无机非金属材料工程专业英语 第7章.ppt

Chapter7 Deformationandstrengtheningmechanisms Whystudydeformationandstrengtheningmechanism Mostofmetallicproductsareproducedthroughdeformingthemetalsomehowtoobtaindesiredshape sizeandstrength Butwhatfactorswouldinfluencetheprocessofdeformation Youmayhaveanexperiencethatanironwirewillgettougheranditwillbreakfinallywhenitisbendedfortimes Doyouknowwhy Studyondeformationandstrengtheningmechanismwillhelpususeanddevelopmaterialsbetter Whatshouldyoubeabletodoafterstudyingthischapter Describedislocationmotionfromanatom sperspective 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 111 slipplane 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 becausethetemperatureatwhichdeformationtakesplaceis cold relativetotheabsolutemeltingtemperatureofthemetal 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 includingtheamountofpriorcoldworkandpurityoftheal