Shape memory alloy
Shape memory alloys (abbreviation FGL , English shape memory alloy , abbreviation SMA ) are special metals that can exist in two different crystal structures. They are often referred to as memory metals . This stems from the phenomenon that they seem to be able to “remember” an earlier shape despite the subsequent strong deformation .
introduction
While most metals always have the same crystal structure up to their melting point, shape memory alloys have two different structures ( phases ) depending on the temperature . The shape transformation is based on the temperature-dependent lattice transformation to one of these two crystal structures ( allotropic transformation). As a rule, there is the high-temperature phase called austenite and the martensite (low -temperature phase ). Both can merge through a change in temperature (two-way effect).
The structural transformation is independent of the rate of temperature change. To initiate the phase transition , the parameters temperature and mechanical stress are equivalent; This means that the conversion can be brought about not only thermally but also by mechanical stress.
A well-known representative for this allotropic transformation is iron . However, iron has no shape memory per se, so another condition must be met. Shape memory alloys need a series of equal shear systems in every crystal system, which result from the spatial symmetry of the unit cell . If all shearings are equally distributed during a transformation, no external change in shape can be seen. However, if only a few shear systems are preferred by external forces, for example, changes in shape are observed.
The first observation of the effect goes back to welding work on sheet metal made of nickel-titanium alloys, which was carried out in the USA in 1953.
Usable effects
Shape memory alloys can transfer very large forces to several 100,000 motion cycles without noticeable fatigue. Compared to other actuator materials, shape memory alloys have by far the greatest specific work capacity (ratio of work done to material volume). Shape memory elements can function for several million cycles. As the number of cycles increases, however, the properties of shape memory elements, e.g. B. a residual strain may remain after conversion.
In principle, all shape memory alloys can perform all shape memory effects. The respective desired effect is the task of manufacturing and materials technology and must be trained by coordinating the application temperatures and optimizing the effect sizes.
Disposable (memory) effect
The one-way effect is characterized by a one-time change in shape when a specimen that was previously pseudoplastically deformed in the martensitic state is heated . It only allows a one-time change in shape. The renewed cooling causes no change in shape, only an intrinsic change in the lattice (austenite to twinned martensite). If you want shape memory alloys for the actuators, z. B. as an adjusting element, the component must be able to return to its "cold form". This is e.g. B. possible with a return element in the form of a spring.
Two-way (memory) effect
Shape memory alloys can also “remember” two shapes - one at high and one at low temperature - thanks to the two-way effect. In order for the component to regain its defined shape when it cools, it must be "trained" through thermomechanical treatment cycles. This causes stress fields to develop in the material, which promote the formation of certain martensite variants during cooling. Thus, the trained shape for the cold state only represents a preferred shape of the martensite structure. The transformation of the shape can only take place with the intrinsic two-way effect if no external forces counteract it. Therefore, the device is unable to do work when it cools down.
Pseudo-elastic behavior ("superelasticity")
In the case of shape memory alloys, in addition to the usual elastic deformation, a reversible change in shape caused by the action of external forces can be observed. This "elastic" deformation can exceed the elasticity of conventional metals up to twenty times. The cause of this behavior, however, is not the bonding force of the atoms, but a phase change within the material. The material must be present in the high temperature phase with an austenitic structure. Under external stresses, the face -centered cubic austenite is transformed into the tetragonally distorted (body-centered or body-centered, tetragonally distorted lattice) martensite (stress-induced martensite). When the load is removed, the martensite converts back into austenite. Since each atom retains its neighboring atom during the transformation, one speaks of a diffusionless phase transformation. That is why the property is called pseudo-elastic behavior . When the load is released, the material returns to its original shape due to its internal tension. No temperature changes are required for this.
This effect is also used in the field of medical technology .
Magnetic shape memory alloys
In addition to the above-described thermally excited magnetic alloys, shape memory alloys exist (engl. Magnetic shape memory alloy , MSMA) showing a magnetically excited change in shape. When an external magnetic field is applied, the twin boundaries are shifted and there is a change in shape and length. The achievable change in length of such alloys is currently in the range of up to 10% with relatively (in contrast to magnetostrictive materials) small transferable forces.
Materials
The materials mainly used as shape memory alloys, which are also called cryogenic materials, are NiTi ( nickel - titanium , nitinol ) and, with even better properties, NiTiCu (nickel-titanium- copper ). Both are most likely used as actuator materials. The conversion temperatures are dependent on an exact stoichiometry (quantitative ratio). If the nickel content is below 50 atomic percent, it is around 100 ° C. If the nickel content of the alloy is varied, it is possible to produce pseudo-elastic or pseudo-plastic behavior as austenite or martensite at room temperature.
