Shape Memory Materials : State of the Art and Requirements for Future Applications

NiTi is only one of many alloy systems that exhibit the shape memory effect. The reason of his success is only the fact that it is the best in many aspects. His large market share created a significant price reduction so that in combination with its good properties, it became a preferential alloy even when compared with Cu-based alloys. Moreover NiTi alloys can be easily tuned to optimal performance by applying the proper combination of deformation and heat treatments. Its most successful applications are related to medical devices. Nevertheless NiTi has also a few drawbacks: 1. In spite of all the efforts to produce a NiTi based alloy with transformation temperatures above 150°C, no successful material has been reported. 2. Because of the high Ni content, surgeons and physicians are still anxious to apply those materials as implants in the human body. 3. For some applications the price remains too high. As a consequence, researchers are looking for alloy systems with high transformation temperatures, stable, reliable and with good strength and ductility. A combination of all those requirements is not realised yet, but significant progress has been made with Cu-Al-Ni and Cu-Al-Be single crystals. New alloy systems such as some B2 Zr-based intermetallics are also now investigated but are too brittle yet for use in appliances. Also some iron-based alloys retain specific attention. For Ni-less shape memory alloys, beta Ti-alloys are now investigated, but for good biocompatibility also V and A1 have to be omitted in those alloys. Further interest remains in exploring new processing routes and even material properties that received only recently more attention.