Brittle-to-ductile transition in NiAl single crystals

160 Pages·1997·5.6 MB·English

by  Shrivastava, Sanjay

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