Ti-Ni Shape Memory Alloys

Figure 10. Stress-strain-temperature diagram for a Ni-Ti (Nitinol) shape-memory alloy showing shape-memory and superelastic characteristics and the deformation behavior of the parent phase above the temperature (above which no martensite can form regardless of the magnitude of the stress). Temperature increases from upper right to lower left...

Finally, metallic fibers find some limited applications as reinforcement in composites. They are generally not desirable due to their inherently high densities and because they present difficulties in coupling to the matrix. Nonetheless, tungsten fibers are used in metal-matrix composites, as are steel fibers in cement composites. There is increasing interest in shape memory alloy filaments, such as Ti-Ni (Nitanol) for use in piezoelectric composites. We will discuss shape-memory alloys and nonstructural composites in later chapters of the text. 

Shape-Memory Alloys. Stoeckel defines a shape-memory alloy as the ability of some plastically deformed metals (and plastics) to resume their original shape upon heating. This effect has been observed in numerous metal alloys, notably the Ni—Ti and copper-based alloys, where commercial utilization of this effect lias been exploited. (An example is valve springs that respond automatically to change in transmission-fluid temperature.) Copper-based alloy systems also exhibit this effect. These have been Cu-Zn-Al and Cu-Al-Ni systems. In fact, the first thermal actuator to utilize this effect /a greenhouse window opener) uses a Cu—Zn-Al spring. 

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. 

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. 

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). 

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. 

Olier, P. Tournie, Y. Roblin, C. Shape Memory Effect and Recovery Stress Measurements in High Temperature Ti-Ni-Hf Shape Memory Alloys. Proc. 6th Int. Conf. on New Actuators, June 17-19, Bremen, Germany (1998)... 

Other uses are as superconductive materials of TiNb, the shape memory alloy of Ti-Ni, the hydrogen occlusion alloy of Ti—Fe, and in computer equipment as nonmagnetic substance, artificial bones, dental roots, cardiac valves and cardiac pacemakers as nontoxic and biocompatible materials [3,5]. 

The recovery forces of shape memory alloys are several tens kg/mm whereas those of polymers are approximately 1 kg/mm. The shape recovery ratio of the alloys is 7% at the maximum compared to those of pol5miers and gels, at an amazingly high 400-500%. The shape recovery temperatures of shape memory alloys can vary by as much as or more than 100°C by several percent variations in composition. This is in contrast to nearly constant recovery ratio in polymers and gels, which depends on the type of materials used. The price of typical Ni-Ti alloys is several hundred thousand yen/kg, which is much more expensive than the several thousand yen/kg that shape memory polymers and gels cost. Alloy density is 6.5 whereas polymer and gel density is 1. 

The intermetallic Ni-Ti system has the imusual property of after being distorted, returning to its original shape when heated. This was the first of the shape memory alloys (SMAs) and was discovered by accident at the Naval Ordnance Laboratory, hence its name Nitinol. Other SMAs include Cu-Al-Ni, Cu-Zn-Al, and Fe-Mn-Si alloys. The shape memory mechanism depends on a martensitic solid-state phase transition that takes place at a modest temperature (50°C—150°C), depending on the alloy. The high temperature phase is referred to as austenite and the low temperature phase is called martensite (following the terminology of the Fe-FeCa system). 

Figure 10 is a stress-strain-temperature diagram for a Ni-Ti shape-memory alloy that summarizes its mechanical behavior. At the extreme rear the stress-strain curve shown in the a-t plane corresponds to the deformation of martensite below Mf. The induced strain, about 4%, recovers between A and Af after the applied stress has been removed and the specimen heated, as seen in the e-T plane. At a temperature above Mj (and Af) SIM is formed, leading to a superelastic loop with an upper and lower plateau, the middle o-e plane. At a still higher temperature and above M, the front a-e plane, no SIM is formed. Instead, the parent phase undergoes ordinary plastic deformation. 

THE MICROSTRUCTURE AND MARTENSITIC TRANSFORMATION IN A (POTENTIALLY) SHAPE MEMORY Ni-AI-Ti-B ALLOY... 

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