Orientation Dependence of the Elastocaloric Effect in Ni54Fe19Ga27 Ferromagnetic Shape Memory Alloy

Abstract

The crystallographic anisotropy of elastocaloric effect (ECE) and relative cooling power (RCP) in Ni54Fe19Ga27 shape memory alloy single crystals are studied via compression tests. Single crystals are studied along the [001], [123], and [011] austenite directions and yield different ECE behaviors and maximum RCPs for various strain levels. A thermodynamic framework using the Helmholtz free energy is employed to analyze the total entropy change as a function of strain. Thermodynamic losses are computed from the mechanical hysteresis of superelasticity experiments to quantify the strain dependent RCP. It is found that the [001] orientation generates the highest maximal RCP of 738 J kg−1 when unloaded from 200 MPa. This is attributed mainly to the large superelastic temperature window of 45 K. However, loading the crystals to stresses higher than 200 MPa causes a multistep transformation in the [011] direction, thus reducing the alloy's overall RCP by 135 J kg−1. This is a consequence of the negative entropy change and large transformation hysteresis generated by the second‐stage transformation in the [011] direction. Interestingly, if only the first‐stage transformation in [011] is employed for the ECE, the [011] direction yields the highest RCP compared to [001] and [123] for any strain up to 3.5%.