Shape Memory Alloys

Timeline of Memory Metals

• 1932 - A. Ölander discovers the pseudoelastic properties of Au-Cd alloy.

• 1949 - Memory effect of Au-Cd reported by Kurdjumov & Kandros.

• 1967 – At Naval Ordance Laboratory, Beuhler discovers shape memory effect in nickel titanium alloy, Nitinol, which proved to be a major breakthrough in the field of shape memory alloys.

• 1970-1980 – First reports of nickel-titanium implants being used in medical applications.

• Mid-1990s – Memory metals start to become widespread in medicine and soon move to other applications.

• Austenite Two Phases

– Hard, firm – Inelastic – Resembles titanium – Simple FCC structure

• Martensite

– Soft – Elastic – Complex structure

Shape Memory Alloy Qualities

• Ability to “remember” its austenite phase – As the metal is cooled to the martensite phase, it can be easily deformed. When the temperature is raised to the austenite phase, it reforms to the original shape of the material.

• Pseudoelasticity – When the metal is changed to the martensite phase simply by strain. The metal becomes pliable and can withstand strains of up to 8%.

• A mix of roughly 50% nickel and 50% titanium is the most common SMA. Also CuZnAl and CuAlNi are widely used.

Shape Memory

The twinned martensite phase resemble the austenite phase from our point of view, but on an atomic level, the structure is different. There are phase planes where the martensite can reconfigure itself with 24 crystallographically equivalent habit planes. This is called twinning because of the symmetry across the planes.

Phase Changes in NiTi

(2001 SMA/MEMS Research Group)

Pseudoelasticity

Pseudoelasticity (superelasticity) occurs when the alloy is above the martensite temperature, but there is a load strong enough to force the austenite into the martensite phase. The alloy will not return to the austenite phase until the loading is decreased or there is a large enough change in temperature.

The figure shows load versus temperature on an SMA. 2001 SMA/MEMS Research Group

The above figure shows the Martensitic transformation and hysteresis (= H) upon a change of temperature. As = austenite start, Af = austenite finish, Ms = martensite start, Mf = martensite finish and Md = Highest temperature to strain-induced martensite. Gray area = area of optimal superelasticity. (Jorma Ryhänen 2000) The figure below shows NiTi’s ability to change its shape along phase planes. Other metals, as we know, slide along slip planes when there is an induced stress.

Flexible Nitinol wires.

University of Alberta Wires have the ability to flex the robotic muscles according to electric pulses sent through the wire.

Nitinol Wires

• Nitinol is generally doped with other materials like Cr, Cu, Al, or Fe.

• Flexinol is a popular brand of SMA wire.

• Flexinol is designed to take more repeated stress cycles than pure NiTi mixes.

• Specifically designed to manufacturer’s needs.

– 0.0010 inch diameter wire can lift 7 grams in 1 second with a 20 mA current.

– 0.010 inch diameter wire can lift 930 grams in 1 second with a 1 A current.

– Wires are also made to change states at different temperatures generally between -30 C and 120 C within 5 C.

Biological Applications

• Bone Plates – Memory effect pulls bones together to promote healing.

• Surgical Anchor – As healing progresses, muscles grow around the wire. This prevents tissue damage that could be caused by staples or screws.

• Clot Filter – Does not interfere with MRI from non-ferromagnetic properties.

• Catheters • Retainers • Eyeglasses

Aircraft Maneuverability

Nitinol wires can be used in applications such as the actuators for planes. Many use bulky hydraulic systems which are expensive and need a lot of maintenance.

USAF Aircraft Pictures

Typical actuator in the wing of a plane.

University of Alberta

Picture of wing with SMA wires. University of Alberta The wires in the picture are used to replace the actuator. Electric pulses sent through the wires allow for precise movement of the wings, as would be needed in an aircraft. This reduces the need for maintenance, weighs less, and is less costly.

Other Applications

• Small incision tweezers • Eyeglass frames • Anti-scalding devices/Fire sprinklers • Household appliances – A deep fryer that lowers the basket into the old at a certain temperature • Underwire bras • Prevent structural damage to bridges/buildings • Robots

Problems With SMAs

• Fatigue from cycling – Causes deformations and grain boundaries – Begin to slip along planes/boundaries • Overstress – A load above 8% strain could cause the SMA to completely lose its original austenite shape • Difficulty with computer programming • More expensive to manufacture than steel and aluminum • Relatively new

References

http://www.mkt-intl.com/tungsten/images/niti_1.jpg

Shape Memory Alloys. University of Alberta 2001 SMA/MEMS Research Group How Memory Metals Shape Product Design. Design News June 1993 Ryhänen, Jorma. Biocompatibility evaluation of nickel titanium shape memory metal alloy. 2000 Lin, Richard. Shape Memory Alloys and Their Applications.

Hornboden, E. Review Thermo-mechanical Fatigue of Shape Memory Alloys. Journal of Material Science. 2004 Martensitic Transformation. Encyclopedia of Materials: Science and Technology. 2001