1. Plastic Deformation

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    Prepared By: Angelo R. Patimo

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    When a metal is stressed, they deformeither temporarily or permanently.This deformation disappears when the

    stress is removed.

    A metal when stressed beyond its

    elastic limit becomes unable to regainits original shape.

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    Plastic deformation in metalstakes place by two mechanisms:

    Slip

    Twinning

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    During plastic deformation, the atoms on acertain crystallographic planes glide overthe other. This causes a permanentdisplacement of one part of the crystalrelated to the other.

    Slip markings appear on the surface of the

    metal crystal as a result of the movement ofcrystal blocks after plastic deformation.

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    These slip markings are called slipbands which indicates the direction ofplanes where the slip occurs.

    The crystallographic planes in whichare in favorable position related to

    shear stress to cause slip are knownas slip planes.

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    Formationof a step

    on thesurface ofa crystalby themotion of

    (a) edgedislocationand (b)screwdislocation

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    Theoretical strengths of perfect crystal weremuch higher than those actually measured.

    This discrepancy in mechanical strength

    could be explained by dislocations.

    On a macroscopic scale, plastic deformationcorresponds to the net movement of largenumbers of atoms in response to an appliedstress.

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    The extra -plane moves along the slipplane.

    Dislocation movement is similar to the waya caterpillar moves. The caterpillar hump isrepresentative of the extra -plane ofatoms.

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    Dislocation movement occurs athigh atomic density. Hence, slipoccurs in high density planes and

    high density directions. Thiscombination of plane anddirection corresponding todislocation movement is calledslip system.

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    Slip Plane most dense atomic packing

    Slip Direction highest linear density

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    Slip systems are characteristics of eachcrystal system. That is, if all metals of similarcrystal structure slip on the samecrystallographic planes and in the samecrystallographic directions.

    CrystalStructure Slip Plane Slip Direction No. ofSlipSystems

    Examples

    BCC {1 1 0}{2 1 1}{3 2 1}

    121224

    W, MoWK

    FCC {1 1 1} 12 Cu, Al, Ni, Ag, Au

    HCP {0 0 0 1}{1 0 1 0}{1 0 1 1}

    336

    Cu, Zn, Mg, Ti, BeTi, Mg, Zr

    Ti, Mg

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    Dislocations are formed during thesolidification process. The number ofdislocations present in a unit area is calleddislocation density.

    Dislocations multiply during the plasticdeformation process indicating sources

    within the crystal which generate newdislocations. One as such is the Frank-Read Source.

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    Diagram of a Frank-Read dislocation source

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    In the previous figure, segment AB ofa dislocation line lies on a slip plane.This segment is bent under the

    influence of shear stress andsuccessively assumes a loop from (a)to (g) until the dislocation loop is

    separated and forming the a newdislocation line, performing the wholeprocess indefinitely.

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    Twinning is a major deformationmost common in HCP metals likeZinc and Tin. Twinning occurs in

    definite crystallographic planesand direction that depend on thecrystal structure.

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    A shear force can produce atomic displacements so that onone side of the plane (the twin boundary), atoms are locatedin mirror image positions to atoms on the other side.

    Twinning may favorably reorient slip systems to promotedislocation movement.

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    Slip TwinningCrystallographic orientation above andbelow the slip plane is the same beforeand after deformation

    There is a crystallographicreorientation across the twin plane.

    All atoms on one side of the slip planemove an equal distance.

    The atoms move distancesproportional to their distances fromthe twinning plane.

    Slip leaves a series of steps on thesurface.

    Twinning leaves a small but welldefines region of the deformed crystal.

    The microscopic appearance of slip islike thin lines.

    The microscopic appearance oftwinning is like broad lines or bands.

    Stress required to produce slip is less. Stress required to produce twinning ishigh.

    Stress necessary to propagate slip isusually higher than the stressnecessary to start slip.

    Stress necessary to propagate twinningis less than the stress required toinitiate it.

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    The mechanism of polycrystalline metals isthe same as that of single crystals. Howeverthe process becomes complicated due to thepresence of large number of randomly

    oriented grains and grain boundaries.

    Dislocations moving on a particular slip plane

    cannot go directly from one grain to anotherin a straight line thus making plasticdeformation difficult.

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