CENTRE FOR NANOTECHNOLOGY CENTRAL UNIVERSITY OF JHARKHAND

RAJVARDHANSEM:VthReg.No:CUJ/I/2013/INT/025Centre for Nanotechnology, Central University of JharkhandE-mail: razvardhan@gmail.com

razvardhan@gmail.com SHAPE MEMORY ALLOYS

CENTRAL UNIVERSITY OF JHARKHAND

Outline

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.

Two Phases•Austenite• Hard, firm• Inelastic• Resembles titanium• Simple FCC structure

•Martensite• Soft• Elastic• Complex structure

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 MemoryThe 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)

PseudoelasticityPseudoelasticity (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.

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.

Flexible Nitinol wires.

University of Alberta

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

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 ManeuverabilityNitinol 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

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

LIMITATIONS• 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

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

Shape Memory Alloys. University of Alberta2001 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. 2004Martensitic Transformation. Encyclopedia of Materials: Science and Technology. 2001

References

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