MechanismofStrainInducedNucleationMartensiticTransformation,应变诱发马氏体相变机理

1、JournaloftheLesCommonMetalsElsevierSequoiaS.A.Lausanne-PrintedinTheNetherlandsAMECHANISMFORTHESTRAIN-INDUCEDNUCLEATIONOFMARTENSITICTRANSFORMATIONSINTRODUCTIONWcareprivilegedtotakepartinthisinternationaltributetothedistinguishedProfessorW.GBurgers,andtakethisopportunitytopresentsomeideasaboutasubject

2、whichhasfascinatedhimformanyyears,namelythenucleationofmartensitictransformations.ProfessorBurgers5interestinthisphenomenonistypifiedbyhisclassicpaperwithA.JBogersonPartialdislocationsonthe110)planesintheb.c.c.latticeandthetransitionofthef.c.cintotheb.c.clalticc"1.ThisremarkablepieceofDedicated

3、toProfessorDr.W.G.Burgersincelebrationofhis75thbirthdayThispaperisbasedonresearchattheMassachusettsInstituteofTechnologysponsoredbytheOfficeofNavalResearchunderContractNo.NOOO14-67-A-O2O4-OO27andbytheNationalScienceFoundationunderGrantNo.GK-26631.G.B.OLSONandMORRISCOHENDepartmentofMetallurgyandMater

4、ialsScience.MassachusettsInstituteofTechnology.Cambridge.Massachusetts(U.S.A)(ReceivedJanuary18th,1972)SUMMARYThepreviousworkofProfessorW.GBurgersandDr.A.J.Bogersisusedtodevelopamechanismofstrain-inducedmartensiticnucleation,involvingtwointersectingshearsystems.Wedistinguishbetweenstrain-inducednucl

5、eationandstress-assistednucleation,thelatterinvolvingthesamesitesandembryosasdoestheregularspontaneoustransformationThestrain-inducednucleation,ontheotherhand,dependsonthecreationofnewsitesandembryosbyplasticdeformation;thisphenomenonmayalsocontributeinamajorwaytoautocatalyticnucleationduringForthec

6、aseofstrain-inducednucleation,itispossibletofocusonspecificintersccting-shcarsystemswhentheausteniticstacking-faultenergyislowand£(h.c.p.)martensitecanformasapartofthesheardisplacements.Itthenbecomesfeasibletoextendtheintersecting-shearmechanismfromthisspecialcasetoalloysofhigherstacking-faulte

7、nergy,wherecisnolongerstablerelativetotheausteniteItshouldbenoted,however,thattheseeventsareveryearlystagesintheformationofmartensiticplatesandrelateprimarilytothegenesisofembryos;theactualgrowthstart-upswhichdeterminetheoperational(measured)nucleationratesmaybecontrolledbysubsequentprocesses.J.Less

8、-CommonMetals.28(1972)NUCLEATIONOFMARTENSITICTRANSFORMATIONS111workhasstimulatedustobuildonit,andwenowwishtoextendtheBogers-Burgersmodeltostrain-inducednuclcation.Althoughmostinvestigationsonthekineticsofmartensiticreactionshavedealtwiththespontaneoustransformationunassistedbyexternallyappliedstress

9、es,mvtautmatvniuanjmuiviatpuiuvuuiduvn11anivimaiiuiidaticaiiniron-basealloys)involvesautocatalyticnucleation2-4.Becauseofthemacroscopicdisplacementswhichaccompanymartensitictransformations,thesurroundingparentphaseisplasticallystrained,anditseemsreasonabletoassumethatstrain-inducednucleationcontribu

10、testotheautocatalysis.Stillanotherimportantaspectofstrain-inducednucleationarisesfromthecircumstancethat,whenthistypeofnucleationissystematicallystudiedthroughintentionally-imposedplasticdeformation,thenucleationsitesandcorrespondingembryoscanbeidentified5"7,atleastinspecialcases,andchanneledin

11、tospecificmodels.Itshouldbeemphasized,however,thatweareconcernedherewithnucleationeventswhichdonotnecessarilydeterminetheobservednucleationrates3.Thelatterquantitiesareoperationalinthesensethatthemeasurementsreflectoniythefrequencyofgrowthstart-upswhichproducemartensiticunitsofavisiblesize8.Therate-

12、con-trollingstepsfortriggering-offsuchgrowtharetreatedelsewhere940.Inthepresentpaper,ontheotherhand,weconsideronlythegenesisofnucleationsitesandembryosbyplasticdeformation,thisbeingaprecursortowhatevermaycontrolthelatergrowthstart-upprocess.STRESS-ASSISTEDVS.STRAIN-INDUCEDNUCLEATIONThecomplexinterre

