Shape-memory alloys transformation temperatures

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...

Figure 3.6 Shape-memory alloys transform from (a) a partially ordered, high-temperature austenitic phase to (b) a mixed austenite-martensite low-temperature state to (c) an ordered mixed-phase state under deformation.

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress (14,15). This hysteresis can be caused by motion of ferroelastic domain walls. This behavior is more compHcated and complex near a martensitic phase transformation. At this transformation, both crystal stmctural changes iaduced by mechanical stress and by domain wall motion occur. Martensitic shape memory alloys have broad, diffuse phase transformations and coexisting high and low temperature phases. The domain wall movements disappear with fully transformation to the high temperature austentic (paraelastic) phase. 

The design of smart materials and adaptive stmctures has required the development of constitutive equations that describe the temperature, stress, strain, and percentage of martensite volume transformation of a shape-memory alloy. These equations can be integrated with similar constitutive equations for composite materials to make possible the quantitative design of stmctures having embedded sensors and actuators for vibration control. The constitutive equations for one-dimensional systems as well as a three-dimensional representation have been developed (7). 

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. 

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. 

It is well known that the martensitic transformation of Al-deficient NiAl (see Sec. 4.3.2) is thermoelastic and produces the shape memory effect. Consequently materials developments have been started which aim at applications as shape memory alloys (Furukawa et al., 1988 Kainuma et al., 1992b, c). The martensitic transformation temperature can be varied within a broad temperature range up to 900 °C, and thus the shape memory efftct can be produced at high temperatures which allows the development of high-temperature shape memory alloys. The problem of low room temperature ductility of NiAl has been overcome by alloying with a third element - in particular Fe - to produce a ductile second phase with an f.c.c. structure. 

The Cu-Al-Ni shape memory alloys are based on the intermetallic phase CujAl with a disordered A2 structure, which is usually known as the P phase, and which is stable only at high temperatures between 567 and 1049 °C with compositions between 71 and 82 at.% Cu (Murray, 1985). At 567 °C this phase decomposes by a eu-tectoid reaction at such a sluggish rate that it can be retained with the then metastable A2 structure by cooling below this temperature. At about 500 °C the P phase undergoes an ordering reaction to form the metastable Pi phase with a DO3 structure. Both phases can be transformed martensit-ically by quenching to form various types... 

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... 

Figure 8.16 The sequence of events taking place during the deformation and recovery of a shape using a shape-memory alloy. Cooling the high-temperature shape below Mf transforms it into a multiply twinned form with the same overall shape. Deformation alters the distribution of the twin boundaries. Reheating the sample above Af causes the material to revert to the high-temperature form. This removes the twins and allows the original shape to be recovered. The temperatures are appropriate to TiNi...

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. 

By combining calorimetry with three-dimensional atom probe analysis, Starink and coworkers [25] examined the aging, at room-temperature, of Al-Cu-Mg-Mn alloys and found that the process is accompanied by a substantial exothermic heat evolution, whereas the only micro-structural change involved the formation of Cu-Mg co-clusters. Tatar and Zengin [26] studied the effects of neutron irradiation on the oxidation behaviour, microstructure and transformation temperatures of CuAlNiMn shape-memory alloy. They showed that irradiation... 

Shape memory alloys work by viitue of their intrinsic switching between two crystalline states, i.e., martensite and austenite (Sherby et al., 2008). At lower temperatures, these alloys adopt the martensite state, which is relatively soft, plastic, and quite easy to shape at a certain higher temperature, they transform into the austenite state, which is a much harder material and not so easy to deform. Figure 1.1 illustrates the principles of shape memory effect in metal alloys. 

Method for Determining the Transformation Temperatures of Shape Memory Alloys... 

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

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. 

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. 

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