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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