Properties. The copper-based and NiTi-based shape memory alloys are considered to be engineering materials. These compositions can be manufactured to almost any shape and size.

The yield strength of shape memory alloys is lower than that of conventional steel, but some compositions have a higher yield strength than plastic or aluminum. The yield stress for Ni Ti can reach 500 MPa. The high cost of the metal itself and the processing requirements make it difficult and expensive to implement SMAs into a design. As a result, these materials are used in applications where the super elastic properties or the shape memory effect can be exploited. The most common application is in actuation.

One of the advantages to using shape memory alloys is the high level of recoverable plastic strain that can be induced. The maximum recoverable strain these materials can hold without permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5% for conventional steels.

The first reported steps towards the discovery of the shape memory effect were taken in the 1930s. According to Otsuka and Wayman (1998), A. Ölander discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Greninger & Mooradian (1938) observed the formation and disappearance of a martensitic phase by decreasing and increasing the temperature of a Cu-Zn alloy. The basic phenomenon of the memory effect governed by the thermoelastic behavior of the martensite phase was widely reported a decade later by Kurdjumov & Khandros (1949) and also by Chang & Read (1951). In the early 1960s, Buehler and his co-workers at the U.S. Naval Ordnance Laboratory discovered the shape memory effect in an equiatomic alloy of nickel and titanium, which can be considered a breaktrought in the field of shape memory materials (Buehler et al. 1967). This alloy was named Nitinol (Nickel-Titanium Naval Ordnance Laboratory). Since that time, intensive investigations have been made to elucidate the mechanics of its basic behavior. The first efforts to exploit the potential of NiTi as an implant material were made by Johnson and Alicandri in 1968 (Castleman et al. 1976). The use of NiTi for medical applications was first reported in the 1970s (Cutright et al. 1973, Iwabuchi et al. 1975, Castleman et al. 1976, Simon et al. 1977). In the early 1980s the idea attained more support, and some orthodontic and mainly experimental orthopedic applications were released. It was only in the mid-1990s, however, that the first widespread commercial stent applications made their breakthrough in medicine. The use of NiTi as a biomaterial is fascinating because of its superelasticity and shape memory effect, which are completely new properties compared to the conventional metal alloys.

General principles. NiTi shape memory metal alloy can exist in a two different temperature-dependent crystal structures (phases) called martensite (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite NiTi and martensite NiTi are notably different.

When martensite NiTi is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called austenite start temperature (A_s). The temperature at which this phenomenon is complete is called austenite finish temperature (A_f). When austenite NiTi is cooled, it begins to change onto martensite. The temperature at which this phenomenon starts is called martensite start temperature (M_s). The temperature at which martensite is again completely reverted is called martensite finish temperature (M_f) (Buehler et al. 1967).

Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTi can have three different forms: martensite, stress-induced martensite (superelastic), and austenite. When the material is in its martensite form, it is soft and ductile and can be easily deformed (somewhat like soft pewter). Superelastic NiTi is highly elastic (rubber-like), while austenitic NiTi is quite strong and hard (similar to titanium). The NiTi material has all these properties, their specific expression depending on the temperature in which it is used.

Hysteresis. The temperature range for the martensite-to-austenite transformation, i.e. soft-to-hard transition, that takes place upon heating is somewhat higher than that for the reverse transformation upon cooling. The difference between the transition temperatures upon heating and cooling is called hysteresis. Hysteresis is generally defined as the difference between the temperatures at which the material is 50 % transformed to austenite upon heating and 50 % transformed to martensite upon cooling. This difference can be up to 20-30 °C (Buehler et al. 1967, Funakubo 1987). In practice, this means that an alloy designed to be completely transformed by body temperature upon heating (A_f < 37 °C) would require cooling to about +5 °C to fully retransform into martensite (M_f).

Thermoelastic martensitic transformation. The unique behavior of NiTi is based on the temperature-dependent austenite-to-martensite phase transformation on an atomic scale, which is also called thermoelastic martensitic transformation. The thermoelastic martensitic transformation causing the shape recovery is a result of the need of the crystal lattice structure to accommodate to the minimum energy state for a given temperature (Otsuka & Wayman 1998).

