Nickel-titanium shape memory alloys

F 2005, Terminology for Nickel-Titanium Shape Memory Alloys... 

F 2063, Specification for Wrought Nickel-Titanium Shape Memory Alloys for Medical Devices and Surgical Implants... 

F 2082, Test Method for Determination of Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery... 

Platinum has been demonstrated as biocompatible for use in an epiretinal array [31] and in cochlear implants [32]. Both titanium and ceramic [32] and platinum-iridium wire [33] have been shown as biocompatible in cochlear implants. Babb and Kupfer [34] have shown stainless steel and nickel-chromium (Nichrome) to be nontoxic. Copper and silver are unacceptable as stimulating electrodes, as these metals cause tissue necrosis even in the absence of current [28, 29, 34, 35, 36]. Nickel-titanium shape memory alloys have good biocompatibility response [37], up to a nickel content of 50% [38],... 

Ryhanen J, KaUioinen M, Tuukkanen J, Junila J, Niemela E, Sandvik P, Serlo W (1998) In vivo biocompatibility evaluation of nickel-titanium shape memory alloy muscle and perineural tissue responses and encapsule membrane thickness. J Biomed Mater Res 41(3) 481-488... 

The use of shape-memory alloys as actuators depends on their use in the plastic martensitic phase that has been constrained within the structural device. Shape-memory alloys (SMAs) can be divided into three functional groups one-way SMAs, tw o-vvav SMAs, and magnetically controlled SMAs. The magnetically controlled SMAs show great potential as actuator materials for smart structures because they could provide rapid strokes with large amplitudes under precise control. The most extensively used conventional shape-memory alloys are the nickel-titanium- and copper-based alloys (see Shape-Memory Alloys). 

As previously mentioned, the nickel—titanium alloys have been the most widely used shape memory alloys. This family of nickel—titanium alloys is known as Nitinol (Nickel Titanium Naval Ordnance Laboratory in honor of the place where this material behavior was first observed). Nitinol have been used for military, medical, safety, and robotics applications. Specific usages include hydraulic lines capable of F-14 fighter planes, medical tweezers, anchors for attaching tendons to bones, eyeglass frames, underwire brassieres, and antiscalding valves used in water faucets and shower heads (38,39). Nitinol can be used in robotics actuators and micromanipulators that simulate human muscle motion. The ability of Nitinol to exert a smooth, controlled force when activated is a mass advantage of this material family (5). 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

Copper-aluminum-nickel, nickel-titanium, and copper-zinc-aluminum are the most common shape-memory alloys. 

An important martensitic transformation occurs in the titanium-nickel (Ti-Ni) system, as it is used in shape-memory alloys, described in Section 8.3.3. The phase in question is TiNi (Figure 8.12), called Nitinol. At temperatures above 1090 °C, TiNi has a bcc structure in which the atoms are distributed at random over the available sites in the crystal. Below... 

The history of these intriguing materials goes back to 1938, when A. Oleander observed the shape memory ability of An—Cd and Cu—Zn alloys (Wayman and Harrison, 1989). Later on other materials such as indium-, nickel-, titanium-, and iron-based alloys were shown to have similar behaviour (Reardon, 2011). Unlike other shape memory alloys, Ni—Ti in particular was found to be very resistant to corrosion and/or degradation and hence ideally suited to implantation in a range of applications, including the human body, albeit more expensive than its other counterparts. Hence the very first temperature-dependent shape memory alloys to be commercialised were nickel—titanium (Bogue, 2009). 

Recently, the integration of actuators into structures has also been researched. Shape Memory Alloys (SMA)-based actuators can be embedded into composites in the form of large diameter, plastically deformed wires. SMA are nickel/titanium alloys with a surprising property if plastically deformed at a low temperature (in a martensitic phase), they can recover the original shape and dimensions though heating above a definite temperature. When SMA are embedded and then heated, the restraints on free deformation imposed by the host composite originate a distributed stress which deforms the structure or modifies its vibrational response. 

At this time all commercial vena cava filters are fabricated firan ehber nitinol (nickel, cobalt, and titanium alloy), phynox (nidcel, cobalt, chromium, iron, and molybdenum alloy), or stainless steel wire in various catfaeter sheath sizes raiigii fiom 7-14 French (23 - 4.7 mm diameter). These metals were chosen because fiiey are biostable and have low thrombogenicity. This means that blood is less likely to form clots cm the surfiice, and cells are not encouraged to grow around the device. Moreover, nitinol is a thermal shape memory alloy that is self-expanding afier deployment and exposed to txxfy temperature(l]. 

The most noteworthy and extensively researched shape-memory materials are shape-memory alloys (SMAs). Olander first discovered and reported in 1932 a novel metallic transformation of gold-cadmium (AuCd) alloy, whose pseudoelasticity triggered imusual macroscopic deformation [12]. The discovery of the SME in equiatomic nickel-titanium (NiTi) alloy, which is well known as nitinol, represented a paradigm shift in the SMA field. 

Another point of comparison between the devices concerns material choice. Nitinol (nickel/titanium alloy) is used in the ASO, the GHSO and the DAW devices. Nitinol is a shape-memory alloy and it reverts back to its previous... 

New materials and designs have been developed for heat protective clothing. For example, improved thermal insulation can be provided by non-wovens made with thin hollowed fibres, and can be made thermo-adaptive with two-way shape memory alloys such as nickel-titanium. Better thermoregulation inside the garment is sought with PCM, either encapsulated ° or incorporated in a matrix. Other solutions use external power, e.g., for liquid coolant circulation or with Peltier cells embedded in the textile. ... 

Shape memory alloys (SMA) undergo solid-to-solid martensitic phase transformations, which allow them to exhibit large, recoverable strains [3]. Nickel-titanium, also known as nitinol (Ni for nickel, Ti for titanium, and nol for Naval Ordnance Lab), are high-performance shape memory alloy actuator materials exhibiting strains of up to 8% by heating the SMA above its phase transformation temperature - a temperature which can be altered by changing the composition of the alloy. 

Martensitic transformations can also occur in other alloys. Of special importance are shape memory alloys. The most commonly used are based on nickel and titanium. In these alloys, a reversible martensitic phase transformation can occur that will be briefly described here. 

Shape memory alloys were the first SMMs studied. Chang and Read (1951) developed the first shape memory alloy, composed from gold and cadmium. However, only after 1963, when an equiatomic nickel-titanium alloy called Nitinol was developed (Buehler et al., 1963), were shape memory... 

A relatively new gronp of metals that exhibit an interesting (and practical) phenomenon are the shape-memory alloys (or SMAs). One of these materials, after being deformed, has the ability to return to its predeformed size and shape upon being subjected to an appropriate heat treatment—that is, the material remembers its previous size/shape. Deformation normally is carried out at a relatively low temperature, whereas shape memory occurs upon heating. Materials that have been found to be capable of recovering significant amounts of deformation (i.e., strain) are nickel-titanium alloys (Nitinol, is their trade name) and some copper-base alloys (Cu-Zn-Al and Cu-Al-Ni alloys). 

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