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UCID-20622-93-1

Chemistry&MaterialsScience

ProgressReport

Weapons-SupportingResearchand

Labo r ato r yDirectedResearch&Development F ir stHalf,FY1993

July1993

i saninformalintendedexternal dis tri bution.Theopinionsandconclusionsstatedarethoseoftheauthorsand mayormaynotbethoseoftheLaboratory. Wo r kperformedundertheauspicesoftheU.S.DepartmentofEnergybythe Law r enceLivermoreNationalLaboratoryunderContractW-7405-Eng-48.

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C&MSProgressReportrSecondHalfFY92Contents

Contents

Foreword............................................................................................................................................................iv

Weapons-SupportingResearch

ThrustAreas...................................................................................................................................................]

Growth,Structure,andReactivityofSurfacesandThinFilms(L.L.Chase)......................................2

AtomisticApproachtotheInteractionofSurfaceswiththeEnvironment(C.A.ColmenaresandT.H.Gouder).............................................................................................3

Molecula

rBeam-SurfaceReactions ( A.V.Hamza,M.Moalem,M.Balooch,andW.J.Siekhaus)......................................................4 ScanningTunnelingMicroscopyofNucleationandGrowthofthe_Phaseof

UraniumonGraphite

(

R.J.Tench,M.Balooch,andW.J.Siekhaus)................................................................................7

Su r face-PhysicsResearch (P

.Bedrossian).....................................................................................................................................8

QuantumElectronTransportthroughUltrathinFilms

(

L.J.TerminelloandE.Tamura).....................................................................................................11

UraniumResearch(L.R.Newkirk)..............................................................................................................12

M ec hanicalandMicrostructuralPropertiesofU-6Nb (

G.Gallegos,P.Johnson,andA.Schwartz)....................................................................................13

P

haseDiagramsofUranium/RefractoryMetals(F.Y.L.Gdnin)....................................................................................................................................14

FundamentalsofthePhysicsandProcessingofMetals(W.H.Gourdin)...........................................16

SoluteSegregationBehaviorinNi3A1-BasedOrderedAlloys(W.H.Gourdin,P.E.Johnson,N.Kioussis,andA.Gonis).........................................................17

StabilityofArtificialIntermetallicSuperlattices:AnExperimentalStudyofTi/XMultilayers (

A.F.Jankowski).................................................................................................................................19

StoichiometricInterlayerBondingofIntermetaliics

(

M.J.StrumandG.A.Henshall).....................................................................................................20

EnergeticMaterials(R.L.Simpson)............................................................................................................22

Expe ri mentalandTheoreticalStudiesofEnergy-TransferDynamicsinEnergeticMaterials (

A.J.RuggieroandL.E.Fried).........................................................................................................22

Chemis

t ryofHigh-PressureReactions (

M.F.Foltz).........................................................................................................................................24

Condensed-

P haseThermalDecompositionofEnergeticMaterialsInvestigatedby AtomicForceMicroscopy(AFM)andSimultaneousThermogravimetricModulated

BeamMassSpectrometry(STMBMS)

(

T.A.Land,R.Behrens,Jr.,andW.Siekhaus)...............................................................................25

SynthesisofNewInsensitiveMaterials

(

C.L.Coon).........................................................................................................................................27

I...31S"[RfBUTtONOFTHISDOCUMENTISUNLIMtTED:'_"-,

,ContentsC&MSProgressReportrSecondHalfFY92

Groups....................................................................................................................................................................29

Interfac

_eScienceandMetal/CeramicInterfacesPreparedUsingUltrahigh-Vacuum DiffusionBonding(W.E.King,G.H.Campbell,A.W.Coombs,G.W.Johnson,B.E.Kelly,

T.C..Reitz,S.L.Stoner,andD.M.Wilson).........................................................................................30

AdvancedSynchrotronRadiationStudyofMaterials(J.Wong,P.A.Waide,andJ.w.Elmer).....34 Wel d ing-InducedSolid-StatePhaseTransformations:ProblemDefinition (

J.W.Elmer,J.Wong,P.A.Waide,andE.M.Larson).................................................................35

FirstEXAFSSpectrawithaYB66Monochromator

( J.Wong,T.Tanal_,Z.Rek,M.Rowen,G.George,I.Pickering,G.Via,andG.Brown,Jr.)..36

Theory,Modeling,andComputation(A.Gonis)......................................................................................39

Elect r onicStructureofMetallicAlloys (

P.P.SinghandA.Gonis).................................................................................................................39

DevelopmentofNewThermoelectricMaterialswithExceptionalFiguresofMerit(TheoryPart) (P

.E.A.Turchi)..................................................................................................................................41

Electronic-St

r uctureCalculations (

J.vanEk,P.A.Sterne,andA.Gonis)............................................................................................43

Real-Space,Multiple-ScatteringTheory

(

E.SowaandA.Gonis).....................................................................................................................45

IndividualProjects....................................................................................................................................47

PropertiesofCarbonFibers

(R.M.Christensen)..................................................................................................................................48

Th ermoelec tr icMaterialswithExceptionalFiguzesofMerit (J.C.Farmer,G.Chapline,M.Dresselhaus,N.Eisner,R.Foreman,L.Hicks,D.Makowiecki, D.O'Brien,M.Olsen,R.Otto,P.Turchi,R.VanKonynenburg,andJ.Yee)................................51

LaboratoryDirected

ResearchandDevelopment

Director'sInitiatives...............................................................................................................................53

Aerogel-BasedElectronicDevices(R.W.Pekala,S.T.Mayer,J.L.Kaschmitter,R.L.Morrison,L.W.Hrubesh,

R.J.Contolini,andA.F.Bernhardt)......................................................................................................54

Molecul

ar -LevelStudiesofEnergeticMaterials

(P.C.Souers)..............................................................................................................................................57

IndividualProjects....................................................................................................................................59

N an o s caleLithographyInducedChemicallyorPhysicallybyModifiedScannedProbeMicroscopy

(M.Balooch,W.J.Siekhaus,andA.V.Hamza).................................................................................60

i i

C&MSProgressReportrSecondHalfFY.92Contents

N anoscaleMagnetics

(J.G.Tobin,G.D.Waddill,A.F.Jankowski,andS.Y.Tong)..........................................................63

InorganicandOrganicAerogels

(L.W.I-Irubesh,T.M.Tillotson,F.M.Kong,andR.W.Pekala)......................................................66

StructuralTransformationandPrecursorPhenomenainAdvancedMaterials ( P

.E.A.Turchi,L.T.Reinhard,andS.C.Moss)................................................................................68

Ductile-

P haseTougheningofRefractory-MetalIntermetallics

(G.A.HenshallandM.J.Strum)..........................................................................................................71

FundamentalStudiesofParticle-SolidInteractions

(T.DiazdelaRubiaandM.W.Guinan).............................................................................................74

Electronic-StructureEvolution_MetalClusters

(V.V.Kresin,M.J.Fluss,R.H.Howell,andW.D.Knight)............................................................77

OsteoporosisResearch

(J.H.Kinney,N.Lane,andG.A.Henshall).......................................................................................79

Solid-StateAmorphizationataCrystallineInterface

(A.F.Jankowski)......................................................................................................................................83

HydrogenChemisorptiononDiamondSurfaces

(R.S.DaleyandR.G.Musket)..............................................................................................................85

iGlennT.SeaborgInstituteforTransactiniumScence............................................87

Overview

(D.C.Hoffman)........................................................................................................................................88

PlutoniumSolubilityandSpeciationinCarbonateSolution

(M.Neu,D.Hoffman,R.Silva,K.Roberts,andH.Nitsche)...........................................................89

BacterialRemediationofActinide-ContaminatedSites

(A.HappelandH.Nitsche)....................................................................................................................90

Cation-CationComplexesofActinides

(N.Hannink,D.Hoffman,R.Silva,andR.Russo)............................................................................91

Two-PhotonSpectroscopyoftheActinideIonsCm3+andAm2+

(J.SytsmaandN.Edelstein)....................................................................................................................93

FullyRelativisticSurfaceGreenFunctionandItsApplications:

TheoreticalStudyonUraniumonPt(111)Surface

(

E.TamuraandA.Gonis)........................................................................................................................95

iii ForewordC&MSProgressReport,FirstHalf,FY93IIIIJ3IIIIIIi!liiiiiii

Foreword

Th eresearchreportedhereinsummaryformwasconductedduringthefirsthalfof FY93undertheauspicesofWeapons-SupportingResearch(WSR)andLaboratory Dire c tedResearchandDevelopment(LDRD). WSRistheprincipalsourceofdiscretionaryfundstosupportfundamentalresearchin theChemistryandMaterialsScience(C&MS)Department.WSRprovidesthescientific andtechnologicalbaserequiredinthelongertermforthesuccessoftheWeaponsPro- gram.Thisyear,themajorchlmgesinourWSRprogramweretoconverttheplutonium thrusttoauraniumthrust,tos.o.paratethetheorycomponentsofeachofourWSRpro- grams,andtorecombinetheseeffortsintoanewtheorythrust.Inthisway,thetheory groupcandevelopmoreeffectivelythestrategiesandcapabilitiesneededtoaddressthe overallWSRprogramneedsasrequirementsandindividualprojectschange. Administratively,workfundedbyWSRisorganizedintothreecategories: •Block-fundedprograms("thrustareas"),eachofwhichtypicallyinvolvesseveral seniorscientistsinacoordinated,focusedapproachtoascientificortechnological problem. •Researchgroupsconsistingoftwoorthreescientists. •Afewsmallerprojectsledbyindividualinvestigators. LDRD- f lm d e dwork,whichbroadenstheexploratoryresearchbaseofC&MS,consists ofseveralcategoriesofdiscretionaryresearchactivities.Ofthese,ExploratoryResearch intheDirectorates(ERD)andDirector'sInitiativesareincludedinthisreport.The formerincorporatesExploratoryF:esearchintheDepartments(Departmental), ExploratoryResearchintheInstitutes,andExploratoryResearchinthePrograms (f o rme r lySR). Theresultsreportedhereareforworkinprogress;assuch,theymaybepreliminary,

fragmentary,orincomplete.Beforequotingorotherwisereferringtoanyreportherein,readersshouldconsultoneofitsauthorstoobtaincurrentinformation.

