Solid-state elastocaloric cooling, exploiting the latent heat yielded by superelastic martensitic transformation, represents a very promising substitute for the conventional vapor-compression ...refrigeration technology. Seeking high-performance bulk elastocaloric materials is of great significance for the efficient energy conversion. Here, we demonstrate the extraordinary elastocaloric properties in a A oriented Ni49Mn33Ti18 polycrystalline alloy prepared by directional solidification. The temperature change induced by compressive stress reaches an extremely high value up to −37.3 K and the stress-induced entropy change can be as large as 51.0 J kg−1 K−1. This colossal elastocaloric response originates from the giant transformation entropy change as well as the strong A texture induced by directional solidification, allowing a reduction in the internal constraints from the differently oriented grains on the lattice deformation and thus a promotion on the release/absorption of latent heat due to enhanced volume fraction of transformed martensite induced by external stress.
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This study aimed to enhance the stability of the superelastic and elastocaloric properties of a Ti49Ni41Cu10 shape memory alloy through a synergistic approach combining compositional design and ...precipitation hardening. The alloy was subjected to a 168-h aging treatment at 400 °C. The aging treatment increased the hardness of the alloy due to the formation of nano-scale C11b Ti(Ni,Cu)2 precipitates. The C11b Ti(Ni,Cu)2 precipitates exhibited a coherent interface with the matrix, contributing to the significant strengthening effect. The thickness of the precipitates was measured to be approximately 2 nm. Additionally, the lattice compatibility between the B2 parent and B19 martensite phases, determined to be 0.997 after aging, indicated a high level of compatibility between the two phases. The high compatibility and precipitate strengthening further contributed to the functional stability of the alloy. The 168-h aged Ti49Ni41Cu10 shape memory alloy showed excellent functional stability during thermal and deformation cycles. The peak temperature of B2 to B19 transformation only decreased by 0.2 °C after 10 thermal cycles and showed limited changes with a decrement of 3.2 °C after 10,000 superelastic cycles. The Ti49Ni41Cu10 SMA showed a large elastocaloric temperature drop of 18 °C at the 1st superelastic cycle and maintained a large temperature drop of 17 °C after 1000 cycles. These findings contribute to a deeper understanding of the microstructural changes and their impact on the functional properties of the Ti49Ni41Cu10 shape memory alloy. This study has demonstrated the potential for developing shape memory alloys with improved stability and performance by utilizing compositional design and precipitation hardening techniques.
•The aged Ti49Ni41Cu10 SMA showed excellent functional stability.•Coherent nanoscale Ti(Ni,Cu)2 formed after aging at 400 °C for 168 h.•Precipitation hardening and B19 martensitic transformation improved the stability.•Highly stable transformation temperatures during 10,000 compressive cycles.•Stable elastocaloric cooling performance (∼17 K) during 1000 cycles.
•The elastocaloric effect appears in a wide temperature range for a strain glass alloy.•An inverse elastocaloric effect is observed in the strain glass alloy with history of zero-field cooling.•The ...temperature-history dependence of elastocaloric effect can be attributed to the slow dynamics of strain nanodomains in response to the external stress.
The singular change of the order parameter at the first order martensitic transformation (MT) temperature restricts the caloric response to a narrow temperature range. Here the MT is tuned into a sluggish strain glass transition by defect doping and a large elastocaloric effect appears in a wide temperature range. Moreover, an inverse elastocaloric effect is observed in the strain glass alloy with history of zero-field cooling and is attributed to the slow dynamics of the nanodomains in response to the external stress. This study offers a design recipe to expand the temperature range for good elastocaloric effect.
