Importance —Shape-Memory Alloys

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. 

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... 

Shape-memory alloys show a thermoelastic martensitic transformation. This is a martensitic transformation, as described above, but which, in addition, must have only a small temperature hysteresis, some 10s of degrees at most, and mobile twin boundaries, that is, ones that move easily. Additionally, the transition must be crystallographi-cally reversible. The importance of these characteristics will be clear when the mechanism of the shape-memory effect is described. 

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. 

Table 6.3 summarizes the most important properties of some commonly used shape memory alloys. 

Shape memory actuators offer a lot of advantages, but there are also some quite serious drawbacks. When comparing SM actuators with other actuator principles (such as piezoelectric stacks or solenoids), it should be taken into consideration that research for improved shape memory materials is relatively young. With shape memory alloys and actuators slowly gaining commercial importance it is expected that in the next few years new SM alloys will emerge that have higher transition temperatures and good effect stability [85,86]. 

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. 

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