High-Temperature Shape Memory Alloys – Recent developments and future perspectives

High-temperature shape memory (HTSMA) alloys that can operate at temperatures well above 100 °C are attractive for many applications in the automotive, aerospace or energy-conversion industries. In addition to elevated phase transformation temperatures, these materials must feature suitable transformation strains, oxidation resistance, and microstructural stability to name but a few. Some of the materials proposed demonstrate good functional properties, but are brittle, and thus difficult to process.

The current presentation will focus on Ti-Ta-based HTSMA. A combination of advanced characterization techniques has allowed shedding light on the processes that govern functional degradation in these alloys. It will be demonstrated how microstructural stability can be improved by alloying and heat treatment. In fact, a stabilization of both the austenite and the martensite can be exploited to minimize functional degradation. Yet, the formation of the so-called ω-phase upon thermomechanical loading has so far always had a negative impact on long-term functional stability. However, from first-principles calculations it became lately possible to derive alloy compositions that do not result in ω-phase formation [1].

Another promising approach are high entropy shape memory alloys [2]. Although research on these materials is still in its infancy, it appears that this concept is also a viable avenue to enhance microstructural stability. The ramifications of the different alloying concepts with respect to microstructure-functional property-relationship and their prospect for transfer into actual applications will also be discussed.

References

[1] A. Ferrari et al., Phys. Rev. Mater., 3, 2019, 103605

[2] G.S. Firstov et al., Shape memory and Superelasticity, 4, 2015, 400.