Shape-memory alloy
Shape memory alloys ( SMAs abbreviation, 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 is due to the phenomenon that they are of an earlier design despite subsequent strong deformation seems to "remember" can.
Introduction
While most metals have always the same crystal structure up to its melting point, have shape-memory alloys, depending on the temperature, two different structures ( phases). The shape change is thus based on the temperature-dependent lattice transformation to one of these two crystal structures ( allotropic transformation ). There are the austenite -called high -temperature phase and the martensite ( low temperature phase). Both can be caused by temperature change into each other ( two-way ).
The structure of conversion is independent of the rate of temperature change. To initiate the phase change, the parameters of temperature and mechanical stress are equivalent; That is, the conversion may be not only thermally, but also be brought about by a stress.
A well-known representative of this allotropic transformation is, inter alia, iron or steel. However, steel has per se a form of memory, it must therefore be still met another condition. Shape memory alloys need in each crystal system a number equal shear systems that arise from the spatial symmetry of the unit cell. Are all shears uniformly distributed at a conversion, no external change in shape can be seen. But if only some shear systems preferably for example by external forces, shape changes are observed.
The first observation of the effect is due to welding on sheets of nickel -titanium alloys, which were carried out in the U.S. in 1953.
Useful effects
SMAs can be transferred to more than 100,000 operating cycles very large forces without striking fatigue. Compared to other actuator materials have FGL by far the largest specific work capacity (ratio of work performed to material volume ). FG elements can operate for several million cycles. With increasing number of cycles, however, deteriorate the properties of shape-memory elements, e.g., may remain after the conversion, a residual elongation.
Basically, 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 a vote of operating temperatures and optimization of effect sizes.
One-way (memory) effect
The one-way effect is characterized by a unique shape change upon heating above the martensitic state to a pseudo- plastically deformed sample. He only allowed a one-time change in shape. The re- cooling causes no change in shape, only an intrinsic change in the lattice ( austenite to twinned martensite). Can you now want to use shape memory alloys also for the actuator, eg as an actuator, the device has to return to its " cold form " return. This is, for example, with a reset element in the form of a spring is possible.
Two-way (memory) effect
Shape memory alloys may be represented by the two-way effect also in two forms - a high and a low temperature - "remember". Thus, the device cools again assumes its defined shape, it must be 'trained' by thermomechanical treatment cycles. This causes the formation of stress areas in the material to promote the formation of certain types martensite on cooling. Thus, the trained shape for the cold condition only represents a preferred form of the martensite microstructure The Transformation of form can take place only when intrinsic two, when no external forces counteract. Therefore, the device is not able to do work on cooling.
Pseudo- elastic behavior ( " super-elasticity " )
Wherein the shape memory alloy caused by external force reversible change in shape in addition to the usual elastic deformation can be observed. This " elastic " deformation may exceed the elasticity of conventional metals up to twenty times, that is, it can be a modulus of elasticity specify which is only one- twentieth of the usual metals value. However, the cause of this behavior is the bonding force of the atoms, but a phase transformation within the material. The material shall this be in the high-temperature phase with austenitic structure. Under external stresses, the face-centered cubic austenite forms around the tetragonal distorted ( body-centered or body-centered cubic, tetragonal distorted grid ) martensite ( stress-induced martensite). Upon relief of the martensite transforms back into austenite. Since during the conversion, each atom retains its neighbor atom, one also speaks of a diffusionless phase transformation. Therefore, the property is referred to as a pseudo- elastic behavior. The material returns upon release by its internal voltage back to its original shape. For any temperature change is required.
Applies this effect, inter alia, in the field of medical technology.
Magnetic shape memory alloys
In addition to the thermally excited alloys described above, magnetic shape memory alloys exist (English magnetic shape memory alloy, MSMA ) showing a magnetically excited deformation. This move by applying an external magnetic field, the twin boundaries and there is a form and change in length instead. The achievable change in length of such alloys is currently in the range up to 10 % at relatively (as opposed to magnetostrictive materials ) small transmittable forces.
Materials
- NiTi ( nickel titanium, nitinol )
- NiTiCu ( nickel -titanium copper) - better properties than NiTi
- CuZn (copper - zinc)
- CuZnAl (copper -zinc-aluminum )
- CuAlNi (copper - aluminum-nickel )
- FeNiAl ( iron-nickel - aluminum)
- FeMnSi (iron - manganese - silicon)
- ZnAuCu (zinc - gold-copper )
Application Examples
- Use as motor or in generators (see eg Thermobile )
- In the automotive industry there is the SMA actuator as the first high-volume application (> 5 million actuators / year) for pneumatic valves
- The latest application is Phone Camera adjustments can find in the market, such as autofocus and close to market introduction, the optical image stabilization.
- The high force is utilized in hydraulic pumps.
- Various applications have been developed as medical implants, for example stents, small structures for stabilizing arteries. At RWTH Aachen miniaturized blood pump was introduced, which is inserted into a compressed form via a catheter into a blood vessel near the heart and unfolds into the active form as a pump in contact with the body warm blood.
- In space technology shape-memory materials are often used to deploy the awning and related activities, while the one-way effect is mainly used.
- Use of high restoring forces as an insert in heat engines
- As actuators, such as springs
- Adaptive modification of wings and winglets on airplanes
- Use in endodontics for root canal treatment strongly curved root canals in which a Exstirpationsnadel would break from stainless steel.
- Use as a wire in fixed braces ( " brackets " )
- As flexible eyeglass frames
Similar materials
- Shape Memory Polymer - Plastics with memory properties