相图的学习合金en概要1

NatureofMetalsandAlloys1ObjectivesUnderstandhowmetal/alloystructuresrelatetotheirspropertiesKnowbasicclassificationofalloysUnderstandasimplephasediagramofabinaryalloyKnowmetal/alloystrengtheningmethods2OutlineStructureofmetalsCrystallizationPhasediagramsAlloymicrostructureAlloystrengthening34AtomicStructure“Cloud〞ofelectronsThemetalionsareheldtogetherbytheirmutualattractiontotheelectroncloud.“MetallicBond〞Excellentelectricalandthermalconductivity5Metalsexistinoneofthe14crystalstructuresatroomtemperature.ExamplesBody-centeredcubic(BCC):e.g.CrFace-centeredcubic(FCC):e.g.Ag,Au,Pd,Co,Cu,NiHexagonalclosed-pack(HCP):e.g.Tiaunitcell6Aspace/crystallattice=combinationofunitcells7PhysicalPropertiesofMetalsAllpropertiesofmetalsresultfromthemetalliccrystalstructureandmetallicbonds.HighdensitytheefficientpackingofatomiccentersinthecrystallatticeGoodelectricalandthermalconductivitythemobilityofthevalenceelectronsinthecrystallattice8Opacityandreflectivetheabilityofthevalenceelectronstoabsorbandre-emitlightMeltingpointmetallicbondenergiesareovercomebytheappliedheat9Physicalpropertieschangealongdifferentcrystaldirectionsbuttheyarenotusuallyobservedinpiecesofmetalorothersolidslargeenoughforpracticaluse.Mostsolidsarepolycrystalline(beingmadeupofalargenumbersofsinglecrystals,called“grains〞).Eachgrainisorientedmoreorlessatrandomwithrespecttoitsneighbors,therefore,thevariationinpropertieswithcrystaldirectionaveragesout.10FormationofGrainsfromamoltenstate:Thegrowthstartsfromthenucleiofcrystallization,andthecrystalsgrowtowardeachother(A-E).Whentwoormorecrystalscollide,theirgrowthisstopped.Finally,theentirespaceisfilledwithcrystals(F).Eachgrowthcrystaliscalleda“grain〞.Grainscontacteachotherat“grainboundaries〞.GrainGrainboundary11GrainSizeIngeneral,thesmallerthegrainsizeofthemetal,thebetteritsphysicalproperties.ControlofGrainSizeNumberofnucleiofcrystallizationThemorerapidlytheliquidstatecanbechangedtothesolidstate,thesmallerorfinerthegrainswillbe.RateofcrystallizationIfthecrystalsformfasterthandothenucleiofcrystallization,thegrainswillbelarger.Slowcoolingresultsinlargegrains.12Theshapeofthegrainsmaybeinfluencedbytheshapeofthemoldinwhichthemetalsolidifies.Squaremold13MechanicalPropertiesofMetalsAlsoaresultofthemetalliccrystalstructureandmetallicbondsGoodductilityandmalleability,relativetopolymersandceramicstheabilityoftheatomiccenterstoslideagainsteachotherintonewpositionswithinthesamecrystallatticeDuctility=abilitytobedrawnintoawireMalleability=abilitytobepoundedintoathinsheet14Istheslidingeasytooccurinperfectmetalliccrystals?15No….iftheywerereally“perfect〞.16LatticeimperfectionsSeveraltypesexistonvariousatomiclevels:PointdefectsLinedefects(Dislocations)GrainBoundariesMacroscopicDefects17PointDefectsThesetypesofdefectsarecontrolledbythesizeoftheforeignatom.Introductionofpointdefectsaltersthelatticedimensionsandchangesthecompositionoftheparentmetalbutdoesnotchangetheoverallcrystalstructureoftheparentatom.18LineDefects(Dislocations)Anextraplaneorlineofatomsexistsintheparentstructure.Thedislocationsactasareasofstressconcentrationandallowatomicplanesto“slip〞overoneanother.Theyprovideamechanismformetalstodeformatmuchlowerstresslevelsthantheorywouldpredict.19DislocationMovements20Edge/ScrewDislocationsEdgedislocationScrewdislocation21Wheneverthedislocationmotionsareimpeded,thematerialbecomesmoreresistanttoslip,makingitstronger.Thepresenceofotherdefectssuchaspointandotherlinedefectshelpstoimmobilizethemovementofthesedislocationsduringstress.22GrainBoundariesGrainboundariesaredefectswhichhavehigherenergythanthegrainsandaremoreactivewithchemicals.Helptostopthedislocation.23MacroscopicDefectsHoles,bubbles,surfaceimperfections,cracks,andmacroscopicimpurities24AlloysandPrinciplesofMetallurgyMetallurgyisthestudyofmetalsandalloys.25Puremetalsareapttobesoftandmanyofthemtendtocorroderapidly.Tooptimizeproperties,mostofthemetalscommonlyusedaremixturesoftwoormoremetallicelements(metal+metalormetal+nonmetal).Asolidmixtureofametalwithoneormoreothermetalsorwithoneormorenonmetalsiscalledanalloy.Binarysystem,ternarysystemHomogeneous(one-phase),heterogeneous(distinctphases)26Whenevertwometalsarenotcompletelymiscibleintheliquidstate,theycannotformanytypeofalloy.e.g.Copper+Lead,Zinc+LeadWhenacombinationoftwometalsiscompletelymiscibleintheliquidstate,thetwometalsarecapableofforminganalloy.Whenthecombinationiscooled,oneofthreepossibilitiesmaytakeplace:asolidsolution,anintermetalliccompoundformation,oraneutecticformation.27Hume-RotheryRulesforAlloyingThelatticeparametersofthetwometalsmustbesimilar.Sametypeofcrystallattice(FCC,..etc)Therelativesizeoftheatomsmustnotexceed15-20%.(>15%multiplephases) Largedifferencesinvalencestateprecludesolubility.Thechemicalaffinityoftheatomsshouldbesimilar.Ahighdegreeofchemicalaffinityformanintermetalliccompoundonsolidification2829CoolingCurvesandPhaseDiagrams30Aphaseisastateofmatterthatisdistinctinsomewayfromthematteraroundit.Eg.Amixtureoficeandwater=2phasesThedistinctionbetweensingle-andmultiplephasealloysisimportanttothestrength,corrosion,biocompatibility,andotheralloyproperties.31CoolingCurvesPureMetalsAlloysMetalAandMetalBAlloysA+BFreezingorMeltingpointSolidificationorFreezingrange32CoolingCurvesand

