Shape Memory alloys& Its applications

SAJITH BABU GEORGE1MTMD 1567204

CHRIST UNIVERSITY

Memory 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.

What is shape memory alloys? A shape-memory alloys (SMA, smart

metal, memory metal, memory alloy, muscle wire, smart alloy) are metal alloys that can be deformed at one temperature but when heated or cooled, return to their “original” shape

The alloy appears to have a memory The most effective and widely used alloys

are NiTi, CuZnAl, and CuAlNi SMA also exhibits superelastic

(pseudoelastic) behavior

Basic working principle.. SMAs have two stable phases :

the high-temperature phase, called Austenite and

the low-temperature phase, called Martensite.

The martensite can be in one of two forms: twinned detwinned

A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs

The shape change involves a solid state phase change involving a molecular rearrangement between Martensite and Austenite

Upon cooling in the absence of applied load the material transforms from austenite into twinned martensite. (no observable macroscopic shape change occurs)

Upon heating the material in the martensitic phase, a reverse phase transformation takes place and as a result the material transforms to austenite.

If mechanical load is applied to the material in the state of twinned martensite (at low temperature) it is possible to detwin the martensite.

Upon releasing of the load, the material remains deformed. A subsequent heating of the material to a temperature above the austenite finish temperature (Af) will result in reverse phase transformation (martensite to austenite) and will lead to complete shape recovery.

(Af: temperature at which transformation of martensite to austenite is complete )

SMA remembers the shape when it have austenitic structure.

So if we need SMA to remember and regain/recover certain shape, the shape should be formed when structure is austenite

Reheating the material will result in complete shape recovery

PSEUDOELASTIC BEHAVIOR

Occurs when an alloy is completely in the Austenite phase

When the load is increased to a point, the alloy transitions from the Austenite phase to the detwinned Martensite phase

Once the load is removed, the alloy returns to its original Austenite shape

Rubber like effect

TEMPERATURE

Mf Ms As Aff s s f

Austenite

Detwinned Martensite(stressed)

NITINOL (Ni-Ti) Was discovered in Naval Ordnance

Laboratory (NOL), Maryland, USA Ni- 50% , Ti- 50%

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.

APPLICATIONS

Biological Applications Bone Plates

Memory effect pulls bones together to promote healing.

Surgical Anchor Clot Filter

Does not interfere with MRI from non-ferromagnetic properties.

Catheters Stent in artries Eyeglasses

Flexible Nitinol wires.

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.

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

Typical actuator in the wing of a plane.

Picture of wing with SMA wires.

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 Anti-scalding devices/Fire

sprinklers Household appliances

A deep fryer that lowers the basket into the old at a certain temperature

Prevent structural damage to bridges/buildings

Robots

ADVANTAGES AND DISADVANTAGES OF SHAPE MEMORY ALLOYS ADVANTAGES

Bio-compatibility Diverse field of application Good mechanical

properties(strong,corrosion resistant) DISADVANTAGES

Expensive Poor fatigue properties overstress

What materials are SMA’s1)Nickel-titanium alloys2) Copper-base alloys such as CuZnAl and CuAlNi3) Ag-Cd 44/49 at.% C4) Au-Cd 46.5/50 at.% Cd5) Cu-Al-Ni 14/14.5 wt.% Al and 3/4.5 wt.% Ni6) Cu-Sn approx. 15 at.% Sn7) Cu-Zn 38.5/41.5 wt.% Z8) Cu-Zn-X (X = Si,Sn,Al) a few wt.% of 9)In-Ti 18/23 at.% Ti10) Ni-Al 36/38 at.% Al11) Ni-Ti 49/51 at.% Ni12) Fe-Pt approx. 25 at.% Pt13) Mn-Cu 5/35 at.% Cu14) Fe-Mn-Si 15) Pt alloys

DO YOU WANT TO ASK ANY QUESTION?

http://en.wikipedia.org/wiki/Shape_memory_alloy

http://www.smaterial.com/SMA/sma.html

Lin, Richard. Shape Memory Alloys and Their Applications

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