Shape Memory Alloys. Steps into the Future

Scientific journal "Russiun engineer", N 1(36) 2013, p. 46-48

The first information about using shape memory alloys relates to the XV—XIII centuries B.C. “…God asked, what was in his hand. And Moses said, “It’s a rod.” “Drop it,” God said, and the rod turned into a snake, but when Moses picked it up, it changed back into the rod…” This is a classic example of using shape memory alloys.

Nowadays legends are becoming a reality. Modern history of shape memory alloys relates to the end of 1940s, when soviet metallurgists V. Kurdumov and L. Handerson noticed, that the alloy they worked on had the shape memory effect. Years later this effect was declared as a discovery and named after Kurdumov. The unique shape memory effect became known around the world and now there are more than 120 alloys with self-recovery ability. These alloys are based on such metal systems as Au-Cd, Cu-Zn-Al, Cu-Al-Ni, Fe-Mn-S, Fe-Ni, Cu-Al, Cu-Mn, Co-Ni, ,Ni-Ti, Ni-Al, etc.

Shape memory effects, reversible shape memory effect and hyper elasticity of the alloys above are conditioned by macroscopic reverberation of micro- and nanostructure transformations of crystal lattice with polymorphous austenitic-martensite first type phase conversion, that’s why these features remain invariable as long as a product is in use. In real life physical processes implementation in a metal is as follows.

By applying a slight mechanical force, products of the same alloy in chilled martensite conditions can take any configuration and be expanded by 7-8 % and in some cases by 12 % over a standard length as if a rubber strap. Such configuration will remain unchanged until heating a product to the temperature of initial austenitic conversion, and during the process of heating to the temperature of final austenitic transformation the alloy doesn’t shift to an austenitic phase, completely regaining its shape and realizing the shape memory effect.

By limiting the exposure on the specifically processed element made of shape memory alloy just by heating and cooling in temperature intervals of final austenitic-martensite transformations, the element will spontaneously flex both by heating and cooling, realizing the shape memory effect. At that, as well as an optimal load-bearing element of any metal constructions, this element can take a shape of a thin rectilinear wire, which works in tension and can infinitely be deformed under heating or cooling by 2% over length ratio, generating in hundreds times greater energy than bimetals of the same weight.

The hyper elasticity effect is realized in the shape memory alloy product, remaining in the temperature area of a stabilized austenitic condition. When deforming the product made of shape memory alloy and stimulating in that way martensite deformation under the constant temperature by coercive force, the element, as if a spring, will entirely take the original shape after removing this exposure. The difference from other springs is that in practice it will have an inexhaustible reserve. With the shape of a rectilinear string it can be subjected to hyperelastic deformation by 7-8% over length ratio, storing ten times greater energy that a regular spring.

The shape memory effect in alloys based on Ni-Ti, for example, is well-defined enough to regulate the temperature range from several units to some tens with fair accuracy by introducing different alloying extra-elements. Besides, the alloys based on Ni-Ti and known all over the world as nitinol have processability of working, corrosion resistance and specific physical and mathematical characteristics. As an example, tensile strength of nitinol ranges between 770 and 1100 MPa, it complies with the same characteristics of the majority of steels, but damping capacity of it is stronger than of iron, the alloy is hyperelastic and has the ability to reduce the shape million times. The surface of nitinol also as well as of many titanium alloys is covered with titanium dioxide, that determines its high level of corrosion resistance to sea-water, brines, most of acids and alkalis. Nitinol semi-finished products are released in the form of bars, wires, tubes and sheets (Fig. 1). The only disadvantage of nitinol is the extremely high cost of semi-finished products manufacture, concerned with high oxidability of titanium, included in its chemical composition.

The unique combination of physical and mathematical features allowed using shape memory alloys in all fields that use metals including medicine, space technology, mining industry, production of different temperature sensors and actuators, robotics to produce thermal-mechanic devices, develop new technologies, etc.

Medicine:

- Gloves, applying in the process of rehabilitation to reactivate the groups of active muscles with functional defect, can be used in intercarpal, elbow, shoulder, ankle and knee joints;

- Contraceptive coils, which take the functional form under the exposure of body temperature;

- Filters for introducing into blood vessels, which take the form of filters with a specified location;

- Weak veins clamps;

- Electric artificial muscles;

- Anchor pins and dynamic latches to fix bone prostheses (Fig. 2);

- Artificial extension device for children’s growing prosthesis;

- Replacement of whirlbone cartilages. The replacing material becomes self-clamping under the spherical form of whirlbone;

- Scoliotones;

- Temporary clamps at the artificial lens implantation;

- Spectacle frame. In the lower part where glasses are fixed with wire. Plastic lenses do not slip out by cooling. A frame doesn’t stretch by wiping and long-term using. Have a hyperelastic effect;

- Orthopedic implants;

- Brackets for a dentition correction;

- Threads for suturing;

- Porous nitinol devices for bone defect replacement and infected wounds healing;

- Herniatomy grids for human and animals;

- A new type of composite materials “bioceramic-nickel titanium”. In these composites the first component nickel-titanium is hyperelastic and has shape memory, and the second one saves bioceramic characteristics;

- Brassiere frames. In 1986 1,5 millions in of frames were produced, it was the first usage of a nitinol wire.

