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Chapter7

Deformationandstrengtheningmechanisms1/51Whystudydeformationandstrengtheningmechanism?Mostofmetallicproductsareproducedthroughdeformingthemetalsomehowtoobtaindesiredshape,sizeandstrength.Butwhatfactorswouldinfluencetheprocessofdeformation?Youmayhaveanexperiencethatanironwirewillgettougheranditwillbreakfinallywhenitisbendedfortimes.Doyouknowwhy?Studyondeformationandstrengtheningmechanismwillhelpususeanddevelopmaterialsbetter.2/51Whatshouldyoubeabletodoafterstudyingthischapter?Describedislocationmotionfromanatom’sperspective.Understandtherelationshipsbetweendislocationmotionandplasticdeformationofmetallicmaterials.Defineslipsystemandexplaintheinfluenceofslipsystemonthedeformabilityofmaterials.Explainmechanismofgrainboundarystrengthening,solid-solutionstrengthening,strainhardening.Describerecoveryandrecrystllizationintermsofboththealternationofmicrostructureandmechanicalcharacteristicsofthematerial.3/51Importanttermsandconcepts:Coldworking冷加工Criticalresolvedshearstress临界分剪切应力Dislocationdensity位错密度Graingrowth晶粒生长Latticestrain晶格应变Recovery回复Recrystallizationtemperature再结晶温度Resolvedshearstress分剪切应力4/51Slip滑移Slipsystem滑移系Recrystallization再结晶solid-solutionstrengthening固溶强化strainhardening应变强化strengtheningbygrainsizereduction细晶强化annealing退火temper回火quench淬火ageing时效5/51naturalageing自然时效artificalageing人工时效dislocationmultiplication位错繁殖misorientation位相差异equiaxedgrains等轴晶粒elongatedgrains拉长晶粒strainfield应变场Stressfield应力场6/51Keyknowledgepoints:

Deformationsmechanismsformetals1.Historical:

1930s,theoryandactuallymeasureddiscrepancyinmechanicalstrengths1950s,dislocationbeexaminedbyelectronmicroscope7/512.BasicconceptsofdislocationsPlasticdeformationcorrespondstothemotionoflargenumbersofdislocations.Anedgedislocationmovesinresponsetoashearstressappliedinadirectionperpendiculartoitsline.

Inanedgedislocation,localizedlatticedistortionexistsalongtheendofanextrahalf-planeofatoms.ThemechanicsofdislocationmotionarerepresentedinFig.7.18/51ThemechanicsofdislocationmotionFig.7.1Atomicrearrangementsthataccompanythemotionofanedgedislocationasitmovesinresponsetoanappliedshearstress.Theextrahalf-planeofatomsislabeledA.ThedislocationmovesoneatomicdistancetotherightasAlinksuptothelowerportionofplaneB;intheprocess,theupperportionofBbecomestheextrahalf-plane.Astepformsonthesurfaceofthecrystalastheextra-planeexists.9/51Slip-------theprocessbywhichplasticdeformationisproducedbydislocationmotionistermedslip.slip

plane-----thecrystallographicplanealongwhichthedislocationlinetraversesistheslipplane.Macroscopicplasticdeformationsimplycorrespondstopermanentdeformationthatresultsfromthemovementofdislocations,orslip,inresponsetoanappliedshearstress.

10/51Drawingacarpetinthewayof(a)and(b)mayhelpyouunderstandthemotionofdislocationbetter.11/51Dislocationmotionisanalogoustothemodeoflocomotionemployedbyacaterpillar(Fig.7.3)

12/51ThemotionofascrewdislocationinresponsetotheappliedshearstressesshowninFig.7.2b.thedirectionofmovementisperpendiculartothestressdirection.Foranedge,motionisparalleltotheshearstress.However,thenetplasticdeformationforthemotionofbothdislocationtypesisthesame.Thedirectionofmotionofthemixeddislocationlineisneitherperpendicularnorparalleltotheappliedstress,butliessomewhereinbetween.13/51Fig.7.2Theformationofasteponthesurfaceofacrystalbythemotionof(a)anedgedislocationand(b)ascrewdislocation.Notethatforanedge,thedislocationlinemoveinthedirectionoftheappliedshearstressτ;forascrew,thedislocationlinemotionisperpendiculartothestressdirection.14/51Dislocationdensity

--------thenumberofdislocationinamaterialisexpressedasthetotaldislocationlengthperunitvolume,orequivalently,thenumberofdislocationsthatintersectaunitareaofarandomsection.Theunitsofdislocationdensityaremillimetersofdislocationpercubicmillimeterorjustpersquaremillimeter.15/51Doyoufindsomerelationshipsbetweendeformationanddensityofdislocation?Whatisit?Whatdoesitmean?DislocationdensitySolidifiedmetalcrystal103mm-2Heavydeformedmetal109to1010mm-2Heattreatingcouldreducethedensitytoontheorderof105to106mm-2Ceramicmaterials102to104mm-2Siliconsinglecrystal0.1to1mm-216/513.CharacteristicsofdislocationWhenmetalsareplasticallydeformed,somefractionofthedeformationenergy(approximately5%)isretainedinternally,theremainderisdissipated(lost消失)

asheat.Themajorportionofthisstoredenergyisasstrainenergyassociatedwithdislocations.Latticestrains

