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Abstract: The effects of rare earth Dy addition on microstructure and martensitic transformation of polycrystalline Ni50Mn29Ga21−x Dy x (numbers indicate at.%) shape memory alloys were investigated by means of scanning electron microscopy, energy dispersive X-ray spectroscopy, differential scanning calorimetry and X-ray diffraction. The results show that the microstructure of Ni–Mn–Ga–Dy alloys consists of the matrix and Dy-rich phase. Small amounts of the Dy-rich phase with 0.1at.% Dy disperse homogeneously in the matrix. With the increase of the Dy content, the Dy-rich phase becomes larger and trends to distribute along the grain boundaries. The one-step martensitic transformation is observed in Ni50Mn29Ga21−x Dy x alloys. With increasing Dy content, the martensitic transformation temperatures of Ni50Mn29Ga21−x Dy x alloys increase remarkably, whereas the martensite structure of the Dy-containing alloys appears to be unchanged exhibiting seven-layered martensite structures at room temperature. [Copyright &y& Elsevier]

Abstract: Ni–Mn–Ga cantilever and tensile microactuators are investigated with respect to their thermoelastic and magnetomechanical characteristics. The base materials are either single-layer Ni–Mn–Ga thin films or Ni–Mn–Ga/Mo thin film composites with different compositions and thicknesses up to 10μm, which have been fabricated by magnetron sputtering. Differential scanning calorimetry measurements of the thin films show the characteristic features of ferromagnetic Heusler alloys exhibiting a martensitic transformation and a ferromagnetic transition well above room temperature. A pronounced temperature-induced shape memory effect is observed in deflection measurements of the microactuators. The maximum recoverable strain is about 0.8%. Magnetostriction measurements of cantilever actuators consisting of Ni–Mn–Ga/Mo thin film composites show a large increase of magnetostrain for decreasing Ni–Mn–Ga film thickness in the submicrometer range. [Copyright &y& Elsevier]

Reversible magnetic field-induced martensitic phase transformation is demonstrated for the first time in Ni2MnGa magnetic shape memory alloy single crystals under constant stress levels and low field magnitudes. Actuation stress levels larger than 25MPa were achieved providing an order of magnitude increase with respect to the values obtained from field-induced martensite reorientation. The necessary magneto-thermo-mechanical conditions for the field-induced phase transformation are discussed. [Copyright &y& Elsevier]

We present the results of an extensive Mössbauer study of the magnetic and martensitic transformation at room temperature of a polycrystalline alloy with a Ni55Fe19Ga26 nominal composition. From calorimetric measurements, we have determined the martensitic transformation temperature of T M ≈ 240 K, in good agreement with the one obtained by magnetic characterization. This sample has a Curie temperature of T C ≈ 287 K. Additional Curie temperatures, belonging to a γ phase, have been also detected. Mössbauer spectroscopy performed at different temperatures monitored all these transformations and the fitting of the obtained spectrum at the highest temperature allow us to give percentages of the different phases in the sample. [ABSTRACT FROM AUTHOR]

We present results of an extensive neutron diffraction study of the martensitic transformation in Shape Memory Alloys of nominal composition Ni_{55-x}Fe_{19+x}Ga_{26} (x=0, 1, 2) at. % transforming close to room temperature. The x=0 sample shows a martensitic transformation temperature T_{M}≈ 250 K and Curie temperature T_{C}≈ 289 K. Neutron measurements for this alloy were performed between 300 K (austenite phase) and 150 K (martensitic phase). At temperatures T>T_{C} we have found the coexistence of two phases: an L2_{1} phase with space group Fm3m and cell parameter a=5.730Å and a γ-fcc with cubic symmetry and with cell parameter a=3.586Å This second phase was also detected by magnetic measurements, giving rise to a second Curie temperature of about 363 K. [ABSTRACT FROM AUTHOR]

