The first-order inter-martensitic transformation induced by temperature change or external stress is frequently observed in Ni-Mn-Ga alloys. Here, the inter-martensitic transformation is exploited to ...strengthen the elastocaloric effect. Through the stress-induced two-step structural transformation, i.e., martensitic and inter-martensitic transformation, giant adiabatic temperature change up to −10.7K is obtained in a directionally solidified Ni55Mn18Ga27 polycrystalline alloy with 〈001〉A preferred orientation, representing the highest value so far in Ni-Mn-Ga alloys. It is demonstrated that microstructure texturing together with introduction of inter-martensitic transformation could be an effective route to improve the elastocaloric properties in polycrystalline NiMn-based alloys.
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In the present study, the reversible adiabatic temperature change (ΔTad) of shape memory alloys was shown to be proportional to the mechanical work released by the reverse martensitic transformation ...during unloading (ΔWu). As there exists a considerable amount of data (ΔWu) on superelastic stress-strain measurements in shape memory alloys, the proposed relationship allows us to predict ΔTad without caloric measurements. The estimated ΔTad from the ΔWu of different Ti-Ni alloys shows good linear relationship with the directly measured values. Moreover, following such a design criterion and by tuning of composition and thermo-mechanical treatment, a group of Ti-Ni binary shape memory alloys with directly measured ΔTad larger than 35 K under tension were achieved. The large ΔTad and ΔWu can be ascribed to the grain refinement and the heterogeneous internal stress fields after the thermo-mechanical treatment, which have enhanced the critical stress of superelasticity and recoverability of martensitic transformation during unloading.
Superelastic behavior and elastocaloric effect were investigated in a Ti-44Ni-5Cu-1Al (at%) alloy subjected to various thermomechanical treatments. The specimen heat-treated at 673 K for 5 min after ...hot rolling and subsequent cold rolling exhibited excellent superelastic strain of 4.9% with a small stress hysteresis of 90 MPa when the maximum tensile stress was 500 MPa. This specimen also exhibited a large elastocaloric effect with a temperature decrease of 17 K when the stress of 500 MPa was removed adiabatically. No remarkable deterioration was observed for the superelastic strain and elastocaloric effect up to 5000 mechanical cycles. The maximum superelastic strain obtained was 6.8% under a tensile stress of 750 MPa. Transmission electron microscope observation and in-situ X-ray diffraction analysis under tensile stress revealed that the average grain size of the specimen is about 40 nm, and the specimen exhibits a successive B2-B19-B19’ transformation.
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In this work, based on composition adjusting and texture control, we demonstrate giant elastocaloric and magnetocaloric effects in a directionally solidified Ni43Mn47Sn10 polycrystalline with strong ...A texture. On removing a low compressive stress of 210 MPa, giant adiabatic temperature change up to −17.3 K is achieved. Such value is much higher than those in some elastocaloric materials reported previously. Besides, giant magnetic field induced entropy changes of 23.5 Jkg−1K−1 and 37.1 Jkg−1K−1 are also obtained under the magnetic field changes of 2 T and 5 T, respectively. Simultaneously achieving giant elastocaloric and magnetocaloric effects actuated by relatively low external field is of great importance to explore multi-caloric or coupled caloric effects in developing efficient and eco-friendly solid-state refrigeration technology.
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We investigate elastocaloric effect by a combination of infrared thermography and digital image correlation technique for the metamagnetic shape memory alloy Ni45Mn44Sn11. A polycrystalline alloy ...containing a 〈111〉 orientated grain and an off-〈111〉 orientated grain was grown by directional solidification method. A large adiabatic temperature change of −9.5 K is observed upon stress releasing. The differently oriented grains demonstrate drastically different elastocaloric cooling behaviors, which can be ascribed to the crystalline orientation dependence of transformation strain. Moreover, large entropy changes of 10.9 and 25.8 J kg−1 K−1 for magnetic field changes of 20 and 50 kOe are obtained, respectively.
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•Functional fatigue occurs in superelasticity and elastocaloric effect of NiTi spring.•A thermo-mechanically coupled constitutive model is established.•The analytical model of helical spring consider ...both torsion and bending.•LATH method is used to solve temperature evolution of SMA spring.•Functional fatigue is well described by the proposed model.
In this work, the evolutions of superelasticity (SE) and elastocaloric effect (eCE) for NiTi shape memory alloy (SMA) helical springs under cyclic deformation are investigated experimentally and theoretically. Experimental results show that the functional fatigue of SE and eCE occur simultaneously, and these phenomena aggravate significantly with the increase/decrease of the loading level/spring index. Then, a thermo-mechanically coupled constitutive model is established. Two major inelastic deformation mechanisms, martensitic transformation (MT) and transformation-induced plasticity (TRIP), are incorporated. In addition to the martensitic and austenitic phases, the martensitic influence zone is also considered in the proposed constitutive model to characterize the high local stress in the region near the austenitic-martensitic interface. To accurately describe the deformation behavior of NiTi SMA springs, an analytical model considering both the torsion and bending deformation modes is employed. Furthermore, the lumped heat transfer analysis method is adopted to derive the evolution equation for the overall temperature of the springs. Finally, to verify the rationality of the newly developed model, predicted results for the functional fatigue of SE and eCE are compared with the experimental data.
