Nanometer

Nanometer Availableonlineatwww.sciencedirect.com cienceDirect JoI瓜NALOFIRONANDSTEELRESEARCH,INTERNATIONAL.2007,14(1):64-68 NanometerStripeMicrostructureofSupersaturatedSolid lutioninFe-CuAlloy RENHui-ping,WANGHai-yan, (CollegeofMaterialandMetallurgyEngineering,Inner Baotou014010,InnerMongolia,China) LIUZong—chang,ANZhi-guo MongoliaUniversityofScienceandTechnology, Abstract:StrengtheningduetoprecipitationofCuinthea-Fematrixisanimportantphenomenonutilizedinthede— signofHSLAsteels.Inthepresentwork,themicrostructureofsupersaturated,solidsolutioninFe-1.18Cubinary alloywasinvestigatedbymeansofhighresolutionelectronmicroscopy.Theresultsindicatedthatthesolidsolution washeterogeneous.therewerelotsofCuatomclusters,whichconsistedofdiffractivestripemicrostructuresimilar totwincrysta1.Orientationdeviationwasobservedbetweentwo(110)planesindiffractivestripes,whichresultsin lightandshadecontrast.Furthermore,formationmechanismsofthenanometrestripemicrostructurewerediscussed intermsoftheinteractionofCuandFeatomsintheFe-Cubinaryal1oys. Keywords:Fe-Cual1oy;supersaturatedsolidsolution}Cu-richcluster;nanometerstripemicrostructure}highreso— lutionelectronmicroscopy Developmentprogramofnewgradehigh strengthlowalloy(HSLA)shipsteelswasbrought outbyAmericaninthe1980s[?.ApplicationofHS— LA一80andHSLA一100steelshasopenedanewarena forstructuralsteels.Decreasingthecarboncontent givesabenefitintermsofweldability,corrosionre— sistanceandsubambientimpacttoughness.Addition ofcopperhasbeenusedtocompensatethestrength lossfromreducingcarboncontentsinHSLA steelsC2--4].Theexcellentperformanceofthissteel derivesfromCuparticlesprecipitationhardeningon aging.Copperalloyinghasthebenefitofsubstantial strengtheningduetoCuprecipitation[引.Moreover, fineprecipitatesofCuperformedanextraadvantage ofhighresistancetofatiguecrackinitiation[引.With theadditionofCuinFe-Cualloys,strengthening effectcanbeobtainedforthereasonofcopperpre— cipitatinginferritematrix.Accordingly,manyre— searcheffortshavealwaysbeendirectedtodevelo.. ping'HSLAsteels.AusefUlmodelalloyforunder— standingtheCuprecipitationstrengtheningmecha- nismistheFe-Cualloysystem[. Manyinvestigationshavebeenfocusedonthe precipitationbehaviorofFe-Cualloy.Thestudyof microstructureinsolidsolutionstatehasnotbeen exploredasyet,butthatshouldbeveryimportant sincesolutiontreatmentisthepreconditionofaging precipitation[.州.Inthepresentstudy.nanometer stripemicrostructureofsupersaturatedsolidsolu— tioninFe-Cualloy(wc=1.18)hasbeeninvesti— gatedbymeansofHREM. IExperimental HighpurityFe—Cubinaryalloywaspreparedby vacuummelting.Thechemicalcomposition(massper— cent,)isC0.005,Si0.031,Mn0.034,Cu1.18, P?0.002,andS?0.005.Thealloywasforgedand rolled,andasamplewiththesizeof10mm×8mmX 8mmwasprepared.Thesamplewasannealedat 850?for2to10htoreachafullysoliddissolved state.Anagingtreatmentwasthenconductedat550 ?foratimerangingfrom100sto100h.Themi— crostructureofsupersaturatedsolidsolutionwasob. servedunderTEMofJE0LJEM一2010. FoandationItem:ItemSponsoredbyNationalNaturalScienceFoundationofChina(503610 01) mog:RENHui-plng(1963 一),Male,Doctor,Professor;E-mail:renhp555(~yahoo.corn;RevisedDate:April9,2006 S , ,, _l?? .? ? No.1NanometerStripeMicrostructureofSupersaturatedSolidSolutioninFe-CuAlloy 2ResultsandDiscussion Fig.1showsthemicrostructureofsolidsolution ofFe-Cualloy,inwhichmanyequiaxedferritecrys— talscanbeobservedinthematrixunderopticalmi- croscopeEFig.1(a)],whileFig.1(b)(TEMpho— tO)showsthattherearelotsofparticlesofabout4 to20nmdispersedintheferritematrix.According totheFe-Cuphasediagram,Fe-Cualloyisinthea— Fesinglephaseregionat850?.sotheseparticles couldonlybeCuatomclusters.Thesupersaturated Cuclustersinthea—FematrixtendstoevolveasCu particlesbyrecrystallizationannealingcourseafter agedat550?[.Manyinvestigationsindicatethat lotsofCu—richclustersexistinferritecrystalsofFe- Cualloy~?.Fig.2showsthattheseclustersconsist ofsomestripes.whichlookliketwincrystalmicro— structureandtakeonbrightanddarkcontrast. Fig.3showsthehighresolutionlatticeimage observedbyhighresolutionelectronmicroscope.It canbeseenfromFig.3(a)thatthosestripesmen— tionedaboveshowcrystallatticeimageafterFourier transformationregeneration.Thecrystalplanes presentedunderTEMshouldbe(110)planes, sincethe(110)spacinginBCCferriteisofmaxi— mum(0.20268nm).Thus,atomstackof(110). canbeobservedclearlyinBandDzoneasshownin Fig.3.Every(110)extendedfromtheferritema— trixtoCuatomclustersalongtheABCDEdirection untilitishamperedbyedgedislocationscoming fromthecrystallatticesaberration. Itiswellknownthatthereisanelectronegative (a)LOM;(b)TEMimage Fig.1Microstructureinsolidsolutionstate differencebetweenFeandCuelement.Moreover, differentcrystallatticeconstantsarealsoimportant factor.Consequently,whenCuatomssoliddissolvedin a—Fe,thecrystallatticeaberrationandmisfitwould Fig.2NanometerstripemicrostructureinCuatomdusters comeintobeing,resultingindiversificationofelec- trondiffractionintensity.A,CandEzonespresent darkfortheweakdiffractiveintensity,whilethe (110).planecanbeseenclearlyinFouriertransfor- mationregenerativeimage.Fig.3showsthatthere areabout2.to4.disorientationbetweenB.Dzone andA,C,Ezone,whichresultsinsharpcontrastof lightanddarkstripes.Thediffractionstripesin clustersindicatethatCuatomsaredistributedregu— larlyandtendtocongregateonthe(001).planesof BCCferritesincetheelasticmodulusin<001>.di— rectionisminima1.Electrondiffractionanalyses provethecorrectnessoftheidea(Fig.4).Cuatoms congregateon(002).and(001).,hencetheyshould alsoclusteron(110).planes,whichhavemaximum planespacing,thatmakelatticeimageof(110) planescanbeobservedclearl3~underTEM. ?66?JournalofIronandSteelResearch,InternationalV01.14 (a)RegenerativeimageafterFouriertransformation; (b)Fouriertransformationpatternofregenerativeimage(a) Fig.3LatticeimageofstripemicrostructureofCu atomclustersinsolutionstate Theselectedelectrondiffractionanalysesindi— catethatCuclusterscouldhaveBCCstructure. Thediffractionpatternswereobtainedindifferent incidentdirections.Fig.4showsthediffractionpat— ternsandindicesforfourdifferentincidentdirec— tionsofelectronbeams.Thebigdiffractionspots arethereflectionsofBCCa-Fe.whilethesmall spotsareCuatomclusters.Therearereciprocal spotsof(001)planearoundthecentralspotswhen theincidentelectronbeamsarein[100],[110], [012-]direction.AscanbeseenfromFig.4(a), somesmall(ooi).planesdiffractionspotswasob— servedregularlyaroundthecentrespots;while