Shape Memory Properties of Wires and Thin Films

Shape memory effects are very interesting functional properties of certain metallic alloys as NiTi, CuZnAl or CuAlNi, to name the most popular ones. After considerable seemingly plastic deformation at low temperatures the elements restore their original shape just by raising the temperature by about 50-100C, thereby seeming to remember their former shape, therefore this is called shape memory. The reasons for this phenomenon are reversible martensitic transformations from one ordered structure to another ordered one with almost the same specific volume. This so called one-way shape memory effect has been thoroughly investigated and is well- understood nowadays and therefore many technical and some medical applications are in use or could be used. The so-called two-way shape memory effect changes the shape of an element not only during heating but also during cooling without external forces. Therefore this effect would be even more attractive for technical applications but it is not quite well understood from a scientific point of view, and this effect is smaller and less stable than the one- way effect. Within this project a two-way shape memory effect was generated in NiTi, NiTiW and CuAlNi alloys and the stability and constancy was investigated up to 10.000 thermal cycles. It was found out under which conditions large effects can be produced and under which conditions the stability would be high enough for certain future technical and medical applications. These investigations have been performed with wire material of several mm in diameter. However, it would be very useful to produce shape memory elements in much smaller dimensions for possible applications as microsensors and microactuators. Since CuAlNi alloys possess the highest phase transformation temperatures and therefore the highest switching temperatures, thin films of this alloy system have been produced by physical vapour deposition, which show shape memory behaviour. This was successful from a scientific point of view but the handling of these extreme small samples proved to be very difficult with respect to generating a very stable two-way effect. Therefore another production route was successfully employed: melt spinning of thin ribbons which are about 50- 100 m thick, about 5 mm wide and several meters long. These thin ribbons exhibit good one-way and two-way effects, the size and stability of which has yet to be optimised with respect to the technical applications as long- term microactuators.