无机非金属材料工程专业英语 第7章

Chapter7,Deformationandstrengtheningmechanisms,Whystudydeformationandstrengtheningmechanism?,Mostofmetallicproductsareproducedthroughdeformingthemetalsomehowtoobtaindesiredshape,sizeandstrength.Butwhatfactorswouldinfluencetheprocessofdeformation?Youmayhaveanexperiencethatanironwirewillgettougheranditwillbreakfinallywhenitisbendedfortimes.Doyouknowwhy?Studyondeformationandstrengtheningmechanismwillhelpususeanddevelopmaterialsbetter.,Whatshouldyoubeabletodoafterstudyingthischapter?,Describedislocationmotionfromanatomsperspective.Understandtherelationshipsbetweendislocationmotionandplasticdeformationofmetallicmaterials.Defineslipsystemandexplaintheinfluenceofslipsystemonthedeformabilityofmaterials.Explainmechanismofgrainboundarystrengthening,solid-solutionstrengthening,strainhardening.Describerecoveryandrecrystllizationintermsofboththealternationofmicrostructureandmechanicalcharacteristicsofthematerial.,Importanttermsandconcepts:,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,Slip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,naturalageing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,Keyknowledgepoints:Deformationsmechanismsformetals,1.Historical:1930s,theoryandactuallymeasureddiscrepancyinmechanicalstrengths1950s,dislocationbeexaminedbyelectronmicroscope,2.Basicconceptsofdislocations,Plasticdeformationcorrespondstothemotionoflargenumbersofdislocations.Anedgedislocationmovesinresponsetoashearstressappliedinadirectionperpendiculartoitsline.Inanedgedislocation,localizedlatticedistortionexistsalongtheendofanextrahalf-planeofatoms.ThemechanicsofdislocationmotionarerepresentedinFig.7.1,Themechanicsofdislocationmotion,Fig.7.1Atomicrearrangementsthataccompanythemotionofanedgedislocationasitmovesinresponsetoanappliedshearstress.Theextrahalf-planeofatomsislabeledA.ThedislocationmovesoneatomicdistancetotherightasAlinksuptothelowerportionofplaneB;intheprocess,theupperportionofBbecomestheextrahalf-plane.Astepformsonthesurfaceofthecrystalastheextra-planeexists.,Slip-theprocessbywhichplasticdeformationisproducedbydislocationmotionistermedslip.slipplane-thecrystallographicplanealongwhichthedislocationlinetraversesistheslipplane.Macroscopicplasticdeformationsimplycorrespondstopermanentdeformationthatresultsfromthemovementofdislocations,orslip,inresponsetoanappliedshearstress.,Drawingacarpetinthewayof(a)and(b)mayhelpyouunderstandthemotionofdislocationbetter.,Dislocationmotionisanalogoustothemodeoflocomotionemployedbyacaterpillar(Fig.7.3),ThemotionofascrewdislocationinresponsetotheappliedshearstressesshowninFig.7.2b.thedirectionofmovementisperpendiculartothestressdirection.Foranedge,motionisparalleltotheshearstress.However,thenetplasticdeformationforthemotionofbothdislocationtypesisthesame.Thedirectionofmotionofthemixeddislocationlineisneitherperpendicularnorparalleltotheappliedstress,butliessomewhereinbetween.,Fig.7.2Theformationofasteponthesurfaceofacrystalbythemotionof(a)anedgedislocationand(b)ascrewdislocation.Notethatforanedge,thedislocationlinemoveinthedirectionoftheappliedshearstress;forascrew,thedislocationlinemotionisperpendiculartothestressdirection.,Dislocationdensity-thenumberofdislocationinamaterialisexpressedasthetotaldislocationlengthperunitvolume,orequivalently,thenumberofdislocationsthatintersectaunitareaofarandomsection.Theunitsofdislocationdensityaremillimetersofdislocationpercubicmillimeterorjustpersquaremillimeter.,Doyoufindsomerelationshipsbetweendeformationanddensityofdislocation?Whatisit?Whatdoesitmean?,Dislocationdensity,Solidifiedmetalcrystal103mm-2Heavydeformedmetal109to1010mm-2Heattreatingcouldreducethedensitytoontheorderof105to106mm-2Ceramicmaterials102to104mm-2Siliconsinglecrystal0.1to1mm-2,3.Characteristicsofdislocation,Whenmetalsareplasticallydeformed,somefractionofthedeformationenergy(approximately5%)isretainedinternally,theremainderisdissipated(lost消失)asheat.Themajorportionofthisstoredenergyisasstrainenergyassociatedwithdislocations.Latticestrains(Fig.7.4)-someatomiclatticedistortionexistsaroundthedislocationlinebecauseofthepresenceoftheextrahalf-planeofatoms.,Fig.7.4Regionsofcompression(dark)andtension(colored)locatedaroundanedgedislocation.,Thestrainsextendintothesurroundingatoms,andtheirmagnitudesdecreasewithradialdistancefromthedislocation.Thestrainfieldssurroundingdislocationsincloseproximitytooneanothermayinteractsuchthatforcesareimposedoneachdislocationbythecombinedinteractionsofallitsneighboringdislocations.