Phase Transformations

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1、MaterialsScience&MetallurgyPtIBPhaseTransformations:CourseALecture1_____________________________________________________________________________________________________________________Lecture1:FundamentalsofDiffusion1.1.BackgroundDiffusionisatransportphenomenoncausedbythephysicalmotionofchemicalsp

2、ecies(molecules,atomsorions),heatorsimilarpropertiesofamedium(gas,liquidorsolid)asaconsequenceofconcentration(or,strictly,chemicalpotential)differences.Thespeciesmovefromhighconcentrationareastolowconcentrationsareasuntiluniformconcentrationisachievedinthemedium.Masstransportgenerallyinvolves:flui

3、dflow–dominantingasesorliquids(e.g.convectioncurrents);viscousflow–flowofaviscousmaterial,generallyamorphousorsemi-crystalline(e.g.glassesandpolymers)duetotheforcesactingonitatthatmoment;atomicdiffusion–principalmechanisminsolidswhichdonotflowreadily(i.e.whichcansupportshearstresses).Alsoinstaticl

4、iquidsasoccursinsolidification.Atomicdiffusionoccursduringprocessessuchas:•annealingofmetalstoreduceexcessvacancies&dislocationsformedduringworking(lecture7);•precipitationhardening,e.g.Al-Cualloys[MMSIA,DH54andlectures9–10];•manufactureofdopedsilicon,e.g.asusedinmanyelectronicdevices;•solidificat

5、ionofmaterials(lectures4-6).Atomicdiffusionis:jumpingofatomsonafixednetworkofsiteswithinthesolidlattice(seenextsectionand2.2);generallytheconsequenceofathermodynamicdrivingforce,i.e.chemicalpotentialgradient(often,butnotexclusively,aconcentrationgradient–seesection2.2).Canconsiderdiffusiononamacro

6、-scale(abovepracticalexamplesandsections1.3-1.6)oratomistically(seenextsectionand2.2).1.2Mechanismsofdiffusion[seealsoMMSIA,DH33]Atomsinthesolid-statemigratebyjumpingintoeitherinterstitialorsubstitutionalvacantlatticesites.interstitialdiffusionsubstitutionaldiffusiondirectexchangeringvacancyAssoci

7、atedtransientdistortionoflatticeduringatomicmovementresultsinbarriertomotion,andmagnitudewillbedifferentinallabovecases(seesection2.2).2005-2006Page1RobWallachMaterialsScience&MetallurgyPtIBPhaseTransformations:C