In NiTi, the relative symmetries between the two phases lead to a highly ordered transformation, where the displacements of individual atoms can be accurately predicted and eventually lead to a shape change on a macroscopic scale. The crystal structure of martensite is relatively less symmetric compared to that of the parent phase.

If a single crystal of the parent phase is cooled below M_f, then martensite variants with a total of 24 crystallographically equivalent habit planes are generally created. There is, however, only one possible parent phase (austenite) orientation, and all martensitic configurations revert to that single defined structure and shape upon heating above A_f. The mechanism by which single martensite variants deform is called twinning, and it can be described as a mirror symmetry displacement of atoms across a particular atom plane, the twinning plane (Buehler et al. 1967, Andreasen et al. 1987).

While most metals deform by slip or dislocation, NiTi responds to stress by simply changing the orientation of its crystal structure through the movement of twin boundaries.

A NiTi specimen will deform until it consists only of the correspondence variant which produces maximum strain. However, deformation beyond this will result in classical plastic deformation by slip, which is irrecoverable and therefore has no “memory effect”. If the deformation is halted midway, the specimen will contain several different correspondence variants. If such a specimen is heated above A_f, a parent phase with an orientation identical to that existing prior to the deformation is created from the correspondence variants in accordance with the lattice correspondences between the original parent phase and each variant. The austenite crystal structure is a simple cubic structure, while martensite has a more complex rhombic structure. This phenomenon causes the specimen to revert completely to the shape it had before the deformation (Andreasen et al. 1987, Gil et al. 1998).

The above phenomenon is the basis of such special properties as the shape memory effect and superelasticity.

Shape memory effect. NiTi senses a change in ambient temperature and is able to convert its shape to a preprogrammed structure. While NiTi is soft and easily deformable in its lower temperature form (martensite), it resumes its original shape and rigidity when heated to its higher temperature form (austenite). This is called the one-way shape memory effect. The ability of shape memory alloys to recover a preset shape upon heating above the transformation temperatures and to return to a certain alternate shape upon cooling is known as the two-way shape memory effect. Two-way memory is exceptional. There is also an all-round shape memory effect, which is a special case of the two-way shape memory effect (Shimizu et al. 1987).

Superelasticity. Superelasticity (or pseudoelasticity) refers to the ability of NiTi to return to its original shape upon unloading after a substantial deformation. This is based on stress-induced martensite formation. The application of an outer stress causes martensite to form at temperatures higher than Ms. The macroscopic deformation is accommodated by the formation of martensite. When the stress is released, the martensite transforms back into austenite and the specimen returns back to its original shape. Superelastic NiTi can be strained several times more than ordinary metal alloys without being plastically deformed, which reflects its rubber-like behavior. It is, however, only observed over a specific temperature area. The highest temperature at which martensite can no longer stress induced is called Md. Above Md NiTi alloy is deformed like ordinary materials by slipping. Below As, the material is martensitic and does not recover. Thus, superelasticity appears in a temperature range from near Af and up to Md. The largest ability to recover occurs close to Af (Duerig et al. 1996).

Limitations of shape memory and superelastic behavior. About 8% strain can be recovered by unloading and heating. Strain above the limiting value will remain as a permanent plastic deformation. The operating temperature for shape memory devices must not move significantly away from the transformation range, or else the shape memory characteristics may be altered. A shape memory NiTi implant must be deformed at a temperature below A_s (usually < +5 °C). Moreover, the deformation limit determined by distinctive implant design (sharp angles, etc.) and the intrinsic strain tolerance of NiTi material must not be disregarded (Otsuka & Wayman 1998).

Mechanical properties of NiTi. For orthopedic biomaterial applications, the two properties of major importance are strength (mechanical) and reactivity (chemical). Generally, there are two basic mechanical demands for the material and design of the implant. Service stresses must be safely below the yield strength of the material, and in cyclic loads the service stress must be kept below the fatigue limit.

