About the epitaxial growth of Mg-subgrains on Al2MgC2 interf

碳纤维增强金属基复合材料

JOURNALOFMATERIALSSCIENCELETTERS18(1999)1671–1674

AbouttheepitaxialgrowthofMg-subgrainsonAl2MgC2interfacialcarbidesinasqueezecastMg-4Al/T300metalmatrixcomposite

C.CAYRON,P.A.BUFFAT

InterdepartmentalCentreofElectronMicroscopy,EcolePolytechniquedeFed´erale´deLausanne,1015Lausanne,SwitzerlandE-mail:cyril.cayron@ep .ch

C.HAUSMANN,O.BEFFORT

SwissFederalInstituteforMaterialsTestingandResearch,3602Thun,Switzerland

Carbon bers,suchasT300 bersforinstance,ex-[7–9],butitwasshownrecentlybyElectronEnergyhibitverygoodmechanicalpropertiessuchastensileLossSpectroscopy(EELS)andHighResolutionElec-strengthandhighelasticmodulus[1]buttheiruseastronMicroscopy(HREM)thatinfactthesephasesarereinforcementtoaluminumbasedalloysisimpairedbyAlastronginterfacialreactionbetweenAlandCleading2MgCvery2carbides,theAlclose[10,11]4C(the3andAlcrystal2MgCstructures2structuresbeingofthetotheformationofAlcarbidesaregivenintheAppendix).Inthepresentmatrix4C3carbidesandthusembrit-tlementofthemetalcomposites(MMCs)[2,3].studytheformationoftheAlMagnesiumwhichfeaturesaverylowdensity,excellent2MgC2phaseiscon- rmedbyTransmissionElectronMicroscopy(TEM)castability[4]andisnotreactivewiththecarbon bersand,moreover,itisshownthattheseternarycarbidesattemperaturesbelow700 CinMMCprocessingcon-aresurroundedbysmallMggrainsnucleatedandgrownditions[5],appearstobeagoodcandidatetoreplaceontheirinterfaceinanepitaxialorientationrelation-aluminum.However,thetransversemechanicalproper-ship.

tiesofunidirectionallyreinforcedMg/CcompositesareThecompositematerialconstitutedofamagne-verypoorbecauseofweakinterfacialbondingstrengthsiumalloycontainingAl4wt%unidirectionallyre-attributedtoatotalabsenceofreactionbetweenMginforcedbyT300carbon berswasproducedasplatesandC.Agoodcompromiseisobtainedbyusinga(100×80×10mm3)usingtheDirectSqueezeCast-MgmatrixcontainingafewpercentofAltocontrolingprocess.Theprocessingparametersusedwereatheextensionoftheinterfacialreaction[6–8].Forin-melttemperatureof820 C,preheatingoftheC- berstance,itwasshownthattheadditionof1wt%ofAltopreformsat750 Cundernitrogenatmosphere,apre-aunidirectionallyreinforcedMg/T300compositeledheatingtemperatureforthediecavityandsqueezeramtoanincreaseoftheUTSby50%to1500MPa[8].of300 Cand nalpressurizationat130MPafor60s.Therefore,theunderstandingandcontroloftheinter-TheTEMinvestigationswereperformedonaPhilipsfacialreactioniscrucialforafurtherdevelopmentofCM20T(LaB6)operatingat200kVandequippedthiskindofcomposite.ThereactionproductinMg-withanenergy-dispersivespectrometer(EDS)NoranAl/T300compositeswasbelievedtobeAl4C3

carbidesVoyagerforstandardlesschemicalanalysis,andon

(a)(b)

Figure1(a)Bright eldimageonthecarbon ber/Mgmatrixinterface.Noticetheplate-shapecarbide(edge-on)onleftsideofthediffractingareamarkedbythearrow.(b)Themicrodiffractionpattern(spotsize100nm)onthediffractingareacorrespondingtotheMgstructurein[110]zoneaxis.

