Reversible elastocaloric effect at ultra-low temperatures in nanocrystalline shape memory alloys

Superelastic shape memory alloys (SMAs) exhibit a reversible elastocaloric effect that originates from a release/absorption of latent heat associated with a stress-induced martensitic phase transformation. In typical SMAs, the conventional elastocaloric effect will vanish when the operating temperature falls below the temperature range in which martensitic phase transformation can be triggered by stress. We report emergence of an unprecedented elastocaloric effect with a decrease of temperature, well below the temperature range of martensitic phase transformation, in a model nanocrystalline NiTi that preserves slim-hysteresis superelasticity at ultra-low temperatures. The new elastocaloric effect emerges at a temperature of ∼90 K, exhibits an opposite sign than the conventional elastocaloric effect, and intensifies gradually with a decrease of temperature to 18 K. At 18 K, a large adiabatic temperature change ΔT_ad of +3.4 K is measured upon rapid release of tensile stress. The measured ΔT_ad are larger and extend over a wider temperature span than the existing electrocaloric, piezocaloric, and barocaloric cryo-refrigeration materials. We show that such low temperature elastocaloric effect originates from an entropic elasticity associated with large non-linear elastic deformations of the nanocrystalline microstructure at ultra-low temperatures. Our study suggests a new avenue to cool ultra-low temperature ambients.