Cryogenic superelasticity with large elastocaloric effect

Abstract

Shape-memory alloys suitable for low-temperature environments are crucially lacking despite their potential applications for aerospace engineering, superconducting technologies, and liquefied-gas-storage technologies. Even for benchmark shape-memory alloys such as Ti–Ni and Ni-based Heusler alloys, superelasticity becomes appreciably difficult to achieve upon cooling because of a drastic increase in stress hysteresis. Here, we report a superelastic strain of more than 7% for a Cu–Al–Mn shape-memory alloy down to 4.2 K with stress hysteresis as small as that at ambient temperature. Furthermore, the transformation entropy change remains as large as that at ambient temperature down to ~50 K. Consequently, an excellent elastocaloric cooling property is simultaneously obtained even at cryogenic temperatures, at which existing shape-memory alloys lose this property because of decreased entropy change and increased stress hysteresis. The developed cryogenic superelastic alloy shows excellent potential as a new class of material combining superelastic properties with sufficiently small dissipated energy and persistent elastocaloric cooling ability even in cryogenic environments. An alloy that recreates its shape even after being warped at cryogenic temperatures can benefit advanced cooling technology. Certain copper-based composites are promising for an environmentally friendly refrigeration method, called elastocaloric cooling, which uses shape-memory effects to extract heat from a system and hence lower its temperature. Kodai Niitsu and colleagues from Tohoku University in Japan now report a means to expand the working temperatures of elastocaloric alloys to near absolute-zero temperatures, which are normally too brittle for metals. By melting and hot-rolling a mix of copper, aluminum and manganese atoms into thin plates, the researchers produced a material with sufficient compatibility between the parent and deformed phases to produce a 7% recoverable transformation strain at 4.2 degrees kelvin. This shape shifting at extremely low temperatures has potential for cooling superconductors and maintaining liquefied gases. Superelastic alloys suitable for low-temperature environments are crucially lacking. Here, we report an excellent superelastic (rubber-like) strain of more than 7% at 4.2 K using a Cu–Al–Mn shape-memory alloy. This alloy also possesses a small stress hysteresis of less than 30 MPa down to 4.2 K, resulting in particularly persistent elastocaloric cooling ability in the low-temperature region. The present Cu–Al–Mn alloy will have significant engineering impact, especially in the fields of aerospace engineering, superconducting technologies, and liquefied-gas-storage technologies, as a new class of materials combining superelastic properties and persistent elastocaloric cooling properties even at cryogenic environments.