J.Wadsworth

iv

Weapons-SupportingResearchThrustAreas--iIii_---

Weapons-SupportingResearch

ThrustAreas

SurfacesandThinFilmsWSRNThrustAreasIIiIiIIlLinilli|iiill Growth,Structure,andReactivityofSurfacesandThinFilms

L.L.Chase,ThrustAreaLeader

Overview

Th eobjectivesofthisnewthrustareaaretoinvestigatethegrowth,structure,and chemi c alactivityofsurfacesandthinfilms,emphasizingactinideelementsandtheir alloys.ThesepropertiesareimportantforDefenseProgramtechnologiesbecausethey arerelevanttoenvironmentalstability,containment,compatibility,firesafety,and

methodsofwillfuldestructionofnucJearweapons.Therearetwosynergisticthemestothisresearch:(1)interactionofsurfacesandthinfilmswiththeenvironmentand(2)

growth,structure,andelectronicpropertiesofthinfilmsandmodifiedsurfacelayers. PreviouslythetopicofaWSRgroupeffort,theinvestigationofelectronicstructureand surfacereactivityofthinfilmsofuraniumonvarioussubstratescontinues.Uranium overlayersweredepositedonpalladium,andtheirmodeofgrowth,theirelectronic stru c ture,thereact_.vityofuraniumwithgasessuchas02andCO,thebulkdiffusionof uranium ,andcompoundsofuraniumproducedbyadsorptionofgaseswere investigated.Ourmainobjectivesaretoevaluatetheusefulnessofthinlayersin studying(1)thestabilityofactinide-containmentmetalinterfacessubjecttoheatand c orrosivegasesand(2)thepossibleuseofactinidesasreactioncentersforcatalysis. Inordertocomplementtheseinvestigationsofsurfacereactionsinambientconditions, amolecularbeamreactivescatteringchamber(whichisnearingcompletion)willbe usedtoobtainkineticinformationregardingthin-filmformationandsurfacereactions.

Nucleation

,growth,andstructureofuraniumdepositedonthebasalplaneofhighly orientedpyrolyticgraphite(HOPG)wereinvestigated.Clusternucleationand "ripening"behavioroccurat<800K,andachangeinstructuretoanapparent_phase ofuraniumoccursat>900K.Thisdemonstratesthatsingle-crystalfilmsof_uranium c anbeformedonHOPG.Thismayprovetobeausefulsubstrateforstudyingthe reactivityandcorrosionofuraniumfilmsbecausedifficultiesininterpretationcaused bydiffusionofreactantsintothebulkareavoidedwiththin-filmsubstrates. Anewsurface-physicsfacilitywascompletedthatincorporatesinsitusample preparation ,electrondiffraction,andatomic-resolutionimagingwithscanning tunnelingmicroscopy.Astudyofdefectcreationonthe(100)surfaceofsiliconbylow- energyxenonirradiationwasinitiated,withthegoalofinvestigatingtheroleof interlayerhoppingonsurfaceandmultilayerannealing. Oneofthemostimportantchallengesforthin-filmtechnologyisthedevelopmentof ultrasmallmicroelectronicdevices.Itisimportanttounderstandhowelectrons propagatethroughsuchsmallstructures.Novelexperimentsinvolvingtheboroncore- levelphotoelectronemissionfromdelta-dopedsilicon:(111):boronareinprogressto investigatethisbehavior. WfiR---ThrustAreasSurfacesandThinFilmsiiiiii.i||iiiii,11iiIIIiiuiiiiiiIIiiiiiiiii AtomisticApproachtotheInteractionofSurfaceswiththeEnvironment

C.A.ColmenaresandT.H.Gouder

Wecontinuedourstudyoftheelectronicstructureandsurfacereactivityofthinlayersof uraniumonvarioussubstrates.Weinvestigateduranium,_verlayersonpalladiumatverylow coverages(belowmonolayerrange),asopposedtoourpreviousconcentrationonmultilayer syst ems .Wecharacterizedthemodeofgrowth,theelectronicstructure,theuraniumreactivity wit hgasmolecules(COand02),andthebulkdiffusionofuraniumanduraniumcompounds formedbygasadsorption.Wehadtwomainobjectives:(1)toinvestigatetheusefulnessofthin layersinstabilizingactinide-containmentmetalinterfacesunderheatandcorrosion,witha possibleapplicationtothelong-termstorageofnuclearwaste,and(2)tostudytheinteractionof disperseduraniumsurfaceatomswithlow-reactivitygasmolecules(CO),withpossible applicat iontotheuseofactinidesaspromotersandlocalreactioncentersincatalysis.Wealso resum edthestudyoftheuranium-carbonsystem. IntroductionpreviouslychemisorbedCO,whereasabovethis concent r ation,COdissociates.Atlowuranium Theinteractionof"surfaces"withthecoverage,COdissociationthusbecomesan "envi r onment "maybestudiedbymeasuringtheactivatedprocess.

effectofadatoms(clusterstofilms)ontheatomicThispassivationofsurfaceuraniumatomsisandelectronicstructuresofsurfacesandontheirattributedtothesolid-stateinteractionbetween

r eactivityandhow,inturn,asurfacemaychangeuraniumandthesubstrateandprovidesthe t hepropertiesoftheadatomsthemselves.Acti-necessaryconditionforinvestigatingactinidesas hideelementsareparticularlyinterestingascatalyticallyactivesubstances.Ontheotherhand, adato m sorsubstratesbecausetheyhave5felec-02adsorptionalwaysleadstotheoxidationof t ronsattheFermi-levelthatmaybedelocalizedsurfaceuranium,atallcoverages,whichshows (aswithuranium)andthusbeavailableforreac-thaturaniumstillretainspartofitsreactivity. tionormaybeclosetolocalization(aswithplu-Wedevelopedanewtechniqueforinvesti- tonium)andnotbereadilyaccessible.Further,gatingchemicalreactionsandbulkdiffusion t hese5felectronsmaybeeasilyprobedbyXPS,ofuraniumoverlayers.Thetechniqueistime- U P S,andsynchrotronradiation,andthuselec-resolved,thusallowingustostudyseveralcon- tr o.r,.icchangesinducedbysubstratesoradatomssecutiveorparalleldiffusionprocesseswhenthey maybemeasuredconveniently,differinactivationenergy.Thiswillbeveryuse- fulforinvestigatingheterogeneoussystemssuch , _soverlayersofmetalsandreactionproductson

TheUranium-PalladiumSystemsubstrates.

WefirstappliedthetechniquetotheU-Pd

Atverylowdosages,uraniumdepositsonsystem.Bulkdiffusionanddissolutionof thetopsurfaceofpalladium.Itneitheragglom-uraniummetalwereshowntoproceedinseveral e r a t esinclustersnordiffusesintothesubstrate,steps.First,reactiveinterdiffusionresultsinthe Alluraniumatomsarelocatedinasimilarchemi-formationofUPd3below200°Candstopsonce calenvironment,andthe5felectronsseemtobeallsurfaceuraniumhasreacted.Thedrivingforce localized.Inthisdispersedphase,theuraniumforthisreactionisprovidedbytheheatof atomslosetheirinitiallyhighchemicalreactivity,formationoftheintermetallic.Amoredetailed asshownbyCOexposurestudies.Belowacriti-studyactuallyallowstheidentificationoftwo calconcentrationofabout0.5monolayer(ML),low-temperaturereactionswithverysimilar heatingofthesampleleadstothedesorptionofactivationenergies,whichwetentatively

SurfacesandThinFilmsWSR--ThrustAreasIiiiiiIIIIii

attributetothereactionofoveflayeraz.dislandReference uraniumwithpalladium.Furtherstudiesare plannedtoconfirmthis.1.R.J.Tench,W.J.Siekhaus,andM.Balooch, Above600°C,theintermetallicdecomposes"AtomicStudyofNucleation,Epitaxy,and anduraniumdiffusesintothebulk.WealsoFilmGrowth,"ChemistryandMateriaLsScience studi edthediffusionofuraniumoverlayersthatResearchReport,LawrenceLivermoreNational weexposedto02.OxygenadsorptionresultsinLaboratory,Livermore,CA,UCID-20622-91-1 theoxidationofallsurfaceuraniumtoU02,and(1991),p.50. thediffusionstudyshowsneitheralloyformation no rdecompositionreactionsbutonlyonediffu- sionprocessathightemperature,whichcoincidesPublication withtheoxidedecomposition.T.GouderandC.A.Colmenares,"ASurface

SpectroscopyStudyofThinLayersofUonPoly-

TheUranium-CarbonSystemcrystallinePalladium,"submittedtoSurf.Sci.