The martensitic magnetothermoelastic transformation induced by a magnetic field in FSMA has been studied. It has been observed that the shape memory effect is associated with the reversible magnetic field-induced martensite at higher temperatures. It has been noted that upon cooling from the high temperature austenite parent phase the ferromagnetic shape memory alloys exhibit transition from paramagnetism to ferromagnetism. The Ni-25Mn-25Ga (at%) shows the thermomechanical martensitic transformation. The micrograph shows the martensitic transformation of the alloy that strongly depends on the applied magnetic field. The mechanisms of the magnetic microstructure causes the martensite bands, which run diagonally from left to right in the micrograph, have been observed. It is proposed that the FSMAs are very promising candidates for biomedical applications as the new smart material. [ABSTRACT FROM AUTHOR]

Ferromagnetic shape memory Ni-Mn-Ga has been shown to exhibit deformations of up to 9.5% when driven with quasistatic fields. This article is focused on the use of Ni-Mn-Ga as the active element in a dynamic transducer consisting of a solenoid and a low-reluctance, closed magnetic path. Despite the absence of a restoring force in this configuration, we have shown in prior studies recoverable compressible strains of λ= -4100 μϵ, which are attributable to internal bias stresses built in the material during manufacture. In this study, we experimentally establish the presence of a modulus defect in Ni50Mn28.7Ga21.3, whereby the elastic modulus increases as much as 255% upon increasing the applied magnetic field from zero to 380 kA/m DC. Experimental measurements are conducted under both mechanical and magnetic excitation, and analyzed in combination with vibratory models for the system. While in our experiments the attractive magnetic forces between the transducer poles may contribute to the total modulus increase, the presence of a modulus change associated with the Ni-Mn-Ga element is substantiated. Dynamic Ni-Mn-Ga transducers offer an attractive mechanism for electrical tuning of the modulus, with potential application in active vibration absorption problems. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic shape memory properties of a single crystal Ni2MnGa alloy were characterized through monitoring magnetic field induced strain (MFIS) as a function of compressive stress, and applied stress induced strain as a function of magnetic field. Compressive stress and magnetic field were applied perpendicular to each other along the [100] and [011] axes, respectively. The critical magnetic fields for variant reorientation, first cycle effect and cyclic evolution of MFIS are reported as a function of stress level. It was revealed that increasing constant magnetic field level significantly increases the stress required for the reorientation, i.e., magnetostress and leads to superelasticity in martensite. Possible microstructural mechanisms, considering the interplay between stress and magnetic field favored martensite variants, magnetic domains and magnetization rotation, are proposed. Moreover, it was observed that the MFIS evolution is field rate dependent as was evidenced by a rate dependent two-stage reorientation where the maximum MFIS magnitude increases as the field rate increases. This effect was attributed to the difference between the nucleation and propagation barrier strength for twin boundary motion in NiMnGa alloys. The magnetostress (5.7MPa), blocking stress (5MPa) and maximum MFIS (5.8%) combination observed in this study is the highest reported to date in NiMnGa alloys. The high blocking and magnetostresses are a consequence of the low test temperature (−95°C) where the magnetocrystalline anisotropy energy is high and detwinning stress is low. Thus, for magnetic shape memory alloys, the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance since both magnetocrystalline anisotropy energy and detwinning stress are a strong function of temperature below the characteristic temperatures. [Copyright &y& Elsevier]

Abstract: Study on Ni–Mn–Ga ferromagnetic shape memory alloys recently keeps active. Intermartensitic transformation was found. Magnetic field enhanced phase transformation strain was discovered, and achieved up to 4%. Fifteen percent super high strain induced by variant reorientation under stress was obtained in non-modulated martensite. Six percent large magnetic field induced strain was achieved, and the temperature dependence was investigated in 5-layered martensite single-variant Ni–Mn–Ga alloys. Several other systems of magnetic shape memory alloys and high temperature shape memory alloy Ni–Mn–Ga are also reviewed. [Copyright &y& Elsevier]

Abstract: Martensite stabilization upon martensite ageing has been observed for many alloys undergoing thermoelastic martensitic transformation. In this paper, it is reported for the first time in a ferromagnetic Ni–Mn–Ga shape memory alloy. The stabilization increases with ageing time and temperature, and the activation energy of the process has been calculated. [Copyright &y& Elsevier]

