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Influence_3

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Influence_3Influe

    Influence

Dec.2008,Volume2,No.12(SerialNo.13)JournalofMaterialsScienceandEngineering,ISS

    Nl93:899’

    ;Influence0fcounter.intuitivebehavioronsurfacenan0crystallizati0n

    ;LINWanming,WEIYinghui,DUHua-yun,HOULi-feng,XUBingshe

    ;(CollegeofMaterialsScienceandEngineering,TaiyuanUniversityofTechnology,Ta&a

    mp;uan030024,China)

    ;Abstract:Nanostructuredsurfacelayerofabout20nmthick ;wasproducedincopperplatebymeansofthesurface ;mechanicala~fitiontreatment(SMAT).BehaviorsoftheSMAT ;samplewereinvestigatedbyusingtransmissionelectron ;microscopy,Vickershardnesstesterandpotentiodynamic ;anodicpolarizationtests.Experimentalresultsshowedthat ;SMATofcoppercanbeusedtocreateananostructuredstirface ;layerthatoccurcounterintuitivebehavior(CIB)phenomena ;afterunloading.Itwasshownthatresidualstressesofafter ;SMATcansignificantlyaffectthepropertiesofcopper. ;Keywords:SMAT;counter-intuitivebehavior;surface ;nanocrystallization;copper

    ;1.IntrOductiOn

    ;Nanocrystalline(nc)materialshaveattractedgreat ;scientificinterestsinrecentyears[1.Numerous

    ;experimentalresultshaveindicatedthatncmaterials ;possespropertiesandbehaviorsthatarefundamentally ;differentfromthoseoftheirconventional

    ;coarsegrainedpolycrystallinecounterparts.-.suchas

    ;highhardnessandstrength,enhancedphysical

    ;properties,improvedtribologicalproperties,and ;superplasticityatlowtemperatures.etc[4-6.

    ;Inmostcases,materialfailuresoccuronsurfaces ;suchasfatiguefracture,frettingfatigue,wearand ;corrosion,etc.Thesefailuresareverysensitivetothe ;structureandpropertiesofthematerialsurface7J.

    ;Optimizationofthesurfacemicrostructureand

    ;propertiesisaneffectiveapproachtoenhancethe ;globalbehaviorandservicelifetimeofmaterials[1. ;Acknowledgement:Theauthorsgratefullyacknowledgethe ;fundingforthisfromtheNationalNatura1ScienceFonndation ;ofChinafNo.5047l070)andtheProjectSponsoredby ;ScienticResearchFoundationfortheReturnedOverseas

;ShartxiProvinceofChina(No.200632).

    ;Corresp0ndingauthor:LINWanruing(1970),male,Ph.D.

    ;candidate,associateprofessor;researchfield:nanomaterials.

    ;E-mail:linwml970@126.com.

    ;SurfacenanocrystallizationrSNC)isahove1 ;methodforimprovingmaterialsproperties.Ithas

    ;beendemonstratedthatamongthedifferenttechniques ;usedtoDroducenanostructuredlayerssurface ;mechanicalattritiontreatment(SMAT),accomplished ;bysurfaceshotpeening,isaneffectivewayofcreating ;localizedplasticdeformationresultingingrain ;refinementdowntothenanometerscalewithout ;changingthechemicalcompositionofthematerial[oJ.

    ;TheSMATprocesshasbeensuccessfullyappliedto ;manymaterialsystemsincludingaluminum[1lJ’iron12

    ;stain1esssteel[1andtitaniumI14-15].

    ;TheCounter-intuitivebehavior(CIB)phenomena ;hasbeenfoundduringsurfacenanocrystallizationby ;SMAT.Counter-intuitivebehavior(ClB1,i.e.thefinal ;deflectionscontrarytothedirectionoftheloadingasa

    .hasbeenstudiedby ;resultofreversesnapbuckingfi5

    ;manyscientists.AseriesofpapersbyGalievprovidea ;reviewofboththeinitialeventandsubsequent ;experimensandanalyses[16J.Theinitialeventoccurred ;duringtestsinwhichanexplosivechargewasdetonated ;inatankofwater,causingshockpressurepulsesto ;impingeondiscsofvariousmetalscoveringacircular ;holeinthethickwallofthetank[17.Thesephenomenas

    ;areassociatedwithchaoticvibrations.Manyoftheresults ;notedabovehavebeenconfirmedbvexperimentsI’’l9J.