Wealsoresumedourstudyofuranium

ove r layersongraphite.ThissubstratewaschosenPresentations becauseitshowsextremelylowinteractionwith ove r

layersandthereforeshouldallowinvestiga-1.T.GouderandC.A.Colmenares,"StudyoftionoftheelectronicstructureofsmallclustersThinLayersofActinides.PresentStatusand

andoverlayersofuraniumwithoutinfluenceofFutureUseofSynchrotronRadiation,"ALSthesubstrate.Workshop:SynchrotronRadiationinTransac-tiniumResearch,LawrenceBerkeleyLabora-Againweconcelttratedoncoveragesbelow1ML.Wefoundclearindicationfortory,Oct.1992.

5flocalizationbyXIX3,andtheuranium4f2.J.G.Tobin,G.D.Waddill,T.Gouder,andemissionsshowedanintensesatelliteat6eVC.A.Colmenares,"TheSearchforX-Ray

andanincreaseinbindingenergy.Atallcover-AbsorptionCircularDichroisminActinide ages,uraniumshowsahighreactivity.Adsorp-Materials:PreliminaryExperimentsUsing tionof10Langmuirs(L)of02resultsintheUFe2andUS,"ALSWorkshop:Synchrotron completeoxidationofallsurfaceuraniumforRadiationinTransactiniumResearch, coveragesaround1ML.LawrenceBerkeleyLaboratory,Oct.1992. C _ . J

rresultsdonotagreewiththosefroma3.T.Gouder,C.A.Colmenares,J.Tobin,andG.D.Waddill,"SynchrotronRadiationStudypreviousSTMstudyofuraniumoverlayerson

g r aphite,1inwhichitwasclaimedthaturaniumoftheSurfaceReactionsofPolycrystallineU staysmetallicunderatmosphericpressureforandUFe2with02,COandH2S,"SSRL19th

10minutes(equivalentto1011Lof02).WhenAnnualUsers'Meeting,Stanford,CA,Oct.

oxidationfinallyoccurs,theauthorsrefertoit1992. as"tipenhanced"surfaceoxidationofuranium. The r emustbeanotherexplanationfortheirSeminar observation,butbecauseofthelackofspectro-

scopicdatawithwhichtoidentifythecomposi-T.GouderandC.Colmenares,"StudyofNeartionofthesurface,itisdifficulttospeculateonSurfaceandBulkDiffusionofUraniumin

t hi sbehavior.PalladiumbyAugerSpectroscopy,"Seminar hostedbyProf.D.Olander,NuclearEngineering Depa r tmen t ,UniversityofCalifornia,Berkeley,CA,Feb.1993. WSR--ThrustAreasSurfacesandThinFilmsIIIIIIIIIIIIIIIIIIIiIIiIIIiI

MolecularBeam-SurfaceReactions

A.V.Hamza,M.Moalem,*M.Balooch,andW.J.Siekhaus

Wehavecompletedtheinitialphaseofconstructionofmolecularbeam-reactivescattering apparat us . TechnicalActivityto<2x10-7Torrbya5000l/sdi_tusionpump.Acollimatingorifice1mmindiameterseparates Th esystemconsistsofthreedifferentiallythesourceandtargetchambers.Thetarget pumpedvacuumchambers(seeFig.1).Thechamberispumpedto<6x10-10Torrbya sourcechamber,whichhousesthereactantgascryopump. sourceandthemodulationassembly,ispumped Ga s_,./inletII"__" /---_---",__,

Miniflangessealed-'--

withgoldloll/_"_;t_'_""source]25 /\ter/ /\'!1-mm pyro m ter

Source

t ube /Chopper/ assembly[)//Detector chamber/

Target&heater

Targe tOuadrupolemass chamberspectrometer(QMS)

Augerelectron

analyzer

In-llneQMS

Figure1.Topview,schematicofthemodulatedmolecularbeamapparatus. *PostdoctoralResearchAssociate,UniversityofCalifornia,Berkeley SurfacesandThinFilmsWSR---ThrustAreasIi!iIiiiiliiiilIIiiliiiiiiiIi

Samplesareplacedonabuttonheater,and2.Constructingandinstallingthesupersonicgasthesurfacetemperatureismonitoredbyaninfra-source.

r edpyrometer.Themolecularbeamisincident3.AddingAugerelectronspectroscopy(AES) onthesampleat45°tothesurfacenormal.Theopticsandanin-line-of-sightmassspectrome_ detec t o rsamplesreflectedordesorbedmoleculester.(BothoftheseitemsareshowninFig.1 atthespecularreflectionangle,-45°.Anextra-butarenotyetacquired.Inanysurface nuclea rquadrupolemassspectrometer(detectionexperiment,aminimumrequirementisto upto1400amu)ishousedinthedetectorcham-characterizethesurfacecomposition.AESis ber

,whichispumpedbyanionpumptoaparticularlyusefulinthisrespect.Thein-linepressure<5x10-l°Tort,andviewstheentiremassspectrometerwillallownecessarysamplearea.Lock-inamplificationtechniquesarecharacterizationoftheincidentbeam.)usedtoobtainthemagnitudeandphaseofthe

fi r

stFouriercomponentofthescatteredsignal.4.Installingabellowsassembly(showninFig.1butnotyetacquired)tovarythedistanceof

Theamplitudeofthescatteredsignalisathedetectorfromthesampletocharacterizemeasureofthestrengthofthereaction,andthe

phaselagisameasureoftheresidencetimeoftheenergyofthescatteredmoleculesanda themoleculeonthetarget,aftercorrectionforrotatingdetectortomeasureangulardistri- transittoandfromthesample,butionsofthescatteredmolecules.

Constructionisbeingaccomplishedinfour

phases:Publication

1.TestingbyscatteringC60fromaconvenient

ta r get.Thisphaseiscompleted.M.Moalem,M.Balooch,A.V.Hamza,W.J.

Siekhaus,andD.R.Olander,"SurfaceMobilityof

C60onSiO2,"J.Chem.Phys.(inpress).

WSR---ThrustAreasSurfacesandThinFilms_IIIIIIIIIIIII,IIIIIIIIIIIIIIIIIIIlllIIllJ iScanningTunnelingMicroscopyofNucleationandGrowthofthe [3PhaseofUraniumonGraphite

R.J.Tench,M.Balooch,andW.J.Siekhaus

Wehavestudiedthenucleationandgrowthofuraniumonthebasalplaneofhighlyoriented pyrolyt icgraphitedepositedbylaserablation(excimerwith308-nmwavelength,20-nsduration, and4×108W/cm2powerdensity)byscanningtunnelingmicroscopy(STM)underultrahigh-vacuumconditions.

Results

Laser-ablateduraniumformedclustersof

varyingsizeonthegraphitesurface.Bymeansof Auge rd_._Tor_spectroscopy(AES)andx-ray photoelectronspectroscopy(XlX3),theuranium depositwasfoundtobecontamination-freeandshowedthatnocarbidehadformedwiththe grap h ite.Surfacediffusionofuraniumwas observ edforcoveragesoflessthanamonolayer asaresultofannealingthesubstrateabove800K, asevidencedbythedecreasednumberdensity andtheincreasedsizeoftheclustersofsamples.Novelformationofordereduraniumthin filmswasobservedforcoveragesgreaterthan twomonolayersafterannealingabove900K.The structureofthesefilms(seeFig.1)displayed u ni formstepheightsofabout5.5A,withsome samplesshowingasmanyasthreestepsinthe uraniumdeposit.Thisvalueof5.5._isverycloseFigure1.Surfacestructureofuraniumfilmon tothecolatticeconstant(5.656./_)ofthe[_phasegraphite.Imageis80./_squaxe. ofuraniumcrystals.

STMimagesshowedfacetededgestothe

plateletswithincludedanglesof120°,whichPresentationagainindicatesexistenceofthe[3phaseof u r anium.R.J.Tench,W.J.Siekhaus,andM.Balooch, Inaddition,itwaspossibletoachievevery"NucleationandGrowthofUraniumandNickel closetoatomicresolutiononthetopofthisonGraphiteStudiedbyScanningTunneling platelet(seeFig.1).Theimagesho_vsanopenMicroscopy,"182ndMeetingoftheElectro- hexagonalstructure,withhexagondiametersofchemicalSociety,Oct.1992. approximately5A.Thisagreesverywellwith the5-A-diameterhexagonsforeithertheBorC laye r softhe]3phaseofuranium.

