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? 1. Corrosion-resistant 2. High strength from low temperature up to 650 o C 3. Low density: 4.5g cm -3 [Al: 2.69 g cm -3 ; Cu: 8.96g cm -3; Fe: 7.88g cm -3] 4. Strength is relatively low when pure 5. A lot stronger when alloyed

Two Crystalline forms of pure Ti 1. Alpha  : (room temperature to 883 o C) - Hexagonal close packed (HCP) - Usually strong and brittle - Not easy to form into various shape 2. Beta  : (>883 o C) - 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 ( %) are largely determined by the oxygen content. Hence, improper hot working such as forging may affect the properties.

Four groups of Ti alloy 1. Alpha titanium alloys: Commercially pure, Ti-Pd 2. 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 3. Alpha-beta titanium alloys: Ti-6Al-4V, Ti-4Al-4Mo-2Sn-0.5Si, Ti-4Al-4Mo-4Sn-0.5Si 4. Beta titanium alloys: Ti-11.5Mo-6Zr-4.5Sn

Purposes of Alloying

Steels Mild Steel Tensile strength: 200 MPa High Strength Alloy Steel Tensile strength: ~ MPa Ultra-High Strength Steel Tensile strength: >1000 MPa

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

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: MPa Fracture toughness: 30-60MPa m -1/2

Near-alpha titanium alloys Almost all alpha phase Small amount of beta phase disperse throughout the alpha Improved creep resistance at temperatures at o C Tensile strength: MPa Fracture toughness: MPa m -1/2

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: MPa Fracture toughness: >50 MPa m -1/2

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

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

What is shape memory materials? Golan Initiatiative Center, Israel

What is shape memory effect (SME)?

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

Shape Recovery (shape memory)

Superelastic Properties (Pseudoelastic)

Typical Loading and Unloading Behavior of Superelastic NiTi Large “ Elastic ” strain compare to most alloys A constant stress platform From:

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:

Non-explosive Release device

Thermo-controller

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

Manipulation of transformation temperatures

The End Q&A