Temperature sensors and alarm:

- Fire alarm (Fig. 3);

- Fire dampers;

- Bathroom alarm devices;

- Power line fuse (electric circuit protection);

- Automatic window opener in the hothouses;

- Heat regeneration boilers;

- Electronic contactor;

- Fuel vapor gases prevention system (in cars);

- Radiator heat removing device;

- Fog lamps switch device;

- Incubator temperature controller;

- Tank for washing with warm water;

- Control valves of coolng and warming devices, heat engines. A sensitive valve of the air-conditioner. Regulates a wind direction into a blow-off of the air-conditioner, intended for heating and cooling;

- Coffee-machine. Determines boiling temperature and controls power of valves and switches;

- Electromagnetic kitchen machine. Heating is implemented by eddy currents, occurring on the bottom under the influence of magnetic force fields. To warn of a burn here goes the signal, which is actuated by nickel-titanium element in the form of a coil;

- Integrated circuits pressurization;

- An electric kitchen stove of a conventional type. A nickel-titanium sensor is used to switch the ventilation at microwave heating and circulating hot-air heating;

- Clamping devices production.

Space technology:

- Satellite-communication antennas;

- Couplings forming vacuum-tight joints for working in outer space. Couplings are also used in aircraft and automotive engineering.

Robotics: For making robots with shoulder bearings, elbow joints, wrists and grippers with 5 degrees of freedom. Wrist flexion, squeezing and releasing of grippers are provided by nickel-titanium spirals, joints and shoulder bearing – by extending and reduction of the wire of the same alloy. Arm position and rate of action are regulated by a straight current transmission from a simulated impulse. The reason for the evenness of robot’s actions is that the given magnitude of a force (the force of shape memory recovering) corresponds to the magnitude of a controlled parameter (current magnitude). Robot’s actions approximate to the actions of a muscled mechanism.

Mining industry: Making of the fully-automatic intelligent borehole system (the system is safe under extreme conditions of pressure and temperature differentials, easy-to-work and has the best compact sizes) is possible due to existent manufacturing capacity of plants and oil-and-gas works, because it comes to a competent engineering of easy-to-produce constructions made of nitinol nanostructured devices and films.

It is important that manufacturing of borehole equipment with fundamentally new characteristics doesn’t need any specific preparations and adds up to only stuff retraining. Due to existent manufacturing capacity it’s becoming possible to produce highly productive downhole devices such as the control valve, selective well completion instruments, packers, hammer drills and tube sealers, manipulators for the tools withdrawal after an accident, pistonless volumetric electro- and heat-mechanic pumps with a bellows frame etc., and also devices which use compact easy-to-produce direct drives made of nitinol elements in the forms of rectilinear wires with the duplicated direct and indirect heating.

Using of intelligent wells with nitinol actuating device will make profitable the development of the new hard-to-reach fields with a complex geology, the renewal of petroleum production in the non-profitable fields, wells temporarily abandoned because of exhaustion, flooding, high oil viscosity, etc.

In turn it will reduce operating costs of oil companies, widen their resource base and increase incomes from the export of carbohydrates.

Other applications:

- Nitinol is used in driving devices of self-recorders. The input transforms into an electric current heating a nitinol wire. Due to its extending and reduction a self-recorder pen puts into operation. Since 1972 some millions of such joints have been produced (data as of the end of XX century). The drive mechanism is simple to use that’s why it breaks extremely rarely;

- Electric kitchen stove of a conventional type. A nitinol sensor is used to switch the ventilation at microwave heating and circulating hot-air heating;

- Hyperelasticity is used to make high-efficient springs, mechanic energy storage devices, hyperelastic spherical segments working at the fixed temperatures, transmit/receive antennas and mobile phones (Fig. 4).

Producing of all devices listed above and beyond is realized by using bars, tubes, wire, nitinol sheets, successfully manufactured by NiTiMet Company (www.niti-met.ru), the only nitinol manufacturer in the Russian Federation.

An amazing shape memory material is becoming an integral part of our life. The range of convenience is increasing from day to day and it promises lots of interesting things. It is safe to say that this is the material of the future.

References:

1. The Old Testament. Plagues of Egypt. Moses’ Rod and the Rod of God’s Wrath (Exodus, chap. 4-1)

2. “Shape memory effects” V. A. Likhachov. Leningrad State University, 1987, p. 216.

3. “The usage of the shape memory effect in modern mechanical engineering”. A. S. Tikhonov. Moscow, Mashinostroenie, 1981, p. 81.

4. “Shape memory”. V. N. Khachin. Moscow, Znanie, 1984, p. 62.

5. “Shape memory alloys”. Ootsuka K., Simidzu K., Sudzuki U., edited by H. Foonakubo. Metallurgiya, 1990, p. 224.

6. Analysis and development of load-bearing units of shape memory alloys. V. Shishkin, N.A. Makhutov. – Izhevsk. Scientific publishing center “Regular and chaotic dynamic”. 2007, p. 412.

7. Shape memory alloys and medicine. V. E. Gunther, V. V. Kotenko, M. Z. Myrgazimov, V. K. Polenichkin, I. A. Vityugov, V.I. Itin, R. V. Ziganshin, F. T. Temerkhanov. – Tomsk. Tomsk University publishing house. 1986, p. 208.

8. Hyperelastic shape memory alloys in science, engineering and medicine. The reference publication. S. A. Muslov, V. A. Andreev, A. B. Bondarev, P. J. Sukhochev. Publishing house “Pholium”, 2010, p. 456.

9. Website. NiTiMet Company. www.niti-met.ru