A Transforming Metal Nanocomposite with Large Elastic Strain(3)

REPORTS

melting(fig.S1).Macroscopicwiresoftheinsitucomposite(nanowireinsitucompositewithSMA,hereinafterreferredtoasNICSMA)withdiametersof0.3to1.0mmweresubse-quentlyfabricatedbyforging,wire-drawing,andannealing(Fig.1B)(19).Thetypicalmicro-structureofNICSMA(Fig.1,CtoE)consistsofNbnanowiresformedinsituwithameandiameterof60nm,welldispersedandwellalignedintheNiTimatrixalongthewireaxialdirection,withwell-bondedinterfaces.Theselected-areaelectrondiffraction(SAED)pattern(Fig.1F)isindexedtobody-centeredcubicNbandB2-NiTiphases.Thephasecomponentsofthecompositewerefur-thercharacterizedbyhigh-energyx-raydiffrac-tion(HE-XRD)(fig.S2)andenergy-dispersivex-rayspectroscopicanalysis(fig.S3).BothSAEDandHE-XRDdemonstratethattheNbnanowiresarewellorientedwithits[110]directionparalleltothewireaxialdirection.Figure1,GandH,showsthemorphologiesoffreestandingNbnanowiresobtainedbyremovingtheNiTimatrixviaelec-trolyticetching(fig.S4),revealingthattheNbnanowireshavelengthsrangingfrom1to100mmandameanaspectratioexceeding100.

InsitusynchrotronHE-XRD(fig.S5)wascarriedoutonNICSMAatroomtemperature.TheevolutionofthediffractionpeaksforB2-NiTi(211)andB19′-NiTi(001)(fig.S6)indicatesthattheNiTimatrixunderwentanelasticdefor-mationfollowedbySIMTduringtensileload-ing.Figure2Ashowstheevolutionofd-spacingstrainwithrespecttotheappliedmacroscopicstrainfortheNb(220)planeperpendiculartotheloadingdirection,illustratingthattheNbnanowiresexhibitedatensileelasticstrainof4.2%whenembeddedintheSIMTmatrix.ThiselasticstrainlimitoftheNbnanowiresiscom-parabletothatoffreestandingnanowires(3–9).Furthermore,theelasticstrainlimitsofthenano-wiresembeddedintheSIMTmatrixincreasegraduallywithdecreasingnanowirediameter.ThemaximumelasticstrainlimitoftheNbnano-wiresobserved(fig.S7)was6.5%(theredcurveinFig.2A).Incontrast,wefoundthat,whenevertheNiTimatrixdeformedbydislocationslipin-steadofbySIMTaftertheinitialelasticdefor-mation(fig.S8),theelasticstrainlimitsoftheNbnanowiresaregreatlyreducedto~1.3%(theblackcurveinFig.2A).Figure2Bshowsacom-parisonoftheelasticstrainlimitsof(a)Nbnano-wiresinthematrixdeformingbydislocationslip(13,14,20–22),(b)NbnanowiresinthematrixdeformingbySIMT,and(c)somefreestandingnanowires(3–9).

Afterpretreatmentwithatensilestraincy-cleof9.5%,thebulkNICSMAexhibitedalargequasi-linearelasticstrainofover6%,alowYoung’smodulusof~28GPa,andahighyieldstrengthofminimum1.65GPawithinthetem-peraturerangeof15°to50°C(Fig.3,AandB).IncomparisonwithotherknownbulkmetalswithlowYoung’smoduli—forexample,Mg,Al,andTialloysandgummetals(1,2,23,24)—theyieldstrengthofNICSMAissuperior.Figure3,Cand

D,showsgeneralcomparisonsoftheelasticstrainlimit,Young’smodulus,andyieldstrengthofNICSMAandothermetals(1,2,23–25)andhu-manbones(23).NICSMAoccupiesauniquespot

onachartofthemechanicalpropertiesofvariousbulkmaterials(fig.S9)andpossessesgoodcyto-compatibility(figs.S10andS11)andcorrosionre-sistanceinaphysiologicalenvironment(fig.S12).

Fig.1.(A)SchematicofthedesignconceptofNICSMA.Schematictensilestress-straincurvesofahigh-strengthmetallicnanowire(I),anSMA(II),andaNICSMA(III).(B)AcoilofNICSMAwirewithadiameterof0.5mm.(C)Transmissionelectronmicroscopy(TEM)imageofalongitudinalsectionofNICSMAwire.NWindicatesnanowire.(D)ScanningTEMimageofthecrosssectionofNICSMAwire(brightregions,crosssectionsofNbnanowires;darkregions,NiTimatrix).(E)High-resolutionTEMimageoftheinterfacebetweentheNbnanowireandtheNiTimatrix.(F)SAEDpatternfromalongitudinalsectionofNICSMAwire.(G)MacroscopicappearanceofabundleoffreestandingNbnanowires.(H)ScanningelectronmicroscopyimageofthefreestandingNbnanowires.

AB

Fig.2.Elasticstrainlimitsofnanowires.(A)Evolutionofthed-spacingstrainwithrespecttotheappliedmacroscopicstrainfortheNb(220)planeperpendiculartotheloadingdirectionintheNICSMAwiresinwhichtheNiTimatrixdeformedbySIMTanddislocationslip.Thegreencurve(withamaximumstrainof4.2%)correspondstothesampleshowninFig.1with60-nm-diameterNbnanowires.Theredcurve(withamaximumstrainof6.5%)correspondstoadifferentsample(fig.S7),withevennarrowerNbnanowires.(B)Comparisonoftheelasticstrainlimitsof(a)Nbnanowiresembeddedinthematrixdeformingbydislocationslip(13,14,20–22),(b)NbnanowiresembeddedinthematrixdeformingbySIMT,and(c)somefreestandingnanowires(3–9

).

VOL339

SCIENCE

11928MARCH2013