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BRITTLE-TO-DUCTILETRANSITIONINNiAlSINGLECRYSTALS By SANJAYSHRIVASTAVA ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1997 TOMYPARENTS ACKNOWLEDGMENTS Iwouldliketoexpressmythankstoanumberofindividualswhohaveinfluenced mythoughtprocessduringandpriortothepursuitofthisdegree. Theseindividuals,in chronologicalorder,areDr.S.Prakash,Dr.A.K.Patwardhan,Dr.K.M.Gupt,Dr.M.N. Shetty,Dr.R.G.Reddy,Dr.M.C.Fuerstenau,Dr.D.A.Jones,Dr.D.Chandra,Dr.S. Labana,Dr.H.Gandhi,Dr.C.Narula,Dr.A.Gangopadhyay,Dr.K.J.R.EUwood,Dr.J. Braslaw,Dr.F.Ebrahimi,Dr.M.J.Kaufman,Dr.R.T.DeHoff,Dr.J.J.Mecholsky,Jr., Dr.B.Sankar,Dr.S.Pearton,andDr.R.Abbaschian. Withouttheinfluence(and friendshipofsome)oftheseindividuals,Iwouldnothavecometothispoint. Iextend mysincerethankstoDr.F.Ebrahimiforgivingashapetothisdissertationandforher advicethroughoutthisstudy. SpecialthanksareduetoDr.M.J.Kaufmanforhissupport andadvice. IamthankfultoDr.R.T.DeHoffforhishelponthermodynamicconcepts involvedinthisworkandDr.J.J.Mecholskyforhishelponfractureaspectsofthisstudy. IwouldliketothankAndreinaGomezforherconstantsupportand encouragementthroughoutthecourseofthisstudy. Anumberofindividualsinthe departmentwhoseinteractionshavebeenusefulandfruitfiilincludeT.Hoyle,D.Fayard, Q.Zhai,D.Kong,I.Kabyemera,T.Matthews,K.Sloan,M.L.Henne,Y.J.Lim,J.Hu, M.Lakshmipathi,T.Adams,R.Kaufman,Dr.M.Weaver,N.Kulkamee,Dr.C.V. Iswaran,Dr.K.T.Hong,Dr.R.Bendale,andS.Subramanyam. ThanksareduetoMary SwansonandDebbieHallintheacademicrecordsoffice,AprilandJulieinthefinance office,andWayneAcreeinMAICformakingmylifeeasier. Financialsupportduring thisstudyfromtheAirForceOfficeofScientificResearch(URIGrantNo.F49620-93-1- 030)isgratefullyacknowledged. Lastbutnottheleast,Iamspeciallythankfultomyparents andelderbrotherwho havebeenaconstantsourceofinspiration. iv TABLEOFCONTENTS ACKNOWLEDGMENTS iii Abstract viii CHAPTER 1 INTRODUCTION 1 CHAPTER2 LITERATUREREVIEW 5 2.1PropertiesofNiAl 5 2.1.1CrystalStructureandPhaseStability 5 2.1.2SlipandFlowBehavior 7 2.1.3Ductility,FractureToughnessAndFracturePath 10 2.1.4EffectofPointDefectsonMechanicalProperties 12 2.2PrinciplesofFractureMechanics 14 2.3TheoriesoftheBDT 18 CHAPTER3 MATERIALSANDEXPERIMENTALPROCEDURES 29 3.1SingleCrystalProcurement 29 3.2ChemicalCharacterizationofSingleCrystals 32 3.3HomogenizationTreatment 32 3.4OrientationCharacterization 33 3.5SpecimenCutting 34 3.5.1FourPointBendSpecimens 35 3.5.2Double-Notched-TensionSpecimens 35 3.6Electropolishing 37 3.7IntroductionofNotch 38 3.8Prestraining 39 3.9HeatTreatments 39 3.10FractureToughnessTesting 40 3.11TensileTesting 42 3.12Microscopy 43 V CHAPTER4 RESULTSANDDISCUSSION 44 4.1TensileProperties 44 4.1.1Stress-StrainCurves 45 4.1.2TemperatureandStrainRateDependenceofFlowStress 50 4.1.3TemperatureandStrainRateDependenceofStrainHardening Exponent 56 4.1.4TensileDuctilityandFractureStress 59 4.1.5CalculationofActivationEnergy 60 4.2Brittle-to-ductiletransition 63 4.2.1EffectofDisplacementRate 68 4.2.2DeterminationofActivationEnergy 76 4.3EffectofPrestrainingonToughness 78 4.3.1PrestrainingProcedure 78 4.3.2SlipTraceAnalysis 81 4.3.3EffectofPrestrainingonRTFractureToughness 83 4.3.4EffectofPrestrainingonBDTT 85 4.4ToughnessAnisotropy 90 4.4.1EffectofAnnealingoftheNotch 91 4.5EffectofHeat-TreatmentsonFractureToughness 94 4.5.1Heat-treatmentinthePrestrainedCondition 98 4.6Fractography 99 4.6.1CrackPropagation 100 4.6.1.1EffectofLoadingMode 104 4.6.1.2EffectofOrientation 107 4.6.1.3EffectofTemperatureandDisplacementRate 109 4.6.2CrackNucleation 115 CHAPTER5 GENERALDISCUSSION 123 