PLASTIC DEFORMATION Dislocations and their role in plastic deformation

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  • PLASTIC DEFORMATIONDislocations and their role in plastic deformation

  • What are dislocations?Dislocations are line defects that exist in metalsThere are two types of dislocations: edge and screwThe symbol for a dislocation is The dislocation density in annealed metals is normally r = 106/cm2

  • Types of dislocationsEdgeScrew

  • Dislocation motionplastic deformationNote: Dislocations normally move under a shear stress

  • How does a dislocation move?

  • Stress field of a dislocation

  • Analog to an electric charge

  • Modes of deformationSlip

    Twinning

    Shear band formation

  • SlipDislocations move on a certain crystallographic plane: slip planeDislocations move in a certain crystallographic direction: slip directionThe combination of slip direction and slip plane is called a slip system

  • Slip..Slip planes are normally close-packed planesSlip directions are normally close-packed directionsRecall for fcc close-packed planes are {111}Close-packed directions are

  • Slip systems

    Crystal system

    Slip plane

    Slip direction

    Total number of slip systems

    Active slip systems

    fcc

    {111}

    12

    5

    hcp

    {0001}

    3

    2/3

    bcc

    {110}

    {100}

    48

    2

  • Dislocation interactionRepulsionAttraction&AnnihilationPositivePositivePositiveNegativeNote: More positive-positive interactions in reality

  • Positive-positive dislocation interactionResults in more stress to move dislocations (or cause plastic deformation):called work hardeningThis type of interaction also leads to dislocation multiplication which leads to more interactions and more work hardening

  • TwinningCommon in hcp and bcc structures

    Limited deformation but help in plastic deformation in hcp and bcc crystals

    Occurs on specific twinning planes and twinning directions

  • Compare slip and twinning

    SLIP

    TWINNING

    HOMOGENEOUS

    LOCALIZED

    COMMON IN FCC

    COMMON IN HCP & BCC

    OCCURS UNDER STATIC LOADING

    OCCURS UNDER SHOCK LOADING

  • Shear band formationLimited non-homogeneous deformation

    Very large localized strain e~1 or 100%

    Occurs especially under high strain rates

    Mechanism of deformation still unclear

  • Plastic deformation movement of dislocationsStrengthening methods

  • Cold workingDeformation at temperatures below 0.4 TmDislocation density increases from 106/cm2 to 1010-12/cm2High dislocation density results in a large number of dislocation interactions which results in high strength and hardness

  • Solid solution strengtheningInteraction between stress fields of alloy atoms and dislocationsThis is the purpose of alloying

  • Grain size refinementSmall grains result in higher strength

    Small grains is equivalent to a large number of grain boundaries in the same volume

    Grain boundaries act as barriers to dislocation motion

  • MechanismStrength is inversely proportional to grain sizes = s0 + kyd-1/2

    Hall-Petch equation

    Smaller grains have more boundary area and hence morebarriers to dislocation motion

  • Precipitation hardeningPrecipitates are second-phase particles

    Hard precipitates act as barriers to dislocation motion

    Applicable only to some alloy systems