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High-efficiency elastocaloric refrigeration requires high-performance elastocaloric materials with both large surface areas to promote heat exchange rate and large elastocaloric effects to increase ...the amount of heat transfer. Ni–Ti shape memory alloys (SMAs) are the most promising elastocaloric materials but they are difficult to process by conventional methods due to their poor manufacturability. Here, we successfully developed Ni–Ti SMAs with large elastocaloric effects by additive manufacturing which has the capability to fabricate complex geometries with large surface areas. The phase transformation temperatures of these additively manufactured Ni–Ti SMAs, fabricated by selective laser melting (SLM), can be tuned by varying the SLM processing parameters and/or post heat treatments and thus tunable large elastocaloric effects were achieved at different temperatures, which can be used for different applications. Owing to its large transformation entropy change and high yield strength as a result of precipitation hardening, the aged SLM fabricated alloy exhibits a remarkably large elastocaloric effect with an adiabatic temperature change as high as 23.2 K, which is among the highest values reported for all Ni–Ti SMAs fabricated by both conventional methods and additive manufacturing. Furthermore, by virtue of the high yield strength and low stress hysteresis of the aged alloy, this large elastocaloric effect shows good stability during cycling. The achievement of such large elastocaloric effects in alloys fabricated by near-net-shape additive manufacturing may accelerate the implementation of high-efficiency elastocaloric refrigeration. This study is instructive for the development of advanced high-performance solid-state refrigeration materials by additive manufacturing.
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Solid-state refrigeration based on the caloric effects has been conceived to be a high-efficient and environmental-friendly alternative to replace the vapor-compression refrigeration technique. The ...implementation of solid-state refrigeration requires that the refrigerants should possess not only remarkable caloric effect but also wide working temperature region. In this work, we demonstrate that various caloric effects can be achieved successively in a multiferroic Ni50Mn35In15 meta-magnetic shape memory alloy prepared by directional solidification, including inverse magnetocaloric effect around inverse martensitic transformation, conventional magnetocaloric effect around Curie transition and elastocaloric effect above Curie transition. Among them, the elastocaloric effect is particular striking, where a giant adiabatic temperature variation up to –19.7 K is achieved on removing a moderate stress of 350 MPa due to the elimination of negative magnetic contribution, with the specific adiabatic temperature change of 56 K GPa–1. Furthermore, through the combination of these successive caloric effects, a broad refrigeration temperature region covering the temperature range from 270 K to 380 K can be achieved. It is demonstrated that the combination of various caloric effects could be a promising way to extend the working temperature range of solid state refrigeration.
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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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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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Superelastic shape memory alloys with elastocaloric cooling ability suitable for working at a broad temperature range are crucially lacking in facing various scenarios of cooling applications. ...Low-cost Cu-Al-Mn shape memory alloys are very promising candidates because of their low temperature dependence of critical driving stress of martensitic transformation. Here, a A oriented Cu68.5Al17.5Mn14 single crystal is developed based on cyclic heat treatments, showing large superelastic response and elastocaloric cooling ability covering an ultra-wide temperature range spanning from 77 K to 450 K. Such enhanced temperature span also contributes to an exceptional refrigeration capacity of 5012 J kg–1, well surpassing those in the elastocaloric materials reported so far.
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•Stress-induced martensitic transformation in the target alloy proceeds in a homogeneous deformation mannar, instead of a conventional Lüders-like deformation.•Microstructural features of the target ...alloy including nanocrystallinity and dispersed nanoprecipitates yield the homogeneous deformation and are achieved by directly aging a cold-rolled Ti-51.5Ni alloy.•Stable room-temperature superelasticity and elastocaloric effect are obtained and ascribed to the low dislocation activity during stress cycling.
Functional stability of superelasticity is crucial for practical applications of shape memory alloys. It is degraded by a Lüders-like deformation with elevated local stress concentration under tensile load. By increasing the degree of solute supersaturation and applying appropriate thermomechanical treatments, a Ti-Ni alloy with nanocrystallinity and dispersed nanoprecipitates is obtained. In contrast to conventional Ti-Ni alloys, the superelasticity in the target alloy is accompanied by homogeneous deformation due to the sluggish stress-induced martensitic transformation. The alloy thus shows a fully recoverable strain of 6% under tensile stress over 1 GPa and a large adiabatic temperature decrease of 13.1 K under tensile strain of 4.5% at room temperature. Moreover, both superelasticity and elastocaloric effect exhibit negligible degradation in response to applied strain of 4% during cycling. We attribute the improved functional stability to low dislocation activity resulting from the suppression of localized deformation and the combined strengthening effect of nanocrystalline structure and nanoprecipitates. Thus, the design of such a microstructure enabling homogeneous deformation provides a recipe for stable superelasticity and elastocaloric effect.
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