    ;Inthispaper,thesynthesisofananostructured ;surfacelayeronflpurecopperplatebyusingSMAT ;wasreported.Themicrostructuralevolutionwas ;characterizedbymeansofdifferenttechniques.And ;thechangeofmechanicalpropertiesaftertheSMAT ;treatmentwasanalyzed.

    ;2.Experimentalprocedures

    ;l9

    ;

    ;Influence

    ;

    ;ofcounterintuitivebehavioronsurfacenanOcrystallizatiOn ;2.1Sample

    ;Thematerialusedinthisworkisapurecopper

;plate(60×100~100inn’linsize),itschemical

    ;compositionscontain(mass,%)0.11C,0.24Si,0.35Mn, ;0.O18Pand0.014S.BeforeSMAT.theplatesurfaces ;werepolishedwithsiliconcarbidepapersandthen ;annealedinvacuumat923Kfor120mintoeliminate ;theef-fectOfmechanicaldeformationandtoobtain ;homogeneouscoarsegrains.Thegrainsizeofthe ;annealedcopperplateisonaverage100-150gm. ;2.2SMAT

    ;ThedetailsoftheSMAThavebeendescribed

    ;previously2o.Inbrief,duringtheSMATprocess. ;Stainlesssteelballs(shots)of8millindiameterwere ;placedatthebottomofacylindershapedvacuum

    ;chamberattachedavibrationgenerator,withwhichthe ;shotswereresonated[2?.Becauseofthehighvibration ;frequencyofthesystem,thesamplesurfacetobe ;treatedwaspeenedbyalargenumberofshotsinashort ;periodoftime.Eachpeeningoftheshottothesurface ;willresultinplasticdeformationinthesurfacelayerof ;thetreatedsample.Asaconsequence,repeated ;multidirectionalpeeningathighstrainratesontothe ;samplesurfaceleadstosevereplasticdeformationin ;thesurfacelayer.ThemainparametersoftheSMAT ;processwerechosenasfollows:thevibration ;frequencyofthechamberdrivenbygeneratoris50Hz; ;theshotdiameteris8ram;andtheprocessingdurations ;are30,45and60min,respectively.

    ;2.3Microstructuralcharacterization

    ;TheTEMexperimentswereperformedona

    ;transmissionelectronmicroscopy(TEMH800)with

    ;anoperatingvoltageof120kV.Thinfoilsamplesfor ;TEMobservationswerethinnedbyusingionmillingin ;avacuumatlowtemperature.

    ;Themicro.hardnessalongthedepthintheSMAT ;CusampleswasmeasuredbyusingaVickershardness ;tester(M-400H1)withaloadof5gandadurationof ;10s,whichwasapproximatelytobe1.3GPa,much ;higherthanthatoftheCGCuofabout0.7GPa. ;20

    ;Thecorrosionbehaviorofsampleswas

    ;determinedbypotentiodynamicanodicpolarization ;tests(PS168A).Theelectrolytewas0.1mol/LCuSOd ;+0.05mol/LH2SO4solutionpreparedbyusingreagent ;gradechemicalsdissolvedindistilledwater.

