材料物理导论-Chapter 4

1.Diffusionphenomenon、essence、studymethodDiffusion:Transportationofatomsormolecules,theonlytransportationfortheatomsinsolidsDiffusionessence:AtomstransportfromoneplacetoanotherbythermalmovementStudymethod：a)representationtheory一Describingthespeedandquantityofthemattertransportationaccordingtothemeasuredparametersb)Atomtheory一Howtotransportoftheatomsduringthediffusionprocess2.DiffusionconditionforsolidmetalsDiffusionismoredifficultforsolidmetalthangasandliquidduetothelargebondingforce,thediffusionconditioncontains:HighTemperature(T):Hightemperatureleadstoalargediffusionactivationenergytoovercomethebindingofthearoundatoms.Fe:diffusiontemperature:500oC;CdiffusiontemperatureinFe:100oCLongtime(t):thelongestdiffusiondistanceofatomsincrystallatticeverytime0.3～0.5nm，1nmdistancerequireaboutonehundredmillionoftransportationAtomsSolutedinmetals:asolidsolubilityfortheatomsinthemetalsrequiredfordiffusionDiffusiondrivenforce:thedirectionaldiffusionneedadiffusiondrivenforce3.Diffusiontypes(1)Dividedbyconcentrationchangeornot:Self—diffusion：diffusioninpurematter,noconcentrationchange,independentondρ/dx,relativewiththermalvibration,onlyinpuremetalsanduniformsolidsolutionse.g.crystalgraingrowthinpuremetals；graingrowthinuniformsolutiosmutual/chemicaldiffusion：concentrationchange,mutualdiffusiondependsonthedifferentatomsolution,thediffusionbetweenthedifferentatomse.g.chemicalheattreatment;materialscompositionuniformity(2)Dividedbythediffusiondirectiondependentondρ/dx(concentrationvarietytrend)Downhilldiffusion：diffusionalongconcentrationdecrease,concentrationuniformitye.g.uniformityannealing,chemicalheattreatmentUphilldiffusion:diffusionalongconcentrationincrease,concentrationchangealongtwodirectionsdecidedbychemicalgrads：a)Effectofelasticitystress.uphilldiffusiondecreasetheenergyinducedbyelasticitydeformation.。b)Absorbatinterface.Decreasethefreeenergyofthewholesystemc)Differentchemicalpotential.Dominatingfactord)Formingcompouds.e.g.Fe3Ce)Specialcondition.e.g.eletricfield,magneticfield。e.g.eutectictransformationforliquidalloy;newphaseformationandgrowthinsolutionsolids(3)DividedbytheappearanceofthenewphaseornotAtomicdiffusion：crystallatticeinvariable,nonewphaseReactiondiffusion：elementconcentrationofthemetalsexceedingthesolubilitylimit,newphaseformation.Aninterfaceoccurbetweenthenewphaseandthemotherphase,thenewphasemaybeinterphaseorsolidsolution.e.g.thenewphasefortheironnitrifiedat520ºCcanbeidentifiedfortheFe—Nphasediagrams,differentWN,differentnewphase.αphase：Ninα—FeforminterstitialsolidsolutionWN0.11%γˊphase：Fe4NasthemotherphaseWN=5.7～6.1%ε相：ε－FexNx=2～3WN=7.8～11.0%Characterofreactiondiffusion：abruptvarietyofthecomponentatthephaseinterface,theabruptconcentrationcorrespondingtothelimitconcentrationonthePhasediagramsFe-NPhasediagramsTemperature/oC4.1representationtheory1.Fick'sfirstlawFick’sfirstlawdescribethediffusionunderthestablecondition,thatistheconcentrationindependentofthetime,onlydependentonthedistance:ThefluxofadiffusingspeciesisproportionaltotheconcentrationgradientJ=－Ddρ/dxρ－concentrationspecies,unit：kg/m3；J－fluxofthediffusingspecies，unit：kg/(m2s)DiffusivityordiffusioncoefficientD:describingdiffusionspeed,thelargertheD,thequickerthediffusion,unit：m2／sMinus:theoppositedirectionofthediffusiondirectionanddρ/dx，thatisatomstransportfromthehighmassconcentrationtothelowone2.Fick’ssecondlawFick’ssecondlaw:describeunstablediffusion，theeverywhereconcentrationvarieswiththetimeanddistanceρ/t=/x(Dρ／x)IfDisindependentofconcentrationρ/t=D2ρ/x2Disisotropicforthree-dimensionaldiffusion(cubicsystem):ρ/t=D（2ρ／x2＋2ρ／y2＋2ρ／z2)3.Application—solveofthediffusionequationThefirstlaw—solvethefirst-orderdifferentialequationsThesecondlaw——obtainthewholesolvebysettingthemiddlevariable(errorfunctionsolve)，theconstantdeterminedbytheinitialandboundaryconditionThediffusioncouplewiththecomponentofthetwoendsnotaffectedbydiffusionweldingprocess：determinetheinitialandboundarycondition→inducingmiddlevariableanderrorfunction→obtainthewholesolve→usingtheboundaryandinitialcondition→obtainthespecialsolveDiffusionbody:thediffusionbodywiththecomponentofoneendnotaffectedbydiffusion－－surfaceheattreatmentprocesssolvemethodasaboveCementite：Thesurfacehardnessenhancedmethodoftheironaccessoryfortheindustrymanufacture,carbonatomsdiffusefromthesurfaceoftheaccessorytotheinteriortoenhancethecarboncontent,themorethecarbonatoms,thehigherthehardnessAustenite：Interstitialsolidsolutionwithcarbonatomsinthe－FewithfccstructureDecaythinfilmsource－－surfacedepositionprocessThesolvemethodasabove4.DiffusioninthesubstitutionalsolidsolutionDuetotheatomicradiusofthesubstitutionalsolutesimilartothatofthematrix,whiletheirdiffusionvelocitydifferent，Kirkendalleffecthappens.Experiment：1947，diffusionannealingat785oCfor56day，thedistanceofMosilkdecreasedby0.25mm,somelittleholesleftonthebrassAnalyse：ifthediffusioncoefficientsame，RZnRCu,shiftonly～1／10，duetothedifferentdiffusionvelocityforthetwoatomsDefining：thesymbolsurfaceshiftinducedbydifferentdiffusio