(Fig.7.4)---someatomiclatticedistortionexistsaroundthedislocationlinebecauseofthepresenceoftheextrahalf-planeofatoms.17/51Fig.7.4Regionsofcompression(dark)andtension(colored)locatedaroundanedgedislocation.18/51Thestrainsextendintothesurroundingatoms,andtheirmagnitudesdecreasewithradialdistancefromthedislocation.Thestrainfieldssurroundingdislocationsincloseproximitytooneanothermayinteractsuchthatforcesareimposedoneachdislocationbythecombinedinteractionsofallitsneighboringdislocations.(Fig.7.5)19/51Fig.7.5(a)Twoedgedislocationofthesamesignandlyingonthesameslipplaneexertarepulsiveforceoneachother;CandTdenotecompressionandtensileregions,respectively.(b)Edgedislocationsofoppositesignandlyingonthesameslipplaneexertanattractiveforceoneachother.Uponmeeting,theyannihilateeachotherandleavearegionofperfectcrystal.20/514.SlipsystemDislocationsdonotmovewiththesamedegreeofeaseonallcrystallographicplanesofatomsandinallcrystallographicdirections.Ordinarilythereisapreferredplane,andinthatplanetherearespecificdirectionsalongwhichdislocationmotionoccurs.Thisplaneiscalledtheslipplane.Itfollowsthatthedirectionofmovementiscalledtheslipdirection.21/51slipsystem-------Thiscombinationoftheslipplaneandtheslipdirection.Theslipsystemdependsonthecrystalstructureofthemetalandissuchthattheatomicdistortionthataccompaniesthemotionofadislocationisaminimum.Foraparticularcrystalstructure,theslipplaneisthatplanehavingthemostdenseatomicpacking,thatishasthegreatestplanardensity.22/51Theslipdirectioncorrespondstothedirection,inthisplane,thatismostcloselypackedwithatoms,thatis,has

thehighestlineardensity.Forexample:FCCcrystalstructure,{111}slipplane,<110>slipdirection,12slipsystem(Fig.7.6andtable7.1)Themoreslipsystem,themoreductileofthemetal.HCPmetalshavefewactiveslipsystems,arenormallyquitebrittle.23/5124/5125/515.PlasticdeformationofpolycrystallinemetalsForpolycrystallinemetals,thedirectionofslipsystemvariesfromonegraintoanother.Foreach,dislocationmotionoccursalongtheslipsystemthathasthemostfavorableorientation(i.e.,thehighestshearstress).(Fig.7.10)Grossplasticdeformationofapolycrystallinespecimencorrespondstothecomparabledistortionoftheindividualgrainsbymeansofslip.(Fig.7.11)26/51Figure7.10sliplinesonthesurfaceofapolycrystallinespecimenofcopperthatwaspolishedandsubsequentlydeformed.27/51Figure7.11Alterationofthegrainstructureofapolycrystallinemetalasaresultofplasticdeformation.(a)Beforedeformationthegrainsareequiaxed.(b)Thedeformationhasproducedelongatedgrains.28/51Duringdeformation,mechanicalintegrityandcoherencyaremaintainedalongthegrainboundaries.Asaconsequence,eachindividualgrainisconstrained,tosomedegree,intheshapeitmayassumebyitsneighboringgrains.Polycrystallinemetalsarestrongerthantheirsingle-crystalequivalents,whichmeansthatgreaterstressesarerequiredtoinitiateslipandtheattendantyielding.29/511.Strengtheningbygrainsizereduction（细晶强化）MechanismsofstrengtheninginmetalsMetallurgicalandmaterialsengineersareoftencalledontodesignalloyshavinghighstrengthsyetsomeductilityandtoughness.Ordinarily,ductilityissacrificedwhenanalloyisstrengthened.Importanttotheunderstandingofstrengtheningmechanismsistherelationbetweendislocationmotionandmechanicalbehaviorofmetals.30/51Macroscopicplasticdeformationcorrespondstothemotionoflargenumbersofdislocations,theabilityofametaltoplasticallydeformdependsontheabilityofdislocationstomove.Restrictionorhinderingdislocationmotionrendersamaterialharderandstronger.Thesizeofthegrains,oraveragegraindiameter,inapolycrystallinemetalinfluencesthemechanicalproperties.(Fig.7.14)31/51Fig.7.14Themotionofadislocationasitencountersagrainboundary,illustratinghowtheboundaryactsasabarriertocontinuedslip.Slipplanesarediscontinuousandchangedirectionsacrosstheboundary.32/51ThegrainboundaryactsasabarriertodislocationmotionfortworeasonsSincethetwograinsareofdifferentorientations,adislocationpassingintograinBwillhavetochangeitsdirectionofmotion;thisbecomesmoredifficultasthecrystallographicmisorientationincreases.Theatomicdisorderwithinagrainboundaryregionwillresultinadiscontinuityofslipplanesformonegrainintotheother33/51Afine-grainedmaterial(onethathassmallgrains)isharderandstrongerthanonethatiscoarsegrained,sincetheformerhasagreatertotalgrainboundaryareatoimpededislocationmotion.

