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    Effect of Cold Work on the Mechanical Behavior of NiTi

    Investigated byInvestigated byMohamed E. Mitwally & Mahmoud FaragMohamed E. Mitwally & Mahmoud Farag

    Science & Technology Research Center (STRC)Science & Technology Research Center (STRC)The American University in Cairo (AUC)The American University in Cairo (AUC)

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    Introduction to SMAsIntroduction to SMAs

    P ropertiesP roperties

    Shape memory effectShape memory effect

    SuperelasticitySuperelasticity

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    Ex pe rime nt al Proc e dur eEx pe rime nt al Proc e dur e

    N iTi (N iTi (5050..77 at% N i)at% N i)

    MMf f == --5353..77ooC, MC, M ss== --3838..99oo C, AC, A ss == --3333..55 ooC and AC and A f f

    ranges fromranges from 5 5 toto 1818 ooCC

    EDM cuttingEDM cutting

    Strips cold rolled toStrips cold rolled to 20 20%,%,3030% & % & 4040% thickness% thicknessreductionreduction

    Material Preparation

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    Ex pe rime nt al Proc e dur eEx pe rime nt al Proc e dur e

    Sh ape memory effectSh ape memory effect

    Three point bending test inThree point bending test incold alcohol bath to strainscold alcohol bath to strainsof of 22..55%,%, 55% & % &1010%.%.

    Springback angle(Springback angle( spsp ))measuredmeasured

    Shape memory recovery %Shape memory recovery %= (= (

    f f / /180180)x)x 100100

    Material Characterization

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    Ex pe rime nt al Proc e dur eEx pe rime nt al Proc e dur e

    HardnessHardness

    An MTS nanoindenter was usedAn MTS nanoindenter was usedto measure nanohardnessto measure nanohardness

    A Berkovich tip was usedA Berkovich tip was used

    Depth was kept constant in allDepth was kept constant in allindentationsindentations

    Material Characterization (cont.)

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    Ex pe rime nt al Proc e dur eEx pe rime nt al Proc e dur e

    S uperelasticityS uperelasticity

    Defined as the ratio betweenDefined as the ratio between

    energy released while unloadingenergy released while unloadingto energy stored while loadingto energy stored while loading

    Both a Berkovich & a sphericalBoth a Berkovich & a spherical

    tip were usedtip were used

    Areas under loading andAreas under loading andunloading curves were computedunloading curves were computed

    to calculate superelasticity asto calculate superelasticity asEE22/E /E11

    Material Characterization (cont.)

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    R e su lts & DiscussionR e su lts & Discussion

    S hape memory effectSh ape memory recovery % vs. Cold rolling %for different strain values

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    R e su lts & DiscussionR e su lts & Discussion

    S hape memory effectS pringback angle( sp ) vs. Cold rolling % fordifferent strain values

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    R e su lts & DiscussionR e su lts & Discussion

    S hape memory effectCombined recovery angle( f) & springback angle( sp ) vs. Cold rolling % for different strain values

    2.5%5%

    10%

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    R e su lts & DiscussionR e su lts & Discussion

    H ardnessHardness values (Gpa) vs. t h ickness reduction

    H = 5.84 R 2

    0.292 R + 3.865

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    R e su lts & DiscussionR e su lts & Discussion

    Su perelasticityS uperelasticity(E2/E1) vs. Cold rolling % usinga Berkovic h and a sp h erical tip

    0.4365 0.938

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    C onc lusionC onc lusion

    Shape memory recovery generally decreases withShape memory recovery generally decreases withcold rolling and applied bending strain.cold rolling and applied bending strain.

    Springback increases with cold rolling but decreasesSpringback increases with cold rolling but decreaseswith applied bending strain.with applied bending strain.

    The contribution of shape memory effect to theThe contribution of shape memory effect to theshape memory recovery increases with appliedshape memory recovery increases with appliedbending strain compared to that of springback.bending strain compared to that of springback.

    Superelasticity of cold rolled samples was lower thanSuperelasticity of cold rolled samples was lower thanthe as received material but increased with coldthe as received material but increased with coldrolling.rolling.

    Hardness of N iTi increases with cold rolling.Hardness of N iTi increases with cold rolling.

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    Thank Thank you you