Martensite thermoelastic transformation

The unique behavior of NiTiNOL is based on the temperature-dependent austenite-to-martensite phase transformation on an atomic scale, which is also called thermoelastic martensitic transformation. The thermoelastic martensitic transformation causing the shape... 

The titanium-nickel alloys show unusual properties, that is, after it is deformed the material can snap back to its previous shape following heating of the material. This phenomenon is called shape memory effect (SME). The SME of TiNi alloy was first observed by Buehler and Wiley at the U.S. Naval Ordnance Laboratory [Buehler et al, 1963]. The equiatomic TiNi or NiTi alloy (Nitinol) exhibits an exceptional SME near room temperature if it is plasticaUy deformed below the transformation temperature, it reverts back to its original shape as the temperature is raised. The SME can be generally related to a diffusionless martensitic phase transformation which is also thermoelastic in nature, the thermoelasticity being attributed to the ordering in the parent and martensitic phases [Wayman and Shimizu, 1972]. Another unusual... 

Figure 5. Electrical resistance changes during cooling and heating Fe-Ni (70 30) and Au-Cd (52.5 47.5) alloys, showing the hysteresis of the martensitic transformation on cooling and the reverse transformation on heating, for nonthermoelastic and thermoelastic transformation, respectively (After Kaufman and Cohen, 1957)...

I. S.Chakravorty, C.M. Wayman, The thermoelastic martensitic transformation in P Ni-A1 alloys, Metall.Trans.7A 555, 569 (1976)... 

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. 

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. 

A Chrysochoos, H Pham, and O Maisonneuve. Energy balance of thermoelastic martensite transformation under stress. Nuclear engineering and design, 162(1) 1—12, 1996. 

Del74] Delaey, L., Krishnan, RV, Tas, H., and Warumont, H., Thermoelasticity, Pseudoelastidty and the Memory Effects Associated with Martensitic Transformations, J. Mater. Sci., Vol 9, 1974, p. 1521-1535... 

Ton75] Tong, H.C. and Wayman, C.M., Tber-modynamic Considerations of SoUd State Engines Based on Thermoelastic Martensitic Transformations and the Shape Memory Effect, Metall. Trans.,Vol 6A 1975, p. 29-32... 

The SME behavior is basically a consequence of a martensitic transformation. When compared, shape-memory alloys are found to have common characteristics such as atomic ordering, a thermoelastic martensitic transformation that is crystallographically reversible, and a martensite phase that forms in a self-accommodating... 

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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