SurfacesandThinFilmsWSRmThrustAreasIiiiIIiii

Surface-PhysicsResearch

P.Bedrossian

Anewsurface-physicsfacilitythatincorporatesinsitusamplepreparation,electrondiffraction, andatomic-resolutionimagingwithscanningtunnelingmicroscopy(STM)wascommissioned inOctober,1992.AstudyofdefectcrealiononSi(lO0)bylow-energy(=200-eV)xenonirradia- t ionwasinitiated,withthegoalofidentifyingatransitiontemperaturefortheonsetofmulti- lay erannealingviainterlayerhoppingofindividualsiliconatoms. TechnicalActivitiesmobileadatomsarecrucialtodevelopingmodels ofion-beamenhancementofepitaxialgrowth. WepreviouslyfoundthationirradiationinFigure1isanSTMimageofSi(100)that thelow-energyregimeresultsindamagethatisresolvestheindividualsilicondimerrowsofthe confinedpredominantlytotheoutermostatomic2x1reconstructionofthatsurface.Irradiation layerofthesubstrateandthatsurfaceevolutionby200-eVxenonat488°C(Fig.2a)and436°C undersuchsputteringisdominatedbytheinter-(Fig.2b)atafluencecorrespondingtoremoval actionofindividualsurfacevacanciescreatedbyofonethirdofamonolayer(ML)resultsinthe thesputteringprocess.Inordertodistinguish,toformationofmonolayer-deepvacancyislands. theextentpossibleinpostmortemSTManalysis,Diminishedmobilityofsurfacevacanciescreated theeffectsofdefectcreationanddefectannealing,bysputteringatthelowertemperatureresultsin singularSi(100)surfaces(misorientation<1/25°)ahigherdensityofsmallervacancyislands.That we r

eused.Processesmediatedbythecreationthedepressionsareexactlyoneatomiclayerdeepandinteractionofsurfacevacanciescouldthere-isverifiedbyFig.2c,adetailofFig.2binwhich

thesilicondimerrowsinsidethedepressionsareforebedistinguishedfromstep-mediatedpro- cesses.Whereasatomicstepsmightbeexpectedperpendiculartothoseoftheoutermostlayer.A toplayasignificantroleintheannealingofion-qualitativechangeinsurfacemorphologyoccurs

induceddefects,alowstepdensitywouldmini-below370°C.TheappearanceofadatomislandsinFig.3,acquiredfollowing200-eVxenonsput-mizetheproportionofdefectsactuallycreatedat

steps,teringofSi(100)at362°C,offersthefirstdirect Adatomgenerationunderhigher-energyexperimentalverificationofadatomcreation sputte ri ng[5-keVargononCu(100)]hasbeenduringionirradiationofasemiconductorsur- p r edict

edfrommolecular-dynamicssimulations.1face.Inhibitionofinterlayerhoppingofsiliconadatomsbelow370°CleadstothenucleationofSimulationshavealsopredictedadatomgenera-

tionbylower-energyions(<20-eVargon)adatomislandsduringsputtering.This,inturn,setsalowerboundonthetemperaturerangeinthroughindividualatomicdisplacementson

bo threconstructedGe(100)andSi(100).2Atsuffi-which200-eVxenonsputteringofSi(100)results cientlyhighsubstratetemperatures,adatomsintwo-layerremoval. gene r at edduringionbombardmentcouldeitherFurtherworkwillexamine inco r po r ateatstepedgesorhopbetweenlevels,*Theeffectofincidentionenergyandmasson annihila ti ngamobilevacancyorfillingavacancythecreationratesforspecificdefects. island.Atsufficientlylowsubstratetempera-*Theidentificationofadisplacementthreshold tu r es ,mobileadatomsthatnolongerpossessforsurfaceatomsbysputtering. suffi ci entkineticenergyeithertoovercome*Thekineticsofsiliconadatomscreatedby po te ntialbarrierstointerlayerhoppingortosputteringonsurfacesroughenedby reachstepedgesarelefttonucleateadatomsputtering.

islands.Experimentalverificationofadatom*Theinteractionofdefectscreatedbylow-creationduringlow-energyirradiationofsiliconenergyionirradiationwithadsorbateatoms

andanunderstandingofthekineticsofthosesuchasgermanium. WSR--ThrustAreasSurfacesandThinFilms.....IllIIIIIIIImallIIMlilllIil Figure1.A700-/_STMimageofatomicallycleanSi(100),showingindividualsilicondimerrows andtwosingleatomicsteps.ThegrayscaleiskeyedtoverticaldisplacementoftheSTMtip. Figure2a.A4200-_,STMimageofsingularFigure2b.A4200-._STMimageofsingular Si(100)followingremovalof=1/3MLby200-eVSi(100)followingremovalof=1/3MLby200-eV Xeionsat488°C.DepressionsareIMLdeep.xenonionsat436°C. SurfacesandThinFilmsWSR--ThrustAreasliiiIIiililiiill Figure2c.A700-_detailofFig.2b,showingindividualsilicondimer_ows.

Figure3.A2000-_STMimageofsingularSi(100)

following_emovalof=1/3MLby200-eVxenonionsat 362
_

C,showingbothmonolayer-deepadatomislands

andmonolayer-highvacancyislands.

ReferencesPresentations

1.D.E.Harrison,Jr.,P.W.Kelly,B.J.Garrison,1.P.Bedrossian,"Low-EnergyIonIrradiationof

andN.Winograd,"Low-EnergyIonImp_::_:SiliconSurfaces,"PhysicsDepartmentCollo- P henomenaonSingleCrystalSurfaces,"Surf.quium,ArizonaStateUniversity,Tempe,AZ, Sc/ .76,311(1978).Feb.1993.

2.M.V.R.MurtyandH.A.Atwater,"Defect2.P.Bedrossian,"AdatomGenerationinIon

Gene r ationandMorphologyof(001)SurfacesIrradiationofSilicon,"SandiaNational du r ingLow-EnergyAr-IonBombardment,"Laboratories,Albuquerque,NM,Feb.1993. Phys .Rev.B45,1507(1992).3.P.Bedrossian,"SputteringandAnnealingofSiliconSurfaces,"GordonResearchConfer- enceonFrontiersofScanningTunneling

PublicationMicroscopy,Ventura,CA,Mar.1993.

4.P.Bedrossian,"VacancyKineticsonSilicon

P .BedrossianandE.Kaxiras,"SymmetryandS_:rfaces,"AmericanPhysicalSocietyGeneral

Stabili

tyofSolitaryDimerRowsonSi(100),"Mo._ting,Seattle,WA,Mar.1993.Phys.Rev.Lett.70,2589(1993). 10 WSR---ThrustAreasSurfacesandThinFilms........IIIIIIIIIIIIIllIlllmm_IlllIIIiiillill,i,==,=, Q uantumElectronTransportthroughUltrathinFilms

L.J.TerminelloandE.Tamura

Oneofthemostimportantchallengesforthin-fi'Imtechnologywillbethefabricationofultra- smallmicroelectronicdevices.Inthelimitofcompleteminiaturization,devicematerialswillbe nomorethanasmallensembleofcarefullyassembledatoms.Inthislimit,theneedwillbegreat t ounderstandhowelectronspropagatethroughsuchsmallstructures. Introductiondownfromthesiliconsurfaceadatom.There- fore,byphotoexcitingtheboronlscoreelectron,

Low-ene

r gy('hot")-electronscatteringthekineticenergyofwhichisselectedwithtun- throughatomscanbestudiedasacomponentofableandmonochromaticsynchrotronradiation, understandingtheclassicalor_miclassicalonecanmeasuretheelectronpropagation modelsofelectrontransportinmaterials.Thethroughthesiliconadatomuniquely.This im por tanceofthisresearchwillberealizedwhenmeasurementwasmadeefficientlyusinga

quantum-effectdevices(devicematerialofonlyamultiple-angleelectronenergyanalyzer.Fromfewatoms)becomeviable,theelectronangulardistributionpatternmeas-

Mostresearchinelectrontransportdealsuredinourinitialexperiment,thescatteringcross withthemacroscopicviewofinelasticprocessessectionforasinglesiliconatomcanbeextracted. governingelectronpropagationinthesolidstate.Ourinitialassessmentoftheexperimental Mostmeasurementsareinterpretedtheoreticallyresultsshowsthatqualitativedifferencesexist usingMonteCarlosimulationmethods.Suchcol-betweenthelow-andhigh-kinetic-energydata culationstreattheelectron-electronandelectron-thatrequireabetterunderstandingoftheorigin phononcomponentsoftheinelasticprocesses,oftheseintensitydifferences,particularlyatlow butignoretheelectron-core-potentialinterac-energy.Toaccomplishthis,wehavebeguna tion.Veryfewexperimentshaveattemptedtomultiple-scattering,local-density-approximation measu r eelectron-atomscatteringcrosssectionssimulationoftheseresults.First,wewillisolate atlowkineticenergies,yetthiscomponentofthecontributionofthesinglesiliconforward- el ec tronpropagationwillbecriticaltounder-scatteringatomtothetotalelectronangular- standingandpredictingthebehavioroffuturedistributionpattern.Second,wewillidentifythe quantum-effectdevices,originoftheoff-normalfinestructurepresentinthelow-kinetic-energymeasurement.

