History

1930s was the first recorded period in timeline involving discovery of shape memory alloys. According to the book “Shape Memory Materials (first published in 1998, written by Otsuka and Wayman), A. Ölander discovered the psuedoelastic behavior of the Au-Cd alloy in 1932. Greninger Mooradian (1938) observed the formation and disappearance of martensitic phase at low and high temperatures of a Cu-Zn alloy. The basic phenomenon of the memory effect governed by the thermoplastic behavior of the martensitic phase was widely reported a decade later by Kurdjumov and Khandros (1949) and also by Chang and Read (1951).

In the late 1950s a researcher at the Naval Ordnance Laboratory named William J. Buehler developed Ni-Ti alloy which was durable enough to be used in nose cone of a missile. He named his discovery Nitinol (Nickel Titanium Naval Ordnance Laboratory). Buehler made up long, thin strips of Nitinol to demonstrate the material by bending it and proving that the strips would not break because they were durable and could be used in missiles. At a management meeting in 1961 at one of the demonstrations, which had been repeated many times until then, a strip of Nitinol was flexed by everyone at the meeting. One of the Associate Technical Directors, the late Dr. David S. Muzzey, heated the compressed Nitinol strip and witnessed that the strip stretched out its original shape.

Not only shape memory alloys but also shape memory polymers, which are also thermally-responsive materials, were developed in the late 1990s.

Recent Examples Of Shape Memory Alloys

With the help of nanotechnology, SMAs such as Nİ-Tİ-Al-Zn, Cu-Ni-Ti and Cu-Al-Zn can be used as fillers in microchips. Although SMAs raise the cost, they make better conductor than traditional fillers which is necessary for us to produce more improved microchips.

Researchers at University of Oxford developed a tiny device made up of SMA that will help brain aneurysm treatment. The device is made from a special laser-cut SMA and will serve as a catheter during surgeries. Device can fit into natural shape of blood vessel. Therefore blood will be diverted away from aneurysm and will allow it to heal.

A new development at Florida Atlantic University (FAU) may be the future of prosthetic devices. By using a heat-sensitive SMA which will bend as a response to core of the finger. Using 3D printers two SMA plates are made and positioned. One them flexes in response to heat and other does the opposite. This contrast between plates makes the device work in harmony. On these grounds, device looks promising to be used as the material of prosthetic devices in the future.

German engineers used thin SMA to form a mechanism of an artificial hand. The thin SMA wires will work like fibers in a muscle of human arm. Similar to prosthetic finger in FAU this material, Ni-Ti alloy in this case, is heat-sensitive and it responses the heat produced by the electric current which flows through the SMA wires.

Applications

Shape memory alloys (SMA) can be used in many areas but amongst those which can be instantly recognizable ones would go as industrial, medicine, optometry, in the engines and many others which I must neglect to write here.

In industry one example is that they are used in aircraft and spacecraft. A variable geometry chevron was developed using Ni-Ti SMA. It has a Variable Area Fan Nozzle (VAFN). In 2005 and 2006 Boeing made a trial and tested it in a flight which was a success.

Scientists are working on the use of SMAs as vibration dampers for launch vehicles and commercial jet engines. SMAs show promise for restraining high vibration that is generated on payloads along with on the fan blades in commercial jet engines. SMAs are also found suitable for other high shock applications.

SMAs are also being used in automotive industry. The first significant usage of SMA in car was a valve that controls low pressure pneumatic bladders in a car seat. The benefits from this usage of SMA was lower noise, lesser weight, lesser power consumption and so on.

In 2014, Chevrolet was the first brand that ever use SMA actuator in the Corvette. SMA actuators, as being a lighter alternative to traditional motorized actuators, made it easier to close the lid since the vent allows the trapped air in the trunk to go out.

SMAs are also suitable for manufacturing anti-shake mobile phone cameras since low-light performance and unsteady videos. The precise usage of the SMA is that SMAs render the possibility of a mechanism called OIS (acronym of “Optical Image Stabilization”) and thanks to this complex mechanism a mobile device can be a rather good camera.

Introduction

Introduction:

Shape memory alloys or in short form SMA are materials that can return its original state after deformation. SMAs are sensitive to heat changes. They can have 2 different crystal states when their temperature is upper or lower than their critical temperature. Materials that can be easily deformed at low temperatures and be easily turned into original states at high temperatures. The most commonly used ones are Ni-Ti and copper based alloys because these alloys can produce significant force during the shape-shifting. In these days, SMAs are used as simultaneous detectors and actuators and so they attract considerable attention.

The most commonly used SMAs in industry are Ni-Ti alloy and copper based alloys. Ni-Ti are perfect with their corrosion resistance and much more ductility when they are compared to copper based alloys as copper based alloys have less thermal determination and are more sensitive to corrosion. However copper based alloys are cheaper, easier to be melted and has a wider range of potential thermal transition. Ultimately, both alloys have their own advantages and disadvantages depending on in what environment they will be used.

Ni-Ti alloy is a dual alloy frame. Such a compound can dissolve extra nickel and titanium in acceptable limits and it has a ductility on a level that can be compared to conventional alloys. Due to this extraordinary ability of dissolving, other element can be add in order to change either the transition pattern or the mechanical properties at will. Even approximately 1% nickel addition will affect the properties of alloy frame. Contaminations, for instance oxygen and carbon, are not wanted in the structure as they change the transition heat and weaken the mechanical properties.

Copper based alloys may consist of three metals, such as CuZnAl and CuAlNi or they may appear in modification as they consist of four that contain manganese. In order to make thinner granular elements such as boron, cerium, cobalt, iron, titanium, vanadium and zirconium are added to the structure. Besides mangan lower the transition heat of both CuZnAl and CuAlNi and it changes the point of eutectoid of the alloys that contain high incidence of aluminum. Mangan is also added instead of aluminum for better ductility. Long duration of heats should be avoided since it causes zinc to vaporize and growth of grain. Feeding water is used for hardening. Cooling outdoor should be enough for alloys that contain high incidence of aluminum. However transition temperatures of cooled parts are generally undetermined and in order to make it become determined ageing should be done at temperature that higher than AF temperature. Thermal determination of copper based alloys are limited by their disintegration kinetics. Therefore the alloys, CuZnAl and CuAlNi, should not be exposed to temperatures higher than 150~200^oC. Ageing at temperature lower than that changes the transition temperatures. Ageing at beta phase will cause similar consequences. Alloys that were aged in martensitic state show martensitic stabilization effect which involves ageing. CuAlNi alloy, at high temperatures, are more determined than CuZnAl alloy. Therefore when strict control of transition temperatures is required in different temperature applications, these factors should be considered.