Austenite shape memory alloy

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.

The behavior of shape memory alloys can be explained on the basis of solid state phase changes that occur within the material. All SMAs exist in one of two phases, known as martensite and austenite, shown in the diagram on page 132. Austenite is the "parent ... 

Lakhani A, Dash S, Banerjee A, Chaddah P, Chen X, Ramanujan RV. Tuning the austenite and martensite phase fraction in ferromagnetic shape memory alloy ribbons of Ni45Co5Mn38Sn12. Applied Physics Letters. 2011 99 242503(l)-242503(3). DOI 10.1063/1.3669510. 

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 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. 

Forward and reverse transformation occur at different temperatures, resulting in a hysteresis as can be seen in Fig. 6.50. The start and end of the transformation from martensite to austenite are given by As (austenite start temperature) and At (austenite finish temperature). The reverse transformation takes place in the temperature interval from Ms to Mt (martensite start and finish temperatures). The shape of the hysteresis curve in Fig. 6.50 strongly depends on the thermomechanical treatment of the shape memory alloy (see also Sect. 6.4.1). 

If the shape memory alloy spring is heated and the temperature surpasses the As temperature, the shape memory material starts transforming to austenite and the coil spring returns to the unstretched form. On reaching the Af temperature, the transformation is completed. It is characteristic for the one-way effect that a shape recovery occurs only when the SM element is heated. There is no shape change when the element is cooled. The cold SM element must be deformed by an external force in order to achieve a movement when heated again. 

AH of these materials have at least two phase transitions that can be described in terms of thermodynamic functions with two ordering parameters (see Appendix B). Ferroic materials are operated near an instabiUty to make domain walls with their associated dipoles and strains moveable, as encountered in PZT or Terfenol . On the other hand, a second type of material involves a partially ordered phase, as in PMN or the shape memory alloys. These materials are operated near a diffuse phase transition with two coexisting phases, a high-temperature austenite-like phase and a low-temperature martensite-Uke phase. A third type of smart... 

A typical shape memory alloy starts in its austenitic phase. When the material is strained, it transforms to the martensitic phase, oriented in a way to produce an elongation in the direction of the load. 

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). 

Recent Advances in Metal-Based Materials The use of shape-memory alloy (SMA) reinforcements is very promising in retrofit and strengthening of existing structures. SMAs have more than one crystal structure. This is called polymorphism. The prevailing crystal structure or phase in polycrystalline metals depends on both temperature and external stress. They are a class of metallic alloys that can remember their initial geometry during transformations (forward and reverse) between two main phases at their atomic level (austenite and martensite). 

Figure 10.38 Typical stress-strain-temperature behavior of a shape-memory alloy, demonstrating its thermoelastic behavior. Specimen deformation, corresponding to the curve from A to B, is carried out at a temperature below that at which the martensitic transformation is complete (i.e., Mf of Figure 10.37). Release of the applied stress (also at Mf) is represented by the curve BC. Subsequent heating to above the completed austenite-transformation temperature Af, Figure 10.37) causes the deformed piece to resume its original shape (along the curve from point C to point D).

The shape-memory effect is observed when the temperature of a piece of alloy is cooled to below that required to form the martensite phase Ms (initial martensite formation) until Mf (martensite formation complete), as seen in Figure 3.25. Upon heating the martensitic material, a reformation of austenite begins to occur at Hg... 

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