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In this online course you will learn the essential facts about NiTi-Shape Memory Alloys (NiTi-SMA).
Equiatomic NiTi (50.0 at.% Ni and Ti each) is the starting point of NiTi-SMA. Deviations from the alloy composition change the mechanical behavior and the transformation temperatures of the functional material within defined parameters. A Ni-Ti-SMA with 50.7 at.% Ni, for example, is super-elastic at room temperature (since the Austenite finish temperature is below RT) and can be used for the production of flexible components or implants like stents.
Through substitution or additional alloying of further elements such as Cu, Fe, Cr, Hf, V, Pt, Pd, Co and several more, the characteristics of NiTi-based SMA can be adapted to the needs of customers and product specifications. Apart from binary NiTi, the alloys known today also include ternary alloys such as NiTiCu (low hysteresis temperature), NiTiFe (R-phase stabilisation), NiTiCr (increase of strength in super-elastic SMA), NiTiNb (wide hysteresis SMA) or also NiTiHf and NiTiPd (high temperature SMA). The next generation of SMAs are currently being developed. It is based on quaternary and higher-grade systems (four and more main elements).
The characteristics of SMA are determined by its microstructure. A solid state phase transformation between Martensite (low temperature phase) and Austenite (high temperature phase) results in different Shape Memory effects. Martensite, in the case of binary NiTi, has a monoclinic crystal structure, whereas Austenite is symmetrical bcc (body-centered cubic). In the martensitic state the material can be deformed pseudoplastically, which means that a seemingly plastic deformation remains after unloading. By heating, the microstructure changes into Austenite and "remembers" its former shape.
If, on the other hand the Austenite is deformed under certain conditions, a stress induced martensitic transformation occurs. The martensitic state remains as long as a mechanical stress is applied. When unloaded, the material returns immediately to its former shape. Within the stress induced transformation, reversible strains between 6-8% can be reached without irreversible deformations. The material is called super-elastic when the Austenite remains stable at ambient temperature and a stress induced transformation inside the Martensite is possible.
For microstructural refinement, the material has to be alternately cold worked and then heat treated. If an adequate cold work has been reached, under specified parameters (temperature and time), a recrystallisation (formation of new `defect free´ grains) can take place. A cold worked structure shows a high dislocation density. These lattice defects lead to inner stresses in the microstructure and strongly affect the material in its phase transformation behaviour.
Therefore, in a cold worked microstructure, Shape Memory characteristics can not be observed at all or only to a limited extent. Through a following heat treatment at temperatures below recrystallisation temperature, recovery processes can be initiated. These processes enable the healing of lattice defects, such as the above mentioned dislocations. In this way, through appropriate heat treatments, Shape Memory characteristics can be adjusted (programmed) specifically.
The programming of a specific shape of a SMA component takes place by a heat treatment under geometrically constrained conditions. In this process, the components, either cold or already warmed up, are transferred into the target shape through a forming tool and are clipped/coupled/connected to it. During the heat treatment the microstructure undergoes different relaxation processes and saves its new shape. A repeated forming is possible, but deteriorates the performance of the components significantly. Resetting, repairing or reconfiguration during operating an SMA element through temporary overheating inevitably leads to a change in microstructure. As a consequence, fatigue properties may deteriorate dramatically. A heat treatment can take place in conventional furnaces, continuous furnaces or in liquid baths. It can also be triggered locally by laser or inductive methods. Usually, NiTi components are quenched with water after heat treatment in order to exclude indefinable cooling processes.
Machining by drilling, turning, and milling of NiTi is basically possible. Due to a high ductility and localized phase transformations, a high toolwear, however, complicates the processing and makes it expensive. It is advisable to use semifinished material geometries for applications. Their geometries can be adapted in functional actuator systems by forming processes or through a combination of several simple geometries.
NiTi can also be structured by laser cutting, wire eroding or water jet cutting. The individual processes offer different advantages and disadvantages. According to the application, they have a different importance. Alternatively, NiTi components can also be produced through powder-metallurgical routes. The metal injection moulding (MIM) as well as the selective laser melting (SLM) are a proper and new way to produce specific geometries with special characteristics. For thin-layer applications, NiTi sputter targets can be used. This is done through physical vapor deposition (PVD) in order to generate functional coatings or micro system actuators with reproducable characteristics.
If NiTi is not heat treated in a vacuum furnace or inert gas, a thin layer of type TiO forms due to oxygen content of the atmosphere. This layer has an auto-passivating function, protects the material against corrosion and improves the biocompatibility. Due to technical problems or aesthetical reasons, surfaces very often have to be reworked. Procedures used here are mechanical grinding and polishing, sandblasting, pickling or electrolytic polishing. Since, as a result of general loading conditions, cracks almost always nucleate on surfaces, a good surface quality is important for cyclic applications. Therefore, the service life of components can be increased significantly by better surface qualities.
NiTi can be used either as a coating material or it can be coated with polymers itself.
