NiTiCu shape memory alloy: Superplastic elongation during thermal cycling

We report on experimental results obtained in the characterization of a Ni45Ti50Cu5 shape memory alloy tested in order to verify its properties at the light of the use in actuators. In this field long term stability of the recovery proper-ties and low hysteresis values are the main requests. Tests were performed in an "ad-hoc" developed device with a maximum number of cycles equal to 5000 and an applied load of 180MPa. Various thermal treatments and cold-working conditions were tested on several specimens in order to identify the combinations that can ensure the best functional properties at the light of the use in actuators. Calorimetric measurements were systematically performed on specimen taken at the beginning and at the end of the cycling procedure. Results confirm that low temperatures and short time thermal treatments account for higher stability in the cycling properties. Surprisingly it has been noted that specimens submitted to complete anneal thermal treatment, in spite of a large plastic deformation during thermomechanical cycling, exhibit only small changes in the recovery properties. A NiTiCu full annealed wire cycled up to rupture can account for a stored plastic deformation up to 60% of the initial length, at the same time demonstrating a complete shape memory effect of 5%. A detailed investigation of the material submitted to this treatment was done in order to clarify its behavior.