;3.Results

    ;3.1j?icrOstructure

    ;Fig.1showstheTEMmicrographsandtheselected ;areaelectrondiffraction(SAED)paRemofthetop ;surfacelayerafterSMATfor30and45min,respectively. ;Themicrostructureofthetopsurfacelayerofthiskindof ;Cusamplewascharacterizedbyultrafineequiaxed ;grains.Thesencgrainspossessrandomcrystallographic ;orientations,asindicatedintheselectedareaelectron ;diffraction(SAED)paRems.Themeangrainsizeofthe ;surfacewasapproximately12nnlandl0nrninthe ;samplespeenedfor30and45min,respectively. ;Fig.1Bright-fieldTEMimagesandthecorresponding ;selectedareaelectrondiffraction(SAED)patternofthetop ;surfacelayeroftheSMATsamplefor30minand45rain ;3.2C0unterintuitivebehavior

    ;Influenceofcounterintuitivebehavioronsurfacenan0crystallizatiOn ;DuringtheSMAT,repeatedmultidirectional ;peeningathighstrainratesontosamplesurfaceleadsto ;severeplasticdeformationinthesurfacelayerand

    cemicrostructure.TheCIB ;refinesthesurfa

    ;phenomenonappearsafterunloading,asshownin ;Fig.2.

    ;.

    ;Fig.2TheCIBphenomenonofpure

    ;copperaftertheSMATfor30min

    ;Shotpeeningasurfacecreatesalarge

    ;compressiveresidualstressonthesurface.Creationof ;suchcompressiveresidualstressissimpleand ;qualitativelyexplainedastheplasticdeformation ;causedbyshot.WithasingleshotwitharadiusofRis ;staticallyincontactwithamemberasshownFig.3the ;relationbetweenthedepthHoftheplasticzonecaused ;bytheshot.andthedepthZoftheindentationis ;expressedbyEquation(1).Themodelconsidersfirsta ;singleshotimpinginguponatargetandonrebounding ;leavingaresiduastressbelowthesurfaceofthetarget. ;hp/R:k,J2/n(1)

    ;,\

    ;/

    ;/,)9(z.

    ;\,

    ;Elastic:Ii

    ;zone,Plasticzone,

;,

    ;fH

    ;,

    ;,

    ;,一.,

    ;,

    ;,

    ;,

    ;4,

    ;Fig.3Schematicofelasticplasticdeformation

    ;Withtherepeatedmultidirectionalpeening,the ;dynamicresponseofcopperplatetransitsfromelastic ;toelastic.plastic.Andthestructuregoesthrough

    ;threestages,i.e.1oading,recoveringafterunloading ;andfinalfreevibration.Itisshownthataxialforcefrst ;reachesthelimitvalueinloadingphase,andthen ;bendingmomentalsoreachesthelimitvalueasthe ;plateswingsnegativedirection.Inthiscase,dissipation ;developingleadstotoolittleenergylefttopositive ;direction.Whentheplategoesbacktonegativeside ;afterunloading,furtherplasticdeformationinthelow ;layerofstructuresleadstoenergydissipationonce ;again.

    ;Experimentalresultsshowedthatthelayersof ;severeplasticdeformationwithnanocrystalline ;structureswerego~enonpurecopperaftertheSMAT. ;Thelongerthepeeningtimecontinues,thethickerthe ;deformationlayers(Fig.4).Thedepthsofplastic ;deformationwerewellcoincidedwiththebuckling ;heightsoftheplanesamples.Andthebucklingheight ;wasinfluencedgreatlybythepeeningprocessing ;parameters.Thebucklingheightincreasedlittleby ;littlewiththeincrementofthepeeningbightinsome ;periodsandthendecreased,thatis,thebucklingheight ;increasedwiththepeeningheightfirst,andthen ;decreasedafterthepeeningheightwasoveracritical ;value.Theeffectofthenumberofinputtedballsonthe ;bucklingbightwassimilartotheinfluenceofthe ;peeningheight,thatis,itincreasedfirstandthen ;decreasedoveracertainnumber.Thebucklingwas ;highwhenthebiggerballsinsizeandhigherinput ;frequencywereemployed.

    ;3.3Surfaceroughness

    ;Therelationshipbetweenthesurfaceroughness

;andthetreatedtimeisshowninFig.5.Theroughness

    ;ofthesurfaceincreasewiththeprocessingtimeuntil

    ;60minofprocessing,then,theroughnessstartsto

    ;decreaseandgraduallyapproachesaconstantvalue.

