清华大学-材料科学基础_08.ppt

材料科学基础FundamentalofMaterials Prof TianMinBoTel 62795426 62772851E mail tmb DepartmentofMaterialScienceandEngineeringTsinghuaUniversity Beijing100084 Lessoneight Chapter TheStructuresofAlloys 3 1Basicconceptsofalloys DefinitionAnalloyisthecombinationofmetal s withotherelementsthroughchemicalbonding Terminology 1 Component orconstituent onecomponentsystemtwocomponentsystembinarysystemthreecomponentsystemternarysystemfourcomponentsystemquarternarysystemfivecomponentsystemquinarysystem multi componentsystem 2 CompositionItcanbeexpressedeitherbyatomicpercentage molfraction XaorbymasspercentageXm 3 PhaseAphaseisahomogeneouspartofthematerialinwhichnoabruptchangeincomposition structureandpropertiesoccurs Analloymaybesinglephaseormulti phasematerial 4 StructureStructureisageneraltermforthecombinationofatomarrangementincludingthetypesamountsanddistributionofalltypesofmaterialaswellasgrainsize defectetc ClassificationofAlloyPhases1 AccordingtostructureSolidSolutionCompoundInSolidSolution atomsofdifferentcomponentshareacommonlatticeinvariableproportion 2 Accordingtopositionofthealloyinphasediagram TerminalS S IntermediateS S orsecondaryS S 3 2FactorsAffectingtheStructureofAlloyPhase 1 WhatissizefactormetallicradiirA rB dionicradiir r dcovalentradiisinglebondradiusVanderWaalsradii Atomicradii CNradii BCC FCC CNofBCC 8 6 a CNofFCC 12 r r 0 12557nm 0 128674nm GoldschmidtatomicradiiistheradiiofatominstructureswithCN 12 Sizefactorisdefinedas 2 WhatisElectrochemicalfactor ElectronegativityXXrepresentstheabilityofanatomofanelementinthecompondtoattractelectronstoitself Pauli sempiricalrule n valencer 1 simplebondradius EAA bondingenergybetweenA AatomsEBB bondingenergybetweenB BatomsEAB bondingenergybetweenA Batoms 3 Electronconcentration e a Electronconcentration e a isthenumberofvalenceelectronsperatomontheaverage i g forCuZn e a 3 2 1 5 3 3SolidSolution Classification1 AccordingthepositionofsoluteatomsinthelatticeofthesolventSubstitutionalS S InterstitialS S 2 AccordingtheregularityofthepositionoccupiedbysoluteatomsOrderedS S DisorderedS S 1 3 12 1 4 4 1 12 Fe12Al4 Fe3Al 3 Accordingtosolidsolubility0 100 continuousseriesofS SS Swithrestrictedsolubility summarysubstitutionalS Sprimary terminal interstitialS Ssecondary intermediate orderedS ScontinuousS SdisorderedS SS SwithrestrictedsolubilityEx Writeoutinfullthecoordinatesofallcationsandanionsinnucleus WurtziteandCaF2referredtoabcaxesoftheanionssublattice DeterminationoftypesofS S istheaverageatomicweighweightedbycomposition Comparenwithno atomsperunitcellofsolvent n no idealsubstitutionalS S n no interstitialS S n no vacantS S Hume RotheryRuleforprimarysolidsolubility1 Sizefactor Sizefactor 100 Ifsizefactor 15 solubilityisverysmall NiOcanbeaddedtoMgOtoproduceasolidsolution Whatotherceramicsystemsarelikelytoexhibit100 solidsolubilitywithMgO FeO MgOsystemwillprobablydisplayunlimitedsolidsolubility CoOandZnOsystemsalsohaveappropriateradiusratiosandcrystalstructures Example 2 CrystalstructureThematerialsmusthavethesamecrystalstructure otherwisethereissomepointatwhichatransitionoccursfromonephasetoasecondphasewithadifferentstructure 3 ElectrochemicalfactorIfthedifferenceinXisgreat thesolubilityisalsoveryrestricted Formationofstablecompoundwillrestrictthesolidsolubility Parameter Electronegativity x SemiemperiesformulasWhere r1 singlebondradiusn valency x0 x0 0 4 x0 0 4 x R R0 0 85R0 1 15R0 Dorken Gurrygraphic 4 Electronconcentrationfactor e ae thenumberofvalenceelectronsa thenumberofatomse a averagenumberofvalenceelectronsperatom Experimentalfindings Zn Ga Ge AsinCu solute solvent Ifcompositionisexpressedintermsofe aratherthanat thesolidsolubilityofallelementsinCuwillberoughlythesame Structurevs e aforCuZnalloysystem CuZn e a 3 2 21 14 Cu5Zn8 e a 21 13 CuZn3 e a 7 4 21 12 PropertiesofS S1 latticeconstantspropertiesVagard slawass ao a ao xforalloyS S a K