Fig.4(b)indicatestheappearanceof(001)and (002)planesdiffractionspots,whichimpliesthe existenceofCuatomclustersat(001).and(002). planes.Ontheotherhand,thediffractionpatterns inFig.4(c)and(d)showsomeadditionaldiffrac— tioncircleswhentheincidentbeamsarein厂111-]and [o12]directionrespectively.' Solidsolutionstateisunevenonatomicscale fromthepointofviewofthdrrhodynamics.When heterogeneousatomstend"t, Ocl6seneighbouring,it behavesshortdistanceoI=din套;onthecontrary, clusterswouldappearh~omogeneousatoms tendtocloseneighbopL'ThatimpliesthereareCu clustersinthesupersaturatedferriteofFe-Cualloy inthermodynamicequilibrium,however,theCu contentofclusterscannotbemeasuredaccuratelyby meansofpresentanalysismethods;. Somediffractivestripeswe'reobservedinCu clusters.theappearanceofwhichlooksliketwin crysta1.buttheformationmechanismhasntbeen describedyet.Latticeaberrationwouldappearwhen Cuatomsaredissolvedintoa—Fe,sincethereare somedifferencesincrystalcharacterandatomdi- mensionbetweenCuandFeatoms.Theelasticmod- Uleofcubiccrystalin<001>directionisthemini- mal,soCuatomstendtoclusteron(001)planesto lessenaberrationin[OOl-]direction.whichresultsin thearrangementofCuatomson{001}planesform- ing(110)crystalplanes.Consequently,greatlat— ticeaberrationwouldoccurinthe[I10]direction andaccordinglyinducecrystallatticetwist.Ascan beseenfromFig.3,distorted(110)planeappeared ontheboundaryofA,CandB,Dzones,inaddi— tion.2.to4.orientationdeflexionandlotsofdislo— cationswereobservedinFig.3,whichcausedbythe accommodatingeffectofself-organizedfunctionon aberrationenergy[引.whichresultsinbrightand shadediffractivestripes.Atomicdensityaremaxi— malin[Tll-]and[1T1-]directionson(110)plane, therefore,theelasticmoduleinthedirectionis maximaleither.ClusterofCuatomscouldbring lotsofdislocationandstackingfaults.whichre— sultsingreatcrystalaberration.When'theaberra— tionreachesacertaindegree,theself-organized functionofsystemwilladjustthestraindirectionto reducestrainenergy.Thisisthepotentialreason why(110).planepresentstwistevery1to3nm (Fig.3). Thenanometerstripemicrostructureinclusters shouldhavegreateffectonCuprecipitation strengtheninginFe—Cualloy,whichhasbeenproved bypreviouswork[.,.,引. 3Conclusions (1)Thesupersaturatedferriteofthetested Fe—-Cualloyisheterogeneousaftersolutiontreat.. ment.TherearelotsofCuatomclustersmicro— structureswithaveragesizeofapproximately2Onm intheequiaxedferritegrains. (2)2.to4.orientationdeflexionwasobserved betweentwo(110)planes,whichcausedbycrystal latticeaberrationofCuclusters.andthisleadsto brightandshadediffractivestripes. (3)Somediffractivestripemicrostructuresim— ilartotwincrystalexistsinCuclusters.anditsfor— marioncanbeexplainedbystrainenergyandsystem self_organizedfunction. N0.1NanometerStripeMicrostructureofSupersaturatedSolidSolutioninFe-CuAlloy?67? (a)B—E1oo];(b)B—E11o];(c)B—E111];(d)B—Eo12] Fig.4Diffractionpatternandindex ?68?JournalofIronandSteelResearch,InternationalVo1.14 TheauthorswouldliketothankInnerMongolia UniversityofTechnologyforprovidingelectronmi, croscope. 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