Withthesesimulations,wewillbeableto

Experimentbetteridentifythecontributionofsingle-atom, low-kinetic-ene rgy ,electronpropagationinsol- Theinitialexperimentsthatwedesignedtoids.Onlywiththiscombinationofexperiment addressthisquestionhavemeasuredtheelectronandtheorycanthisvaluableinformationbe angulardistributionpatternoftheboronlsobtained.Furtherexperimentationwillmeasure photoemissionat29and176eV.Thismeasure-theboronlsphotoelectronangulardistribution mentwasmadeusingsynchrotronradiation-patternsforkineticenergiesbelow25eV.These basedphotoemissionandtookadvantageofthewillbecomparedtothehigherkinetic-energy u ni quegeometryofthedelta-dopingcompoundpatternsmeasuredat29and176eVandcalcu- ofSi(111):boron.Theuniquesurfacegeometryoflotionsatisoenergetickineticenergies. thissystemplacesasingleboronatomtwolayers 11 UraniumResearchWSR--ThrustAreasmlI--IIIIIIIIIIIIIIIIIIIIIIIII'1--

UraniumResearch

L .R.Newkirk,ThrustAreaLeader

Overview

B a s i cresearchintothemetallurgicalpropertiesofuranium'alloysrepresentsa red i rec t ionofouractinideresearchactivities.LawrenceLivermoreNationalLaboratory wasrecentlynaa,,edLeadLaboratoryforUraniumFabricationintheNuclearWeapons ComplexReconfigurationProgram,arolethatcarrieswithittheneedtosupportbasic r ese ar c hinuraniummetallurgy.Thisnewresponsibility,combinedwithpractical limitationsontheproductivityofbasicplutoniumresearch,ledustorealizethatour limitedresourcesforbasicresearchinactinidemetallurgywouldhavemaximum scientificandprogrammaticimpactifredirectedtowarduraniumalloys. U r a ni umanduranium-alloycomponentsplayanimportantroleinnuclearweapon designandfabrication.Anunderstandingofthebasicmetallurgicalbehaviorofthese materialsisanecessaryandimportantunderpinningfordevelopingandensuringtheir performancewithinthefabricationconstraintsthatwillbeassociatedwiththemodern reconfiguredweaponscomplex.Theseconstraintswillcombinetheenvironmentalcon- cernassociatedwithprocessingradioactivematerialswiththeneedtoguaranteespe- cificaspectsofperformanceinaweaponenvironment.Asaresultofover40yearsof rese archonuraniumanduraniumalloys,bothwithintheweaponscomplexandelse- where,agreatdealofbasicinformationisalreadyavailable;however,themetallurgical complexityofthesematerialshasleftmanyscientificallyinterestingandimportant questionstobeaddressed. Altho ug hnotascomplexasplutonium,pureuraniumisametallurgicallycomplicated material.Itexistsinthreesolidphases:ahigh-temperature,bccphase(7);anintermedi- ate,highlycomplex,tetragonalphase([3);andalow-temperature,orthorhombicphase ( c _).Thesephaseshavedramaticallydifferentmechanicalproperties,andthephase transformationfrom_too_canbecomplex.Asalloyingelements(particularlyrefrac- torymetals)areaddedtouranium,themetastablephasebehaviorbecomesincreasingly compli c ated.Uponquenchingfromthe7(bcc)region,variousnewphasesareobserved. Thesephases(c_',(z",yO,andys)arevariousdistortionsofthebasicc_phase, ( c _ 'and(x'),andthe3'phase,(T°andys),someofwhichformthroughverycomplex m arte n sitictransformations.Thesevariousphaseshavedifferentandsometimes seem i nglyanomalousmechanicalproperties.Inaddition,thecorrosionsensitivityofthe a l loywillbei_ighlydependentonthealloyingelement,itsconcentration,andpossibly th eresultingphasestructure. Theresearchprogramconsistsoftwodistinctareasofeffort.Thelargerandmore ambitiousofthesefocusesonthemechanicalpropertiesandphasetransformations 12 ! WSR---ThrustAreasUraniumResearchJ]iiiiiiiii|ii_[i111111iiiIii|iiiiiiiiiiiiiiiiiii___ a ss ociat edwithanarrowbutextremelyimportantcompositionalregioninthe uranium / niobiumsystem.Byvirtueofitsstainlessproperties,thisisaveryimportant alloysystem,andimprovingourbasicunderstandingofitsconstitutiverelationships willultimatelyassistourdevelopmentofalternatefabricationtechnologiessuchasspin- forming.Thesecondandsmallerareaofresearchwillfocusoncharacterizingthephase relationshipsinseveraluranium/refractorymetalalloysystems.Thiswillprovide insightinevaluatingpotentiallyinterestingbinaryuraniumsystemsforeithercorrosion resistanceorothervaluablemetallurgicalproperties.

MechanicalandMicrostructuralPropertiesofU-6Nb

G.Gallegos,P.Johnson,andA.Schwartz

B ecau seofitsuniqueproperties,U-6Nbhasimportantweaponsapplicationsand,becauseofits unu sualmechanicalbehavior,isalsoofscientificinterest.Atlowstrains,forexample,a"double- kneeeffect"isobservedinthestressstrainrelationshiptypicalofashape-memoryalloy.The deformati onmechanismatlowstrainshasbeenattributedtotwinningandintertwinning (neitherofwhichhasbeenproven).Thisdeformationmechanismeventuallybecomesexhausted wh enthevariantsarealignedwithrespecttotheimposedstrainorbecomeimmobilizedby colli s ion swithgrainboundaries.Atthispoint,itisassumedthatnormalplasticityby dislocationmotionbegins,andsubsequentstrainingcanpermanentlyalterthestructuresuch thattherecoverablestrainduringreversionisreduced.Thesecondyieldpoint,correspondingto theonsetofdislocationmotion,israthersoft---orgradual--inthisalloy.Thismayindicatethata cont in uou sl yincreasingfractionoffullydetwinnedgrainscontributestowo:'_ardeningin whatappearstobethedetwinningregime.Thesemechanismsmayoperatedifferentlyinthe ort horho m bicormonoclinicU-Nbphases(a'ando_')thaninthetetragonalphase(y°). ApproachWeexpectthatthebehaviorduringthisdeformationmode,ifaccomplishedbythe U-6Nbtypicallyisusedinanagedconditionmotionoftwinboundaries,shoulddependon afte rquenchingfromthehigh-temperatureytheinitialvariantsizeandorientationsandthe phase.Althoughevidenceofdislocationsandamountofstrainthatcanbeaccommodatedper subg r ainshasbeenobserved,thisisbelievedtounitmotionofthetwinboundaries.Complete betheresultofthequenchingoperation.Thisreversiontotheinitialmicrostructureispossi- projectfocusesonenhancingourunderstandingble,inprinciple,byheatingintothehigher- ofthedeformationbehaviorofU-6Nbinthe¢z"temperature_,phasefield,followedbycooling and_ophases.Specifically,weexpecttobettertoretransformtotheinitialmicrostructure, unde r standtheeffectofshearstressesonboththerebyrecoveringthestrain. thelow-strain(double-kneearea)andhigh-strain deformation.High-straindeformationisof inte r estforformabilitystudies,butdataareProgress limi t edtostudiesintension.Theapproachfor t hi sprojectistoconducttorsiontestsintheMostoftheeffortonthisl:'_o._thasbeen appropriateregimesandtocombinethesewithtooptimizethetorsionspecimengeometryfor microst r ucturalcharacterizationbytransmissionobtainingthedesireddata.Torsiontestingwas electronmicroscopy(TEM).chosentoallowthework-hardeningbehaviorto bestudiedtohighstrainlevels,beyondthe30-to 13 UraniumResearchWSR--ThrustAreas_lUmliiIIIINIIIIIIIIIInlllII_.----I--__IIIIIIIIIIII[ 50-pe
r

centlimitcausedbygeometricsofteninginprocessedtoobtainpredominantly_",withaatensiletest.smallfractionof_,o.

