REPORTS

InsitusynchrotronHE-XRDwasusedtocharacterizethedeformationandphasetransfor-mationevolutionsoftheNbnanowiresandtheNiTimatrix,duringthepretreatment(Fig.4A,inset)andthesubsequenttensilecycle(Fig.4C,inset).Afterthepretreatment,theNbnanowiressustainedanelasticcompressivestrainof–1.4%(pointD),whereastheNiTimatrixsustainedanelastictensilestrainof1%(pointE)(Fig.4A).ThereisalsosomeretainedB19′phaseinthematrix(Fig.4B).Theseresultscanbeunderstoodasfollows.Uponremovalofthepretreatmentload,theplasticallydeformedNbnanowires(AtoBinFig.4A)hinderedtherecoveryoftheNiTimatrixbecauseoftheB19′→B2transfor-mation(15,16),whichcausedlargeresidualstrainsinthenanowiresandtheSMAwithsomeretainedB19′phase.Thisdemonstratesthatstrongcouplingbetweenthenanowiresandthematrixtookplaceduringthepretreatment.Inthesubse-quenttensilecycle(Fig.4C),theelasticstrainachievedintheNbnanowireswasupto5.6%(AtoB),consistingofthepreexistingelasticcompressivestrainof–1.4%(OtoB)andanelastictensilestrainof4.2%(OtoA).TheNiTimatrixwentthroughcontinuousSIMTthrough-outthetensileloadingandexhibitedanultralowtangentialeffectivemodulus(Fig.4,DandE)ratherthanundergoinganinitialelasticdefor-mationfollowedbyanabruptSIMTtransition,aswouldoccurinamonolithicSMA(16).ThecontinuousSIMTcanbeascribedtothecontri-butionofthepreexistinginternaltensilestressandtheretainedB19′phaseinthematrix.Uponunloading,theNiTimatrixunderwentareversetransformationfromthestress-inducedmartens-itetotheparentphase(Fig.4,DandE),in-troducingasmallhysteresisinthestress-straincurveresultingfromenergydissipationduringtheprocess.TheexperimentalevidencepresentedabovedemonstratesthattheNbnanowiresex-periencedanultrawideelasticstrainof4.2%–(–1.4%)=5.6%,whichcloselymatchesthephasetransformationstrainof~7%ofNiTi.ThismatchingofelasticandtransformationstrainsresultsintheextraordinarypropertiesofNICSMA.

ReferencesandNotes

1.M.F.Ashby,MaterialsSelectioninMechanicalDesign(Butterworth-Heinemann,Burlington,VT,2005).2.T.Saitoetal.,Science300,464(2003).

3.E.W.Wong,P.E.Sheehan,C.M.Lieber,Science277,1971(1997).

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6.G.Richteretal.,NanoLett.9,3048(2009).7.L.Tianetal.,NatCommun.3,609(2012).8.K.Kozioletal.,Science318,1892(2007).

9.D.A.Waltersetal.,Appl.Phys.Lett.74,3803(1999).10.Y.Dzenis,Science319,419(2008).

11.P.Podsiadloetal.,Science318,80(2007).

12.J.N.Coleman,U.Khan,Y.K.Gun'ko,Adv.Mater.18,

689(2006).

13.L.Thillyetal.,ActaMater.57,3157(2009).14.V.Vidaletal.,Scr.Mater.60,171(2009).

15.K.Otsuka,C.M.Wayman,Eds.,ShapeMemoryMaterials

(CambridgeUniv.Press,Cambridge,1998).

16.K.Otsuka,X.Ren,Prog.Mater.Sci.50,511(2005).17.S.Ogata,J.Li,S.Yip,Science298,807(2002).

18.M.Piao,S.Miyazaki,K.Otsuka,Mater.Trans.Jpn.Inst.

Met.33,337(1992).

19.SeesupplementarymaterialsonScienceOnline.20.C.C.Ayd ner,D.W.Brown,N.A.Mara,J.Almer,

A.Misra,Appl.Phys.Lett.94,031906(2009).

21.L.Thillyetal.,Appl.Phys.Lett.88,191906(2006).22.C.Scheuerlein,U.Stuhr,L.Thilly,Appl.Phys.Lett.91,

042503(2007).

23.M.Niinomi,M.Nakai,Int.J.Biomater.2011,1(2011).24.M.Niinomi,Metall.Mater.Trans.APhys.Metall.Mater.

Sci.33,477(2002).

25.D.C.Hofmannetal.,Nature451,1085(2008).Acknowledgments:WethankG.H.Wu,H.B.Xu,andY.F.ZhengforvaluablediscussionsonthedeformationmechanismofNICSMA.ThisworkissupportedbythekeyprogramprojectofNationalNaturalScienceFoundationofChina(NSFC)(51231008),theNational973programsofChina(2012CB619400and2009CB623700),andthe

NSFC(51071175,51001119,50831001,and10825419).J.L.alsoacknowledgessupportbyNSFDMR-1008104andDMR-1120901.X.D.H.acknowledgessupportbytheBeijingHigh-levelTalents(PHR20100503),theBeijingPXM201101420409000053,andBeijing211project.

