Materials-science-and-engineering--an-introduction

•311Chapter10PhaseTransformationsinMetals:Thedevelopmentofasetofdesirablemechanicalcharacteristicsforamaterialoftenresultsfromaphasetransformationthatiswroughtbyaheattreatment.Thetimeandtemperaturedependenciesofsomephasetransformationsareconvenientlyrepresentedonmodifiedphasediagrams.Itisimportanttoknowhowtousethesediagramsinordertodesignaheattreat-mentforsomealloythatwillyieldthedesiredroom-temperaturemechanicalproperties.Forexample,thetensilestrengthofaniron–carbonalloyofeutectoidcomposition(0.76wt%C)canbevariedbetweenapproximately700MPa(100,000psi)and2000MPa(300,000psi)dependingontheheattreatmentemployed.WHYSTUDYPhaseTransformationsinMetals?Aphotomicrographofapearliticsteelthathaspartiallytransformedtospheroidite.2000.(CourtesyofUnitedStatesSteelCorporation.)DevelopmentofMicrostructureandAlterationofMechanicalProperties1496T_c10_311-35712/31/059:55Page3112ndREVISEPAGES10.1INTRODUCTIONOnereasonfortheversatilityofmetallicmaterialsliesinthewiderangeofme-chanicalpropertiestheypossess,whichareaccessibletomanagementbyvariousmeans.ThreestrengtheningmechanismswerediscussedinChapter7—namely,grainsizerefinement,solid-solutionstrengthening,andstrainhardening.Additionaltech-niquesareavailablewhereinthemechanicalpropertiesarereliantonthecharac-teristicsofthemicrostructure.Thedevelopmentofmicrostructureinbothsingle-andtwo-phasealloysordi-narilyinvolvessometypeofphasetransformation—analterationinthenumberand/orcharacterofthephases.Thefirstportionofthischapterisdevotedtoabriefdiscussionofsomeofthebasicprinciplesrelatingtotransformationsinvolvingsolidphases.Inasmuchasmostphasetransformationsdonotoccurinstantaneously,considerationisgiventothedependenceofreactionprogressontime,orthetrans-formationrate.Thisisfollowedbyadiscussionofthedevelopmentoftwo-phasemicrostructuresforiron–carbonalloys.Modifiedphasediagramsareintroducedthatpermitdeterminationofthemicrostructurethatresultsfromaspecificheattreat-ment.Finally,othermicroconstituentsinadditiontopearlitearepresented,and,foreach,themechanicalpropertiesarediscussed.PhaseTransformations10.2BASICCONCEPTSAvarietyofphasetransformationsareimportantintheprocessingofmaterials,andusuallytheyinvolvesomealterationofthemicrostructure.Forpurposesofthisdiscussion,thesetransformationsaredividedintothreeclassifications.Inonegrouparesimplediffusion-dependenttransformationsinwhichthereisnochangeinei-therthenumberorcompositionofthephasespresent.Theseincludesolidificationofapuremetal,allotropictransformations,and,recrystallizationandgraingrowth(seeSections7.12and7.13).Inanothertypeofdiffusion-dependenttransformation,thereissomealterationinphasecompositionsandofteninthenumberofphasespresent;thefinalmi-crostructureordinarilyconsistsoftwophases.Theeutectoidreaction,describedbyEquation9.19,isofthistype;itreceivesfurtherattentioninSection10.5.LearningObjectivesAftercarefulstudyofthischapteryoushouldbeabletodothefollowing:1.Makeaschematicfractiontransformation-versus-logarithmoftimeplotforatypicalsolid–solidtransformation;citetheequationthatdescribesthisbehavior.2.Brieflydescribethemicrostructureforeachofthefollowingmicroconstituentsthatarefoundinsteelalloys:finepearlite,coarsepearlite,spheroidite,bainite,martensite,andtemperedmartensite.3.Citethegeneralmechanicalcharacteristicsforeachofthefollowingmicroconstituents:finepearlite,coarsepearlite,spheroidite,bainite,martensite,andtemperedmartensite.Now,intermsofmicrostructure(orcrystalstructure),brieflyexplainthesebehaviors.4.Giventheisothermaltransformation(orcontin-uouscoolingtransformation)diagramforsomeiron–carbonalloy,designaheattreatmentthatwillproduceaspecifiedmicrostructure.transformationratephasetransformation1496T_c10_311-35712/31/059:55Page3122ndREVISEPAGESThethirdkindoftransformationisdiffusionless,whereinametastablephaseisproduced.AsdiscussedinSection10.5,amartensitictransformation,whichmaybeinducedinsomesteelalloys,fallsintothiscategory.10.3THEKINETICSOFPHASETRANSFORMATIONSWithphasetransformations,normallyatleastonenewphaseisformedthathasdifferentphysical/chemicalcharacteristicsand/oradifferentstructurethantheparentphase.Furthermore,mostphasetransformationsdonotoccurinstanta-neously.Rather,theybeginbytheformationofnumeroussmallparticlesofthenewphase(s),whichincreaseinsizeuntilthetransformationhasreachedcomple-tion.Theprogressofaphasetransformationmaybebrokendownintotwodistinctstages:nucleationandgrowth.Nucleationinvolvestheappearanceofverysmallparticles,ornucleiofthenewphase(oftenconsistingofonlyafewhundredatoms),whicharecapableofgrowing.Duringthegrowthstagethesenucleiincreaseinsize,whichresultsinthedisappearanceofsome(orall)oftheparentphase.Thetrans-formationreachescompletionifthegrowthofthesenewphaseparticlesisallowedtoproceeduntiltheequilibriumfractionisattained.Wenowdiscussthemechan-icsofthesetwoprocesses,andhowtheyrelatetosolid-statetransformations.NucleationTherearetwotypesofnucleation:homogeneousandheterogeneous.Thedistinctionbetweenthemismadeaccordingtothesiteatwhichnucleatingeventsoccur.Forthehomogeneoustype,nucleiofthenewphasefor