Role of thermal expansion anisotropy on the elastocaloric effect of shape memory alloys with slim-hysteresis superelasticity

We reveal the mixed-mode mechanism of elastocaloric effect in the course of slim-hysteresis superelasticity in a severely deformed NiTi shape memory alloy with tailored (negative/zero/positive) thermal expansion anisotropy. It is shown that in addition to the latent heat of phase transformation, the reversible heat from elastic deformation (known as the thermoelastic effect) plays a significant role in the overall elastocaloric response. The magnitude of the adiabatic temperature change associated with the thermoelastic effect $|\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}|$ can reach 3.1 K, which is comparable to that from the phase transformation latent heat ($|\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{PT}}|=2.9$ K). The sign and magnitude of the $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}$ scales with the sign and magnitude of the coefficient of thermal expansion (CTE) along the stressing direction. For the directions with negative CTEs, the $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}<0$ while for those with positive CTEs the $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}>0$ upon rapid release of tensile stresses. As such, the cooling performance is strongest when the material is tensioned along the direction of the strongest negative CTE $(--14.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ owing to the synergistic interplay of $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{PT}}$ and $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{\mathrm{TeE}}$. The results reveal the importance of thermal expansion property on the elastocaloric effect.