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    ;vebehavioronsurfacenanocrystallization

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    ;Fig.4MicroscopiccrosssectionalobservationofpurecopperaftertheSMAT ;for(a)30min,(b)45min,and(C)60rainrespectively ;020406080100120140

    ;Treatedtime(min)

    ;Fig.5Variationofthesurfaceroughness

    ;withtheSMATduration

    ;Suchageneraltrendoftheroughnessvalueasa ;nctionoftheprocessingtimecanbeunderstood ;talitativelybydividingtheimpactingprocessinto ;reestages.Stage1-theroughnessincreasestage: ;tringthisstagetheindentsontheplatecreatedbythe ;lpactingballsareseparated.Eachoftheseindents ;

    ;,neratesapairofthepeakandvalley,i.e.,thebottom ;‘theindentbecomesthevalley,whereasthepileup

    ;artheedgeoftheindentbecomesthepeak.In ;[dition,repeatedimpactsmayhaveoccu~edinsome ;cationseventhoughtheentiresurfaceoftheplatehas ;)tbeencoveredbyindents.Asaresult.theroughness ;,ntinuestoriseduetothenewlycreatedindentsand

    ;bsequentlytherepeatedimpactsinsomeofthe ;tlleysandridgesbetweenindents.Stage2-the ;roughnessdecreasestage:inthisstagetheentireplate ;surfacehasbeencoveredbyindents,andsomeregions ;havebeenimpactedseveraltimes.Thus,thereisno ;moreflatsurfaceforgeneratingnewindents. ;Furthermore,theheightofthepeakregionsis ;continuouslyreduced,whereasthedepthofthevalleys ;willbenotaffectedmuchbyrepeatedimpacts.Asa ;result,theroughnessdecreases.Stage3-the ;steady?statestage:thisisthelaststageinthesurface ;roughnessevolution.Inthisstagetheroughnessdoes ;notchangemuchbecausetherateofgeneratingpeaks ;andvalleysisindynamicequilibriumwiththerateof ;reducingtheheightofpeaks.

    ;3.4Microhardnesss

    ;Themicrohardnessofthecopperplatepeenedby ;SMATwasexamined.Thedistributionsofthe ;measuredhardnessinthethicknessdirectionaregiven ;inFig.6,showingtheinitialhardeningeffectofthe

    ;shotpeeningonthecoppersubstrate.Hardnessinthe ;shotpeenedstateissignificantlyhigherinthe ;nearsurfaceregion.

    ;Themicrohardnessofthetopsurfacelayerare

    ;1.723and1.752GPafor45and60min,respectively, ;whichareabouttwotimeshigherthanthatofthe ;matrix.Theincrementofmicrohardnessmaybe

    ;associatedwithtwomainfactors.Firstly,theincrease ;ofmicrohardnessmaybeattributedtograins

    ;Influenceofcounter.intuitivebehaviorOilsurfacenan0crystallizati0n

    ;refinementonthesurfacelayerfollowingtheclassical ;HallPetchrelationshipbyEquation(2).

    ;o-:+Kd/.(2)

    ;Where6istheyieldstress,60theyieldstressofa ;singlecrystal,Kaconstantanddisthegrainsize.The ;valueofKincreaseswithincreasingtheTaylorfactor. ;Therelationshipisalsosimilarlyapplicablefor ;hardness.Secondly,theenhancementof

    ;microhardnessisprobablyattributedtotheCIB. ;0200400600800l000

    ;Depth(vm)

    ;Fig.6Hardnessevolutionalongthe

    ;depthoftheSMATedsample

    ;3.5Corrosion

    ;Fig.7showstypicalanodicpolarizationcurves ;obtainedfromcoarsegrain,nccoppersamples,

    ;respectively.Thesecurveshavequalitativelysimilar ;behaviorbutwithdifferentvaluesoftheir

    ;electrochemicaldata.Incomparisonwithcoarsegrain

    ;copper,Recopperexhibitedlowercorrosionresistance. ;PrimarypassivepotentialEofnccopper(E=150mV)

    ;wasmorenegativethanthatofcoarse-graincopper ;(E174mV)indicatingthattheformerwaseasierto ;passivitythanthelatter.Itisevidentthatnccopper ;exhibitsaloweractivationenergyforpassivitythan ;thatofcoarse-graincopper.PassivitycurrentdensityI ;ofnccopperwasmuchgreaterthanthatofcoarsegrain

    ;copperindicatingthatthencstructureenhancedthe ;kineticsofanodicdissolutionthatresultsinagreater ;dissolutionratefornccopper.