T oobtainasensitivemeasurementoftheCharacterizationofmicrostructuralchanges double-yieldbehavior,itisnecessarytomaintainisoffto.asolidstart.Beginningwiththerolled thestrainvrofileasuniformlyaspossibleacrossU-6Nbplatematerial,3-mm-diameterrodswere thecrosssectionofthetorsionspecimen.Thisextractedalongthethreeprincipaldirections: r equi r esahollowspecimenconfiguration,alongtherollingdirectioninplane,perpendicu- Thin-walledtorsiontestingrequiresaverylartotherollingdirectioninplane,andtrans- sho r tspecimenlengthtoavoidbuckling,whichverse.Thematerials-handlingprocedureswere r aisesquestionsastotheinfluenceofendanddevelopedtoobtainthe3-ram-diameterdisks su r faceeffects.Forthisinvestigation,therefore,requiredforTEM. wehavedecidedtotestthick-walled,hollow,Afteranumberoffailedelectropolishing to r sionbars.attemptsusingdifferentsolutions,asuitable Anexactsolutionforthestressattheouterelectrolytehasbeendiscoveredthatproduces fibe rofsuchaspecimenexistsintheformofareasonableamountsofelectron-transparent modifi edField-Backofenequation.Wehavetriedmaterial.Furtheradjustmentsintheelectro- fou rdifferentgeometriesoftorsionspecimenspolishingparameters--includingvoltage,cur- andgripsinordertooptimizethegeometrytorent,temperature,andelectrolytecomposition-- meettherequirements.Themostrecentspecimenshouldleadtoevenbetterthinfoils. geometryappearstomeetourcriteria,althoughPreliminaryTEMhasrevealedatwo-phase bucklingcontinuestopresentaproblem.Thismicrostructure.Thematrixphase,whichcon- specimendesignwillberefined,andtestingonstitutesabout99%ofthethinarea,hasamedium U-6Nbshouldbeginshortly.Thedataacquisi-densityoftwinsbutfewvisibledislocations.The tionprogramforthesetestshasbeencompleted,secondphaseappearstobeanorderedinter- WehaveobtainedaU-6Nbplatetobeusedmetalliccompound.Phaseidentificationand fo rthesestudies.Theplatehasbeencharacter-detailedmicrostructuralcharacterizationare i z edby_hemicalanalysis,andx-ray-diffractionbeginningatthistimeandwillprovidethebasis studi esareunderwaytodeterminethetextureofforfurtherstudyintothedeformationmech- theplate.Thefirstsetoftestswillbeperformedanismsactiveinthisalloy. onaU-6Nb(U-14at.%Nb)alloysamplethatwas

PhaseDiagramsofUranium/RefractoryMetals

F.Y.L.G_nin

Thegoalofthisstudyistoreviewselectedbinaryphasediagramsofuraniumrefractory-metal alloysinordertoidentifysystemswithpotentiallyinterestingmetallurgicalorcorrosion- resist antproperties.TherefractorymetalsofinteresthereareCr,Hf,Mo,Nb,Ta,Ti,V,W,and Z r .ThestudywillfirstconcentrateonNb,V,W,andMo. dXdX Pr o gre s s('_)solidus-('dT')liquidus-A/']fusi°nRV2m.(1) Ini ti ally,thephasediagramswereexamined fo rviolationsofgeneralthermodynamicrules,Theresultsofthisevaluationfortheuranium suchastheconsistencyoftheinitialslopesoftherefractory-metalsystemsaregiveninTable1. solidusandliquidusinrelationtotheVan'tHoff eq uation: 14 WSR---ThrustAreasUraniumResearch--]Iilfll--iiii__HillIIII.II1,1IIIIIIIIII--IIIIIIIlUIIII Table1.Resultso_phase-diagramexaminationforadherencetothermodynamicrules. "................Difference-......._fusio._._n--

SystemSolidusslope"Liquidusslopea(absolu[evalue)RTm....i,,,.Mo-U---10,290.....-m0.......................2460--Ta-U-99,000-396041252465

V-U-20,130-107011301750

W-U.-oo-348034803200

"Slopes_meC/lO0at.%. F o rmolybdenum,thevalueofAHfusionasdeter-TocalculatetheGibbsfreeenergiesoftheliquid minedfromthephasediagramishighbyafac-alloys,anidealentropyofmixingbehaviorwas

torof2,andthemolybdenum-richsolidusshouldassumed.Theparameters01and@2aregiveninconnectwiththeuranium-richsolidus,whichTable2.

indicatesthatthecurvatureofthemolybdenum solid u sisincorrect.Theoppositeistruefortan-SolidAlloys talum ;i.e.,AHfusionistOOlOW.Forvanadium,the l iquiduscurveseemstoosteep.ThisobservationThechemicalenthalpyofformationforthe issupportedbythefactthattheliquidifromeachbccphasewasassumedtobethesameasthatof si d

eofthephasediagramshouldjoin.Fortheliquidphase.Miedemahaspointedoutthatanelasticcontributiontotheheatofformationfortungsten,thedifferenceofslopesisconsistent

wi ththeprediction,thesolidalloysmustbeaddedtothechemical

Thethermodynamicparametersforthecontribution.Theelasticcontribution,AfHelas,wascalculatedbyusingthemethodoutlinedby

liquidandsolidsolutionshavebeencomputedMiedema.ThetotalenthalpyofformationwasfortheU-NbandU-Tasystems.fittedtothefollowingexpression:

LiquidAlloysAfHsolid(J/mol)=AfHchem+AfHelas=

Pu r euraniumandrefractorymetalwerex(1-x)(@2+@4x+@5x2)•(3)takentobethestandardstatesinthepresent calculation.Thechemicalenthalpyofformation, Af H ch e m,fortheliquidalloyswascalculatedTheparameters@3,@4,and05aregivenin usingthemodelproposedbyNiessenetal.TheTable2.Usingtheseparameters,wecomputed calculatedvaluesofAfHchemwerefittedtothethesolidiandliquidi,whichdidfitwellwith followingexpression,wherexisthemolefractionexperimentalresults.Ontheotherhand,exper- ofrefractorymetal:imentaUydeterminedthermodynamicparame-tersdonotfitthemodel.Experimentshave Af H chem(J/tool)=x(1-x)(O1+@2x)•(2)thereforebeenplannedfortheU-NbandU-V systemstovalidateexistingexperimentaldata. Table2.Thermodynamicparametersforliquidandsolidsolution.i,_i...... S ys te m-@1"....02a@3"0'4"@s_' U-l _ b-14,927i874.8.....27,407--272.0-i731.8

U-Ta!2,3181547......25,302....1314.......-7874.2

15

WSl_---ThrustAreasFundamentalsofthePhysicsandProcessingofMetalsIII....--_--_'--_"'"""IIII--Iiiiiii........

FundamentalsofthePhysicsandProcessingofMetals

W.H.Gourdin,ThrustAreaLeader

Overview

InthefirsthalfofFY93,theMetalsResearchProgramhascontinuedtoshiftitsfocusto thepropertiesandprocessingoforderedintermetallicmaterials.Thischangeispartofa long- r angeplantograduallyshifttheemphasisoftheprogramandmakeitmore directed.TheprojectonthestabilityofmetallicsuperlatticesledbyMarcelSluiterwas te r minatedearlyintheyear,butAlanJankowskifabricatedalimitednumberofTi2/Cr2 multilayerstoverifytherelativestabilityof(111)and(110)layerorientations.Using gr azing-incidencex-raydiffraction,hedemonstratedthatmultilayerswith(111) o r ientationdecaymoreslowlythanthosewith(110)orientation,inqualitative agree m entwiththecalculationsofSluiterandTurchi. OureffortstounderstandtheroleofboronandothersolutesinNi3AIhavemadegood prog r ess.Asstatedinpreviousreports,ourgoalistounderstanddetailsoftheroleof solutessuchasboroninpromotingenhancedductilityinthehopethattheknowledge wegaincanbeappliedtoother,moretechnologicallyimportantsystems.Our th eo r eticalstudieshaveshownasharpcontrastbetweenthechangesinducedbya ductilityenhancersuchasboronandanembrittlingagentsuchashydrogen.Difference plotsofelectrondensityshowthatboronpromotesamoreuniformdistributionof ch a r gethroughouttheNi3AIlattice.Thisdistributionisofthesamegeneralcharacteras wefoundpreviouslyinpureNi3A1,whichsuggeststhatthepresenceofboronmay improvecohesionwhenthematrixstructureismodifiedasitisnearagrainboundary. Co n versely,hydrogentendstodepletethechargeintheinterstitialregionsandto modifytheNi-AIbonding.Thisdisruptionofthebondstructuresuggestsreduced cohesion.Experimentalworkhasconcentratedonpreparingformechanicaltestsof bicrystalspecimens,bothnaturalandartificial,andattheendofMarch,equipmentand softwarewereinplacetodeterminethecrystallographicorientationandmechanical p r ope rti esofthespecimens.WealsoestablishedacollaborationwiththeUniversityof CaliforniaatIrvinetostudygrain-boundaryslidinginbicrystalspecimens,whichwe willprovide. Ifintermetallicmaterialsareevertohaveasignificantimpactonthegas-txtrbine indus t ry,methodsmustbedevelopedtojointhembothtothemselvesandtoother supe r alloys.Straightforwardapplicationoftraditionalmethods,however,isfrequently unsatisfac t o r ybecauseofthehighmeltingpointsofthesematerialsandthefactthat theyarebrittle.MikeStrum'sstudyofstoichiometricinterlayerbondingof intermetallics,anewprojectthisyear,isexploringonepossiblenovelapproachto joiningNiA1,analloyofparticularinterestforengineapplications. 16 FundamentalsofthePhysicsandProcessingofMetalsWSR---ThrustAreas--lllIllIllll----_--IIIIllllIlllII_--- SoluteSegregationBehaviorinNi3A1-BasedOrderedAlloys

W.H.Gourdin,P.E.Johnson,N.Kioussis,*andA.Gonis

Theobjectiveofthisprojectistoprovideafundamentalunderstandingoftheeffectsofternary solut esonintergranularcohesionandductilityinL12orderedintermetallicalloys.

