http://personal.cityu.edu.hk/~appchung

Ti and Shape Memory Alloys:

Ir. Dr. Jonathan C.Y. ChungAssociate ProfessorDepartment of Physics and Materials ScienceCity University of Hong Kong

Why we choose Ti alloy?

1. Corrosion-resistant

2. High strength from low temperature up to 650oC

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 883oC)- Hexagonal close packed (HCP)- Usually strong and brittle- Not easy to form into various shape

2. 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.

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 SteelTensile strength: 200 MPa

High Strength Alloy SteelTensile strength: ~500-1000 MPa

Ultra-High Strength SteelTensile strength: >1000 MPa

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

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

Near-alpha titanium alloys

Almost all alpha phase Small amount of beta phase disperse t

hroughout the alpha Improved creep resistance at temperat

ures at 450-500oC Tensile strength: 855-1040 MPa Fracture toughness: 50-70 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 ductilit

y Poor fatigue performance Tensile strength: 1220-1450 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 weldin

g processes can also be used Resistance spot and seam welding is only used wh

en 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 platformFrom: www.sma-inc.com

Superelastic DevicesNiTi 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/

Non-explosive Release device

Thermo-controller

Transformation temperature of SMA: As, Af, Ms

Heating: As, Af Cooling: Ms, Mf Superelastic pr

operties is the best around Af

-100 -50 0 50 100

Cooling

Heating

As Af

RsRf

MsMf

Manipulation of transformation temperatures

0

10

20

30

40

50

60

200 300 400

30 mins ageing with FC (a)30 mins ageing with WQ (b)60 mins ageing with FC (c)60 mins ageing with WQ (d)

The End

Q&A