D.E.B.acknowledgessupportbytheInstituteforNanoScience,Engineering,andTechnology(INSET)ofNorthernIllinois

eoftheAdvancedPhotonSourcewassupportedbytheU.S.DepartmentofEnergy,OfficeofScience,undercontractno.DE-AC02-06CH11357.

SupplementaryMaterials

/cgi/content/full/339/6124/1191/DC1MaterialsandMethodsFigs.S1toS12References(26–30)

9August2012;accepted10January201310.1126/science.1228602

TerrestrialAccretionUnderOxidizingConditions

JulienSiebert,1*JamesBadro,2DanieleAntonangeli,1FrederickJ.Ryerson2,3

Theabundanceofsiderophileelementsinthemantlepreservesthesignatureofcoreformation.

Onthebasisofpartitioningexperimentsathighpressure(35to74gigapascals)andhightemperature(3100to4400kelvin),wedemonstratethatdepletionsofslightlysiderophileelements(vanadiumandchromium),aswellasmoderatelysiderophileelements(nickelandcobalt),canbeproducedbycoreformationundermoreoxidizingconditionsthanpreviouslyproposed.Enhancedsolubilityofoxygeninthemetalperturbsthemetal-silicatepartitioningofvanadiumandchromium,precludingextrapolationofpreviousresults.WeproposethatEarthaccretedfrommaterialsasoxidizedasordinaryorcarbonaceouschondrites.Transferofoxygenfromthemantletothecoreprovidesamechanismtoreducetheinitialmagmaoceanredoxstatetothatofthepresent-daymantle,reconcilingtheobservedmantle

vanadiumandchromiumconcentrationswithgeophysicalconstraintsonlightelementsinthecore.hedepletionofsiderophile(i.e.,“iron-loving”)elementsinEarth’smantlerelativetochondritescanconstraintheredoxstateofaccretingmaterialsduringterrestrialaccretionandcoredifferentiation(1–4).Forexample,metal-InstitutdeMinéralogieetdePhysiquedesMilieuxCondensés,UniversitéPierreetMarieCurie,UMRCNRS7590,InstitutdePhysiqueduGlobedeParis,75005Paris,France.2InstitutdePhysiqueduGlobedeParis,UniversitéParisDiderot,75005Paris,France.3LawrenceLivermoreNationalLaboratory,Livermore,CA94551,USA.

*Towhomcorrespondenceshouldbeaddressed.E-mail:julien.siebert@impmc.upmc.fr

1

T

silicatepartitioningexperimentsatatmospher-icpressureindicatethattheobserveddepletionofslightlysiderophileelements(SSEs)suchasVandCrcanonlybeproducedatconditionsmorereducingthanthoserequiredtoaccountfortheabundanceofmoderatelysiderophileelements(suchasNi,Co,andW)orhighlysiderophileele-ments(5).Usingmetal-silicatepartitioncoefficientsobtainedatpressuresupto25GPa,homogeneousaccretionmodelspositthatmetal-silicateequilib-riumtookplaceatthebaseofadeepterrestrialmagmaoceanatasingleoxygenfugacity(fO2)(6–8).However,thepressure-temperature(P-T)

VOL339

SCIENCE

conditionsrequiredtoproducetheobservedde-pletionsforVandCr(atthepresent-dayfO2)requiretemperaturesthatgreatlyexceedthatofthemantleliquidus(2,4,9,10).Suchcondi-tionsarephysicallyinconsistentwiththemagmaoceanhypothesis,wheretheP-Tconditionsatthebaseofamagmaoceannecessarilyliebe-tweenthemantlesolidusandliquidus,therebycreatingarheologicalboundarythatenablesthemetaltopondandequilibratewiththesilicatemelt.TosatisfythisrheologicalconstraintandSSEabundancepatterns,recentmodelsofcorefor-mationconstrainmetal-silicateequilibrationtotheP-TconditionsoftheperidotiteliquidusandinvokeearlyaccretionofhighlyreducedmaterialswithaFeO-poorsilicatecomponent(2,4,10,11).TheseinitiallylowfO2conditions(~IW-4,cor-respondingto4logfO2unitsbelowtheiron-wüstitebuffer)enhancethesiderophilecharacteroftheSSEsattherelevantP-Tconditions.Subsequent,gradualoxidationofthemantleto~IW-2overthecourseofcoreformationisre-quiredtoaccountformoderatelysiderophileele-mentabundancesand,mostimportant,toreachthecurrentmantleFeOcontent[8weightpercent(wt%)FeOinsilicate].Underreducingcon-ditions,siliconislikelytobetheonlylightele-mententeringthecoreinlargeamounts(10–12).ThisscenarioreliesonextensivepressureandtemperatureextrapolationofSSEpartitioningdata,asexistingresultsarerestrictedtorela-