Episode 12: Lightweight shape memory alloy retains superelasticity at 4 K

In this podcast episode, MRS Bulletin’s Sophia Chen interviews Sheng Xu from Tohoku University, Japan about his lightweight shape memory alloy that retains superelasticity at temperatures as cold as 4 K and as hot as 400°C. This range is about 5 times wider than commercial shape memory alloys. Shape memory alloys are needed for extreme environments such as part of machines in space or deep sea. Xu also sees uses for biomedical applications or for storage containers for liquid fuels like liquid hydrogen, which must be kept at very cold temperatures. This work was published in a recent issue of Nature.

Transcript

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Sophia Chen. Rubber is elastic. You pull on a rubber band, and it lengthens, and once you let go, it springs back to its original shape. Metals, on the other hand, typically aren’t very elastic. You can’t stretch a loop of wire much, and it doesn’t spring back. But Sheng Xu and his colleagues have created an unusual metal. You can stretch it like rubber.

SHENG XU: If you deform it, and then you release the force, it can spring back.

SOPHIA CHEN: Xu is a materials researcher at Tohoku University in Japan. His new material qualifies as a type of material known as a shape memory alloy. These are metals that you can deform and stretch, but then when you release the force, they return to their original shape. This quality is known as superelasticity. Metals that exhibit superelasticity could be useful as part of machines in extreme environments, such as in space or in the deep sea, where typical elastic materials like rubber wouldn’t work. 

SHENG XU: My motivation is to develop such shape memory alloy that can be operational in wider temperature range. If you use such a flexible alloy you can make airless tire that’s operational on the surface of the moon.

SOPHIA CHEN: Xu’s shape memory alloy exhibits superelasticity ranging from as cold as 4 Kelvin to above the boiling point of water, a range spanning approximately 400 degrees Celsius. This range is about 5 times wider than commercial shape-memory alloys, which typically operate between 273 and 353 Kelvin. In fact, Xu says it’s the only known lightweight shape memory alloy to retain superelasticity at 4 Kelvin. The mechanism behind shape memory alloys’ superelasticity comes from phase transitions. As you stretch the material, its crystal structure changes, causing a phase transition in the material. If you release the force, the crystal structure goes back to the original configuration. Xu’s material is an alloy composed predominantly of titanium and aluminum, with some chromium as well. Xu’s team came across the specific formulation using phase diagrams of titanium and aluminum alloys. The phase diagram indicated that this formulation would allow for the desired starting phase found in many shape memory alloys. Still, creating the material was challenging. 

SHENG XU: There’s no specific rule to discover such an alloy. We have done numerous experiments to find such material. 

SOPHIA CHEN: Xu says they spent about three years to figure out the right composition for the alloy.

His team then studied the phase changes in the alloy using neutron diffraction measurements at room temperature. The measurements indicated the expected crystal structure. However, they still don’t know the specific reasons why the alloy exhibits this superelasticity at such a wide temperature range.

SHENG XU: We’re just very lucky to discover such a phenomenon.

SOPHIA CHEN: In future work, Xu plans to develop applications for this material. One possibility is through collaboration with JAXA, the Japan Aerospace Exploration Agency, for applications in space. He also thinks that the material could be useful for biomedical applications or for storage containers for liquid fuels like liquid hydrogen, which must be kept at very cold temperatures. This work was published in a recent issue of Nature. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.