Shape memory effect deformation temperature

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. 

Schematic illustration of the difference between shape memory and superelastic effects. For shape memory, the deformation occurs at a temperature for which the material is martensitic. A superelastic effect occurs when the deformation occurs just above the Af temperature. From J. A. Shaw, Int. J. Plasticity 16 (2000) 542. 

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. 

The phenomenon of shape memory effect in SMPs is brought about by large changes in elastic modulus, E, above and below the transition temperature. Figure 1.3 shows a typical modulus behaviour of SMPs with temperature. At a temperature above the transition, the polymer enters a rubbery elastic state, and hence the elastic modulus of the polymer is much reduced. Consequently the polymer can be easily deformed by application of an external force (Bar-Cohen, 1999 Liu et ah, 2007). If the material is allowed to cool below its transition temperature, under reasonable strain, its temporary deformation becomes hxed. At this stage, the polymer lacks its rubbery elasticity and displays a high modulus. This state is called the glassy state. This deformation can be recovered when the polymer is heated above the transition (Hu, 2007). 

Macroscopically, the shape memory effect can be described and evaluated using a cyclic tensile deformation experiment under a temperature programming, represented as a three-dimensional (3-D) plot of strain versus temperature and force as depicted in Figure 4.2. The sample is deformed to a certain level of strain at T > and then can be fixed during cooling,... 

In thermodynamic parameters, the thermal-responsive shape memory effect can be explained by the fact that at a temperature higher than the shape transition temperature (Ttrans). thc blcud material tanporarily changes its shape when cooling below the Ttrans) thc material retains the deformed shape and it can return to the initial nonde-formed shape by reheating at more than... 

Fig. 3. Schematic demonstration of the molecular mechanism of the thermally induced shape-memory effect for a multiblock copolymer, Ttrans = Tm. If the rise in temperature is higher than Ttrans of the switching segments, these segments are flexible (marked red, here) and the polymer can be deformed elastically. The temporary shape is fixed by cooling down below Ttrans (marked blue, here). If the poljrmer is heated up again the permanent shape is recovered.

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