Processing and Properties of Ti-Ta High Temperature Shape Memory Alloys

Due to their high martensite start temperature and excellent workability, Ti-Ta shape memory alloys (SMAs) are very attractive for application as high temperature SMAs. In the present work, we provide results on processing and functional stability of Ti-Ta SMAs. We show how Ti-Ta alloys can be produced through arc melting and subsequent thermal and thermomechanical treatments. Most importantly, sufficient mixing of Ti and Ta during melting and decomposition during solidification are important factors that affect the quality of the ingot. After melting, it is necessary to anneal Ti-Ta to remove Ta concentration gradients. The functional stability of Ti-Ta was evaluated through thermal cycling using differential scanning calorimetry (DSC). It was found that binary Ti-Ta only exhibits a relatively poor stability due to the formation of omega phase at relatively low temperatures. Therefore, slow heating/cooling rates, as well as holding steps at elevated temperatures during thermal cycling strongly affect cyclic stability, as they allow for omega phase nucleation. Nevertheless, the functional stability of Ti-Ta can be improved through Al additions. Alloying Al slows down kinetics of Omega phase precipitation and thus allows for a more stable transformation behavior.