NiTiHfNb High-Temperature Shape Memory Alloys

Research on HTSMAs has accelerated during the last decade. Ni-rich NiTiHf alloys have recently been investigated to achieve higher transformation temperatures and open new application opportunities in aerospace, automotive and energy industries. However, there are still many challenging aspects that must be considered to have a viable, robust, economic, and reliable engineered solution. First, depending on the application requirements, tuning of the transformation temperature and transformation behavior through chemistry and processing is desirable. Another critical factor in determining a new HTSMAs viability is whether or not the alloy can be thermo-mechanically processed for good work output and dimensional stability. In this regard, the poor workability of NiTiHf alloys is a serious problem. Some ternary element additions to binary NiTi, including Hf, increase hardness and decrease cold workability and ductility. Control of the chemistry and improvement of the cold workability of these alloys is crucial for manufacturing them into suitable sizes and shapes and for microstructure tuning.

In this work, we will present results on NiTiHf alloys with addition of Nb as suitable element to improve cold workability. The effects of such addition on chemistry, as well as on functional shape memory properties will be described. Microstructural features unique to these quaternary alloys will also be discussed.