Summaryorientationonthelocalstrainaccommodation

andcohesivestrength--withandwithout Attentionhasfocusedrecentlyontheelec-boron---ofsingle,well-definedgrainboundaries. tronicstructureofhighlyorderedintermetallicTestswithstraightbarspecimenslatelast compound scontainingtransitionmetalelementsyearandearlythisyearmadeitplainthatasim- and,inparticular,onunderstandingtherelation-piefrictiongriparrangementwouldnotbesatis- s hi pbetweentheelectronicstructure,macro-factoryforthesetests.Evenspecimensheat- scopicfracturebehavior,andsolutecontent.Thetreatedtominimizeboronsegregationshoweda exceptionalhigh-temperaturestrengthsexhibitedsurprisingamountofductility,andthestresses bymanyofthesematerialsarethoughttoorigi-duringdeformationbecamelargeenoughto nateinthenatureandextentoflocalelectronic-producesignificantslipping,therebycompromis- chargetransfer,whichisalsoresponsiblefortheingmeasurementsofstrain.Weredesignedthe s ta bilityofthesecompounds,gripstoaccommodateminiatureshoulder("dog- TheroleofboroninpolycrystallineNi3Albone")specimenswitha1-×1-x3-ramgauge hasbecomeaparadigmforsoluteeffectsinlengthandhadseveralspecimensfabricated.We whichtheappropriateselectionofaninterstitialalsoacquired,calibrated,andinstalledaminia- dopantcancompletelysuppressbrittlebehavior,tureclip-onstraingaugetoallowustodirectly Theabilityoflowdopantlevelstosegregatemeasurethestrainsinthevicinityofthegrain tograinboundariesanddominatethefractureboundary°Suchmeasurementsareimportant modeundoubtedlydependsonthelocaldefectbecausewewanttodistinguishbetweenthe structureitself,aswellasthespecificmodifica-effectsofboronontheboundary'scohesive tionoftheelectronicstructureprovidedbythestrengthandthelocalaccommodationofstrain presenceofthedopant.However,recentresultsneartheboundary.Attheendofthefirsthalfof intheliteratureindicatethatboronmayalsoFY93,wewerereadytobegil_testingwiththe mitigatetheeffectsofhydrogenintroducedintomodifiedspecimensandapparatus. thelatticefromtheambientatmosphere.Tobemostinformative,theorientationof Weareaddressingtheseissuestheoreticallythetwohalvesofeachbicrystalmustbewell wi thaseriesoffirst-principlescalculationsonknown.Toaccomplishthenecessarymeasure- bulkNi3Al,withandwithoutboron,andexperi-mentsusingelectronbackscatteringpatterns mentallythroughasetofteststhatallowsimul-(EBSP),weacquiredandlearnedtooperatea taneousmeasurementsofthechemistry,struc-computer,data-acquisitionhardware,andneces- ture,andplasticbehaviorofindividualgrainsarysoftware.Thisequipmentwillbeusedin bo unda r ies,conjunctionwithascanningelectronmicroscope eq uippedwithanEBSPcameraattheUniversity

Prol,ressofCalifornia,SantaBarbara.Theductilityofthestraightbarspecimens,evenwhentreatedtominimizetheamountof

Onaminiatureloadframe,wearemeasur-boronpresent(accordingtoconventionalwisdom ingthemechanicall:_haviorofbicrystalscutfoundintheliterature),ispuzzling.Toinvesti- f r omalarge-grainedboule.Theseexperimentsgate,wehydrogen-chargedaspecimenandfrac- willexploretheeffectsofcrystalandboundarytureditimmediatelyinaninertenvironment ( argon).Augermeasurementsshowedlittleifany *CaliforniaStateUniversity,Northridgeboronattheboundary,althoughaccurateabso- 1 7

WSR--ThrustAreasFundamentalsofthePhysicsandProcessingofMetals--_---.,-------iiiiii_irlliiii__iiiiiii_[__.....

lutemeasurementsaredifficulttoobtain.Never-density-differenceplots,whicharethedifference theless,thisobservationisconsistentwithourbetweentheelectron-charge-densitycontours notionthatsomeboundariesmaybeintrinsicallycalculatedwithasolutepresentandthecharge- s t r

ong,regardlessofthepresenceofboron,densitycontourscalculatedforthe"pure"inter-ProfessorJ.Earthman,U.C.Irvine,willmetallicandanisolatedsoluteatom.

studytheeffectofboronontheslidingofplanarSuchdifferenceplotsforNiaAIwithboron grainboundariesinbicrystalsfabricatedonourandhydrogenareshowninFig.2,andthe just-co m pleteddiffusion-bondingmachine.Hisdiiferenceintheinfluenceofthesesolutesis measu r ementswillcomplementourtensiles'a'iking.BoroncontributeschargeintheNi-Ni experi m entsonthesameboundaries.Initialplansin_sticeandgenerallyproducesanexaggerated a r etostudya£3{110}twin,aY.,5{210}boundaryversionofthebondstructureweobservedinthe ( al r eadyonhand)anda"random"boundaryofpurecompound.Thissuggeststhatitmay thesameorientationasanaturaloicrystalcutcontributetoenhancedcohesionatdefectssuch fromthelarge-grainedboule,asgrainboundaries.Hydrogen,incontrast, Becausemanyboundariesconsistofregulardepleteschargeintheinterstitialregions,which a rr aysofsimplepolyhedra,webelievethatwhatisapparentlydonatedtoamoredirectionalNi-Al welearnabouttheeffectsofboroninvariousbond,suggestiveofamorebrittlestructure.[In sitesintheNi3Allatticewillalsoapplytoitstheplots,negativevalues(chargedepletion)and effectsatboundaries.Ourpreviouscalculationspositivevalues(chargeenhancement)areshown s how edthatwhenboronisplacedinoctahedralasdashedandsolidlines,respectively.] inte r stices,itspstateshybridizestronglywith su rr o undingnickel-atomdstates.Further,this h yb r idiza t ionisdiscouragedbythepresenceofPresentationaluminumatomsin&ecoordinatingoctahedron. Inthefirsthalfofthisyear,wecompletedH.Watanabe,N.Kioussis,R.G.Hemker,A. calculationsofpurenickelandpurealuminumGonis,W.H.Gourdin,andP.Johnson,"Effectof fo rcomparisonwiththeorderedcompound.WeHydrogenandBoronImpuritiesontheElectronic alsostudiedtheeffectsofhydrogen,aknownStructureandMechanicalPropertiesofNi,Al, embrittlingagent,tocompareitsbehaviorwithandNi3AI,"AmericanPhysicalSocietyMeeting, ou rpreviousresultsforboron,aductilizer.ToSeattle,WA,Mar.1993. emphasizethepossibleeffects,weusedcharge- Figure2.Chargetransferon(110)byaddingBat1/2(111}(a)andon(110)byaddingH2at1/2(111)(b). 18

,Fundamentals,ofthe,Phys!csandProcessin_ofMetalsWSR--ThrustAreas_____II1._iiiiiiiiiiiII-_...._iillli,i_IIIiii

StabilityofArtificialIntermetallicSuperlattices:

AnExperimentalStudyofTilXMultilayers

A.F.Jankowski

Theobjectiveofthisstudyistotestthetheorythatspecificartificialsuperlatticesarethermo- dynamicallystableorunstable.

SummaryTherelativestabilityoftheTi2/Cr2super-

latticespecimenscanbecomparedfromaplot Existenceofthermodynamicallystableandofthenormalizedreflectedintensityatgrazing u S table,body-centered-cubic(bcc)superlatticesincidenceasafunctionofhomogenizationtime hasbeenpredictedbySluiterandTurchi.1,2Corn-(seeFig.1).Thecompositionprofilehasclearly putationsbasedonchemicaldisorderhavebeendecayedmorerapidlyinthe[110]growthdirec- p r oposedforhypotheticaltitanium-basedbinarytionthaninthe[111]direction.Theseresultsat systems.Specifically,formultilayerpairsconsist-200°Careinqualitativeagreementwiththepre- ingoftwomonolayersoftitaniumandtwodictionbySluiterandTurchiforthebehaviorof m o nolayersofX(eitherchromiumorvanadium),thesuperlatticesat0K.Adetailedanalysisofthe energet i cstabilityispredictedfor(111),whereincrystallographic-dependentinterdiffusivitiesisin theorderingenergyzXEORDislessthanzero.Anprogress,alongwithcomputationofthesystem experi m e n talstudyisinitiatedtosynthesizearti-activationenergyfromadd,itionalhomogeniza- ficialsuperlatticesofTi2/X2usingphysicalvaportionsconductedat400°C depo s iti o n.Grazing-incidencex-raydiffraction isusedtomonitorthedecayofthecomposition profileforahomogenizationseriesofisothermal1.0c---....---....................... annealtreatments.TherelativestabilityofTi2/X2 m ultilaye r swithdifferentgrowthtexturesis compa r edwiththeoreticalpredictions.