13、lationshipsamongappliedstress,plasticstrain,andmartensitictransformationshavebeenstudiedextensivelybyBollingandRichman11-14inFe-Ni-C久rdFeNi-CrCaIIavqThevwere久hietndefine久temnerAtureM；(lyingaboveMJ,belowwhich"yielding”underappliedstressisinitiatedbytheonsetofmartensiteformation,andabovewhichyiel

14、dingunderstressisinitiatedbyregularslipprocessesintheparentphaseAccordingly,thetemperaturedependenceoftheyieldstressisnegative(normal)aboveAff,butispositivebelowInthelatterregime,thestresstostartthemartensitictransformation(andhenceyielding)approacheszeroasthe胚temperatureisapproachedTheserelationshi

15、psareillustratedschematicallyinFig.1AttemperaturesunderAC.theyieldingwhichaccompaniesthemartensitictransformationoccursbelowtheregularyieldstress(<rjoftheaustenite(asextrapolatedfromhighertolowertemperatures).Here,theobservedyieldstressfollowsatemperaturedependenceconsistentwiththedependenceof陆on

16、appliedelasticstress16.Weregard:henucleationundertheseconditionsasbeingstress-assistedAbovehowever,theappliedstressmustreachorexceeduyinordertoinitiatethemartensitictransformation.AsindicatedbyFig.1,thereisevidence(serratedstress-straincurves)14,15thatthestressatwhichthetransformationisinitiatedatte

17、mperaturesjustabovefollowstheregularyieldstrengthorAtstillhighertemperaturestheinitiatingstressrisesaboveuntilatemperaturelimitisreachedatAldThesymbolhasitsusualmeaninghere,beingthetemperatureabovewhichmartensiteisnotproducedbyplasticdeformation.Itisimportanttonotethat,inthetemperaturerangeaboveMf,t

18、heTemperotureFig.1.Schematicrepresentationofinterrelationshipsbetweenstress-assisted(belowM：)andstrain-induced(aboveM：)nucleationof/martensiteinFe-Ni-Calloys.AfterresultsofBollingandRichmanu14.stressatwhichtransformationisinitiatedliessignificantlybelowtherelatively-steepstress=ass:stedtransformatio

19、nlineextrapolatedupfrombelowTherefore,wepostulatethatthenucleationoccurringaboveisdifferentinkindfromthestress-assistednucleationmanifestedbelowThisdistinctioniseasilyrationalizedonthesuppositionthatplasticdeformationatayandaboveintroducesnew,highiy-potentsiteswhichallownucleationatappreciablylowers

20、tressesthaninthecaseofstress-assistednucleation.Hence,weconsiderthenucleationaboveAlftobe(plastic)straininducedWccannowsummarizethisoverallsituation.AttheusualM.thechemicaldrivingforceissufficientlylargeforthepre-existingnucleationsitesorembryosintheparentaustenitetobecomeoperativewithouttheapplicat

21、ionofstress.AttemperaturesbetweenandM,suchnucleationcanstilloccur,butonlywiththeaidofappliedstress.Thisisthesircss-assisieairarisformaiionregime.Here,therequiredinitiatingstressisintheclasticrange(beloway),butincreaseswithincreasingtemperaturebecauseoftheconcommitantdecreaseinchemicaldrivingforceAtM

22、：、theinitiatingstressfornucleation,basedontheoriginalsitesandembryos,reachesayandplasticstrainingentersthepicture.Evidently,theresultingstrain-inducednudeationcanbeactivatedatlowerstressesthanthestress-assistednucleation,andsotheinitiatingstresstendstofollowjustaboveM：Atstillhighertemperatures,howev

23、er,thefurtherreductioninchemicaldrivingforcencccbilatcbaddilionalplasticstraininginordertoproducedetectableamountsoftransformation;thisrequirestheinitiatingstresstoriseaboveay.Theforegoingdescriptionappliesparticularlytoparamagneticaustenites.BoiiingandRichman12haveshownthat,beiowthestress-assistedt

24、ransformationcharacteristicsarerathersimilarforbothparamagneticandferromagneticaustenites.Ontheotherhand,abovethestrain-inducedtransformationtendstobesuppressediftheausteniteisferromagneticAlthoughthjscuriousbehaviorrequiresfurtherelucidation,itappearsthattheferromagneticstatehasastronginhibitingeff

25、ectonstrain-inducednucleationincontrasttoitsnegligibleeffectonstress-assistednucle-auuii,iiii5vanuumKCiiasuuuiiiuiiaicviucnuciuiuicbcpaiaiciiaiuicuimeiwuiypcbofnucleation.Neutronirradiationexperiments17onausteniticstainlesssteeldemonstrateconvincinglythatthestrain-inducedtransformationisprimarilydep

26、endentonplasticstrainratherthanontheactingstress.Theamountofmartensitictransformationasafunctionofplasticstrainwasfoundtobethesameinboththeannealedandirradiatedmaterials,eventhoughthelatterwasmuchstronger(duetoradiationdamage)thantheformerSPECIALSITESFORSTRAIN-INDUCEDNUCLEATIONTheformationofc(n.c.p.

