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Highly precise and efficient characterization of thermophysical parameters associated with martensitic transformation (MT) in shape memory alloys (SMA) is challenging based on conventional calorimetry methods. Moreover, existing methods for evaluating the elastocaloric effect of SMA typically require a series of tests and calculations. In addition, the present method cannot evaluate the nonreversible part during MT. This work proposed a technique rarely mentioned in previous studies on martensitic transformation of metals and alloys, i.e., utilizing the modulated differential scanning calorimetry (MDSC) to superimpose a sinusoidal signal over an underlying DSC ramp. By adjusting appropriate measurement parameters, the reversible and nonreversible parts of thermal events during MT of SMAs were revealed. Furthermore, a series of thermal parameters useful for the study of MT can be obtained by this method and thus may provide a perspective for studying the MT process. Based on MDSC technique, we took Ni-Mn-Sn-(Cu) alloys, a kind of ferromagnetic shape memory alloy, as an example to demonstrate the study of the MT process as well as the elastocaloric effect. From the perspective of energy dissipation, we analyzed the intrinsic relationship between nonreversible component and thermal hysteresis in the MT process. Conventional DSC test and experimental results on the adiabatic temperature change (ΔTad) were also provided to verify the MDSC prediction results. [ABSTRACT FROM AUTHOR]

Solid-state refrigeration based on elastocaloric materials (eCMs) requires reversibility and repeatability. However, the intrinsic intergranular brittleness of ferromagnetic shape memory alloys (FMSMAs) limits fatigue life and, thus, is the crucial bottleneck for its industrial applications. Significant cyclic stability of elastocaloric effects (eCE) via 53% porosity in Ni-Fe-Ga FMSMA has already been proven. Here, Ni-Fe-Ga foams (single-/hierarchical pores) with high porosity of 64% and 73% via tailoring the material's architecture to optimize the eCE performances are studied. A completely reversible superelastic behavior at room temperature (297 K) is demonstrated in high porosity (64–73%) Ni-Fe-Ga foams with small stress hysteresis, which is greatly conducive to durable fatigue life. Consequentially, hierarchical pore foam with 64% porosity exhibits a maximum reversible ∆Tad of 2.0 K at much lower stress of 45 MPa with a large COPmat of 34. Moreover, it shows stable elastocaloric behavior (ΔTad = 2.0 K) over >300 superelastic cycles with no significant deterioration. The enhanced eCE cyclability can be attributed to the pore hierarchies, which remarkably reduce the grain boundary constraints and/or limit the propagation of cracks to induce multiple stress-induced martensitic transformations (MTs). Therefore, this work paves the way for designing durable fatigue life FMSMAs as promising eCMs by manipulating the material architectures. [ABSTRACT FROM AUTHOR]

Homogeneous superelastic behavior in shape memory alloys (SMAs) has been confirmed to be crucial for functional and structural fatigue properties, and more importantly, for stable elastocaloric effect (eCE). Here, the Ti–22Nb–4Zr–2Ta plate was prepared by successive cold rolling at 98.5%, followed by annealing at 1023 K for 30 min, forming a strong recrystallized texture, denoted as {112} β < 110> β. Such favorable texture contributed to a completely recoverable superelastic strain of ∼2.4% with narrow hysteresis of 35/45 MPa under isothermal/quasi-adiabatic conditions, which resulted in homogeneous martensitic transformation (MT) and adiabatic temperature changes (ΔT adi) of 4.8 K upon loading and 4.4 K upon unloading. The observation on the evolution of space-resolved strain and temperature throughout the sample revealed that the intrinsic feature of diffuse transformation along with favorable texture contributed to meso-scopically homogeneous transformation and related eCE. A standard deviation of strain was determined to be only 0.05%, corresponding to a small temperature deviation of 0.2–0.3 K, which quantitatively demonstrated the homogeneity, potentially high solid-state cooling performance efficiency, and long structural and functional fatigue life in Ti-based SMAs. • A Ti-based alloy with {112}<110> texture by 98.5% cold rolling & 1023 K annealing. • A 2.4% recoverable strain with hysteresis of 35/45 MPa at slow/fast rates. • Diffuse transformation & beneficial texture contribute to homogeneous transition. • Standard deviation of strain distribution was 0.05% at strain amplitude. • ΔT adi was +4.8 K/-4.4 K with standard deviation of 0.2–0.3 K. [ABSTRACT FROM AUTHOR]

