2材料科学基础英文版课件_(10)

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1、LinearDefect–DislocationsFeatures:onedimensionalClassificationandformationEdgedislocationandscrewdislocationCompressivestressesTensilestressesEdgedislocationRepresentedbysymbol-positivedislocation;-negativeScrewdislocationRepresentationOpencircles–atompositionsa

2、bovetheslipplane;solidcircles–atompositionsbelowtheslipplaneAB–dislocationlineMixeddislocationBurgersvectorUsedtoindicatethedirectionandmagnitudeofthelatticedistortioncausedbyadislocationDenotedasbForedgedislocations,bisperpendiculartothedislocationline;forscrewdi

3、slocations,bisparalleltothedislocationline;formixeddislocations,bisneitherperpendicularnorparalleltothedislocationlineOnedislocationjusthasonebFormetals,bnormallypointsinaclose-packedcrystaldirectionanditsmagnitudeistheinteratomicspacingbecausetheslipdirectionisno

4、rmallyintheclose-packeddirectionEdgedislocationScrewdislocationMixeddislocationEffectsofdislocationsonthepropertiesofmaterialsPlayacrucialroleintheplasticdeformationofmaterialsDislocationstrengtheningisoneofthemajorstrengtheningmechanismsformetallicmaterialsThepro

5、pertiesofLEDdependconsiderablyonthedislocationdensityinlightemissionmaterialssuchasGaNandSiC(thelower,thebetter)ObservationofdislocationsDarklines-dislocationsPlaneDefectsFeatures:twodimensionalOutlineExternalsurfacesGrainboundariesTwinboundariesStackingfaultsPhas

6、eboundariesExternalSurfacesSurfaceatomsarenotbondedtothenearestneighborsabovethesurface,leadingtoahigherenergystate,i.e.,asurfaceenergyTobestable,materialsneedtoreducethesurfaceenergy.Toreducethesurfaceenergy,thematerialstendtominimizethetotalsurfaceareaGrainBound

7、ariesInpolycrystallinematerials,agrainboundaryistheboundarybetweentwoadjacentgrainswhichhavedifferentorientationsFeaturesofgrainboundaries2~3atomiclayersthick(0.5~1nm)Withintheboundary,thereissomeatomicmismatchandthedensityislower,sothegrainboundaryisinahigherener

8、gystate,leadingtoagrainboundaryenergyToreducetheenergyofthesystem,grainstendtogrowtoreducethetotalgrainboundaryarea.Degreeofmisorientation–lowanglebound