Shape-memory alloys superelasticity

Figures 20.6 and 20.7 illustrate the relation between shape memory and superelasticity. Shape memory occurs when the deformation takes place at a temperature below the Mf. Superelasticity occurs when the deformation is at a temperature above the Af. For both the memory effect and superelasticity, the alloy must be ordered, there must be a martensitic transformation, and the variant boundaries must be mobile.

Ferromagnetic materials exhibiting shape memory effects and superelasticity under action of a magnetic field turn out to have a great potential for applications. This problem was discussed in a talk by A. Vasiliev (Shape memory alloys of the Ni-Mn-Ga type). 

Figure 10.5 Schematic engineering stress ngineering strain ia-s) curve for a shape-memory alloy showing superelasticity...

Cobalt chrome alloys, gold alloys, mercury amalgams, nickel-chrome alloys, nitinol alloys (shape memory and superelastic), stainless steels, tantalum, titanium, and titanium alloys... 

Furuya, Y. Matsumoto, M. Matsumoto, T. Mechanical Properties and Microstructure of Rapidly Solidified TiNiCu-Alloy. Proc. Int. Conf. on Shape Memory and Superelasticity, March 7-10, Pacific Grove, CA, USA (1994), pp. 905-910... 

Fe, Al, Cr, Co, and V tend to substitute for nickel, but sharply depress Mg (Ref 14 to 16), with V and Co being the weakest suppressants and Cr the strongest. These elements are added to suppress Mg while maintaining stability and ductility. Their practical effect is to stiffen a superelastic alloy, to create a cryogenic shape memory alloy, or to increase the separation of the R-phase from martensite. 

Figure 9 is a stress-strain curve for a single-crystal specimen of a Cu-39.1Zn shape-memory alloy deformed in tension at about 50 °C above its temperature (Schroeder and Wayman, 1979). Yielding at an essentially constant stress (upper plateau) corresponds to the formation of 9R stress-induced martensite (SIM) from the B2 parent. At about 9 7o strain the specimen becomes fully martensitic. When the stress is released, the strain follows the lower plateau and fully recovers as the SIM reverts to the parent. This behavior corresponds to a mechanical (as opposed to a thermal) shape memory. A stress-strain relationship such as that shown in Figure 9 is frequently referred to as a superelastic stress-strain loop. The stress necessary to... 

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. 

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...

Superelasticity Minimal stress increase beyond the initial strain region resulting in very low modulus in the region for some shape memory alloys. 

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