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Ti and Shape Memory Alloys:

Ti and Shape Memory Alloys:. Ir. Dr. Jonathan C.Y. Chung Associate Professor Department of Physics and Materials Science City University of Hong Kong. Why we choose Ti alloy?. Corrosion-resistant High strength from low temperature up to 650 o C

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Ti and Shape Memory Alloys:

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  1. Ti and Shape Memory Alloys: Ir. Dr. Jonathan C.Y. Chung Associate Professor Department of Physics and Materials Science City University of Hong Kong http://personal.cityu.edu.hk/~appchung

  2. Why we choose Ti alloy? • Corrosion-resistant • High strength from low temperature up to 650oC • Low density: 4.5g cm-3[Al: 2.69 g cm-3; Cu: 8.96g cm-3; Fe: 7.88g cm-3] • Strength is relatively low when pure • A lot stronger when alloyed

  3. Two Crystalline forms of pure Ti • Alpha: (room temperature to 883oC)- Hexagonal close packed (HCP)- Usually strong and brittle- Not easy to form into various shape • Beta: (>883oC)- Body Centered Cubic (BCC, the same as steel at room temperature)- Less strong but not so brittle- Slightly easier to form into different shapes However, the properties of commercially pure titanium (99-99.5%) are largely determined by the oxygen content. Hence, improper hot working such as forging may affect the properties.

  4. Four groups of Ti alloy • Alpha titanium alloys:Commercially pure, Ti-Pd • Near-alpha titanium alloys:Ti-11Sn-5Zr-2.25Al-1Mo-0.2Si, Ti-6Al-5Zr-0.5Mo-0.25Si,Ti-5.5Al-3.5Sn-3Zr-1Nb-0.25Mo-0.3Si • Alpha-beta titanium alloys:Ti-6Al-4V,Ti-4Al-4Mo-2Sn-0.5Si, Ti-4Al-4Mo-4Sn-0.5Si • Beta titanium alloys:Ti-11.5Mo-6Zr-4.5Sn

  5. Purposes of Alloying

  6. Steels • Mild SteelTensile strength: 200 MPa • High Strength Alloy SteelTensile strength: ~500-1000 MPa • Ultra-High Strength SteelTensile strength: >1000 MPa

  7. Alpha titanium alloys • Strong • High strength at high temperatures (<883oC) • Good weldability • Difficult to work • Non-heat treatable • Tensile strength: 330-860 MPa • Fracture toughness: >70MPa m-1/2

  8. Alpha-beta titanium alloys • Appreciable amount of beta phase at room temperature • Can be solution treated, quenched and aged to give higher strength • Tensile strength: 990-1330 MPa • Fracture toughness: 30-60MPa m-1/2

  9. Near-alpha titanium alloys • Almost all alpha phase • Small amount of beta phase disperse throughout the alpha • Improved creep resistance at temperatures at 450-500oC • Tensile strength: 855-1040 MPa • Fracture toughness: 50-70 MPa m-1/2

  10. Beta titanium alloys • Entirely beta phase at room temperature after quenching (fast cooling), or sometimes even upon air cooling • Ready for cold working (forming) • Can be solution treated, quenched and aged to give higher strength • In high strength condition the alloys have low ductility • Poor fatigue performance • Tensile strength: 1220-1450 MPa • Fracture toughness: >50 MPa m-1/2

  11. Weldability • Commercially pure titanium,  and near- titanium alloys have good weldability • Some - alloys are weldable: e.g. Ti6Al-4V •  alloys are generally not weldable • O and N can cause a lot of problem during welding at high temperatures • TIG weld is the most widely used process • Electron-beam, laser, plasma arc and friction welding processes can also be used • Resistance spot and seam welding is only used when fatigue life is not important

  12. Shape memory Alloy (SMA) • TiNi or NiTi •  titanium alloys

  13. What is shape memory materials? Golan Initiatiative Center, Israel

  14. What is shape memory effect (SME)?

  15. Why there is shape memory properties? Martensitic transformation: Formation of non-equilibrium phase non-diffusion transformation

  16. Shape Recovery (shape memory)

  17. Superelastic Properties (Pseudoelastic)

  18. Typical Loading and Unloading Behavior of Superelastic NiTi • Large “Elastic” strain compare to most alloys • A constant stress platform From: www.sma-inc.com

  19. Superelastic Devices NiTi superelastic devices are used for applications which demand the extraordinary flexibility and torqueability of NiTi. NiTi has the ability to absorb large amounts of strain energy and release it as the applied strain is removed. The elasticity of NiTi is approximately ten times that of steel. NiTi also has excellent torqueability and kink resistance, which are important for medical guidewires. Further, superelastic NiTi alloys provide a constant force over a large strain range. This has been exploited in the field of orthodontics where a constant force enhances tooth movement with greater patient comfort. Examples of superelastic devices include: • Vascular, Esophageal and Biliary Stents • Medical Guidewires • Medical Guidepins • Surgical Localization Hooks • Flexible, Steerable and Hingeless Laparoscopic Surgical Instruments • Remote Suturing and Stapling Devices • Bone Suture Anchors • Eyeglass Frames • Endodontic (Root Canal) Files • Orthodontic Arches • Brassiere Underwires • Cellular Telephone Antennas • Damping Devices From: http://www.sma-inc.com/

  20. Non-explosive Release device

  21. Thermo-controller

  22. Cooling • Heating Transformation temperature of SMA: As, Af, Ms • Heating: As, Af • Cooling: Ms, Mf • Superelastic properties is the best around Af Rs Rf Ms Mf As Af

  23. Manipulation of transformation temperatures

  24. The End Q&A

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