We have studied the effect of Co-doping on the microstructure, martensitic transformation, and magnetocaloric effect (MCE) of a Ni50Mn38Sb9Si3 alloy. Here, we further study the effect of Co-doping on the microstructure, martensitic transformation (MT), and MCE of Ni50Mn38Sb9Si3 alloy. The results show that phases α, β, and γ coexist in Ni50−xCoxMn38Sb9Si3 (x = 6, 12) alloys. The MT temperatures remain almost unchanged with the addition of Co element. Above the room temperature (RT), phase α exhibits MT under external magnetic fields up to 50 kOe, and possesses a great enhancement of ΔM from 0.008 emu/g in Ni50Mn38Sb9Si3 alloy to 21.4 emu/g in Ni38Co12Mn38Sb9Si3 alloy as well as an improvement of ∂M/∂T (obtained from M-T curves) that benefit ΔSM. The MT temperature intervals of M-T curves are enlarged from 3 K in the un-doped alloy to 11 K in the water-quenched Ni38Co12Mn38Sb9Si3 alloy attributed to the reduced coupling between magnetic and structural transition. This contributes to a magnetic entropy change (ΔSM) of 4.62 J/kg K and net refrigeration capacity (RCnet) of 18.5 J/kg under a magnetic field of 50 kOe, which is superior to that of the ferromagnetic compounds containing secondary precipitates. [ABSTRACT FROM AUTHOR]

Bulk magnetic refrigerant materials with first-order character usually possess hysteresis losses and concentrated refrigeration working temperature interval ( δT FWHM ), which are detrimental for magnetic refrigeration applications. Here, we report on Ni-Mn-Ga micro-particles with size ~38.5–45 μm fabricated by grinding and subsequent stress relief annealing (SRA). The microstructure, martensite transformation (MT), hysteresis loss and magnetocaloric effect (MCE) of the SRA particles were studied in comparison with the bulk alloy counterpart. The thermal and magnetic hysteresis losses in SRA particles ~1.3 K and ~1.9 J/kg were significantly reduced compared to those in the bulk alloy ~5.5 K and ~9.7 J/kg, respectively, which is related to the reduction of neighboring grain constraints on martensite/austenite phase boundary motion. The MT transformation temperature range of the SRA particles ~26.5 K was higher than that of the bulk alloy ~5.5 K because of the high specific surface area of the particles and the residual internal stress after SRA. A magneto-structural partly-coupled state was attained in the SRA particles, which is responsible for the wide δT FWHM ~25 K and the high refrigeration capacity ( RC net ) ~117 J/kg. Based on these results, a possible approach for the high magnetic refrigeration efficiency is expected and discussed. [ABSTRACT FROM AUTHOR]

The effects of Fe doping on the microstructure, martensite transformation and superelasticity in melt-extracted Ni 50 Mn 25 Ga 25− x Fe x ( x =1–6) microwires were investigated. The unique solidification process during melt-extraction creates the micron-sized diameter wires with small grains and semicircular cross-section. At ambient temperature Ni 50 Mn 25 Ga 25− x Fe x ( x <4) microwires are austenite phases with a cubic L2 1 structure, while microwires with x >5 are martensitic phases with seven-layered modulated (7M) structure. The results point out that martensite transformation temperatures are strongly related to Fe content due to the change of valence electron concentration ( e / a ). Reversible superelastic strains of 0.92% and 0.75% are obtained in Ni 50 Mn 25 Ga 21 Fe 4 and Ni 50 Mn 25 Ga 20 Fe 5 microwires, respectively. It is demonstrated that the temperature dependence of stress-induced martensite (SIM) stress follows the Clausius–Clapeyron relation. The temperature dependence of SIM stress in Fe-doped Ni–Mn–Ga microwires is 10.5 MPa/K. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reduced dimensions elastocaloric materials (i.e. films, ribbons, wires and foams) are reviewed, which are expected to potential candidates for miniaturized refrigeration. • Based on various properties,e.g. ∆σ hys , ∆T ad /∆σ cr and ∆T ad /∆ε tr their elastocaloric performances (∆T ad / ∆S iso) are explored. • For their commercialized application, the fatigue life and cooling efficiency (COP mat and COP mat / COP Carnot) are demonstrated. • The challenges/proposed solutions and recent achievements for micro cooling eCE demonstrators/prototypes based on small-sized eCMs are ultimately outlined. Elastocaloric cooling technology has obtained huge attraction over vapor compression systems because it is more efficient with negligible environmental impact. The elastocaloric effect (eCE) is generally concerned with superelasticity and latent heat of shape memory alloys (SMAs), related to the martensitic transformation (MT) under uniaxial loading/unloading. There are various small-scale applications such as microelectromechanical systems, medical devices and lab-on-chip systems, where active local cooling with precise temperature control is essential for their proper functioning. To explore potential eCE for miniaturized active refrigeration, reduced dimensions SMAs (i.e. thin films, microwires, ribbons and foams) may be attractive for micro cooling devices by providing a large surface to volume ratio and thus high heat transfer capacity, low thermal hysteresis and high working frequency. Therefore, the eCE properties of the state-of-the-art reduced dimensions elastocaloric materials (eCMs) are thoroughly reviewed and comparatively discussed with their bulk counterparts. The most appropriate eCMs for miniaturized eCE refrigeration are revealed on the basis of large adiabatic temperature change (△ T ad), low stress hysteresis (△ σ hys), high cyclic stability and large coefficient of performance of material (COP mat). The challenges and recent achievements for micro cooling eCE devices/prototypes based on small-sized eCMs are summarized. [ABSTRACT FROM AUTHOR]