27、)martensiteiny(f.c.c.)austeniteofiowstacking-iauitenergyiswell-established5.Therearealsosomeinstancesinwhich£hasbeenobservedtoprovidefavorablenucleationsitesfortheformationofaz(b.c.c.orb.c.t.)martensite.Figure2,takenfromastudybyVenables5,showsarformedattheintersectionoftwo£platesinanausten

28、iticstainlesssteel(Type304)deformedat78KThisisclearlyacasewhereanucleatingsitehasbeencreatedbyplasticstraining.Thedthusgeneratedmaybeuonslrucdasaslrain-induucdembryo;ithasnotIriggered-ofTintoacountablemartensiticplateinthesenseofoperationalnucleation,asdiscussedearlierL.dguuuuig-liasrcpuricauidimein

29、icrcuiioiisoiaunvcsupsysicmswiin£platesmayalsoproducefavorablesitesfor&nucleation.ManganonandThomas7foundthatacanbenucleatedbytheintersectionoftwoeplatesorbytheintersectionofaneplatewithatwinorgrainboundaryintheausteniteNotwithstandingtheseinstancesinwhich£intersectionsobviouslyplayade

30、finiteroleinthestrain-inducednucleationofarmartensite,itcanbearguedthattheFig.2.Nucleationof&(b.c.c.)martensiteattheintersectionoftwot(h.c.p.)platesinaustenitic(f.c.c.)stainlesssteel.Venables5ephaseisnotnecessaryforthispurpose,eveninthosealloyswhereecanform.Lagncborg6hasnoted,laterconfirmedbyGoo

31、dchildetal.3ythatacanbegeneratedinausteniticgrainswhicharesoorientedrelativetothetensile-strainingaxisthat£formationissuppressed.Similarly,BreedisandKaufman19havealsoconcludedthatcannucleateindependentlyofthe£,particularlyinthosecaseswheretheeisnoteventhermodynamicallystablewithrespecttoei

32、thertheyora.ThereseemstobearealcorrelationinFc-Ni-Cralloysbetweendecreasingstacking-fauitenergyofthef.c.causteniteandincreasingeaseof&formationwithplasticstrain20-21.Insuchexperiments,£isfoundinthealloysofloweststackingfaultenergy,butthereisnoaccompanyingdiscontinuityintheobservedtrendofotf

33、ormationwithstacking-faultenergy.Thus,althoughstrain-inducednucleationisfavoredbylowstacking-faultenergy,theformationofedoesnotappeartobea叱s邮condition.Theobservationsofanucleationattheintersectionsof£plateswithothercplates,withactiveslipsystems,orwithtwinboundaries,allhaveacommonfeature.Despite

34、theselimitations,the£intersectionmodeofstrain-inducednucleationisworthyofdetailedattentionbecausethesiteandassociatedcrystallographyhavebeenwelldeterminedItisalsoconceivablethat,evenininstanceswherecisnotdetected,£orfaulted£mayactuallyparticipateinsometransientway,andsotheccasemayoffe

35、rtangiblecluesconcerningotherpossibleintersectionmechanisms.Moreover,thereisadynamicaspecttosuchintersectionswhichshouldnotbeoverlooked.Whenausteniticstainlesssteelisplasticallystrained(about15%)at60°C,somecisformed,andthenconsiderablymoreappearsoncoolingunderloadto30°C.buttheresultingconv

36、ersiontoismuchsmallerthanwhentheausteniteisplasticallystrainedto15%directlyat30°C6.Clearly,strain-inducednucleationtothedeformationINTERSECT1NG-SHEARMECHANISMSFORANF.C.C.-B.C.C.TRANSFORMATIONsheared,andfurther,theelementsofbothshearsareofthetype111112仏Weshallnowexaminethisdouble-shearfeatureasa

37、mechanismfortransformingaustenitetomartensiteBogersandBurgers1havealreadybuiltanexcellentfoundationforthisprooiemwunmcirliigenioubnaru-spucicmvuci,tupiuuuucunctumi峠.Figure3(a)and(d)*illustratesanf.c.c.packingofsphereslyinginlllfccplanes.Beforedealingwiththetransitiontoab.c.cstructure,itisinstructive

