recent development of in situ characterization during microgravitysolidificationexperimentsopensnewdoorsandwillcertainlygiveanewimpetustoresearchinspace.

Acknowledgements

ThisstudywaspartlysupportedbytheXRMONproject(AO-2004-046;Grantno:20288/06/NL/VJ)oftheMAPprogramof theEuropeanSpaceAgency(ESA)andbytheFrenchNationalSpaceAgency(CNES;conventionno150515/00).Theauthors express their gratitude to Dr.Olivier Minster andPr. BernardZappoli for their continuous support.The authorswish to acknowledgealltheinternationalpartnerswhowereinvolvedintheseprojects:thesescientificresultscouldnothavebeen achievedwithouttheircollaboration.

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