Shape-memory alloys stress-strain curve

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

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 10.38 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).

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