Pseudoelastic coats can absorb oscillations (at bearings, waves or rotating tools) and prevent wear (particularly cavitation wear). But NiTi can as well be coated with polymers such as polyurethane or PTFE (teflon). For this purpose, if necessary, a special surface treatment is required, for example mechanical roughening and / or the application of chemical binders.
Binary NiTi-SMA show ultimate tensile strengths of over 1200 MPa and ultimate elongations of over 10%.
Pseudoelastic NiTi-SMA show a mechanical hysteresis during the tensile test at room temperature. Here, the upper plateau stress is above 380 MPa.
An overview of the main mechanic and physical characteristics is given here.
Temperatures at which a phase transformation between Martensite and Austenite begins or ends are marked as "transformation temperatures".
If the structure is martensitic and heated, the transformation into Austenite begins at the Austenite start temperature. It is finished when the Austenite finish temperature is reached. The transformation from Austenite to Martensite begins at the Martensite start temperature and is finished at the Martensite finish temperature. The transformation peaks don`t lie on top of each other and a thermal hysteresis can be observed, which must be considered for specific service operations.
For microstructural reasons, SMA have a functional fatigue when they are subjected to cyclic activations.
The extent of fatigue depends on the alloy composition, the processing parameters of the material and the thermomechanical conditions during operation. Through optimization of the alloy composition, the processing and appropriate components design, fatigue can be significantly reduced. By resistance-controlled heating of the SMA component, the fatigue can also be controlled electronically. Today, a comprehensive understanding of SMA allows good predictions about fatigue behavior.
NiTi-SMA is licensed as implant material and is used, among other things, for stents, as wires for braces or other guide wires, e.g. for catheters.
The biocompatibility of NiTi-SMA is based on the formation of a thin but dense oxide layer. Through the high proportion of Ti in the alloy, a TiO layer forms in an oxygen-rich atmosphere, or even in water. It protects against further oxidation and, furthermore, is a barrier between liquids and base material. Although NiTi-SMA normally contain approx. 50 at.-% Ni, it can be used as implant material. In a special way, the reactivity of Ti binds the Ni to the metal matrix and prevents the release of nickel ions which may cause allergic reactions.
NiTi-SMA can be produced in different shapes. Depending on the route of production, the material can be delivered as wire, band or tube.
Through further processing steps, such as heat treatments, shape setting or cutting processes like water jet or laser cutting, 2D- or 3D-shaped parts can be produced. These are, for example, pressure sleeves, springs, bending or torsion actuators. The delivery status of the semi-finished parts is either cold worked (no SMA characteristics – needs to be heat treated on the customers´ side) or already heat treated / annealed and straightened. The surface can be oxidized, grinded, polished, sandblasted, pickled or electrolytically polished. The latter is often needed for components which are used in the field of medical applications. The cost for surface processing increases in the same order.
Actuator components may further be cut on length, contacted, pre-strained and pre-cycled to be fatigue optimized and ready for assembly.
SMA actuators can be applied using different working principles. With the selection of suitable semi-finished material and the right geometry, they allow the use under tension, pressure, bending or torsion. Wires are predestined for tension actuators. They can be installed space-saving and energy-efficient. Pressure can be applied / brought up by thick-walled tubes or shims. Thicker wires or sheet metal strips are suitable, with restrictions, for bending. Torsion can be realized by thicker wires or sheet metal strips. Through the combination of several different principles, numerous possibilities are given for the development of actuators.
In contrast to the design of structural materials, besides dimensioning and the choice of the alloy, when developing SMA actuators, special factors / influences have to be taken into account. These are, among others, production conditions, load sensitivities, temperature and additional dependencies of the operation conditions.
That`s why the design process is rather complicated and needs a high level of skills and expertise in handling SMA.
Actuator strokes and forces are very important when SMA actuators shall be / are used. The stroke marks the expansion or contraction an actuator can reversibly do when it remembers its former shape due to the temperature induced phase transformation. The actuating force is the force that the actuator exerts against a counterpart (weight or spring) when heated. It results from a suppressed deformation when the possible stroke is blocked by a counteracting force.
The maximum stroke can be observed when an unloaded actuator is heated. The actuating force is at maximum when the deformation of the actuator is completely blocked (typically used for clamping applications). Actuator technologies are often about the right combination and synchronization of stroke and force to reach a maximum performance / actuator technology for the system. Both are complementary, what - according to a correct interpretation – determines the operating points.
