On the Influence of Alloy Composition on the Microstructures, Workability and functional Stability of NiTiZr Shape Memory Alloys

NiTiZr alloys provide a shape memory effect at high temperatures. While the costs for Zr are relatively low as compared to those of other elements (like Pt, Pd or Hf) which can be added to yield high phase transformation temperatures, the limited workability of NiTiZr represents a general drawback of this alloy system. In the present work, we show how microstructures, workability and functional stability of NiTiZr depend on the alloy composition. A variety of shape memory alloys (SMAs) on the basis of NiTiZr with various Zr and Ni levels was characterized by electron microscopy, compression testing and differential scanning calorimetry (DSC). It was found that higher Zr concentrations result in elevated transformation temperatures but also in strongly decomposed microstructures with brittle secondary phases. The Zr level strongly affects the ductility of the alloy. As a consequence, the benefit of elevated transformation temperatures has to be well balanced against a sufficient workability. The functional stability of the alloys was evaluated by thermal cycling. It was found that the stability of solution annealed NiTiZr SMAs is lower than in the case of binary NiTi but can be significantly improved by aging in Ni-rich alloys. In the present study, we identified one alloy composition which shows elevated phase transformation temperatures, an acceptable workability and a wide thermal hysteresis comparable to conventional NiTiNb SMAs. This alloy might by attractive for coupling devices which only require a small number of cycles.