38、tovisualizetheconfigurationofspheresafteraregularf.c.ctwinningshear,showninFig.3(c)and(f)whereinsuccessivelayersparalleltotheclose-packedplanePVQhaveshiftedbva一/6112、inthedirectionQTperpendiculartoPV.TheresultofthistwinningshearisthattheshearplanePVQanditsconjugateplanePVSremainasclosc-packcd111fccp

39、lanes,whereastheothertwoclosepackedplanesQSVandQSPbecome(100；fccplanes.Thus,the6(fangledenotedinFig.3(a)isenlargedto90°inFig.3(c).*Figure3(dH0arcsectionsthroughthehard-spheremodelinFig.3(aH6correspondingtoJ110|fccplanes.SjchplanesarenormaltotheshearplanePVQandcontainboththesheardirectionQTandth

40、edilatationaldirection.J.Less-CommonMetals.28(1972)NUCLEATIONOFMARTENSITICTRANSFORMATIONS113(a)Originaltrtr<hrdmlarTAngrrnentwithfour111pUimi.b)Intermediatepositionafter1/SofncrmaitwinThrcit<nrvbrtwrcn(111)plaw*tMrallrlk>P*V'“awdPTQ*ha*in«*rMM«dby5.4%.(C)FimdptwfiiM)ah«Tr

41、«mplrt1<rin«frar.(d)(e)(tl-ef)OwmvliontbnHighth*110planvTQN|n«m<iirnhrtoPVFig.3.Hard-spher*modelillustratingtwinningshear条，/6112inausteniteandtheintermediatepositioncorrespondingtoafcc/18<112>wheretheinitiallyclose-packedplaneQSVtakesonthegeometryofa110应planeinQSV：Bogersan

42、dBurgers1.（0）Fig.4.StackingofllOlc-typeplanesinhard-spheremodel(a)aftertheafcc/18<211>shearinFig.3(b)and(e),and(b)afterthesuccessiveshearingofthese1lOj-typeplanestoachievetheproperstackinginaregularb.c.c.structureBogersandBurgers1.Ifwenowtracethepathstakenbythehardspheresduringthetransitionfro

43、meachspheremustrideuptoasaddle-oointpositionaau(cc/18112/«ctsshowninF;g.3(b)sncl(t).3(b)furtherindicatesthiiibetweentheinitialandfinalstates.Henceadilatationalcomponentnormaltotheshearplaneisinvolved;thisexpansionamountsto5.4%atone-thirdofthetwinningtheshearplanePVQanditsconjugateplanePVSarcsti

44、lldimensionallyunchanged,buttheplanesQSVandQSParcdistortedinsuchawaythatthe6(fangleinFig.3(a)isenlargedto70°32'inFig.3(b),thusattainingthegeometryofa110bccplane.Althoughthestackingsequenceoftheseplanesisnotcorrectforatrueb.c.c.structure,thelattercanbeobtainedifsuccessiveplanes(paralleltoQSV

45、orQSP)areshearedaccordingtoFig.41.Referredtotheb.c.cstructure,thisshearcorrespondstoadisplacementofdbcc/8<110>oneachplane,whereasreferredtothef.c.cstructure.Again,becauseofthehard-spherepackingthereisadilatationnormaltotheshearplaneSinceeachoftheaboveshearsentailsadilatationalcomponent,theyarc

46、moreaccuratelydescribedasinvariani-pianestrains.AccordingtotheBogers-Burgersmodel,then,ab.c.c.structurecanbegeneratedfromanf.c.cstructurebytwoinvariant-planestrains(eithersuccessivelyorsimultaneouslywhichcanbethoughtofasone-thirdandone-halff.c.c.twinningshears.Forconvenience,weshallcallthesetheT/3an

47、dT/2shears.BogersandBurgers1pointoutthattheirfirstshear(T/3)mustinvolvedisplacvrnciilboneachj111fccplaneofufcc/18<I12>,thelallctbeingunc-thiidtheBurgersvectorofaShockleypartialdislocation,afcc/6<l12TheyakosuggestthepubiuniiynidibuunpuiuuiuibpiaucinciiiliiigmuuuuiuyuicspicdunigviaShockleypar

48、tialdislocationoveranumberofsuccessive111fccplanes.Thisidea,althoughseeminglystrangeatfirstthought,isnotdifficulttoimagine.Intheconventionalglidemotionofanafcc/6<l12>partialdislocation,theatomspassthroughappropriatepositionsfortheT/3shearproposedintheBogers-Burgersmodel.Underconditionsofsuffic