0261–8028

C1999KluwerAcademicPublishers1671

碳纤维增强金属基复合材料

(a)(b)

Figure2(a)ADSTEMimage(spotsize1.5nm)oftheinterfaceshowingtheAlpresenceintheMgmatrixand(b)correspondingEDSspectraofthe

plates.

(a)

(b)

(c)(d)

Figure3(a)Bright eldimageontwoAl2MgC2carbidesandonaMgsubgrainbetweenthemincoherencewiththeuppercarbide.Corresponding

microdiffractions(spotsize10nm)of(b)thisAl2MgC2phasein[11

¯0]zoneaxisand(c)theMgsubgrainwiththespotsofthecarbideinthebackground,bothin[11¯0]zoneaxis.(d)DiffractionsimulationofthetwocoherentAl2MgC2andMgstructures(emptycirclesrepresentMgspots

inextinctionconditionduetothestructurefactor).

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碳纤维增强金属基复合材料

(a)

(b)

(c)

Figure4(a)Dark eldimageofaAl2MgC2carbideincoherencewithaMggrain.TheMoir´efringesrevealthesmallmis tassociatedtothe(110)

planes.(b)Correspondingselectedareadiffractionpatternin[11¯1]Mg=[22¯1]Al2MgC2zoneaxesand(c)diffractionsimulation.

aHitachiHF2000-FEGforAnnularDarkScanning

carbidesisalsorevealedtobeaMgsubgraininanori-ElectronMicroscopy(ADTEM).TEMfoilspecimensentationrelationshipwiththeuppercarbide(Fig.3c).werepreparedbymechanicaldimplingdownto20µmTheelectronbeamwassituatedonthesubgrainnearwithoutanyliquid,followedbyargonionmillingonatheuppercarbidesothatitsspotsarealsovisibleintheGatanDuo-Millmachine,operatingatanacceleratingbackground.ThesimulationisgiveninFig.3d.Thevoltageof5kVand10 angle,withaliquidnitrogenorientationrelationshipbetweentheMgsubgrainandcoolingstage.

theAlAttheinterfacebetweenthe bersandthematrix,2MgC2carbideisdirectlydeduced:

sharpedge-onplate-shapedzones(300nm×20nm)[001]Mg//[001]Al2MgC2

and

arevisible(Fig.1a).Surprisingly,theelectronmi-crodiffractionsonthesezoneswithaspotsizeof(110)Mg//(110)Al2MgC2

100nmarenotinagreementwiththeAlThisorientationrelationship,correspondingtodirectturebutwiththeMgstructure(Fig.1b).2MgCNevertheless,2struc-hexagonallatticebasevectorparallelismisexplainedADSTEMimages(Fig.2a),whicharesensitivetothebythesmallmis tassociatedtotheMgandAlatomicnumberandthustothechemicalnatureofel-(100)planes(respectively0.278and0.295nm:2MgC5.7%).2ements,andEDSspectra(Fig.2b),revealthepres-Themis tassociatedtothe(001)Mgand(002)enceofAlinthesezones.Infact,asmallspotsizeAl(10nm)isrequiredinthemicrodiffractionforabetter2MgC2planes(respectively0.521nmand0.615nm)ishigher(15.3%).Thisorientationrelationshipiscon-understanding.Indeed,microdiffractionsontwoedge- rmedonalltheotherobservedMggrains.Inthisway,ondiffractingplates(Fig.3a)revealtheirAltheMoir´efringes(D=3.57nm)visibleonthedarkstructure(themicrodiffractionoftheuppercarbide2MgCis2 eldimageofFig.4acanbeexplainedbyitscorre-showninFig.3b).Thediffractingzonebetweenthese

spondingselectedareadiffractionpatterninFig.4b

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碳纤维增强金属基复合材料

showingtheproximityofthe(110)Mgand(110)Al2MgC2spots:

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