Comparative study of degradation effects in different shape memory alloy systems

Abstract
The presented investigations aim at clarifying the influence of microstructure and applied training procedure on the stability of the created intrinsic two-way shape memory effect (TWSME). The investigations are carried out on three different alloy systems in order to get an appropriate variation of the microstructure with respect to dislocation density, second phase particles and grain size. A uniaxial TWSME is induced by a thermomechanical treatment in wire specimens of the alloys NiTi, CuAlNi and a two phase alloy composed of a NiTi matrix and dispersoids of tungsten. Subsequently, thermal cycles are carried out, continuously observing the changes in the deformation behaviour. The results suggest that the training must result in both, a large pseudoplastic strain as well as a pronounced irreversible strain in order to obtain a TWSME of high magnitude. The degradation of the TWSME in the NiTi-based alloys was found to consist of two characteristic stages, which can always be recognised, independent of the preliminary heat treatment. The obvious strengthening effect of the particles in the two-phase alloy NiTi-W has a very positive effect on the stability of the created TWSME. This effect can even be improved by preliminary cold working. The stability of the effect in the CuAlNi alloy is far better then in the binary NiTi alloy. No dramatic change in the degradation rate is observed throughout the investigated period of 5000 thermal cycles.