(Fig.7.5),Fig.7.5(a)Twoedgedislocationofthesamesignandlyingonthesameslipplaneexertarepulsiveforceoneachother;CandTdenotecompressionandtensileregions,respectively.(b)Edgedislocationsofoppositesignandlyingonthesameslipplaneexertanattractiveforceoneachother.Uponmeeting,theyannihilateeachotherandleavearegionofperfectcrystal.,4.Slipsystem,Dislocationsdonotmovewiththesamedegreeofeaseonallcrystallographicplanesofatomsandinallcrystallographicdirections.Ordinarilythereisapreferredplane,andinthatplanetherearespecificdirectionsalongwhichdislocationmotionoccurs.Thisplaneiscalledtheslipplane.Itfollowsthatthedirectionofmovementiscalledtheslipdirection.,slipsystem-Thiscombinationoftheslipplaneandtheslipdirection.Theslipsystemdependsonthecrystalstructureofthemetalandissuchthattheatomicdistortionthataccompaniesthemotionofadislocationisaminimum.Foraparticularcrystalstructure,theslipplaneisthatplanehavingthemostdenseatomicpacking,thatishasthegreatestplanardensity.,Theslipdirectioncorrespondstothedirection,inthisplane,thatismostcloselypackedwithatoms,thatis,hasthehighestlineardensity.Forexample:FCCcrystalstructure,111slipplane,slipdirection,12slipsystem(Fig.7.6andtable7.1)Themoreslipsystem,themoreductileofthemetal.HCPmetalshavefewactiveslipsystems,arenormallyquitebrittle.,5.Plasticdeformationofpolycrystallinemetals,Forpolycrystallinemetals,thedirectionofslipsystemvariesfromonegraintoanother.Foreach,dislocationmotionoccursalongtheslipsystemthathasthemostfavorableorientation(i.e.,thehighestshearstress).(Fig.7.10)Grossplasticdeformationofapolycrystallinespecimencorrespondstothecomparabledistortionoftheindividualgrainsbymeansofslip.(Fig.7.11),Figure7.10sliplinesonthesurfaceofapolycrystallinespecimenofcopperthatwaspolishedandsubsequentlydeformed.,Figure7.11Alterationofthegrainstructureofapolycrystallinemetalasaresultofplasticdeformation.(a)Beforedeformationthegrainsareequiaxed.(b)Thedeformationhasproducedelongatedgrains.,Duringdeformation,mechanicalintegrityandcoherencyaremaintainedalongthegrainboundaries.Asaconsequence,eachindividualgrainisconstrained,tosomedegree,intheshapeitmayassumebyitsneighboringgrains.Polycrystallinemetalsarestrongerthantheirsingle-crystalequivalents,whichmeansthatgreaterstressesarerequiredtoinitiateslipandtheattendantyielding.,1.Strengtheningbygrainsizereduction（细晶强化）,Mechanismsofstrengtheninginmetals,Metallurgicalandmaterialsengineersareoftencalledontodesignalloyshavinghighstrengthsyetsomeductilityandtoughness.Ordinarily,ductilityissacrificedwhenanalloyisstrengthened.Importanttotheunderstandingofstrengtheningmechanismsistherelationbetweendislocationmotionandmechanicalbehaviorofmetals.,Macroscopicplasticdeformationcorrespondstothemotionoflargenumbersofdislocations,theabilityofametaltoplasticallydeformdependsontheabilityofdislocationstomove.Restrictionorhinderingdislocationmotionrendersamaterialharderandstronger.Thesizeofthegrains,oraveragegraindiameter,inapolycrystallinemetalinfluencesthemechanicalproperties.