Elasticity shape memory alloy

Fig. 6.53. Stress-strain diagram of pseudo-elastic shape memory alloys...

Resonant US spectroscopy is also the technique of choice for studying the vibration behaviour of the metastable p phase near the martensic transformation of a single crystal Cu-27.96 at.% AI-3.62 at.% Ni shape memory alloy. The elastic constants C, of the anysotropic material and the internal friction Q (Cj) were determined, and the elastic constants C and C44 found to be the quantitities controlling the vibrational response of the p phase near the martensic transformation on the other hand, the evolution of the internal friction was found to be correlated with the elastic behaviour of the alloy [49],... 

Shape memory polymers are here to stay, not only because of their unique ability to display double existence under the influence of a triggering mechanism, but also because, unlike shape memory alloys, their elastic deformation and recoverable strains are huge, and their transition dependence can be tailored to fit specific requirements as well as having excellent biocompatibility, nontoxicity, ease of manufacture, and, perhaps most importantly, low cost of manufacture. 

The properties of the shape memory alloy vary with its temperature. Above the transition temperature, the alloys crystallic structure takes on the austenitic state. Its structure is symmetric and the alloy shows a high elastic modulus. The martensitic crystalline structure will be more stable for thermodynamical reasons if the materials temperature drops below the transformation temperature. Martensite can evolve from austenitic crystals in various crystallographic directions and will form a twinned structure. Boundaries of twinned martensite can easily be moved for that reason SM elements can be deformed with quite low forces in the martensitic state. 

Above the transition temperature an extraordinary elasticity can be observed in the shape memory alloys. Figure 6.53 shows a uniaxial stress-strain diagram of a pseudo-elastic (or super-elastic) SMA. 

Among shape memory materials, shape memory alloys and bimetals are well known. Compared to these metallic compounds, SMP have a lower density, high shape recoverability, easy processability, and lower cost. Similar to conventional thermoplastics, SMP can be easily molded by the common methods, such as injection, extrusion, compression, and casting. In addition, their shape recovery temperature can be set at any value in the range room temperature 50 C, which allows a wide variety of applications. Also, SMP can be colored if desired because they are transparent. However, since the retractive force of the polymer is based on the small entropy elasticity, the SMP applications differ from those of metallic alloys. The basic differences between shape memory polymers and alloys are listed in Table 1. 

The design of shape-memory devices is quite different from that of conventional alloys. These materials are nonlinear, have properties that are very temperature-dependent, including an elastic modulus that not only increases with increasing temperature, but can change by a large factor over a small temperature span. This difficulty in design has been addressed as a result of the demands made in the design of compHcated smart and adaptive stmctures. Informative references on all aspects of SMAs are available (7—9). 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

Next-generation metallic biomaterials include porous titanium alloys and porous CoCrMo with elastic moduli that more closely mimic that of human bone nickel-titanium alloys with shape-memory properties for dental braces and medical staples rare earth magnets such as the NdFeB family for dental fixatives and titanium alloys or stainless steel coated with hydroxyapatite for improved bioactivity for bone replacement. The corrosion resistance, biocompatibility, and mechanical properties of many of these materials still must be optimized for example, the toxicity and carcinogenic nature of nickel released from NiTi alloys is a concern. ... 

TiNi is an alloy composed of titanium and nickel. This alloy shows very high elastic deformation and a shape memory effect, which are not possessed by other types of conventional metallic alloys. These properties along with their superior ductility, fatigue strength, and corrosion resistance have resulted in many applications for MEMS. 

Currently all commercial vena cava filters are fabricated from either nitinol (nickel and titanium alloy), phynox (nickel, cobalt, chromium, iron, and molybdenum alloy), or stainless steel wire in various catheter sheath sizes ranging from 7 to 14 French (2.3-4.7 mm diameter). These metals have been chosen because they are biostable and have low thrombogenicity. This means that blood is less likely to form clots on the surface, and cells are not encouraged to grow around the device. Moreover, nitinol is a thermal shape-memory and super-elastic alloy that is self-expanding after deployment. For this particular application the nitinol will be tuned to expand on exposure to body temperature." ... 

In this section some examples of precision engineering prototypes are presented that apply electrically heated shape memory actuators as driving elements. Further on flexure hinges of pseudo-elastic SM alloys will be presented. 

Metals share excellent mechanical and conductivity (electrical and thermal) properties ideal for high stress apphcations such as heart valves. Titanium—nickel alloys have become the most common material for metaUic cardiovascular applications (stents and valves) due to unique properties shape memory effect, super-elasticity, high degree of biocompatibility moreover, they are almost completely inert and nonmagnetic. 

Falvo A, Futgiuele FM, Maletta C (2005) Laser welding of a Nili alloy Mechanical and shape memory behavior. Materials Science and Engineering A. 412 235-240 Wu MH (2001) Fabrication of Nitinol materials and components. Proceedings of the international conference on shape memory and super-elastic technologies, Kunming, China, 285-292... 

Nitinol is a nickel-titanium alloy known as memory metal. The name nitinol is derived from the s)mnbols for nickel (Ni), titanium (Ti), and the acronym for the Naval Ordinance Laboratory (NOL), where it was discovered. If an object made out of nitinol is heated to about 500 °C for about an hour and then allowed to cool, the original shape of the object is "remembered," even if the object is deformed into a different shape. The original shape can be restored by heating the metal. Because of this property, nitinol has found many uses, especially in medicine and orthodontics (for braces). Nitinol exists in a number of different solid phases. In the so-called aus-terite phase, the metal is relatively soft and elastic. The crystal structure for the austerite phase can be described as a simple cubic lattice of Ti atoms with Ni atoms occupying cubic holes in the lattice of Ti atoms. What is the empirical formula of nitinol and what is the percent by mass of titanium in the alloy ... 

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