Abstract: The effect of the fourth element Ta on the microstructure, crystal structure and martensitic transformation of (Co45Ni25Ga30)1−x Ta x (x =0,0.5,2,5) alloys was investigated. Some Ta-rich precipitates were observed in the higher Ta-addition samples, which lead to the decrease of the martensitic transformation temperatures and thermal hysteresis. In addition, 1423K annealing temperature is not high enough to impede the presence of the γ phase in Co–Ni–Ga–Ta alloys, and 1473K heat treatment temperature is needed. [Copyright &y& Elsevier]

Abstract: Three Ni–Fe–Ga ferromagnetic polycrystalline alloys having a martensitic transformation close to room temperature have been produced. Some results on their stress–strain behaviour under compression will be presented in this work. At temperatures below A f (finish of the reverse martensitic transformation), multiple steps in the stress–strain curves indicate the possibility of stress-induced martensite to martensite transformations. In this temperature domain, the critical stress to induce the variant reorientation and intermartensitic transformations is nearly temperature independent. In addition, the alloys show almost perfect strain recovery on unloading, that is the so-called rubber-like effect. As the temperature is increased the multiple step stress–strain curves evolve to a single-step transformation corresponding to stress-induced martensite from the parent phase. [Copyright &y& Elsevier]

This paper proposes a method to calculate the uniaxial stress–strain relationship of polycrystalline Fe–Pd, which has a martensite structure produced by cooling. Strain is caused by changes in the fractions of the three Bain corresponding variants which form the martensite structure within each grain. Internal stress and elastic energy are accumulated as straining proceeds and are caused by differences of eigenstrain (transformation strain) between differently oriented grains. The stress and elastic energy are evaluated on the basis of micromechanics. The stress acting on a grain due to surrounding grains is calculated using a mean field method. In this procedure, an averaging method is introduced, by which the mean field of grains having the same tensile direction, but having random lateral directions, are written in a closed form. The averaging method facilitates computations. It is shown that tension and compression differ in the stress–strain relationship. [Copyright &y& Elsevier]

The microstructure and texture evolution of 10M Ni-Mn-Ga melt-spun ribbons were thoroughly evaluated by high-energy synchrotron radiation and electron backscatter diffraction. The as-spun ribbons were subjected to annealing treatment in order to tailor microstructure, atomic order degree, and crystallographic texture. The optimum annealing treatment at 1173 K for 72 h produced a homogenous <100> fiber texture and induced grain growth to the size that spans the entire ribbon thickness. This in turn reduced microstructural constraints for twin variant reorientation in the direction perpendicular to the ribbon surface. On the other hand, a homogenous radial microstructure ensured in-plane stress/strain compatibility giving rise to strain accommodation during variant reorientation. Particular attention was also given to the evaluation of atomic order, which to the largest extent controls the characteristic transformation temperatures. It also lowered the twinning stress to a level sufficiently low for martensitic variant reorientation under magnetic field. As a result, 1.15% magnetic field-induced strain without the aid of mechanical training in the self-accommodated state was achieved. [Display omitted] [ABSTRACT FROM AUTHOR]

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K—which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples. [ABSTRACT FROM AUTHOR]

Polycrystalline Ni 50.4 Mn 27.3 Ga 22.3 alloy with<001>texture was prepared by conventional casting. The alloy exhibited enhanced magnetocrystalline anisotropy after superelastic training. Decent conventional magnetocaloric effect (MCE) with magnetic-field-induced entropy change Δ S = 9.2 J/kg·K under a magnetic field 30 kOe and inverse MCE with Δ S = 2.5 J/kg·K under 5 kOe were demonstrated when the field was applied parallel and perpendicular to<001>, respectively. As a result, large rotating MCE with Δ S = 4.2 J/kg·K under a low field of 20 kOe was achieved at room temperature. This shows that polycrystalline Ni-Mn-Ga alloys may act as promising magnetic refrigerants. [Display omitted] • The textured Ni 50.4 Mn 27.3 Ga 22.3 alloy obtained strong magnetocrystalline anisotropy after superelastic training. • The trained alloys showed decent inverse magnetocaloric effect 2.5 J/kg·K under 5 kOe perpendicular to< 001 >. • The large rotating magnetocaloric effect 7.3 J/kg·K obtained under 30 kOe. • The large RMCE was attributed to magnetostructural transition and strong uniaxial magnetocrystalline anisotropy. [ABSTRACT FROM AUTHOR]

