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.
Elastocaloric refrigeration technology is a solid-state alternative to the traditional vapor compression technology. It is considered to be highly efficient, energy-saving and environmentally ...friendly. Out of the materials with elastocaloric effect, NiTi shape memory alloy is the most researched due to its excellent comprehensive mechanical and functional properties. However, it has some problems like large superelastic hysteresis, relatively high transformation stress and poor cyclic stability, which hinder its commercial application. This article explains the mechanism of elastocaloric effect and its characterization methods. Relevant research progress including the effects of considering elemental doping, mechanical processing, heat treatment, surface finishing, optimization of preparation methods and gradient structure on elastocaloric effect in NiTi alloy are summarized. In addition, the development and current status of elastocaloric refrigeration based on NiTi alloy are analyzed in this paper.
In this paper, we report that the superelasticity and the elastocaloric effect of Ti-50.8Ni (at.%) alloy are drastically improved by a combination of precipitation and grain refinement strengthening. ...The process is as follows: first, aging at 773 K after solution treatment to form coherent fine precipitate of Ti3Ni4; second, cold working followed by annealing at 673 K to form nanocrystalline grain containing Ti3Ni4. The decay of the superelasticity and the elastocaloric effect by 100 stress cycles is <10%.
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We investigate the temperature-dependent mechanical properties and elastocaloric effects of different multiphase nanocrystalline NiTi alloys with grain sizes of 10 nm, 15 nm, 20 nm, 27 nm, 34 nm and ...55 nm. These multiphase nanocrystalline NiTi alloys mainly consist of B2, B19′ and R phases and can undergo the thermally induced phase transition over a wide temperature range from 208 K to 353 K. Their superelastic deformation (loading to a strain of 4%) is contributed by combinations of the elastic deformation of B2 phase, the R → B2 phase transition, the reversible martensite reorientation and the B2 → R and B2/R → B19' phase transitions. Such complex deformation mechanism together with the microstructure evolution with temperature brings strong temperature-dependences of the mechanical properties and elastocaloric effects. As results, at the grain sizes of 15 nm ∼ 20 nm, the stress under a constant superelastic strain can show a significant V-shaped variation with temperature. At the grain size of 55 nm, the adiabatic temperature drop can even change from −8.5 K to −29 K ∼ −35.8 K as the ambient temperature rises from 293 K to 333 K. By comparison of the mechanical properties and elastocaloric effects of the different multiphase nanocrystalline NiTi alloys, it is found that the one with the grain size of 20 nm exhibits the best comprehensive cooling performances, which has a soft near-linear superelasticity (a moderate driving force), a very wide refrigeration temperature window (> 245 K) with considerable adiabatic temperature drops (∆Tmax= −17.1 K), and a good functional stability. This work provides an experimental basis for using the multiphase nanocrystalline NiTi alloys as solid-state refrigerants.
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•Multiphase nanocrystalline NiTi alloys are fabricated by thermomechanical treatment.•Their mechanical properties and elastocaloric effects depend strongly on temperature.•Microstructure evolution with temperature causes this strong temperature dependence.•The sample with 55 nm sized grains has an adiabatic temperature drop of up to −35.8 K.•The one with 20 nm sized grains exhibits the best comprehensive cooling performance.
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Exploring elastocaloric materials with high transformation entropy change (ΔS) is a key mission for the development of elastocaloric refrigeration technology. Here, we show an ...adaptive design strategy, tightly coupled a machine learning (ML) with theoretical calculations to accelerate the discovery process of multi-component Cu-Al-based shape memory alloys (SMAs) with high ΔS. Based on a linear regression model, Al, Co, Fe, Ni are the elements that are beneficial to the significant promotion of ΔS in the Cu-Al-based alloys. In our results, Cu72.2Al20.2Ni6.2Co0.7B0.7 is discovered with the highest ΔS of 1.88 J/mol K from a potential space of ~500,000 compositions, which is higher than the highest ones found in ternary Cu-Al-Mn and reported experimental value by 9.9% and 17.5%.
This paper reports the elastocaloric effect of two Cu-based shape memory alloys: Cu68Al16Zn16 (CuAlZn) and Cu73Al15Mn12 (CuAlMn), under compression at ambient temperature. The compression tests were ...conducted at two different rates to approach isothermal and adiabatic conditions. Upon unloading at a strain rate of 0.1 s−1 (adiabatic condition) from 4% strain, the highest adiabatic temperature changes (ΔTad) of 4.0 K for CuAlZn and 3.9 K for CuAlMn were obtained. The maximum stress and hysteresis at each strain were compared. The stress at the maximum recoverable strain of 4.0% for CuAlMn was 120 MPa, which is 70% smaller than that of CuAlZn. A smaller hysteresis for the CuAlMn alloy was also obtained, about 70% less compared with the CuAlZn alloy. The latent heat, determined by differential scanning calorimetry, was 4.3 J g−1 for the CuAlZn alloy and 5.0 J g−1 for the CuAlMn alloy. Potential coefficients of performance (COPmat) for these two alloys were calculated based on their physical properties of measured latent heat and hysteresis, and a COPmat of approximately 13.3 for CuAlMn was obtained.
This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.
The elastocaloric effect and functional fatigue life of the polycrystalline Ni50Ti45.3V4.7 shape memory alloy have been studied. Temperature changes on rapid loading and unloading of a mechanically ...trained specimen were directly measured at different compressive stress levels. It was found that the maximum elastocaloric strength was accomplished at the stress corresponding to the end point of a transformation plateau. In the fatigue test, no significant degradation occurred in the elastocaloric cooling ability after 5000cyclic loadings. Compared to pure NiTi, the present alloy showed improved efficiency and endurance limit of the functional stability.
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The elastocaloric alloys Ni‐Ti and Cu‐Zn‐Al in a regenerative refrigeration device can produce outstanding results with up to 20‐times larger specific cooling powers compared to gadolinium, a ...benchmark magnetocaloric material, with comparable coefficient‐of‐performance (COP) values (≈5). These results can open up a new way of making cooling devices with more compact systems and provide the possibility to avoid expensive rare‐earth materials.
Elastocaloric signature of nematic fluctuations Ikeda, Matthias S; Worasaran, Thanapat; Rosenberg, Elliott W ...
Proceedings of the National Academy of Sciences - PNAS,
09/2021, Letnik:
118, Številka:
37
Journal Article
Recenzirano
Odprti dostop
The elastocaloric effect (ECE) relates changes in entropy to changes in strain experienced by a material. As such, ECE measurements can provide valuable information about the entropy landscape ...proximate to strain-tuned phase transitions. For ordered states that break only point symmetries, bilinear coupling of the order parameter with strain implies that the ECE can also provide a window on fluctuations above the critical temperature and hence, in principle, can also provide a thermodynamic measure of the associated susceptibility. To demonstrate this, we use the ECE to sensitively reveal the presence of nematic fluctuations in the archetypal Fe-based superconductor Ba(Formula: see text)
Formula: see text By performing these measurements simultaneously with elastoresistivity in a multimodal fashion, we are able to make a direct and unambiguous comparison of these closely related thermodynamic and transport properties, both of which are sensitive to nematic fluctuations. As a result, we have uncovered an unanticipated doping dependence of the nemato-elastic coupling and of the magnitude of the scattering of low-energy quasi-particles by nematic fluctuations-while the former weakens, the latter increases dramatically with increasing doping.
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