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•Buckling stability and fatigue life of thin-walled Ni-Ti tubes were studied•Ni-Ti tubes withstood over a million full transformation compressive fatigue cycles•Adiabatic temperature change of 27 K ...was measured after one million fatigue cycles•This is the largest directly measured durable elastocaloric effect to date•Thin-walled Ni-Ti tubes are ideal candidates for efficient elastocaloric devices
Elastocaloric cooling is emerging as one of the most promising alternatives to vapor-compression cooling technology. It is based on the elastocaloric effect (eCE) of shape memory alloys (SMAs), which occurs due to a stress-induced martensitic transformation (superelasticity). In recent years, several elastocaloric proof-of-concept devices have been developed and the best of them have already achieved commercially relevant cooling characteristics. However, the proposed devices are not yet ready for commercialization, mostly due to their short fatigue life, which is a consequence of the tensile loading. The fatigue life can be significantly improved if the material is instead subjected to compressive loading, but mechanical instabilities (buckling) and the poor heat transfer of bulky geometries (favorable for compression) are the major challenges to overcome when designing compressed elastocaloric elements. Here, we show for the first time that thin-walled Ni-Ti tubes, which allow for the rapid heat transfer, can withstand more than 106 compressive loading cycles without any degradation of the eCE while maintaining high efficiency (coefficient of performance) and adiabatic temperature changes as high as 27 K. This is the largest, directly measured, durable eCE for any elastocaloric material in the high-cycle fatigue regime to date, and so opens up new avenues in the development of durable and efficient elastocaloric devices.
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Elastocaloric effect in shape memory alloys relies on the latent heat associated with stress-induced martensitic transformation, which can be exploited for solid-state cooling applications. However, ...large stress hysteresis inherent to the first-order transformation greatly restricts the energy conversion efficiency and working temperature window. Here, by utilizing compositional gradient engineering to tailor mechanical hysteresis and microstructure texturing to promote elastocaloric response, a composition-graded Ni50Mn31.5Ti18.5 alloy with A preferred orientation has been fabricated through magnetic field-assisted directional solidification. Owing to the composition segregation induced by a transverse magnetic field applied during solidification, considerable amounts of preexisting martensite domains are embedded in the austenite matrix, which contributes to the reduced critical driving stress and stress hysteresis of martensitic transformation. In combination with large cooling capacity favored by highly preferred orientation, the material's coefficient of performance has been greatly improved. Moreover, a broad refrigeration temperature span of 200 K covering 263 K to 463 K is also realized, with a maximum adiabatic temperature variation of –18.4 K. We attribute the enhanced elastocaloric properties to the synergy of preferred orientation and compositional gradient, which can be developed as an effective route towards performance improvement of elastocaloric materials.
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ThCr2Si2-type intermetallic compounds are known to exhibit superelasticity associated with structural transitions through lattice collapse and expansion. These transitions occur via the formation and ...breaking of Si-type bonds, respectively, under uniaxial loading along the 0 0 1 direction. Unlike most ThCr2Si2-type intermetallic compounds, which have either an uncollapsed tetragonal structure or a collapsed tetragonal structure, SrNi2P2 possesses a third type of collapsed structured: a one-third orthorhombic structure, for which one expects the occurrence of unique structural transitions and superelastic behavior. In this study, uniaxial compression and tension tests were conducted on micron-sized SrNi2P2 single crystalline columns at room temperature, 200 K, and 100 K, to investigate the influence of loading direction and temperature on the superelasticity of SrNi2P2. Experimental data and density functional theory calculations revealed the presence of tension-compression asymmetry in the structural transitions and superelasticity, as well as an asymmetry in their temperature dependence, due to the opposite superelastic process associated with compression (forming P-P bonds) and tension (breaking P-P bonds). Additionally, following thermodynamics, the observations suggest that this asymmetric superelasticity could lead to an opposite elastocaloric effect between compression and tension, which could be beneficial potentially in obtaining large temperature changes compared to conventional superelastic solids that show the same elastocaloric effect regardless of loading direction. These results provide an important fundamental insight into the structural transitions, superelasticity processes, and potential elastocaloric effects in SrNi2P2.
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Nitinol (NiTi), a shape memory alloy (SMA) of nickel and titanium, exhibits two unique properties: the shape memory effect and superelasticity. It is a material of choice for applications demanding ...extraordinary flexibility and motion. It is subjected to greater fatigue strains compared to ordinary metals. The structural and functional fatigue properties are important for assessing the fatigue life and reliability of the superelastic NiTi. The advances made in the experimental analysis to improve the structural and functional fatigue resistance of superelastic NiTi are reviewed in this paper. Various aspects of fatigue behaviour of NiTi in biomedical and cooling applications, along with fatigue failure mechanism, are elaborated under structural fatigue. Importance of functional fatigue and its connect with structural fatigue performance of NiTi is discussed citing recent research literature. Furthermore, the effect of processing parameters involved in additive manufacturing on the fatigue performance of NiTi is also discussed.
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