Recent Developments in High Temperature Shape Memory Alloys

In the last couple of decades, shape memory alloys (SMAs), particularly NiTi-based alloys, have drawn a great deal of attention since they exhibit excellent functional and mechanical properties near room temperature. However, new challenges in automotive, aerospace and energy exploration industries require enhanced operating efficiency of mechanical components at elevated temperatures (>100 ^oC) where NiTi binary SMAs are not suitable for use. Several NiTiX alloy systems (X: Au, Pd, Pt, Pd, Hf, Zr), with increased transformation temperatures compared to binary NiTi, have been characterized to explore possible applications in the aforementioned fields.

Among these, NiTiPd and NiTiPt alloys were extensively studied in the recent past. However, as a consequence of the increasing need for relatively inexpensive, lightweight and high performance engineering materials, the trend has moved to the design, processing and characterization of cheaper alternatives such as NiTiHf and NiTiZr SMAs. Initial studies carried out on the Ni-rich compositions of NiTiHf and NiTiZr systems, showed that they can provide promising shape memory response while still maintaining relatively high transformation temperatures.  We will present the current state-of-the-art on HTSMAs for their utility as solid state actuators. Different alloy systems will be compared for their actuation stresses, strains, work outputs, efficiencies, and fatigue responses while also discussing current materials challenges and opportunities in various applications.