TiNi Shape-Memory Alloys

The shape-memory effect is a complex function of composition, martensite start temperature Mg, stress, strain, microstructure, texture, and aging treatment. It consists essentially of a reversible transformation strain and the associated macroscopic shape change. At low numbers of transformation cycles (e g., up to 100), the 

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Gloanec, A.-L., Cerracchio, P., Reynier, B., Van Herpen, A., Riberty, P., 2010. Fatigue crack initiation and propagation of a TiNi shape memory alloy. Scripta Materialia 62, 786—789. Acta Materialia Inc. Published by Elsevier Ltd. 

Fabrication of various microfeatures of TiNi shape memory alloy... 

T. Mineta, E. Makino, Characteristics of the electrochemical etching of TiNi shape memory alloy in a LiCl-ethanol solution, J. Micromech. Microeng 20 (2010) 125012, http //dx.doi.org/10.1088/0960-1317/20/... 

Five groups of materials based on Ti may be distinguished [1.51-53] commercially pure (i. e., commercially available) Ti (cp-Ti), low-alloy Ti materials, Ti-base alloys, intermetallic Ti-Al materials, and highly alloyed functional materials TiNi shape memory alloys, Nb-Ti superconducting materials (Sect. 4.2.1), and Ti-Fe-Mn materials for hydrogen storage. 

Walker, J., K. GabrieL and M. Mehregany. 1990. Thin-film processing of TiNi shape memory alloy. 

An important martensitic transformation occurs in the titanium-nickel (Ti-Ni) system, as it is used in shape-memory alloys, described in Section 8.3.3. The phase in question is TiNi (Figure 8.12), called Nitinol. At temperatures above 1090 °C, TiNi has a bcc structure in which the atoms are distributed at random over the available sites in the crystal. Below... 

Figure 8.16 The sequence of events taking place during the deformation and recovery of a shape using a shape-memory alloy. Cooling the high-temperature shape below Mf transforms it into a multiply twinned form with the same overall shape. Deformation alters the distribution of the twin boundaries. Reheating the sample above Af causes the material to revert to the high-temperature form. This removes the twins and allows the original shape to be recovered. The temperatures are appropriate to TiNi...

Shape memory alloys (e.g. CuZnAl-, FeNiAl-, TiNi-alloys) are already being used in biomedicine as cardiovascular stents, guidewires and orthodontic wires. The shape memory effect of these materials is based on a martensitic phase transformation. 

The discovery of the shape memory effect in TiNi by Buehler et al. at the Naval Ordinance Labs occurred during an investigation of the alloy for possible use as a corrosion-resistant knife for underwater activities. The investigators called the alloy nitinol for Nickel, Titanium, and Naval Ordinance Labs. 

TiNi is an alloy composed of titanium and nickel. This alloy shows very high elastic deformation and a shape memory effect, which are not possessed by other types of conventional metallic alloys. These properties along with their superior ductility, fatigue strength, and corrosion resistance have resulted in many applications for MEMS. 

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... 

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