Other copper-based materials are CuZn ( copper - zinc ), CuZnAl (copper-zinc- aluminum ) and CuAlNi (copper-aluminum-nickel). Although they are cheaper, they have both higher transformation temperatures and a poorer shape memory. They are used in particular in medical technology. FeNiAl (iron-nickel-aluminum), FeMnSi (iron-manganese-silicon) and ZnAuCu (zinc- gold- copper) are less common .
Application examples
- Use as a motor or in generators (see e.g. Thermobile )
- In the automotive industry, the shape memory actuator is the first application in large numbers (> 5 million actuators / year) for pneumatic valves
- The latest applications on the market are telephone-camera adjustments, such as autofocus and, shortly before the market launch, optical image stabilization .
- The high actuating force is used in hydraulic pumps .
- Various applications as medical implants have been developed, for example for stents (small structures for stabilizing arteries). A miniaturized blood pump was presented at RWTH Aachen University , which is inserted into a blood vessel near the heart in a compressed form by means of a catheter and unfolds into the form effective as a pump in contact with the body-warm blood.
- Applications in the field of bioanalysis, e.g. B. Lab-on-a-Chip systems.
- Flat, thin bending actuators based on shape memory alloy wires
- For switching micro valves
- In space technology, shape memory materials are often used for deployment of solar panels and similar activities, mainly using the one-way effect.
- Use of the high restoring forces as an application in heat engines
- As actuators , such as springs or meander-shaped film actuators
- Adaptive change of wings and winglets on aircraft
- Use of nitinol in endodontics for root canal treatment of severely curved root canals in which a stainless steel extirpation needle would break.
- Use as wire in fixed dental braces (" brackets ")
- In rods of flexible glasses frames
Similar materials
- Shape memory polymer - plastics with memory properties
literature
German:
- D. Stöckel: Alloys with shape memory. Industrial use of the shape memory effect. expert-Verlag, 1988, ISBN 3-8169-0323-1 .
- M. Mertmann: NiTi shape memory alloys for actuators in gripper technology . VDI Verlag, 1997, ISBN 3-18-346905-7 .
- J. Spielfeld: Thermomechanical treatment of copper alloys with shape memory . VDI Verlag, 1999, ISBN 3-18-355705-3 .
- P. Gümpel (Ed.): Shape memory alloys . expert-Verlag, 2004, ISBN 3-8169-2293-7 .
- S. Langbein, A. Czechowicz: Construction Practice: Shape Memory Technology . Springer Vieweg, 2013, ISBN 978-3-8348-1957-4 .
English:
- TW Duerig (Ed.): Engineering Aspects of Shape Memory Alloys. Butterworth-Heinemann, London 1990.
- YY Chu: Shape memory materials and its applications. Trans Tech Publ., Zurich 2002, ISBN 0-87849-896-6 .
- VA Chernenko: Advances in shape memory materials. Trans Tech Publ., Zurich 2008, ISBN 978-0-87849-381-4 .
Web links
Individual evidence
- ↑ A. Sozinov, AA Likhachev, N. Lanska, K. Ullakko: Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase . In: Applied Physics Letters . tape 80 , no. 10 , 2002, p. 1746 , doi : 10.1063 / 1.1458075 .
- ↑ a b Gunther Eggeler, E. Hornbogen: Materials with shape memory . In: The magazine . Volume 9, No. 1 , 1998, ISSN 0938-4081 ( online ).
- ↑ Christina Elmer: Memory metal: Alloy always returns in its original form . Spiegel Online, October 2, 2013, accessed October 2, 2013.
- ↑ Jean-Pierre Joosting: Shape memory alloy optical image stabilizer debuts in smartphone. In: Microwave Engineering Europe. January 13, 2015, accessed July 18, 2017 .
- ↑ A strong memory. May 2, 2019, accessed June 12, 2019 .
- ↑ Smart materials improve aerodynamics in cars and airplanes. Retrieved August 15, 2019 .
- ↑ Mini-Valves: Special solution for the smallest medical products. February 12, 2018, accessed on June 12, 2019 (German).
- ↑ SMAterial.com - Applications (Eng.)
- ^ R. DesRoches, J. McCormick, M. Delemont: Cyclic Properties of Superelastic Shape Memory Alloy Wires and Bars . In: Journal Of Structural Engineering . tape 130 , no. 1 , 2004, p. 38-46 , doi : 10.1061 / (ASCE) 0733-9445 (2004) 130: 1 (38) .