Strain hardening behavior of phase reversion-induced NGUFG a

MaterialsScience&EngineeringA613(2014)60–70ContentslistsavailableatScienceDirectMaterialsScience&EngineeringAjournalhomepage:www.elsevier.com/locate/mseaStrainhardeningbehaviorofphasereversion-inducednanograined/ultra?ne-grained(NG/UFG)austeniticstainlesssteelandrelationshipwithgrainsizeanddeformationmechanismV.S.A.Challaa,X.L.Wana,M.C.Somanib,L.P.Karjalainenb,R.D.K.Misraa,nLaboratoryforExcellenceinAdvancedSteelResearch,CenterforStructuralandFunctionalMaterials,UniversityofLouisianaatLafayette,LA70504-4130,USAbMaterialsEngineering,FacultyofTechnology,UniversityofOulu,90014Oulu,FinlandaarticleinfoArticlehistory:Received22May2014Receivedinrevisedform17June2014Accepted18June2014Availableonline27June2014Keywords:StainlesssteelNanograinedDeformationmechanismStrainhardeningabstractTheconceptofphasereversioninvolvingseverecolddeformationofmetastableaustenitetogeneratestrain-inducedmartensite,followedbytemperature–timeannealingsequence,wasusedtoobtainvaryinggrainsizefromnanograined/ultra?ne-grained(NG/UFG)tocoarse-grained(CG)regimewiththeobjectivetoelucidatetheinterplaybetweenstrainhardeningbehavioranddeformationmechanismsinconjunctionwiththemechanicalproperties.Thestudyunderscoresthatirrespectiveofthegrainstructureandoperatingdeformationmechanisms(twinningversusstrain-inducedmartensite),thegenericnatureofstrainhardeningisunaltered.However,thereweresubtledifferencesinthethree-stagestrainhardeningabilitythatweregovernedbynoticeablydifferentdeformationmechanisms.TherewastransitionindeformationmechanismfromessentiallynanoscaletwinninginNG/UFGandsub-microngrained(SMG)structurestostrain-inducedmartensitetransformationinthe?ne-grained(FG)andCGstructures,abehaviorrelatedtotheincreaseinthestabilityofaustenitewithdecreaseingrainsize.&2014ElsevierB.V.Allrightsreserved.1.IntroductionCurrently,thereisastronginterestindevelopinghighstrengthsteelstoreduceweightinawiderangeofstructuralapplications.Austeniticstainlesssteelsalsobelongtothiscategorybecauseoftheirexcellentmechanicalpropertiesincludinghighstrainhard-eningability.Theyieldstrengthofausteniticstainlesssteelwithcoarsegrainstructureisratherlowandisintherangeof350–450MPa.Theconventionalapproachofgrainre?nementcon-tinuestobeanattractivepropositionforenhancingtheyieldstrengthofengineeringmetalsandalloys[1,2].Ingeneral,thermo-mechanicallycontrolledprocessing(TMCP)isviewedasoneoftheprimaryapproachestoobtaingrainre?nementinawidegroupofmetalsandalloysincludingcarbonsteels.Forinstance,ferritegrainsizeoflessthan5μmcanbeobtainedincarbonsteelsthroughacombinationofTMCPandoptimizeduseofmicroalloyingelementssuchasNbandV[3–6].SevereplasticdeformationprovidesanavenuetoovercomethelimitationsofTMCPtowardgrainre?nementsuchthatsubmicronorultra?negrainsizecanberealized[7,8].InthecontextofobtaininggrainnCorrespondingauthor.Tel.:t13374826430;fax:t13374821220.E-mailaddress:dmisra@louisiana.edu(R.D.K.Misra).re?nementinausteniticstainlesssteels,wehaverecentlydevel-opedaningeniousapproachofdevelopingnanograined/ultra?ne-grained(NG/UFG)structuresinmetastableausteniticstainlesssteelsthatinvolvescontrolledphase-reversionannealingofthestrain-inducedmartensiteformedfromcold-deformedaustenite.Here,anappropriaterollingreduction(typically45–75%)ofausteniteatroomtemperatureleadstostrain-inducedtransfor-mationofaustenite(face-centeredcubicγ)tomartensite(bccα0).Uponannealing,thestrain-inducedα0-martensiterevertsbacktoausteniteeitherthroughamartensiticshearordiffusionalrever-sionmechanism[9–13].NG/UFGstainlesssteelofType301LNwascharacterizedbyacombinationofhighyieldstrengthandelonga-tionoftheorderof900–1000MPaand30–40%,respectively,whichgreatlyexceededtheyieldstrengthof350–450MPaandelongationof$40%ofitscoarse-grained(CG)counterpart[9–13].Austeniticstainlesssteelsexhibitdifferenthardeningmechan-ismsunderdeformation,dependingontheirchemicalcomposi-tion,suchastransformation-inducedplasticity(TRIP)[14]ortwinning-inducedplasticity(TWIP)[15].Thenucleationofbothmartensiteandtwinsduringdeformationactsasaconcomitantsourceofstrongobstaclesforsubsequentmovementofdisloca-tions,inamannersimilartograinboundaries.Theactivationofthesemechanismsisgovernedbystackingfaultenergy(SFE),sothatTRIPeffectisobservedinsteelswithSFEr15mJ/m2,bothhttp://dx.doi.org/10.1016/j.msea.2014.06.0650921-5093/&2014ElsevierB.V.Allrightsreserved.V.S.A.Challaetal./MaterialsScience&EngineeringA613(2014)60–7061TRIPandTWIPwithSFE15–20mJ/m2andTWIPabove20mJ/m2[16,17].Grainsizeisanotherimportantfactorthatin?uencesmicrostructuralevolutionduringthedeformationprocess.Forinstance,grainsizeaffectsthestabilityofausteniteaswellasnucleationsiteandtherateofmartensitictransformation[14].Recently,wehavedemonstratedthatinphasereversion-inducedmetastableausteniticstainlesssteels(Type301LN),thestrainaccommodationmechanismchangesfromTRIPtoTWIP,whenthegrainsizeisdecreasedfromCGtoNG/UFGstructure[13].Theunderlyingreasonforsuchabehaviorwasattributedtoincreaseinthestabilityofaustenitewithdecreaseingrainsize.MotivatedbythechangeindeformationmechanismwithchangeingrainsizefromCGtoNG/UFGregime,itisnowintriguingtostudytheircorrespondingstrainhardeningbehavior.Thus,theobjectiveofthestudydescribedhereistoexplorethestrainhardeningcharacteristicsusingthedataacquiredfromthetensiletestsandconductanin-depthanalysisofstrainhardening.ThestudyisuniquefromtheviewpointthatitanalyzesanddifferentiatesthestrainhardeningbehaviorinasinglesteelthatexhibitsbothTRIPandTWIPeffects,dependingonthegrainsize.2.ExperimentalprocedureThestartingmaterialwasacommercialtype301LNausteniticstainlesssteelof$1.5mmthicknessandthechemicalcomposi-tion(inweightpercent)ofsteelispresentedinTable1.Thestripswerecoldrolledinalaboratoryrollingmillto$62%reductioninthicknessandsubsequentlyannealedinaGleeble1500thermo-mechanicalsimulator.Theannealingexperimentswerecarriedoutonstripsof120mm?10mm(thickness$0.6mm).Theheatingratetotheholdingtemperaturewas2001C/s.Theannealingtemperaturewasvariedintherangeof700–10001Candholdingtimevariedbetween1and100s.Thechangeinannealingtemperature–timesequenceenabledustoobtainawiderangeofgrainsizefromtheNGtoCGregime[13].Followingannealing,allsampleswerecooledinairblowwithcoolingratesatleast2001C/sdowntoabout4001C.Theannealingconditionsweresoselectedthattherewascompletereversionofmartensitetoaustenite.Themeasurementofgrainsizeinvolvedtwomethods.Inthe?rstapproach,theaverageaustenitegrainsizeofthesampleswasdeterminedbyanalyzingatleast100grainsinthemicrographsusingtheImageJsoftwaretodeterminethemeanlinearinterceptgrainsize,d.Thesecondmethodinvolvedapplicationofanapproachpreviouslyusedbyoneoftheco-author's(Karjalainen)group[18].Theobjectiveofthisapproachwastotakeintoconsiderationgrainsizedistributionandtherebymeasureaweightedaveragegrainsize,dw.Here,about100grainsweredistributedinbinsof250nm(0.25μm)insize.Whileasmallbinsizeisexpectedtoresultinpoorstatisticalaccuracy,alargebinsizemaymasktheeffectofsmallgrains.Thus,anoptimumbinsizeof250nm(0.25μm)wasselectedforastatisticallyrelevantdistribution,keepinginviewthegrainsizerangeofsamples.Denotingthenumberofgrainsintheithbinasnianddividingitbythetotalnumberofgrains,N,theweightoftheithbinis[18]wii?nNe1TTable1Chemicalcompositionofausteniticstainlesssteelinwt%.CSiMnCrNiMoN0.0170.521.2917.36.50.150.1500Moreover,thesquarerootofthearealmeanofnigrainsintheithbingivestheaveragegrainsize,di,fortheithbin.Knowingdiandwi,theweightedaveragegrainsizeofthesampleiscalculatedby[18]Ndw?∑widie2Ti?1Tensiletestswereconductedbymachiningphasereversionannealedspecimenswithapro?leof25?25mm2and20mmgagelength.Thisnon-standardgeometryisbecauseofthenarrowuniformannealedzoneinGleeblesamples.Tostudythedeformedmicrostructureasafunctionofstrain,thetensiletestswereinterruptedatselectedengineeringstrainof0.02,0.1and0.2(i.e.,truestrainof0.02,0.096,and0.182).Areaclosetothehighlystressedregionwithinthegagelengthwasusedfortheprepara-tionoftransmissionelectronmicroscope(TEM)foilsfromanumberoftensile-testedspecimensforeachcondition.Thespeci-menswereexaminedinTEM(HitachiH7600)operatedat120kV.Thinfoilswerepreparedbytwin-jetelectropolishingof3mmdisks,punchedfromthespecimens,usingasolutionof10%perchloricacidinaceticacidaselectrolyte.Anumberoffoilswereexaminedforeachexperimentalcondition.3.ResultsMicrographsofselectedgrainstructureobtainedviadifferentannealingtemperature–timecombinationsarepresentedinFig.1[13].Thegrainsizedataisrevisitedheretodevelopanunder-standingofstrainhardeningbehaviorinconjunctionwithgrainsizeanddeformationmechanisms.Thedifferentgrainsizesarereferredasnanograined/ultra?ne-grained(NG/UFG),sub-microngrained(SMG),?ne-grained(FG)andcoarse-grained(CG)(Table2).3.1.TensilebehaviorThetensilepropertiesarelistedinTable2.Thetruestress–truestrain(σT–εT)curvesplottedfromtensiledataarepresentedinFig.2,whicharecharacterizedbyanin?exioninalltheplotsbecauseofthemicrostructuralevolutionduringplasticdeforma-tion,andaredifferentfromthoseofmostsingle-phasemetals/alloyswithoutphasetransformation[20].However,theplateauatlowstrainsismoreobviouswithNG/UFGandSMGstructuresthaninFGandCGones.ThetensilebehaviorofCGspecimenappearstoshowacontinuousincreaseintruestresswithincreaseinstrain,whileNG/UFGandSMGstructuresshowasmallplateau(indicatedbyarrowsinFig.2)atsmallstrainspriortoacontinuousincrease.TheFGstructurerepresentsatransitionbetweenthetwodifferenttensilebehaviors.Thus,atthestartoftensilestraining,thetruestressincreasesatarelativelyslowerratewithincreaseinstrainforNG/UFGandSMGstructuresthanforCGandFGones.3.2.StrainhardeningbehaviorThestrainhardeningrates(SHR;dσT/dεT)computedfromthetruestress–strainplotsinFig.2areplottedasafunctionoftruestraininFig.3forsampleswithdifferentgrainsizes.InthecaseofNG/UFGandSMG,theSHR?rstdecreasessteeply,reachingaminimumat0.02–0.03strain,followedbyarapidincreasetoahighdσT/dεTlevelatrelativelylowstrains($0.15and0.175,respectively),andthena?naldecreaseuntiltheonsetofneckingtofracture.AsregardsFGandCGsamples,theirSHRsdecreasemoreslowlytoaminimum,followedbyarapidincreasetoaSHRpeaklevelatsomewhathighstrains($0.225and0.25,62V.S.A.Challaetal./MaterialsScience&EngineeringA613(2014)60–70Fig.1.