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 designed a (Ni,Cu)-rich Ti49Ni41Cu10 shape memory wire annealed by pulsed electric treatment of 60 A/mm2 for 0.3 sec. Granular and strip-type Ti(Ni,Cu)2 precipitates, formed before ...application of the electric current treatment, facilitated the plastic deformation during the wire drawing process. The wire demonstrated stability in superelasticity and elastocaloric cooling capability. The accumulated residual strain was only 0.15 % after 50 superelastic cycles with an applied strain of 3.5 % at 338 K. The elastocaloric temperature drop of the wire remained at about 9.4 K for 50 cycles. Moreover, under 3 % applied strain, the wire exhibited elastocaloric temperature drops of 5.3 K to 8.1 K within the application window of 323 K to 373 K. TEM observations confirmed that the grain size of the wire was about 20–30 nm, which led to the homogeneous superelasticity performance. Furthermore, the superior compatibility between the B2 and B19 phases reduced plastic deformation at the interfaces during martensitic transformation. The stable functionality and elastocaloric cooling capacity of the wire originated from the combination of nanocrystalline structure and superior compatibility between the B2 and B19 phases. These results demonstrated that the Ti49Ni41Cu10 SMA wire could be fabricated due to the presence of ductile Ti(Ni,Cu)2 precipitates, and the subsequent pulsed electric current treatment caused rapid annealing to achieve a stable functional performance.
•Ti49Ni41Cu10 wire was annealed with pulsed electric current of 60 A/mm2 for 0.3 s.•Ductile Ti(Ni,Cu)2 improves workability and facilitates wire drawing process.•The accumulated residual strain was only 0.15 % after 50 superelastic cycles.•The elastocaloric temperature drop of the wire remained at 9.4 °C for 50 cycles.•The annealed wire showed stable functionality and elastocaloric cooling capacity.
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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Cyclability of elastocaloric effect is of crucial importance for practical applications of elastocaloric refrigeration which is a promising alternative to the conventional cooling technology based on ...vapor compression. The well-known Ni-Mn-based magnetic shape memory alloys exhibit fascinating multicaloric effects (including elastocaloric, magnetocaloric and barocaloric effects), but they are intrinsically brittle because of weak grain boundary cohesion, which results in poor cyclic stability of elastocaloric effect. Here we demonstrate that microalloying with boron is very effective in enhancing the mechanical properties and cyclic stability of elastocaloric effect in Ni-Mn-In intermetallic magnetic shape memory alloys. The elastocaloric effect of the boron-free Ni51.5Mn33In15.5 alloy degrades rapidly after only ∼20 cycles; in contrast, that of the boron-doped (Ni51.5Mn33In15.5)99.7B0.3 alloy remains stable with almost no degradation for more than 150 cycles. The enhancement of mechanical properties and cyclic stability of elastocaloric effect is mainly attributed to the increase of gain boundary cohesion and grain refinement resulting from microalloying with boron. Furthermore, by virtue of its enhanced mechanical properties, a high adiabatic temperature change up to 6.6 K (under 550 MPa), a large stress-induced entropy change of 20.0 J kg−1 K−1 (under 300 MPa) and a high coefficient of performance of 18 were successfully achieved in the boron-doped (Ni51.5Mn33In15.5)99.7B0.3 alloy showing a working temperature just around room temperature. These advantages make this alloy promising for room-temperature elastocaloric refrigeration. This study is instructive for designing high-performance elastocaloric materials for solid-state mechanocaloric cooling applications.
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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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Structural fatigue is the major obstacle that prevents practical applications of the elastocaloric effect (eCE) in cooling or heat-pumping devices. Here, the eCE and fatigue behaviour of Ni-Ti plates ...are systematically investigated in order to define the fatigue strain limit and the associated eCE. Initially, the eCE was evaluated by measuring adiabatic temperature changes at different strain amplitudes and different mean strains along the loading and unloading transformation plateaus. By comparing the eCE with and without pre-strain conditions, the advantages of cycling an elastocaloric material at the mean strain around the middle of the transformation plateau were demonstrated. In the second part of this work, we evaluated the fatigue life at the mean strain of 2.25% at the loading plateau and at the unloading plateau after initial pre-straining up to 6% and 10%, respectively. It is shown that on polished samples, durable operation of 105 cycles can be reached with a strain amplitude of 0.50% at the loading plateau, which corresponds to adiabatic temperature changes of approximately 5 K. At the unloading plateau (after initial pre-strain of 10%), durable operation was reached at a strain amplitude of 1.00%, corresponding to adiabatic temperature changes of approximately 8 K. The functional fatigue was analysed after the cycling and it is shown that once the sample has been stabilized there is no further degradation of the eCE, even after 105 cycles. These results present guidelines for the design and operation of efficient and durable elastocaloric devices in the future.
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The coefficient of performance of material (COPmat, the ratio of caloric effect to energy dissipation), a key figure-of-merit for solid-state refrigerants, is significantly degenerated by the ...inherent hysteresis of materials. In this work, a low-hysteresis NiTi refrigerant with gradient structures (grain sizes varying from ~ 10 nm to ~ 3500 nm) is fabricated for elastocaloric cooling by localized laser surface annealing on a severely-deformed substrate (50% thickness reduction). The obtained gradient-structured (GS) NiTi exhibits more than 83% improvement in COPmat with a comparable adiabatic temperature change ΔTad compared to the homogeneous coarse-grained NiTi and extends the lower limit of operational temperature from above 283 K to 243 K. Furthermore, the large specific cooling capacity (~ 4.5 K/1%), narrow stress hysteresis (~ 60 MPa) and robust mechanical properties (high strength, high ductility and high stability) make the GS NiTi superior to most elastocaloric materials in refrigeration capability and efficiency. Such significantly enhanced cooling and mechanical performances of the GS NiTi originate from the unique gradient structure, which possesses a sound synergetic strengthening effect and an overall uniform phase transformation mode. The work proposes a promising strategy for optimization of thermomechanical performances of elastocaloric materials and demonstrates a great industrial potential of the GS NiTi in solid-state cooling.
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The stress-induced transformation from austenite phase to R phase in NiTi and TiNi-based shape memory alloys has a small stress hysteresis and a high stability to cycling across the transformation ...due to a small lattice distortion. However, the elastocaloric effect is limited due to the intrinsic small transformation strain and this significantly hinders its application in solid-state refrigeration. Here, we report a macroscopically inhomogeneous stress-induced R-phase transformation in Ti50Ni48.5Fe1.5 wire with a stress plateau in the stress-strain curve, showing difference from the conventional macroscopically homogeneous transformation with an associated continuous increase of the stress. This novel transformation behavior permits reaching a temperature decrease of 5.8 K with a transformation strain of only 0.4 %, which brings a near 100 % increase in the elastocaloric effect of R-phase transformation compared to Ni50.8Ti49.2 and an excellent cycling stability. Infrared tensile tests and in-situ X-ray diffraction measurements prove the existence of transformation bands, confirming the localized deformation of this transformation behavior. Transmission electron microscopy results show that the microstructure condition for inducing such a macroscopically inhomogeneous R-phase transformation in Ti50Ni48.5Fe1.5 is the limited hindrance effect from the local stress induced by doped Fe atoms. The stress-induced R-phase transformation with an enhanced cooling capacity and an excellent fatigue property makes Ti50Ni50-xFex alloys a new alternative material for solid-state refrigeration.
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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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