Small-sized materials with large surface to volume ratio favor heat transfer during magnetic refrigeration cycling and thus may help enhancing the refrigeration efficiency. Here, high Fe content Ni 44.9 Fe 4.3 Mn 38.3 Sn 12.5 polycrystalline microwires were prepared by a melt-extraction technique. The as-extracted microwires were annealed at 1173 K for 60 min, leading to significant grain growth and formation of a secondary Fe-rich γ phase. The annealed microwire exhibits larger magnetization difference ( ΔM ) between the austenite and martensite phases and smaller thermal hysteresis compared to the as-extracted microwire. The annealed microwire possesses a magnetic transition to austenite at 299 K, followed by a martensitic transformation (MT) from a ferromagnetic austenite to a weak-magnetic martensite at 208 K upon cooling. Under a magnetic field of 50 kOe, the annealed microwires show a maximum magnetic entropy change ΔS m of 6.9 J/kg·K and an effective refrigeration capacity RC eff of 78.0 J/kg over a broad working temperature span ΔT FWHM of 20 K around the MT. In addition, magnetic transition of the austenite gives rise to ΔS m − 3.7 J/kg·K and RC mag 232.5 J/kg with ΔT FWHM of 85 K under 50 kOe. [ABSTRACT FROM AUTHOR]

Ni-Mn-Sn alloys exhibit high magnetocaloric effect (MCE) during the magnetic-field-induced martensite to austenite phase transformation. However, the low magnetization of the austenite, narrow working temperature interval and high magnetic hysteresis reduced their magnetic refrigeration capacity. Here, the effects of Fe doping on the structural and magnetic transitions of Ni 50-x Fe x Mn 38 Sn 12 alloys were investigated. The doping of Fe higher than 2.9% induced the occurrence of a secondary γ phase, creating a dual phase alloy. The martensite transformation temperature significantly decreased with the occurrence of the secondary phase. However, the Curie temperatures of both austenite and martensite changed slightly with the formation of the secondary phase. As a result, the magnetostructural coupling, i.e. overlap of the structural and magnetic transitions between the ferromagnetic austenite and the paramagnetic martensite, occurred in the alloys with x = 0.5 - ∼4.2. Furthermore, the magnetization of the austenite and martensite both increased with increasing Fe content from x = 0 to x = 5.5, with the magnetization difference between the austenite and martensite saturating at x = 4.2. The dual phase Ni 45.8 Fe 4.2 Mn 38 Sn 12 alloy, exhibiting a magnetic field dependence of martensite transformation temperature ΔT M / μΔH - 0.28 K/kOe, showed a wider working temperature interval than that of the un-doped Ni 50 Mn 38 Sn 12 alloy. This demonstrated that the Ni 45.8 Fe 4.2 Mn 38 Sn 12 alloy might exhibit a high magnetic entropy change, wide working temperature interval and low magnetic hysteresis, and thus may be a promising magnetic refrigeration material. [ABSTRACT FROM AUTHOR]

Bamboo-grained Cu 71.1 Al 17.2 Mn 11.7 microwires with diameter ∼150 μm were created by Taylor-Ulitovsky method and subsequent annealing. The grain architecture dependent superelasticity (SE) and elastocaloric effects (eCE) were confirmed. The bamboo-grained microwires exhibited low stress hysteresis loss during SE due to the reduced constraints caused by grain boundary, which is favorable for the mobility of martensite-austenite interface. Consequently, a large isothermal entropy change ∼ 21 J/kg K with a wide temperature range 90 K was achieved. The produced adiabatic temperature change (ΔT ad) reached −11.9 K under a maximum stress 400 MPa. A stable ΔT ad of −5.6 K showing little degradation in 200 eCE cycles was found with applied stress 325 MPa. This work reveals a fact that the bamboo-grained Cu–Al–Mn microwire may act as a desirable material for miniature solid-state refrigeration devices. Image 1 • Bamboo-grained Cu 71.1 Al 17.2 Mn 11.7 microwires were successfully prepared with reduced stress hysteresis (∼40 MPa). • A large isothermal entropy change ∼ 21 J/kg K with a wide temperature range of 90 K was achieved. • A large reversible adiabatic temperature change ΔT ad of −11.9 K was obtained at room temperature. • A stable ΔT ad of −5.6 K showing no degradation in 200 elastocaloric cycles was achieved at room temperature. [ABSTRACT FROM AUTHOR]