(a–c)TEMmicrographsofphasereversionannealed301LNtypeaustenitestainlesssteelwithvaryinggrainsizefromNG/UFGregimeto?ne-grained(FG)regimeand(d)lightmicrographsofCGsteel.Theaverageweightedgrainsizedwwasdeterminedfromanumberofmicrographsandisindicatedoneachofthemicrographs.NG/UFG:nanograined/ultra?ne-grained,SMG:sub-micron-grained,FG:?ne-grained,andCG:coarse-grainedsteels(adaptedfromRef.[13]).Table2Tensilepropertiesofphasereversion-inducedausteniticstainlesssteelwithdifferentgrainsizes.WeightedaveragegrainsizeNG/UFGSMGFGCG320nm757nm2132nm22μmAverageyieldstrength,MPa768722667350%averageelongation34384140TheNG/UFGtoCGregimewasobtainedbycoldworkingto62%reductionfollowedbyphasereversionannealinginthetemperaturerangeof973K(7001C)to1273K(10001C).Fig.2.Truestress–truestrainplotsforphaserevisionannealedausteniticstainlesssteelsprocessedtoobtaineddifferentgrainsizefromNG/UFGtoCGregime.respectively)andthena?naldecreaseuntiltheonsetofneckingtofracture.Thus,basedonaboveoutlinedobservationsinFigs.1–3,wecanconcludethattherearesubtlebutdistinctdifferencesinthetensilebehaviorofsampleswithdifferentgrainsizesinaregimeextendingovertensofnanometerstoseveralmicrometers,wherebehaviorofNG/UFGandSMGstructuresbelongtoonecategory,FGrepresentsthetransitiongrainsize,andCGcanbereferredasbelongingtothesecondcategory.ThestrainhardeningbehaviorwasfurtherunderstoodintermsofCrussard–Jaoul(C–J)analysis(ln(dσT/dεT)vs.ln(εT)plot),showninFig.4.AchangeintheslopeofalinesegmentintheC–Jplotcanprovideafurtherinsightintopossibledifferencesindeformationmechanisms.InearlierstudiesconductedbyotherresearchersusingC–Janalysis[15,19–22],thenaturallogarithmicvaluesofSHRs(ln(dσT/dεT))wereseparatedintodifferentstagescharacter-izedbyadecreaseorincreaseinln(dσT/dεT)aswellasnearlyconstantplateausorpeaklevel.Fromthepointofviewofclarity,inFig.4bwepresentananalysisforNG/UFGandCGstructures.Wede?ne?vestagesandthedataissummarizedinTable3.InstageAoftheCGstructure,thelogarithmicvalueofSHR(ln(dσT/dεT))decreasesrapidly.ThisisfollowedbyashortstageB,whenthe(ln(dσT/dεT))remainsalmostconstant.Withincreaseinstrain,thelogarithmicvalueofSHR(ln(dσT/dεT))increasesgradually,V.S.A.Challaetal./MaterialsScience&EngineeringA613(2014)60–7063attainingahighstrainhardeningcoef?cient(stageC).Withfurtherincreaseinstrain,logarithmicvalueofstrainhardeningrate(ln(dσT/dεT))remainsnearlyconstantoveranarrowstrainrange(stageD).Inthelaststagetheln(dσT/dεT)decreasesrapidly,markingtheonsetofneckingleadingtofracture(stageE).IntheNG/UFG(alsoSMG),stageBdoesnotexistbecausetherewaswell-de?nedyieldpoint.StageCiscloselyrelatedtothesmallplateausinthetruestress–truestrainplotsmarkingtheyieldbehavior(Fig.2).Thedifferencesbetweenthetwogroupsofstructuresviz.,NG/UFG/SMGandCG,withFGrepresentingthetransitiongrainsizecanbesummarizedasfollows:(a)InstageA,theSHRdecreasesrapidlyforNG/UFGstructure(andalsoSMG),whilethecorrespondingdecreaseinCGstructureisatarelativelyslowerratesuchthatstageAisextendingoverashortstrainforNG/UFG(andSMG)andlargestrainforCG.