Chapter 7 - Shape memory alloys: existing and emerging applications

Shape Memory Alloys (SMA) can be included in a particular class of advanced materials - sometimes called smart materials - alongside with other types of active materials, such as piezoelectric and magnetostrictive materials. They are also known as multifunctional materials, due to their multiple functional properties, such as: one and two way shape memory effects (SME), pseudoelastic behavior, which includes the superelastic effect (SE) and the rubber-like effect, high damping capacity and biocompatibility (for certain compositions), or magnetostriction. There are several types of SMA defined as a function of the main alloying elements and exhibiting a reversible thermoelastic martensitic transformation. The NiTi alloy system is the most widely studied and used in various applications, although the functionality has been observed in numerous other alloys. The martensitic transformation can be induced under specific conditions, either by thermal or thermomechanical control, or even by magnetic fields. Although used since the 70's, some difficulties related to SMA must be overcome, so that they can reach their full potential. They are still expensive, mainly due to manufacturing aspects related to oxygen intake of Ti based alloy and the difficulties to control the parameters of functionality, which is often a well guarded company technical secret. Despite these aspects and as a consequence of intensive research activity, shape memory alloys had been used with increasing success in a number of fields, mainly biomedical, automotive and aeronautical, taking advantage of a favorable force / weight output. This chapter presents an overview of SMA and existing types, the technologies for producing and transforming these alloys including joining. The martensitic transformation, responsible for the shape memory effect is briefly discussed followed by the relevant properties of these alloys. Existing and emerging applications are detailed in the final part.