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Shape Memory Alloys

Shape Memory Alloys. Theresa Valentine ENMA490 Fall 2002. Shape Memory Effect. Martensite-austenite transformation Austenite is parent, high-temperature phase, cubic structure Martensite is low-temp phase, usually tetragonal [A]  [twinned M] on cooling, diffusionless shear transformation

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Shape Memory Alloys

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  1. Shape Memory Alloys Theresa Valentine ENMA490 Fall 2002

  2. Shape Memory Effect • Martensite-austenite transformation • Austenite is parent, high-temperature phase, cubic structure • Martensite is low-temp phase, usually tetragonal • [A]  [twinned M] on cooling, diffusionless shear transformation • Deformation of martensite moves twin boundaries; recovered on heating and transformation to austenite

  3. Shape Memory Effect Shape memory effect mechanism, showing (a) undeformed parent crystal, (b) martensite, (c and d) deformed martensite through twin boundary movement, and (e) reversion to the parent phase after heating. From Otsuka (1998), p.37, fig. 2.11.

  4. Shape Memory Effect Free-energy versus temperature curves for the parent (Gp) and martensite (Gm) structures in a shape memory alloy. From Otsuka (1998), p.23, fig. 1.17. Martensite-austenite phase transformation in shape memory alloys. From http://www.tiniaerospace.com/sma.html.

  5. Superelasticity • Stress-induced martensite formation above transition temperature • Martensite immediately reverts to austenite once stress is removed • Large recoverable deformation From http://www.sma-inc.com/SMAandSE.html

  6. Property Value Transformation temperature -200 to 110 C Latent heat of transformation 5.78 cal/g Melting point 1300 C Specific heat 0.20 cal/g Young’s modulus 83 GPa austenite; 28 to 41 GPa martensite Yield strength 195 to 690 MPa austenite; 70 to 140 MPa martensite Ultimate tensile strength 895 MPa annealed; 1900 MPa work-hardened % Elongation at failure 25 to 50% annealed; 5 to 10% work-hardened Nickel-Titanium • Near-equiatomic NiTi most widely used SMA today From http://www.sma-inc.com/NiTiProperties.html

  7. Nickel-Titanium Parent β (austenite) phase with B2 structure Martensite phase with monoclinic B19’ structure B2 (cesium chloride) crystal structure. From http://cst-www.nrl.navy.mil/ lattice/struk/b2.html B19’ crystal structure. From Tang et al., p.3460, fig.5.

  8. Nickel-Titanium • Intermediate R phase can nucleate in B2, then B19’ phase grows from R • 1 and 2 show single dislocations in B2 from which an R phase grows Nucleation of R-phase in an alloy of Ti-48Ni-2Al from dislocations. From Otsuka (1998), p.56, fig. 3.7.

  9. Shape Memory Alloys Today • Shape memory effect means deforming at low temperature, changing back at high temperature • Shape memory alloys (SMAs) first discovered 1951 • NiTi SMA discovered 1963 • Macroscale applications as: • Tube couplings • Air-directing flaps • Spring actuators

  10. Macroscale SMAs • “Magic flower” http://www.kobico.co.kr/english/ek3.html

  11. Macroscale SMAs • “Climbing koala” http://www.kobico.co.kr/english/ek3.html

  12. Macroscale SMAs • Eyeglass frames – superelastic NiTi http://www.flexon.com/HTML2001/flexon01.html

  13. MEMS Applications for SMAs • TiNi pneumatic microvalve http://www.sma-mems.com

  14. MEMS Applications for SMAs • NiTi microbubble (UCLA) http://aml.seas.ucla.edu/

  15. MEMS Applications for SMAs • Flow control http://www.afrlhorizons.com/Briefs/Sept02/OSR0203.html

  16. MEMS Applications for SMAs • Surgical micro-wrapper http://www.afrlhorizons.com/Briefs/Sept02/OSR0203.html

  17. MEMS Applications for SMAs • One-axis translation stage http://dmtwww.epfl.ch/isr/hpr/sma.html

  18. MEMS Applications for SMAs • Micro-gripper http://dmtwww.epfl.ch/isr/hpr/sma.html

  19. Future Research • Thin films for micro-actuators, micro-devices • Compositional changes during sputtering • Accurate phase diagram necessary • Two-way shape memory effect • Alloying NiTi with small percentages of other metals (Cu, Fe)

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