(Fig.7.14),Fig.7.14Themotionofadislocationasitencountersagrainboundary,illustratinghowtheboundaryactsasabarriertocontinuedslip.Slipplanesarediscontinuousandchangedirectionsacrosstheboundary.,Thegrainboundaryactsasabarriertodislocationmotionfortworeasons,Sincethetwograinsareofdifferentorientations,adislocationpassingintograinBwillhavetochangeitsdirectionofmotion;thisbecomesmoredifficultasthecrystallographicmisorientationincreases.Theatomicdisorderwithinagrainboundaryregionwillresultinadiscontinuityofslipplanesformonegrainintotheother,Afine-grainedmaterial(onethathassmallgrains)isharderandstrongerthanonethatiscoarsegrained,sincetheformerhasagreatertotalgrainboundaryareatoimpededislocationmotion.Hall-Petchequation:y=0+kd-1/2,Fig.8.15Theinfluenceofgrainsizeontheyieldstrengthofa70Cu-30Znbrassalloy.Notethatthegraindiameterincreasesfromrighttoleftandisnotlinear.,2.Solid-SolutionStrengthening(固溶强化),High-puritymetalsarealmostalwayssofterandweakerthanalloyscomposedofthesamebasemetal.Increasingtheconcentrationoftheimpurityresultsinanattendantincreaseintensileandyieldstrengths.solid-solutionstrengthening-impurityatomsgointoeithersubstitutionalorinterstitialsolidsolutionimposelatticestrains.,Alloysarestrongerthanpuremetalsbecauseimpurityatomsthatgointosolidsolutionordinarilyimposelatticestrainsonthesurroundinghostatoms.Latticestrainfiledinteractionsbetweendislocationsandtheseimpurityatomsresult,andconsequently,dislocationmovementisrestricted.Fig.7.167.17,7.18,3.strainhardeningorworkhardeningorcoldworking(形变强化，冷作硬化）,Strainhardeningisthephenomenonwherebyaductilemetalbecomesharderandstrongerasitisplasticallydeformed.Sometimesitisalsocalledworkhardening,or,becausethetemperatureatwhichdeformationtakesplaceiscoldrelativetotheabsolutemeltingtemperatureofthemetal,coldworking.Mostmetalsstrainhardenatroomtemperature.Itissometimesconvenienttoexpressthedegreeofplasticdeformationaspercentcoldworkratherthanasstrain.Percentcoldwork(%CW)isdefinedaswhereA0istheoriginalareaofthecrosssectionthatexperiencesdeformation,andAdistheareaafterdeformation.,Fig.7.16Variationwithnickelcontentoftensilestrength,yieldstrength,andductilityforcopper-nickelalloys,showingstrengthening.,Fig.7.17(a)Representationoftensilelatticestrainsimposedonhostatomsbyasmallersubstitutionalimpurityatom.(b)Possiblelocationsofsmallerimpurityatomsrelativetoanedgedislocationsuchthatthereispartialcancellationofimpurity-dislocationlatticestrains.,Fig.7.18(a)Representationofcompressivestrainsimposedonhostatomsbyalargersubstitutionalimpurityatom.(b)Possiblelocationsoflargerimpurityatomsrelativetoanedgedislocationsuchthatthereispartialcancellationofimpurity-dislocationlatticestrains.,Thedislocationdensityinametalincreasewithdeformationorcoldwork,duetodislocationmultiplication(繁殖,增加)ortheformationofnewdislocations.Ontheaverage,dislocation-dislocationstraininteractionsarerepulsive.Thenetresultisthemotionofadislocationishinderedbythepresenceofotherdislocations.,Recovery,recrystallization,andgraingrowth,Plasticallydeformingapolycrystallinemetalspecimenattemperaturesthatarelowrelativetoitsabsolutemeltingtemperatureproducesmicrostructureandpropertychangesthatinclude(1)achangeingrainshape(2)strainhardening(3)anincreaseindislocationdensity.,Somefractionoftheenergyexpandedindeformationsisstoredinthemetalasstrainenergywhichisassociatedwithtensile,compressiveandshearzonesaroundthenewlycreateddislocations.Thesepropertiesandstructuresmayrevert(return)backtotheprecold-workedstatesbyappropriateheattreatment.Suchrestorationresultsfromtwodifferentprocessesthatoccuratelevatedtemperatures:recoveryandrecrystallization,whichmaybefollowedbygraingrowth.,Recovery,Duringrecovery,someofthestoredinternalstrainenergyisrelievedbyvirtueofdislocationmotion(intheabsenceofanexternallyappliedstress),asaresultofenhancedatomicdiffusionattheelevatedtemperature.Thereissomereductioninthenumberofdislocations,anddislocationconfigurations(similartothatshowninFigure5.12)areproducedhavinglowstrainenergies.Inaddition,physicalpropertiessuchaselectricalandthermalconductivitiesandthelikearerecoveredtotheirprecold-workedstates.,Recrystallization,Recrystallizationistheformationofanewsetofstrain-freeandequiaxedgrainsthathavelowdislocationdensitiesandarecharacteristicofthepre-cold-workedcondition.(Fig.7.21)Afterrecrystallization,themetalbecomessofter,weaker,yetmoreductile.Recrystallizationtemperature-thetemperatureatwhichrecrystallizationjustreachescompletionin1h.Typically,itisbetweenonethirdandonehalfoftheabsolutemeltingtemperatureofametaloralloyanddependsonseveralfactors,includingtheamountofpriorcoldworkandpurityoftheall