PhaseDiagram100%100%33PhaseDiagramPhaseofafamilyofalloysofageneralmetalcompositionaredefinedbytheTemperature-Composition(Phase)diagramforthatfamilyofalloys.34ClassificationofAlloySystems1.SolidSolutions2.IntermetallicCompound3.EutecticAlloy…35SolidsolutionsTwometalsarecompletelymiscibleintheliquidstate,andtheyremaincompletelymixedonsolidification.LSAsingle-phasesystemAlwayshavearangeofpossiblecompositionse.g.thesolidphaseinthecopper-gold(Cu-Au)systemhasawiderangeofcompositionsbetween100%Cuand100%Au36PhaseDiagramof

aSolidSolutionAllcompositionsabovetheliquiduslineareliquid,andthosebelowthesoliduslinearesolid.Solidandliquidexistintheareabetweenbothline.Thesolidhasonlyonephase.TemperatureMetalA(100%)MetalB(100%)%compositionLIQUIDSOLIDliquidussolidusLiquid+Solidalloy1alloy237IntermetallicCompoundsTheresultingphasehasafixedchemicalcompositionoranarrowrangeofcompositions.e.g.inanamalgamalloy,73.2%Agand26.8%SnAg3Sn(onephase)Silverandtinatomsoccupyingdefinitepositionsinthespacelattice.38PhaseDiagramof

anIntermetallicCompound

Ag3Sn,73.2%Agand26.8%Sn39EutecticAlloysThemetalsaresolubleintheliquidstate,butseparateintotwophasesinthesolidstate.LS1+S2