The reaction time of an actuator depends on how fast the phase transformation between Austenite and Martensite can be initiated. Four essential aspects are important: The transition temperature within the material, the surrounding conditions (such as ambient temperature and medium), the actuator geometry and the mechanical stress-state of the actuator. The higher the temperature gradients are, the faster heating (cold actuator in warm air flow) or cooling processes (electronically warmed actuator in air) can take place. An actuator which has a small thermal hysteresis (Af – Mf or Ap – Mp) can be actuate faster between Mf and Af, for example, as an actuator whose core values are wider apart. If Af, for example, is far above the surrounding temperature, a large temperature gradient is given for faster cooling. On the other hand more, (electric) energy for heating is needed to ensure fast switching. Introduced heat within the actuator (for example by resistance or induction heating) subsequently is released to the environment through its surface (convection and radiation) and contact points (heat conduction). The higher the surface-to-volume ratio, the faster the cooldown process. An actuator with a great material volume stores more heat and therefore needs more time to cool down. The preload of an actuator (applied load) also shifts the thermodynamic equilibrium. Consequently, transition temperatures in the material under tension are different to those in an unloaded state. This way, the cooling speed can also be constructively influenced by the degree of mechanical preloading. The higher the preloading, the less usable actuator activity remains. For the appropriate design of an actuator, all parameters and their interactions need to be coordinated.
As with all cyclic stressed metallic materials the working life of SMA components also depends on how it`s used.
For actuator technology the design of SMA components is done in a way that plastic deformation are avoided. The achievable cycle duration primarily is a function resulting from the relation between actuating force and stroke. The less the material volume which undergoes a phase transition the higher is the working life. The number of cycles for thermal activated SMA components (apart from super-elastic components) can therefore vary between few 10.000 or 100.000 cycles. But they can also significantly exceed more than 1 million cycles. For purely mechanically stress of super-elastic components considerably higher cycle numbers can be reached.
NiTi can be connected with other materials. The elements that are used most often are crimp connectors. These are suitable for heavily uniaxially stressed actuator wires. But NiTi can also be connected to parts of the same alloy or with different materials (for example with stainless steel) by welding. Bonding and soldering of NiTi involve a great effort and generally achieve significantly lower connection forces.
Usually, the tensile test is used for the measurement of the material parameters which also matter for conventional structural materials. It is primarily about flexibility, strength and ultimate elongation. These properties are rather secondary for an actuator behavior, but they are of utmost importance for the fundamental evaluation of processing of ingots, for semi-finished parts and components.
The Differential Scanning Calometry (DSC) supports the thermal analysis of the material. It allows for the measurement of the transformation temperatures under load-free condition. With the DSC, the start- and finish-temperatures of the phase transformation during heating or cooling are detected. The area surrounded by the baseline and the DSC-curve marks the transformation enthalpy and gives the experts hints about the present microstructure. In the material development the DSC method, for example, is used to quantify the degrees of deformation and parameters for heat treatment.
The macroscopic characteristics of SMA are defined through their microstructure and therefore through the structural constitution of matter. This can be examined on different length scales with different microscopic techniques. The devices that are used in this field are the light microscope, the scanning electron microscope (SEM) or, within fundamental research, the transmission electron microscope (TEM). With methods of microscopy, grain sizes, precipitates, textures and crystal structures which are of importance for the materials characteristics, can be analyzed.
For the characterization of superelastic components, especially when they are to be used in the medical field, examinations according to specific criteria which are defined in a small number of ASTM norms, are necessary. Superelasticity means that a reversible deformation is possible which exceeds conventional elastic behavior. The reason for this behavior is a stress-induced phase transformation which is almost completely reversible. For characterization, once again important ultimate strength and elongation are important. The following test is about the determination of plateau stress and strain levels, as well as about the residual strains due to functional fatigue.
For the actuator behavior, several tests are necessary to evaluate the maximum actuator force (strain-controlled test) and the maximum actuator stroke (stress-controlled test). Of major importance is the stress for detwinning of a martensitic structure. Beside force and stroke, the test also records, after a deformation at constant stress or after unloading, the temperature changes. As a consequence, three-dimensional diagrams can be plotted. These information figures represent a comprehensive statement on the actuator behavior.
SMA actuators can be heated through Joule heating, based on their electrical resistance. The resistance depends on the alloy composition, the microstructure and crystal structure of the alloy. It also depends on the ambient conditions and the service lifetime reached so far. When the material undergoes a phase transformation, this can also be detected by the resistance. For this purpose, special electronics, as the EMS developed by Ingpuls, are necessary. Control units for actuator systems which we develop for or together with our customers are based on this method.
In different tests and applications, the temperature and load dependency always should be considered. Temperature as well as mechanical loading affect the thermodynamic equilibrium and can lead to a significant shift of transformation temperatures or other material properties. Special characteristics give important hints which also must be taken into consideration for SMA component design.
The cyclic behavior is measured in endurance tests. The goal of these tests is to determine the degradation of actuator properties such as actuation force or stroke over the course of the actuator's life cycle. Also, the displacement of the transformation temperatures during cycling or the maximal possible number of cycles, i.e. the service life of an actuator. Due to this data, electronics can react to the service life and then power the SMA actuators properly to maximize their performance. An excessive supply of power ("over-heating") decreases the functional stability significantly and hence, also dramatically reduces the device`s service life.
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