(b)Second,stageBispresentinFGandCG,butabsentinNG/UFGandalsoSMGstructures.(c)Third,theminimumvalueofSHRinstageBisgreaterforCG(andFG)andlowerforNG/UFG(andSMG)structures.TheminimumSHRcorrespondstotheintersectionofstagesAandCinNG/UFGandSMGstructuresandis$465MPaand$1346MPaforNG/UFGandCGstructures,respectively(Table4).(d)Withincreaseingrainsize(NG/UFGtoCG),theplotofstageCgraduallymovestoright,i.e.,towardshigherstrain.(e)ThemaximumSHRsforallstructureswithdifferentgrainsizeinstageDwerehigh(Table4).(f)Asaconsequence,stageEalsograduallymovedtoright,i.e.,toahigherstrainwithincreaseingrainsize(NG/UFGtoCG).Allthesevariationswithdifferentgrainsizesarerelatedtomicrostructuralevolutionanddeformationmechanisms,andarediscussedinnextsection.Fig.3.Strainhardeningrateasafunctionoftruestrainasafunctionofgrainsize.3.3.MicrostructureevolutionduringdeformationThemicrostructuralevolutionasafunctionofselectedstrainfordifferentgrainsizeispresentedinFigs.5–8.IntheNG/UFGstructure,therearenumerousstackingfaults(SF)atlowstrain(ε?0.02)andextendedstackingfaults(SF)intwosystemsinter-sectedoneanother,anddislocationswereinhibitedbystackingfaults(Fig.5b).Withincreaseinstrain(ε?0.1),mechanicaltwinsofnanoscalethicknesswerealreadypresent(Fig.5c).Insomegrains,twinsgrewindifferentdirectionsandmutuallyintersectedoneanother(Fig.5d).Dislocationglideduringplasticdeformationisinhibitedbytwinboundaries.Atappreciablyhigherstrainofε?0.2,therewasanincreaseintwindensitysuchthatthetwinsorganizedinbundles(Fig.5eandf).Thebundlesinsomecaseswere$300nmlarge.Thediffractionpatterncon?rmedthepre-senceofdeformationtwins.Fig.6summarizestherepresentativeTEMmicrographsofthedeformationprocessesassociatedwiththetensilestrainingfortheTable4Themaximumandminimumstrainhardeningratesofsamples.SampleMinSHRinstageB,MPaMaxSHRinstageD,MPaFig.4.TheC–Janalysis(ln(dσ/dε)vs.lnε)forsamplesasafunctionofgrainsize.NG/UFG4652864SMG4893168FG9753156CG13462710Table3ThevalueofplasticstrainofeachstageofthesampleswithdifferentgrainsizesbasedonC–Janalysis.ThedatawasextractedfromFigs.2and4.SampleNG/UFGSMGFGCGStageAεo0.014εo0.032εo0.066εo0.093StageB––0.066oεo0.0960.093oεo0.133StageC0.014oεo0.1330.032oεo0.1500.096oεo0.1910.133oεo0.202StageD0.133oεo0.1560.150oεo0.1770.19oεo0.2330.202oεo0.271StageE0.165oεo0.300.177oεo0.330.233oεo0.380.271oεo0.4164V.S.A.Challaetal./MaterialsScience&EngineeringA613(2014)60–70Fig.5.Representativebright?eldtransmissionelectronmicrographsofNG/UFGstructureillustratingtensilestrain-induceddeformationstructureat(a,b)engineeringstrainof0.02(truestrain?0.02),(c,d)0.1(truestrain?0.096),and(e,f)0.2(truestrain?0.182).Thediffractionpatterncon?rmedtwins.SMGstructure.WenotefromFig.6aandbthattherearenumerousstackingfaultsinsideagrainatalowstrain(ε?0.02).Withincreaseinstrain(ε?0.1),themainmicrostructuralfeaturesareformationoftwinswithdifferentgrowthdirectionsandtheymutualintersecteachother(Fig.6candd).Athighstrain(ε?0.2),twinsintersectedeachother,separatingthegrainintomanysmallregions(Fig.6e).Someofthetwinswereorganizedinbundles(Fig.6f).WenowdescribethebehaviorofphasereversionannealedFGsteelincomparisontoNG/UFGandSMGstructures.InFGsteel,

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