Shape memory alloys (SMAs) are a fascinating group of metals that have two remarkable properties, the shape memory effect and superelasticity.1,2 Shape memory refers to the recovery of shape (i.e., strain) after apparent ''permanent'' deformation (induced at relatively cold temperatures) by heating above a characteristic transformation temperature (often near room temperature [RT]). Superelasticity refers to the isothermal recovery of relatively large strains during a mechanical load-unload cycle that occurs at temperatures above a characteristic transformation temperature. A large number of SMAs have been discovered since the mid-1900s to late 1900s, and the list continues to grow.3 Many of these alloys, while scientifically interesting, consist of precious metals or only exhibit useful properties as single crystals, which do not lend them to practical use in commercial applications. A few alloys, however, have emerged as commercially viable for novel devices. These include certain copper alloys (CuAlZn) and nickel-titaniumbased alloys, such as near-equiatomic NiTi, known as Nitinol (first discovered in the early 1960s) and some ternary alloys such as NiTiCu and NiTiNb (see Fig. 1). To date, it is fair to say that NiTi-based SMAs have the best memory and superelasticity properties of all the known polycrystalline SMAs. The NiTi family of alloys can withstand large stresses and can recover strains near 8% for low cycle use or up to about 2.5% strain for high cycle use. This strain recovery capability can enable the design of novel devices in either a thermally active mode or an isothermal energy absorption mode. NiTi SMAs have other advantages in terms of corrosion resistance, fatigue resistance, and biocompatibility, thereby making them the preferred material system for most shape memory applications being considered today.The materials science and mechanics literature regarding SMAs are vast, and we will not attempt a complete review here (see Otsuka and Ren 4 for a recent review). The field remains an active area of research, and the understanding of the mechanisms involved at all scales from the crystalline lattice to the macroscopic scale has progressed significantly, even during the past decade or so. Furthermore, advances in materials processing have resulted in production of Nitinol SMAs with good quality control, with reproducible properties, and in relatively large quantities. Nitinol wire, in particular, is being produced with excellent properties and is relatively inexpensive compared to most other forms. Applications of SMA wire are now being seriously considered even in costsensitive engineering sectors.5 Consequently, our focus in this article is on the thermomechanical behavior of polycrystalline Nitinol wire under uniaxial tensile loading.Shape memory alloys exhibit some rather surprising phenomena as well as extreme sensitivities to testing conditions, which can create pitfalls in material testing and interpretation for someone who is new to SMAs. As we like to say-it is not an amateur sport. N...
