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 Ni 54 Fe 19 Ga 27 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 À1 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 À1 . 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%.

Influence of microstructure on the application of Ni-Mn-In Heusler compounds for multicaloric cooling using magnetic field and uniaxial stress

“…The recovery of the sample to its initial length upon heating to 310 K after the measurement implies no austenite slip resulting in irreversibility. A similar material response was reported for Ni-Fe-Ga single crystals and can result from the friction behavior of the shape-memory alloy [47]. Additionally, the reverse transformation is limited below A f and fully hindered below A s by the thermal hysteresis.…”

An Analytical Approach for Computing the Coefficient of Refrigeration Performance in Giant Inverse Magnetocaloric Materials

“…In Equation (7), the isothermal entropy change has been derived for any single phase exposed to the intensive force y i in G. It is important to note that, in experiments where strain drives the caloric effect and is thus the independent variable, the Helmholtz free energy expression should be employed, instead of G, to insure consistency between the measured and predicted caloric behaviors [19].…”

“…Interestingly, the assumption of commutativity, described above, no longer holds exactly at T Curie due to the inability to quantify the derivative of G. However, at temperatures above and below T Curie , Equation ( 7) is applicable for determining magnetic field-induced ∆S iso . energy expression should be employed, instead of 𝐺, to insure consistency between the measured and predicted caloric behaviors [19].…”

“…Equation ( 19) is applicable to magnetocaloric materials that exhibit first-order magnetostructural phase transitions, such as NiMnIn, where y i = µ 0 H and ∆X i = ∆M M→A ; thus, ∆S M→A = −∆M M→A ξ•d(µ 0 H)/dT 0 and ξ = 1 for a complete transformation. Equation (19) appears to be similar to Equation ( 7); however, these two expressions were derived under dissimilar thermodynamic conditions and therefore provide the ∆S iso for fundamentally different processes. There has been much debate in the literature whether Equation ( 7) and its discretized form [56] are applicable to quantify ∆S M→A across first-order transitions [57].…”

On the instability of the giant direct magnetocaloric effect in CoMn0.915Fe0.085Ge at. % metamagnetic compounds

“…Above critical transformation temperatures, the MMSMA is typically characterized by a cubic and ferromagnetic austenite phase, and below them, a non-magnetic non-cubic martensite phase. During the martensitic transformation, macroscopic strain 3 , 17 – 19 , crystallographic volume 20 , 21 , bulk magnetization 22 , 23 , and electric resistivity 24 are all subject to large fluctuations. The changes observed in these physical properties are explained through free-energy analysis and equilibrium thermodynamics.…”