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    ;Fig.7Polarizationcurvesoftheoriginalsample ;andtheSMATsamplesfor60min

    ;Thecorrosionresistanceofmaterials1sdependent ;uponmanvfactors[.Forncmetalithasbeen

    ;demonstratedthatitscorrosionbehaviorisgreatly ;affectedbyreducingtheaveragegrainsizetothe ;nanocrystallinerange.Reductionofgrainsizeresultsin ;enhancingtheactivityofsurfaceatomsandintergranular ;atoms,givingrisetoanincreaseinpassivityabilityand, ;atthesametime,indissolutionrateofpassivefihn.In ;additiontothespecimendefectcausedbysynthesis ;techniquealsoplaysanimportantroleonthecorrosion ;performanceofbulknccopperexceptforgrainsize.Itis ;impossibletosynthesizenondefectnecopperbulkusing

    ;IGCWCmethod.Microdefectssuchasmicrogap

    ;usuallyoccurontheinterfacebetweengrains.These ;areaswithdefectswouldbecorrodedpreferentialto ;otherareas.Thusthecorrosionofnccopperisgreatly ;relatedtothesedefectsthataresusceptibletocorrosion. ;Inotherwords,defectofncmaterialsfacilitatethe ;corrosion.Inthepresentcase,toacertainextent,the ;lowerresistanceofnccoppertocorrosionistraceableto ;itsdefect.Thesefactorsmentionedaboveleadtoa ;decreaseinthecorrosionresistanceofnccopper. ;4.Conclusions

    ;(1)Thecounterintuitivebehavior(CIB)

    ;phenomenonhasbeenfoundafterunloadingbySMAT. ;Thelayersofsevereplasticdeformationwith ;nanocrystallinestructureswerego~enonpurecopper

    ;aftertheSMAT.Thelongerthepeeningtimecontinues, ;23

    ;8765432

    ;

     ;Influenceofcou

    ;nter-intuitivebehavioronsurfacenan0crystaIIizatiOn ;thethickerthedeformationlayers.Thedepthsofplastic ;deformationwerewellcoincidedwiththebuckling ;heightsoftheplanesamples,andthebucklingheightwas ;influencedgreatlybythepeeningprocessingparameters. ;(2)Theroughnessofthesurfaceincreasewiththe ;processingtimeuntil60minofprocessing,then,the ;roughnesssta~stodecreaseandgraduallyapproaches ;aconstantvalue.

    ;(3)Hardnessintheshotpeenedstateis

    ;significantlyhigherinthenear.surfaceregion.The ;micro-hardnessofthetopsurfacelayerare1.723and ;1.752GPafor45and60min,respectively,whichare ;abouttwotimeshigherthanthatofthematrix. ;(4)PrimarypassivepotentialEofnccopperwas ;morenegativethanthatofcoarse-graincopperindicating ;mattheformerwaseasiertopassivitythanthelatter. ;References:

    ;11WANGT.S.,LUB.,ZHANGM.,eta1.

    ;Nanocrystallizationand0martensiteformationinthe

    ;surfacelayerofmediummanganeseausteniticwear-

    ;resistantsteelcausedbyshotpeening.MaterialsScience ;andEngineeringA.,2007,458:249252.

    ;2]LlUG.,LUJ.andLUK..Surfacenanocrystallizationof ;3l6Lstainlesssteelinducedbyultrasonicshotpeening. ;MaterialsScienceandEngineeringA..2000.286:91.95. ;f31WANGZ.B.,LUJ.andLU.K..Chromizingbehaviors ;ofalowcarbonsteelprocessedbymeansofsurface ;mechani

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