Shape memory alloys properties

Fig. 1. Schematic of the hysteresis loop associated with a shape-memory alloy transformation, where M. and Afp correspond to the martensite start and finish temperatures, respectively, and and correspond to the start and finish of the reverse transformation of martensite, respectively. The physical property can be volume, length, electrical resistance, etc. On cooling the body-centered cubic (bcc) austenite (parent) transforms to an ordered B2 or E)02...

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. 

Heusler alloys have a rich variety of apphcations, owing to some of their unique properties. Some of these phases are half-metallic ferromagnets, exhibiting semiconductor properties for the majority-spin electrons and normal metallic behavior for the minority-spin electrons. Therefore, the conduction electrons are completely polarized. The Ni2MnGa phase is used as a magnetic shape memory alloy and single crystals of Cu2MnAl are used to produce monochromatic beams of polarized neutrons. 

As their name implies, shape-memory alloys are able to revert back to their original shape, even if significantly deformed (Figure 3.24). This effect was discovered in 1932 for Au-Cd alloys. However, there were no applications for these materials until the discovery of Ni-Ti alloys (e.g., NiTi, nitinol) in the late 1960s. As significant research has been devoted to the study of these materials, there are now over 15 different binary, ternary, and quaternary alloys that also exhibit this property. Other than the most common Ni-Ti system, other classes include Au-Cu-Zn, Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. 

In Japan, several commercial projects have been reported in the literature. For example, at the National Research Institute for Metals, the NiTi shape-memory alloy is produced by combustion synthesis from elemental powder for use as wires, tubes, and sheets. The mechanical properties and the shape-memory effect of the wires are similar to those produced conventionally (Kaieda et ai, 1990b). Also, the production of metal-ceramic composite pipes from the centrifugal-thermite process has been reported (Odawara, 1990 see also Section III,C,1). 

How can the eyeglass frames shown in the photo "remember" their original shape How do braces move your teeth These items may be made of a shape-memory alloy that has a remarkable property. It reverts to its previous shape when heated or when the stress that caused its shape is removed. 

Melting is the transition of a material from a solid to a liquid. Transitions from one phase to another also can take place within a solid. A solid can have two phases if it has two possible crystal structures. It is the ability to undergo these changes in crystalline structure that gives shape-memory alloys their properties. 

From the heterostructures that make possible the use of exotic electronic states in optoelectronic devices to the application of shape memory alloys as filters for blood clots, the inception of novel materials is a central part of modern invention. While in the nineteenth century, invention was acknowledged through the celebrity of inventors like Nikola Tesla, it has become such a constant part of everyday life that inventors have been thrust into anonymity and we are faced daily with the temptation to forget to what incredible levels of advancement man s use of materials has been taken. Part of the challenge that attends these novel and sophisticated uses of materials is that of constructing reliable insights into the origins of the properties that make them attractive. The aim of the present chapter is to examine the intellectual constructs that have been put forth to characterize material response, and to take a first look at the types of models that have been advanced to explain this response. 

Some of these binary and ternary phases have been studied to a larger extent because of special physical and/or mechanical properties. Of particular interest are the Cu phases CuZn, Cu j +1, 3,Zn i 2.3,Al, and (Cu,Ni)3Al, on which the Cu-Zn-Al and Cu-Al-Ni shape memory alloys are based and which are the subject of the following sections. In addition, the Cu-Au phases CU3AU and CuAu and the Cu-Sn phases Cu3Sn and Cu Snj will be addressed, which are important constituents of Cu-Au alloys and amalgams for dental restorative applications. 

Table 1.2 Properties of shape memory alloys compared with shape memory polymers...

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. 

Dong, Z., U. E. Klotz, C. Leinenbach, A. Bergamini, C. Czaderski and M. MotavaUi (2009). A novel Ee-Mn-Si shape memory alloy with improved shape recovery properties by VCprecipitation.A vfl ce E g >Jee > gMflfer flZsll(l-2) pp.40-44. 

TABLE 16.2 Comparison between Two Mechanical Properties of Different Actuating Materials Skeletal Muscles, Thermomechanical (Thermal Liquid Crystals and Thermal Shape Memory Alloys), Electrochemomechanical (Conducting Polymers and Carbon Nanotubes) and Electromechanical (Ionic Polymer Metal Composites, Field Driven Liquid Crystal Elastomers, Dielectric Elastomers)... 

SMP have many advantages compared to metallic shape-memory alloys [4]. SMP are lightweight and allow substantially higher elongations, which are enabling properties for various technical applications. The variation of shuctural parameters of the molecular architecture enabled tailoring of SMP to the demands of specific... 

Titanium and its alloys have many biomedical applications due to their high strength and corrosion resistance, and are commonly incorporated in replacement hip joints and items such as bone pins [1]. Porous Ti foams have been explored for biomedical uses due to their enhanced adhesion to host tissue [15]. Surface-treatment of Ti and Ti alloys to enhance material properties, such as wear resistance, in a biomedical context has been examined [16]. In addition, titanium nitride-based materials could potentially serve as coatings for biomedical implants [17]. NiTi-based shape memory alloys are attractive candidates for biomedical materials due to their shape retention and pseudoelasticity, however, manufacturing and processing these memory alloys for biomedical apphcations is typically not straightforward [18]. 

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