(=2solidsolutions)40PhaseDiagramof

aEutecticAlloyLa-solidsolution+b-solidsolutionThelowesttemperatureatwhichanyalloycompositionisentirelyliquid=“EutecticTemp〞(779.4°C,E)TheeutectictemperatureislowerthanthefusiontemperatureofeitherAgandCu.Ateutecticpoint,thereisnosolidificationrange.(~puremetal)Ateutecticcomposition(72%Ag+28%Cu),thetwophasesoftenprecipitateasveryfinelayersofonephaseovertheotherone.Silver-coppersystemEutecticcomposition41Silver-coppersystemAg-richphaseCu-richphaseEutecticcompositionfinelayersofaphaseoverbphaseb+eutectica+eutectic42Ex.Lead-TinAlloyEutecticcompositionHightincontentb+eutectic43Howtoreadasimplephasediagram?(1)CompositionofLiquidandSolidPhasesatVariousTemp.(2)AmountofLiquidandSolidPhasesatVariousTemp.44CompositionofLiquidandSolidPhasesatVariousTemp.Alloy(80%A+20%B)Ifanalloyof80%Aand20%Bismeltedandthencooled,itremainsliquiduntil760°C,atwhichpointthesolid(97%A,3%B)beginstoprecipitate.Furthercoolingto680°Cresultsinthesolidwiththecomposition90%Aand10%Bandtheliquidwiththecomposition60%Aand40%B.Whenthetemperaturereaches560°C,thesolidis80%Aandtheliquidcontains40%A;below540°Cthereisnoliquidandthesolidis80%A.45AmountofLiquidandSolidPhasesatVariousTemp.Therelativeamountsofthetwophasesintheliquid-solidregioncanbedeterminedatagiventemperaturebytheinverseleverrule.At560°Cfor60%Aand40%BcompositionLiquid=XY/XZSolid=YZ/XZ46Ex.Silver-PalladiumSystem**47AlloyMicrostructureCastMicrostructureWroughtMicrostructureRecrystallizationandGrainGrowth48CastMicrostructureGrainsareusuallyvisible.Sizeofgrainscoolingrate(fastratesmallgrains)Fine-grained(“equiaxed〞=uniforminsizeandshape)alloysaregenerallymoredesirablefordentalapplications.moreuniformproperties49WroughtMicrostructureMetalingotshot/coldworking(rolling,swaging,orwire-drawing)produceseveremechanicaldeformationofthemetalE.g.orthodonticwiresandbandsGrainsarebrokendown,entangledineachother,andelongatedtodevelopafibrousstructure.Ingeneral,mechanicalpropertiesaresuperiortothoseofthesamecastalloys.50RecrystallizationandGrainGrowthThereappearanceofthegrainorcrystallinestructurewhenheatedorannealed(usuallymoreobviousinthewroughtmass).Degreeofrecrystallizationisrelatedto:AlloycompositionandmechanicaltreatmentTemperatureandthedurationoftheheatingoperation51A,thefibrousmicrostructureandarrowsindicateresidualstresses.

B,Minimalheatleavesthefibrousstructureintactbutrelievesthestresses.Thelatticeremainsdistorted.C,Annealingwithmoreheatallowsthelatticedeformationtoberelieved.

DandE,Furtherheatingcausesalossofthefibrousstructureandgrowthofthegrains,whichincreaseinsizewithincreasingapplicationofheat.grossviewmicrostructurecrystalview52PropertiesofAlloys53PhaseStructurevs.PropertiesThestrengthofamaterialexistinginatwophasedstructureisnormallygreaterthanthatofasinglephasedstructure.54SolidSolutionAlloysOftenhavehigherstrengthandhardnessandlowerductilitythaneitherpuremetal.Thealloyingatomsareabsorbedintodislocation,therebypreventingdislocationmovement.Possessmeltingrangesandalwaysmeltbelowthemeltingpointofthehighestfusingmetal.Havehighercorrosionresistancethanmulti-phasedalloys,andinsomecaseshigherthanthepuremetal(e.g.Cr+Festainlesssteel).55EutecticAlloysAreusuallyharderandstrongerthantheparentmetalsareoftenquitebrittle.Possesameltingpointattheeutecticcomposition.OftenhavepoorcorrosionresistanceGalvanicactionbetweenthetwophasesatamicroscopiclevelcanacceleratecorrosion.56IntermetallicCompoundsAreusuallyveryhardandbrittle.Propertiesrarelyresemblethoseofparentmetals.E.g.Ag2Hg3indentalamalgamhaspropertiescompletelydifferentfromthoseofpuresilverormercury.57StrengtheningofMetalsandAlloysPrinciple:Increasedinteractionofdislocationswillincreasethestrengthofthematerials.(1)GrainsizealterationsSmallgrains

reducedductilitybutincreasedstrength,toughnessandpolishability.Canbeachievedfrom:Quenching(quickcooling)UseofnucleatingagentsUseofgrainrefinerse.g.Irencourageevennucleation(withoutsacrificingductility)Plasticdeforming(coldworking)58(2)Cold-workingWork-hardeningorstrain-hardening:rolling,wiredrawingmechanicallydeformthealloyTheshapeofthegrainischangedfromequiaxedtolongandthin.IncreaseshardnessandyieldstrengthaswellaschemicalreactivityDecreasesductilityandcorrosionresistanceTheharmfuleffectofcold-workingmayberemovedbyheattreatment,recrystallization,andgraingrowth.59(3)AnnealingHeatingthealloytotemperaturessufficienttoaltergrainsize(1/3-1/2meltingtemperature)RecrystallizationandgraingrowthThegrainsconvertfromlongandthintoequiaxed(convertthecoldworkingresult)(4)Solute-hardeningAddingsoluteorimpurityatomswhichwillinteractwithdislocations.60(5)PrecipitationoragehardeningReliesontheab