Progress_

S yn t hesisofthesuperlatticesamples,x-ray di f f r acti o ncharacterization,andhomogenization p r ocessingarecompletefortheTi2/Cr2system. Abccchromiumepitaxiallayerhasinducedthe•(111) gro w thofthe_-titaniumphaseintheTi/Crmul-o(1"1O)

tilayer.The0.2-1am-thickfilmsarepreparedby0.1....-'--....._.....thesputterdepositionofthermalizedneutrals048--98144

usingplanarrnagnetronsourcesontoroomtemp-Time(hr) eratu r esubstrates.Apowderdiffractometer operatedinthe0/20modeusingcopperK(xFigure1.Intensityofthefirst-ordergrazing- r adiationrevealsthetextureofthefilmdeposits,incidencereflectionfortheTi2/Cr2(111)and F ilmsofTil.84/Cr2.04(110)andTil.90/Cr2.07(111)Ti2/Cr2(110)filmsisplottedasafunctionof aregrownonsubstratesofamorphoussilicaandtimeat200°C.Thenormalizedintensityvalues c r ystalli n esapphire,respectively.Thecomposi-indicateamorerapidinitialdecayinthe[110] tionofthefilmsisdeterminedfrommatchingdirectionthaninthe[111]growthdirection,in calibratedquartz-crystalmonitorreadingswithqualitativeagreementwiththeSluiter-Turchi total-thic k nessprofilometermeasurements.Mea-predictionsforTi/Cr. su r e m entofthefilmconcentrationisconfirmed byusingAugerelectronspectroscopycoupled withdepthprofiling. 19 I WSR---ThrustAreas_FundarnentalsofthephysicsandProcessing_ofMetals ReferencesMetallicSuperlattices,"MRSSymp.Proc.238,623(1992).

1.M.SluiterandP.Turchi,"TheRoleofChemi-2.M.SlulterandP.T_'rchi,"PhaseStabilityof

ca]InteractionsintheStabilityofArtificialArtificialSuperlattices:Chemical-OrderEffect,"Phys.Rev.B46,25_5(1992).

StoichiometricInterlayerBondingofIntermetallics

M.J.StrumandG.A.Henshall

Theobjectiveofthisstudyistodevelopanunderstandingofisothermalthin-filmbonding processesandapplysuchmethodstothejoiningofrefractoryorderedintermetallics. Summary•Isothermalsolidificationandhomogeniza- tiontoformtheparentphasebyreaction Ou

rpresentgoalistodevelopapredictivesynthesisofthepre-placedfilms.understandingoftheisothermalbondingprocess

inwhichelementalfilmsplacedatthejointinter-Bothreactionsarebeingevaluated,withempha- facesreacttoformajointwiththesamestoichio-sisondevelopingapredictivemodeloftheiso- m erryastheparentmaterial.Thisjoiningthermalsolidificationreactior,kinetics. me th od,conceivedbytheauthors,isamodifica- tionofconventionalliquid-assisteddiffusion-Progressbonciingprocessesbutwithouttheadditionof t raditionalmelting-pointdepressants.ThroughInthianewlyfundedproject,wehave thismethod,weexpecttoobtainjointcomposi-selectedanappropriatematerialssystem(NiAI), tionsandpropertiesapproachingthoseofthearrangedacollaborationwithGeneralElectric parentmaterial.ItisparticularlysuitableforAircraftEnginestoproducesinglecrystalsforthe jo ininghigh-temperaturematerials,includingexperiments,preparedaluminum-coatednickel inte r metallicsandceramics,whichhavepoorsubstratesforpreliminarystudies,initiated weldabilitybyconventionalfusionweldingmetallurgicalbondingexperiments,completed me th ods,isothermalsolidificationexperimentsonnickel P r io rtobonding,astoichiometriccompositesubstratesthathaveverifiedourabilitytouse inte r laye rconsistingofelementalcomponentsisreactionsynthesistoformNiAl,andidentified p r e-placedontheparentmaterialinterfacesbyaseveralkineticmodelstoassessand,perhaps, processsuchasvapordeposition,asillustratedinimprove. F

ig.3forthejoiningofNiAl.ThecompositeisTheselectionoftheintermetallicphase,thenheatedinvacuumtoasuitabletemperatureNiAl,asthematrixmaterialwasbasedonits

a bo vethemeltingpointofthelow-temperaturephasediagram,itstechnologicalimportanceasa constituent.Theresultingliquidlayerisallowedhigh-temperaturestructuralmaterialofthe toreactwiththeneighboringconstituentandfuture,anditslackofconventionalweldability. iso th e r mallysolidifestoformthehigh-melting-AdvantagesoftheNi-Alsystemincludethelow pointparentcompound.Thisprocessofstoichio-meltingtemperatureofaluminum,thehigh m et r icinterlayerbondingcomprisestwoseparatesolubilityofnickelinliquidaluminum,andthe r eac ti ons:abilitytoavoidintermediatephasesinthe °Theaccomplishmentofmetallurgicalaluminum-richconstituentatreasonablejoining bondingbyawettingreactionbetweenthetemperatures(above1133°C).Single-crystalNiA1 t_ a nsientliquidphaseandtheparentwasselectedtoencourageaplanarreactionfront mate r ial,byavoidingenhancedreactionratesatgrainboundaries. 20 FundamentalsofthePhysicsandProcessingofMetalsWSR---ThrustAreas-illIililil ___Whereasthe03-mmandthickerlayersincreasethesolidificationtimesbyincreasingthe transportlengths,theyarebeingusedtofacilitatemicrostructuralcharacterization.Theintermetal-

NIAINIAIlicphasesproducedintheseexperimentswere

substrat esubstraleidentifiedbyEDXcompositionalanalyses.

Complet

edisothermalsolidificationtreatments_J_JresultedinanouterlayerofNiA1separatedfrom

___RN!._thenickelsubstratebyathinNi3Alinterlayer._.:_.........,.__Al____':'_":_'_"___,_._,_::,._:..or__......Thenickelsubstrate,unlikeNiAl,providesa_'_'._,_:'.'_"-_'_"_'_':_;_semi-infinitesourceofnickelandtherefore

' . _ % _ . _ , * _ ' , _ .

__'_'__._...:_:_..,:.:.:_,_._._.___,_,"_ai_%I__%_encouragesNi3A1formation."'....__........._'_'_"''.'._i.__.,_.,,._.,_%_I._:_%'.!_:Xa_,_'__:_:_.__._:_:_Interruptedisothermalsolidificationpro-I_._.':_._."._;_:_!_._.'_1_'::.::':_ducedsimilarintermetalliclayersextendingfrom

/itheoriginalNi/AIinterfaceoutwardintotheNiAINIAIaluminumlayerbutterminatingina,divorcedIsubstrateeutecticlayernearthesurfaceofthealuminumcoating.Defectsp;oducedduringthereactionhavebeenidentifiedasKirkendallvoidsand

intermetallicstressfractures.Thesensitivityof

defectformationtoprocessingmethodswillcontinuetobeevaluatedinfuturestudies.Figure3.Stoichiometriccompositefilmsare

pre-placedbetweenthecomponentstobeContinuingWorkjoined.Inthiscase,filmsofnickelandalumi- numontheNiAIparentmaterialareillustrated.Uponreceiptofnewmaterial,ourcontinu- ingexperimentswillfocusoncharacterizingtheevolutionofintermetallicformationoncoated

Initialexperimentsofisothermalsolidifica-single-crystalNiAlsubstratesforinputintoationwereconductedoncommerciallypurenickelkineticmodel.

( N i201)substratescoatedwithhigh-purityalu-Thesecondconcurrenttaskistoevaluatethe minum.Thecoatings,appliedbyelectronbeamwettabilityofthematinginterfaces.Itiswell eva por ation,adheredwellandreactedwiththeknownthatwettingofasurfacebyaluminum nickelsubstrateuponheating.Completeiso-requiresspecialconsiderationoftheneedto thermalsolidificationwassuccessfullyachieveddisruptthenativesurfaceoxide.Inorderof fo

ra0.3-mm-thickaluminumlayerinapproxi-preference,methodsbeingevaluatedforthismately145minutesat1200°C.Theseresultspurposearesuperheating,applicationof

ap pe a rpromisingonatechnologicallevel,pressure,andtheuseofthinreactivecoatings.

especiallyifoneconsidersthatthinnerlayerscanTheseexperimentsareinprogress.beusedtofurtherdecreasethereactiontimes.

21
EnergeticMaterials...........................WSR---ThrustAreas

EnergeticMaterials

R.L.Simpson,ThrustAreaLeader

Overvi

e w Thisthrustseekstoestablishafundamentalunderstandingbetweenthemolecular structureofenergeticmaterialsandtheirsensitivityandperformance.Theeffortcon- sistsoffouractivities.Thefirstisanexperimentalandtheoreticalefforttodetermine energy-transportpropertiesbetweencrystal-latticephononmodes(sudlasthosegen- eratedinshockedmaterials)andvibronmodes.Inthesecondarea,reactionkineticsand ignitionsensitivityarebeingexaminedintermsofmolecularstructureandconforma- ti on.Thethirdactivityinvolvesdeterminingphysicalandchemicalsolid-statedecom- posi ti onprocessesofenergeticmaterialsandcorrelatingthemtomicromorphological chan gescausedbythermalcyclingandcharacterizedbyatomicforcemicroscopy ( A .F M).Thisactivityhasdirectrelevancetoappliedprogrammaticactivitiesbecause materialdefectsresulti

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