Hall-Petchequation:σy=σ0+kd-1/234/51Fig.8.15Theinfluenceofgrainsizeontheyieldstrengthofa70Cu-30Znbrassalloy.Notethatthegraindiameterincreasesfromrighttoleftandisnotlinear.35/512.Solid-SolutionStrengthening

(固溶强化)High-puritymetalsarealmostalwayssofterandweakerthanalloyscomposedofthesamebasemetal.

Increasingtheconcentrationoftheimpurityresultsinanattendantincreaseintensileandyieldstrengths.solid-solutionstrengthening---impurityatomsgointoeithersubstitutionalorinterstitialsolidsolutionimposelatticestrains.36/51Alloysarestrongerthanpuremetalsbecauseimpurityatomsthatgointosolidsolutionordinarilyimposelatticestrainsonthesurroundinghostatoms.Latticestrainfiledinteractionsbetweendislocationsandtheseimpurityatomsresult,andconsequently,dislocationmovementisrestricted.Fig.7.167.17,7.1837/513.strainhardeningorworkhardeningorcoldworking(形变强化，冷作硬化）Strainhardeningisthephenomenonwherebyaductilemetalbecomesharderand

strongerasitisplasticallydeformed.Sometimesitisalsocalledworkhardening,

or,becausethetemperatureatwhichdeformationtakesplaceis‘‘cold’’relativeto

theabsolutemeltingtemperatureofthemetal,coldworking.Mostmetalsstrain

hardenatroomtemperature.Itissometimesconvenienttoexpressthedegreeofplasticdeformationas

percentcoldworkratherthanasstrain.Percentcoldwork(%CW)isdefinedaswhereA0istheoriginalareaofthecrosssectionthatexperiencesdeformation,and

Ad

istheareaafterdeformation.38/51Fig.7.16Variationwithnickelcontentoftensilestrength,yieldstrength,andductilityforcopper-nickelalloys,showingstrengthening.39/51Fig.7.17(a)Representationoftensilelatticestrainsimposedonhostatomsbyasmallersubstitutionalimpurityatom.(b)Possiblelocationsofsmallerimpurityatomsrelativetoanedgedislocationsuchthatthereispartialcancellationofimpurity-dislocationlatticestrains.40/51Fig.7.18(a)Representationofcompressivestrainsimposedonhostatomsbyalargersubstitutionalimpurityatom.(b)Possiblelocationsoflargerimpurityatomsrelativetoanedgedislocationsuchthatthereispartialcancellationofimpurity-dislocationlatticestrains.41/51Thedislocationdensityinametalincreasewithdeformationorcoldwork,duetodislocationmultiplication(繁殖,增加)ortheformationofnewdislocations.Ontheaverage,dislocation-dislocationstraininteractionsarerepulsive.Thenetresultisthemotionofadislocationishinderedbythepresenceofotherdislocations.42/51Recovery,recrystallization,andgraingrowthPlasticallydeformingapolycrystallinemetalspecimenattemperaturesthatarelowrelativetoitsabsolutemeltingtemperatureproducesmicrostructureandpropertychangesthatinclude(1)achangeingrainshape(2)strainhardening(3)anincreaseindislocationdensity.43/51Somefractionoftheenergyexpandedindeformationsisstoredinthemetalasstrainenergywhichisassociatedwithtensile,compressiveandshearzonesaroundthenewlycreateddislocations.Thesepropertiesandstructuresmayrevert(return)backtotheprecold-workedstatesbyappropriateheattreatment.Suchrestorationresultsfromtwodifferentprocessesthatoccuratelevatedtemperatures:recoveryandrecrystallization,whichmaybefollowedbygraingrowth..44/51RecoveryDuringrecovery,someofthestoredinternalstrainenergyisrelievedbyvirtueofdislocationmotion(intheabsenceofanexternallyappliedstress),asaresultofenhancedatomicdiffusionattheelevatedtemperature.Thereissomereductioninthenumberofdislocations,anddislocationconfigurations(similartothatshowninFigure5.12)areproducedhavinglowstrainenergies.Inaddition,physicalpropertiessuchaselectricalandthermalconductivitiesandthelikearerecoveredtotheirprecold-workedstates.45/51RecrystallizationRecrystallizationistheformationofanewsetofstrain-freeandequiaxedgrainsthathavelowdislocationdensitiesandarecharacteristicofthepre-cold-workedcondition.(Fig.7.21)Afterrecrystallization,themetalbecomessofter,weaker,yetmoreductile.Recrystallizationtemperature----thetemperatureatwhichrecrystallizationjustreachescompletionin1h.Typically,itisbetweenonethirdandonehalfoftheabsolutemeltingtemperat