Shape Memory Alloys

Vormgeheugenlegeringen

Alliages à mémoire de forme

Formgedächtnislegierungen

prof. dr. ir. Jan Van Humbeeck

MTM-KULeuven BioTiNet Leuven March 6, 2012

Requirements

• A (thermoelastic) martensitic transformation

Shape deformation by reorientation of martensite variants (Martensite phase).

Shape deformation by strain-induced martensite (Beta phase)

Transformation temperatures below 150°C

Small Hysteresis

Transformation Temperatures

martensite

martensite

beta

beta

As Af

Ms Mf

T

heating

cooling 10 K

Reorientation

Shape recovery after reorientation

Alloy systems exhibiting SME

• NiTi alloys and its derivatives

• Cu-based alloys (Cu-Zn-Al, Cu-Al-Ni, Cu-Al-Mn, Cu-Al-Be)

• Fe-based alloys with ε-martensite (Fe-Mn-Si, Fe-Ni-Co-Ti)

• CuZr-based alloys

• β-Ti alloys

• Magnetic Field Driven SMA (Ni-Mn-Ga)

Transformation temperatures as function on Ni-content. Lines denoted with 400°C and 500°c give the TT after annealing at those temperatures.

Functional Properties

• One Way Shape Memory Effect

• Two Way Shape Memory Effect

• Actuator

• Constrained Recovery Force

• Superelasticity

• High Damping Capacity

Biomedical Applications

• Orthodontic devices

• Guidewires

• Non-invasive surgical instruments

• Orthopaedic implants

• Stents

• Filters

• Exo-prosthesis

Securing the Human Body

Requirements for Medical Implants (biomaterials)

• The reliability of the mechanical functions and functional properties

• The chemical reliability (the resistance to deterioration of their properties in a biological medium, the resistance to expansion, dissolution, corrosion)

• Biological reliability-biological compatibility, lack of toxicity and carcenogenicity, resistance to the

formation of thrombus and antigens.

Biocompatibility of NiTi

• NiTi is bio-inert (osteopermissive)

– intermetallic compound

– Ti20 surface layer

• Deviation from stoichiometric composition (with Ni contents exceeding or depletion on ~ 2at% and more) leads to a raise of electrochemical activity and to a lowering of the corrosion resistance

• Alloying with elements of the Pt-group (Ru, Rh, Os, Ir, Pd, Pt) or Mo increases corrosion resistance. TiNi-Mo has a higher passivity

• Alloying with Cu, Fe, Mn, Al results in a slight drop of the corrosion resistance

• Ionic beam treatment, TiN and TiCN, increases the corrosion resistance and suppresses the dissolution of Ni from NiTi alloys

Corrosion resistance

Orthodontic Applications

•Steerable biliary guidewire (Director)

Guidewires: -high flexibility and high kink resistance

Superelastic NiTi endodontic files for root canal surgery

A Simon filter, shown in longitudinal and transverse views of the deployed state

A 1-mm-diam urological grasper (Bacher, Tuttlingen, Germany) demonstrates kink resistance. The shaft comprises a nitinol wire concentrically placed in a nitinol tube; the distal end is a hinged, stainless-steel grasper, which opens to an approximately 90° included angle.

Biorthex porous NiTi for bone reconstruction

Stents

• Self-expanding stents based on the superelastic properties

• Balloon-expandable stents

• Expansion by the shape memory effect

• Drug delivery stents

• Temporary Stents (removable)

Stents produced by laser-cutting from tubes

Wire-stent

Self-expandable stent (Angiomed)

“Stents”

Conclusions

• (NiTi-) Shape memory alloys are well established functional materials.

• The main application sector is medical.

• Since the value of the functional properties is very much dependent on the thermomechanical history, it is difficult to provide correct design data on the material.

• Creative design might open new application possibilities