49、ientchemicaldrivingforce,theatomsmaytendto"stickintheb.c.c.positions"correspondingtotheafcc/18<l12>displacement(Fig.3(b)and(c)ratherthancontinuethroughtothefullBurgers-vcctordisplacementof£ifCC/6<112>(Fig3(c)and(f).However,thefullBurgersvectorcanbeconservediftheatomsinanad

50、jacentplaneareconcurrently4<draggedalong''totheproper“b.c.c.positions".Inthisway,theT/3shearoftheBogers-Burgersmodelcanbeachievedbyanarrayofdfcc/6<ii2)partiaidislocations,averagingoneoneverythirdiii;f<<5plane.Aproblemarisesinconnectionwiththesecond(T/2)shearBogersandBurgerss

51、uggestthatthisshearmayinvolveb.c.cpartialdislocationswithabcc/8<H0>vectorssincethesehavebeenpreviouslyproposedtoexplainb.c.c.twinning22.However,theexistenceofsuchpartialdislocationsisnotgenerallyaccepted.Wecancircumventthisdifficultybyconsideringthesecondshear(T/2)relativetothef.c.c.insteadoft

52、heb.c.c.structure.Thiscomesaboutnaturallyintheshear-intersectionmechanism(obedescribedbelow.Aschematicintersectionofpartial-dislocationarraysforthestrain-inducedformationofab.c.c.embryoinf.c.causteniteisillustratedinFig.5.Theplaneofthisdiagramisparallelto(l10)fcc,andperpendiculartoVQinFig.3.Then,pla

53、nes(Til)fccandcorrespondtoPVQandQSV,respectively.TheT/2shear(thesecondoftheBogers-Burgersshears)canbeaccomplished(withspreading)byanarrayofJ.icss-CommonMetals,28(1972)NUCLEATIONOFMARTENSITICTRANSFORMATIONS#/、ooiFig.5.Schematicillustrationofintersectingshearsduetotwoarraysofafec/6<112>partialdi

54、slocationsinausteniteOnearray(T/3)haspartialdislocationsoneverythird111仏planeandaveragesone-thirdofatwinningshear,whiletheotherarray(T/2)haspartialdislocationsoneverysecondplaneandaveragesone-halfofatwinningshear.Theresultingdoubly-faultedintersectionhasanexactb.c.c.structure.afx/621Tpartialdislocat

55、ionsoneverysecond(lTl)fccplane,justastheT/3shear(thefirstoftheBogers-Burgersshears)canbeaccomplished(withspreading)byanarrayofafcc/6211partialdislocationsoneverythird(Til)plane.EachoftheseBurgersvectorshasacomponentoutoftheplaneofthediagramThepartialdislocationsareshownintheirpositions(丄)aftertheint

56、ersectionprocess,andthestackingfaultsleftbehindoutsidetheintersectedregionaredenotedbyheavylines.(Ofcourse,thesefaultswillbeboundedattheirotherendsbyotherpartialdislocations.)TheBogers-Burgersdislocation-spreadinghypothesisismostlikelytobevalidwithintheintersectedvolumeandduringtheintersectioneventU

57、ndertheseconditions,theatomscanattaintheirtrueb.c.c.positionswhich,aswehaveseen,happentolieintermediaicbetweenthefulldisplacementsinvolvedintheregularglidingofflfCC/6<l12>partialdislocations.Whenreferredtothef.c.c.structure,theintersectedvolumeis"doublyfaulted''(lightersolidlinesi

58、nFig.5),butthisregionnowhasatrueb.c.c.structure.112>oneverysecond：111fccplaneWcnowexaminetheintersectionofaT/3arraywiththecoherentinterfaceofancplate.OnecanwellimaginethatabarrierofthistypewillcausetheT/3setofpartialdislocationstopileupattheinterface.Alikelyplaneinthe£forthepassageofItismore

59、realistictopicturetheintersectionevent,notasacollisionoftwofrontsofmovingpartialdislocations,butastheintersectionofonemovingarraywithastationarypacketofstackingfaultsleftbehindbythepriorpassageofanotherarrayofpartialdislocations.ThissituationisreadilyvisualizedfromthefactthatthesetofstackingfaultsleftbehindbytheT/2dislocationarrayproducesexactlyanc(h.c.p.)plate;inotherwords,theh.c.pstructurecanbemodeledasashearofthesedislocationswouldbe仃OllkEvervsecondbasalolaneintheh.c.D.structureisintheproperpositionforaT/2shear(exceptforthedilatation),andallthatisnecessary