Elastocaloric cooling capacity of shape memory alloys – Role of deformation temperatures, mechanical cycling, stress hysteresis and inhomogeneity of transformation

Elastocaloric (EC) effect refers to the rapid cooling in shape memory alloys (SMAs) during reverse transformation from martensite to austenite under adiabatic conditions. We present a very comprehensive study of the EC response far extending the existing literature by studying the effect of loading states (tension and compression), long-term cycling, strain localization, and deformation temperatures in several SMAs including CuZnAl, NiTi, NiTiCu, Ni₂FeGa and NiTiHf_13.3. We found a temperature change of 14.2 °C in CuZnAl, 18.2 °C in NiTi, 15.2 °C in NiTiCu, 13.5 °C in Ni₂FeGa, and 6.95 °C in NiTiHf_13.3 upon reverse transformation depending on the entropy change (as high as 60 J/kg K), the stress hysteresis, the inhomogeneity of the transformation and the number of superelastic cycles. A gradual deterioration of the EC effect in tension develops, while in compression the EC effect can be sustained much longer (in excess of 10⁴ cycles). The Ni₂FeGa SMAs possess an operational EC temperature window of nearly 200 °C, which is the widest among the chosen SMAs. With over one hundred experiments reported in one study, this paper represents an authoritative summary of the EC capabilities of a wide range of SMAs.