5.1BDTT 123 5.2Prestraining 126 CHAPTER6 CONCLUSIONS 130 LISTOFREFERENCES 133 APPENDIXA EFFECTOFNOTCH-TYPEONFRACTURETOUGHNESS 138 APPENDIXB vi DETERMINATIONOFSTRAINHARDENINGEXPONENT 143 BIOGRAPHICALSKETCH 149 vii AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy BRITTLE-TO-DUCTILETRANSITIONINNIALSINGLECRYSTALS By SanjayShrivastava May1997 Chairperson:Prof.FereshtehEbrahimi MajorDepartment:MaterialsScienceandEngineering NiAlisapotentialcandidateforhightemperaturestructuralapplicationsbecause ofitshighmeltingpoint,highthermalconductivity,relativelylowdensity,andgoodhigh temperatureoxidationresistance. However,itsuffersfromahighbrittle-to-ductile transition(BDT)temperatureandalowroomtemperaturetoughness. Thepresentstudy focusesonunderstandingthemechanismoftheBDTandinvestigatingtheeffectsof strainrateandprestrainingontheBDTinstoichiometricNiAlsinglecrystals. TheresultsofthisstudyindicatedthattheBDTtemperature(BDTT)correlates withtheonsetofnet-sectionyieldinginthedouble-notchedtensionspecimens.Astrong dependenceoftheBDTTontheapplieddisplacementratewasobserved. Itwasproposed thattheBDTarisesfromareductioninstrainhardeningrateaswellasinyieldstrength withtemperature. Thesimilarityfoundinactivationenergyvaluescalculatedbasedon thestrainratedependenceoffracturetoughnessandtensiledatasupportsthisproposal. viii Whiletheprestrainingwasfoundtoincreasethetoughnessatroomtemperature nearlytwo-fold,italsoincreasestheBDTTofNiAlsinglecrystals. Thisobservation suggestedthattheBDTofNiAlisnotlimitedbythedislocationdensity. Basedonthe fractographicanalysis,itwasfoundthatlocalizationofstrainisresponsibleforcrack nucleationinNiAlsinglecrystals. Itwassuggestedthatthecrackinstabilityis "nucleation-controlled"atlowtemperatureswhereasitis"propagation-controlled"at hightemperatures. Basedonthissuggestion,theincreaseinroomtemperaturetoughness uponprestrainingwasattributedtothereductioninthelocalizationofstrainupon prestrainingachievedduetotheenhanceddislocationdensity. TheincreaseintheBDTT uponprestrainingwasattributedtotheincreasedyieldstresscausedduetoprestraining. j ThecleavageplaneforNiAlsinglecrystalswasfoundtobe{511}forthe specimengeometryandorientationusedinthestudy. Theobservationofthiscleavage planeisinagreementwiththepreviouslyreportedresults. Thecleavageplanewas independentofheat-treatment,prestraining,andtemperature;however,thesizeofthe largestcleavagefacetwasfoundtodecreasewithincreasingtemperature. ix CHAPTER 1 INTRODUCTION TheintermetalliccompoundNiAIpossessesahighmeltingtemperature(1911K), highthermalconductivity(fourtoeighttimesthatofNi-basedsuperalloys),relatively lowdensity(twothirdsoftheconventionalNi-basedsuperalloys)andgoodhigh temperatureoxidationresistance. ThelowdensityofNiAIprovidesoneofthemajor benefitsofNiAIforaircraftengineapplications. Thedecreaseddensityresultsinlower selfinducedstressesinrotatingturbineairfoils,andtheturbinedisksmaybedown-sized toreflecttheloweroperatingstressesimposedbythereducedmassoftheblades. The totalreductioninweightfortheturbinerotorstage(bladesplusdisk)isprojectedtorange 30-40%. Besidestheadvantageofthelowdensity,anequallyimportantpayoffcomes fromthehighthermalconductivityofNiAI. Thehighthermalconductivityprovidesan improvedcoolingefficiency. Thesepropertiesmakeitapotentialcandidateforhigh temperaturestructuralapplications.'"^ Besidestheabovementionedadvantages,forNiAI tobeasuccessfiilhightemperaturestructuralmaterial,agoodcombinationofhigh- temperaturestrengthandroomtemperatureductilityandfracturetoughnessisrequired. However,stoichiometricNiAIsuffersfromlowroomtemperatureductilityandfracture toughness. 1