材料科学导论(英文课件)--青岛科技大学 strength mechanisms

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1、Chapter8Dislocations,Strength&DuctilityHowaredislocations,strength,andductilityrelated?¾Onsetofdislocationmotionlimitsstrength;¾Strengtheningtechniquesimpededislocationmotion;¾Heattreatmenttechniquesrestoreductility.Purpose:Todesignandtailorthemechanicalproperti

2、esofmaterials.Motivation•Explorevariousdeformationmechanismstoexplainthedeformationbehaviorsofmaterials.•Describetechniquesusedtostrengthenvarioustypesofmaterials.¾Allmetalsandalloyscontainsomedislocationsthatwereintroducedduringsolidification,duringplasticdefor

3、mation,andasaconsequenceofthermalstressesthatresultfromrapidcooling.¾Dislocationdensity(#ofdislocationinamaterial)isexpressedasthetotaldislocationlengthperunitvolume.dislocationdensity(mm-2)Carefullysolidifiedmetalcrystals103Heavilydeformedmetals109to1010Ceramic

4、s105to106Siliconsinglecrystals0.1to1Characteristicsofdislocations•Strainfieldsthatexistarounddislocations.Compressive,tensileandshearstrainsexistaroundaedgedislocation.•Neighboringdislocationsmayinteractbecauseoftheexistenceofthestrainfields.Mutualrepulsiveforce

5、AttractiveforceSlipsystems•Slip:theprocessbywhichplasticdeformationisproducedbydislocationmotion.•Slipplane:thecrystallographicplanealongwhichthedislocationlinetraverses.•Slipdirection:thecrystallographicdirectionalongwhichthedislocationlinemoves.•Slipsystem:the

6、combinationoftheslipplaneandtheslipdirection.SlipplaneSlipdirection(a)A{111}<110>slipsystemshownwithinanFCCunitcell.(b)The(111)planefrom(a)andthree<110>directionswithinthatplanecomprisepossibleslipsystems.¾MetalswithFCCorBCCcrystalstructureshavearelativelylargen

7、umberofslipsystems(atleast12).Thesemetalsarequiteductilebecauseextensiveplasticdeformationisnormallypossiblealongthevarioussystems.¾HCPmetals,havingfewactiveslipsystems,arenormallyquitebrittle.•Theabilityofametaltoplasticallydeformdependsontheabilityofdislocatio

8、nstomove.•Restrictingorhinderingdislocationmotionrendersamaterialharderandstronger.Strengtheningprinciple:¾movementdirectionofdislocationchangedwhenpassthroughgrainbo