The mechanical properties of NiTi depend on its phase state at a certain temperature (Buehler et al. 1967, Van Humbeeck et al. 1998). Fully austenitic NiTi material generally has suitable properties for surgical implantation. The common mechanical properties of martensitic and austenitic NiTi are presented in Table 2-1. There are some exceptional properties that might be useful in surgery. NiTi has an ability to be highly damping and vibration-attenuating below A_s. For example, when a martensic NiTi ball is dropped from a constant height, it bounces only slightly over half the height reached by a similar ball dropped above the A_f temperature. From the orthopedic point of view, this property could be useful in, for example, dampening the peak stress between the bone and the articular prosthesis. The low elastic modulus of NiTi (which is much closer to the bone elastic modulus than that of any other implant metal) might provide benefits in specific applications. NiTi has unique high fatigue and ductile properties, which are also related to its martensitic transformation. These properties are usually favorable in orthopedic implants. Also, very high wear resistance has been reported compared to the CoCrMo alloy (Sekiguchi 1987). NiTi is a non-magnetic alloy. MRI imaging is thus possible. Electrical resistance and acoustic damping also change when the temperature changes.

Table 2-1. Selected mechanical properties of NiTi, implant stainless steel (316LVM), titanium (cp-Ti, grade IV) and Ti-6Al-4V alloy.

Effect of alloy composition, heat treatment and mechanical working on NiTi properties. It is feasible to vary the critical transition temperatures either by small variations of the Ti/Ni composition or by substituting metallic cobalt for nickel. Lowering of Af is possible by adding nickel. If nickel is added above 55.6 Wt%, a stable second phase (Ti-Ni3) forms and the NiTi properties are lost. To avoid this problem, the cobalt substitution can be used to lower the TTR. The properties of NiTi can also be greatly modified by mechanical working and through heat treatment (time and temperature) (Buehler et al. 1967).

Fabrication. Solid NiTi alloys are manufactured by a double vacuum melting process, to ensure the quality, purity and properties of the material. After the formulation of raw materials, the alloy is vacuum induction melted (1400°C). After the initial melting, the alloy transition temperature must be controlled due to the sensitivity of the transition temperature to small changes in the alloy chemistry. This is followed by vacuum arc remelting to improve the chemistry, homogeneity and structure of the alloy. Double-melted ingots can be hot-worked (800°C) and cold-worked to a wide range of product sizes and shapes (Andreasen et al. 1987).

Porous NiTi can be made by sintering or using self-propagating high temperature synthesis, also called ignition synthesis. The possibility to make composite SMA products (combination with polymers) is under investigation (Brailovski et al. 1996).

Programming. The use of the one-way shape memory or superelastic property of NiTi for a specific application requires a piece of NiTi to be molded into the desired shape. The characteristic heat treatment is then done to set the specimen to its final shape. The heat treatment methods used to set shapes in both the shape memory and the superelastic forms of NiTi are similar. Adequate heat treatment parameters (temperature and suitable time) are needed to set the shape and the properties of the item (Otsuka & Wayman 1998). They must usually be determined experimentally for the requirements of each desired part. Rapid cooling of some kind is preferred, such as water quenching or rapid air cooling.

The two-way shape memory training procedure can be made by SME training or SIM training. In SME training, the specimen is cooled below M_f and bent to the desired shape. It is then heated to a temperature above A_f and allowed freely to take its austenite shape. The procedure is repeated 20-30 times, which completes the training. The sample now assumes its programmed shape upon cooling under M_f and to another shape when heated above A_f.

In SIM training, the specimen is bent just above Ms to produce the preferred variants of stress-induced martensite and then cooled below the Mf temperature. Upon subsequent heating above the A_f temperature, the specimen takes its original austenitic shape. This procedure is repeated 20-30 times.

Summarizing task to the module:

Choose one of the following topics and write an essay.

Essay Topics

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Date: 2016-04-22; view: 772