Large and reversible elastocaloric effect induced by low stress in a Ga-doped Ni-Mn-Ti alloy

The Solid-state cooling based on caloric effects is considered a potential alternative to conventional refrigeration technology, which uses ozone-depleting gases. Several shape memory alloys have attracted attention for solid-state cooling since they present a high reversibility of the caloric effect, which depends mainly on thermal hysteresis and sensitivity to the applied field. In the present work, a study substitution of Mn by Ga in the Ni₅₀Mn₃₄Ti₁₆ alloy led to diminished thermal hysteresis in the martensitic transformation by 9 K. The elastocaloric effect, thermal and microstructure properties of a polycrystalline Ni₅₀Mn₃₂Ti₁₆Ga₂ alloy have been characterized. The elastocaloric effect was obtained indirectly from the length change as a function of temperature at constant stress. An isothermal entropy change (ΔS_ISO) of 23.0 J kg⁻¹ K⁻¹ during heating and 22.0 J kg⁻¹ K⁻¹ during cooling was observed for applied stress of 160 MPa. In addition, the ΔS_ISO is reversible for a temperature span between 287 and 319 K, reaching a maximum of 20.5 J kg⁻¹ K⁻¹ at 299 K. The thermal hysteresis changed slightly while the applied stress increased up to 160 MPa since the sensitivity of the martensitic transformation temperatures to stress was 0.150 K/MPa during cooling and 0.160 K/MPa during heating. The X-ray diffraction analysis revealed a mixture of B2-type cubic austenite, 5M modulated martensite, and a second intermetallic phase identified as Ni₃Ti. All these results were obtained around room temperature.