Change in entropy (ΔS) has been estimated at the first order martensitic transition in selected Ni-Mn-X (Ga, Sn, and In) based ferromagnetic shape memory alloys (FSMAs). During the first-order martensitic structural phase transition, discontinuity in entropy leads to a large ΔS. ΔS can be estimated from differential scanning calorimeter curves. Magnetic characterization reveals that the magnetic contribution ΔSM, is dominant contributor to ΔS. ΔSM is also responsible for the giant magnetocaloric effect observed in these FSMAs. By controlling the valence electron to atom ratio (e/a), it is possible to realize a large entropy change near ambient temperature in these alloys. Estimation of ΔS and ΔSM in a variety of promising FSMAs provides a means to identify alloys suitable for applications such as magnetic sensors, actuators and magnetic refrigeration. [ABSTRACT FROM AUTHOR]

Ferromagnetic shape memory alloys (FSMA) have the possibility to induced a strain by applying a magnetic field. The main advantage of the FSMA is that the strain cycling frequency is two orders of magnitude higher than coventional shape memory alloys. The best alloy showing this effect is the Ni-Mn-Ga system, with a high mobility of its martensite variants and high magnetocrystalline anisotropy constant. Nevertheless, due to the high brittleness of this alloy, other systems (Ni-Fe-Ga, Co-Ni-Al, Co-Ni-Ga, ...) are being investigated as an alternative to Ni-Mn-Ga. In the current work, Ni-Fe-Ga-Co single crystals have been studied. In spite of the formation of L10 martensite (low mobility of the variants), the [001] crystals exhibited magnetic-field-induced strains (in tension) larger than 2%, under an assisting tensile stress around 16 MPa and fields below 15 kOe. In martensitic samples previously compressed, application of a constant tensile stress along the same axis together with a perpendicular magnetic field produces the elongation of the sample by variant reorientation, as one of the variants rotates its c axis from the field direction to the stress-axis direction. An estimated magnetostress of ~0.8 MPa is in good agreement with the theoretical value given by the ratio of magnetocrystalline anisotropy constant and twinning shear.

Kinetic arrest behavior is an interesting phenomenon that magnetic martensitic transition is suppressed during field cooling. A metastable "austenite" is arrested at very low temperatures upon withdrawing magnetic field after field cooling. However, why such a kinetically arrested "austenite" only appears in limited compositions has not been interpreted so far in ferromagnetic shape memory alloys. To settle this issue, the phase transforming properties and phase diagram of Ni 37 Co 11 Mn 52-x Sn x (x=8.5~10) system are explored. We found that kinetically arrested "austenite" is a special frozen strain glass state, which only appears within the martensite/strain glass phase boundary composition undergoing two-step transitions, i.e., austenite to strain glass and to martensite transition sequences. The appearance of kinetic arrest behavior is due to the comparative competition between the kinetic hindrance and martensitic driving force. The comprehensive understanding of the kinetic transforming properties of these ferromagnetic shape memory alloys is beneficial for tuning their functionalities. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this work, we studied the effect of doping with copper to seek improvement in mechanical and photo induced micro actuation (PIMA) properties of a Co-Ni-Al ferromagnetic shape memory alloy (FSMA). Unique and unexplored PIMA property of FSMA system has opened up the scope of various laser controlled micro-mechanical engineering applications in miniature scale. Transmission Electron Microscopy and X-ray diffraction technique were used to characterize and correlate the evolved microstructure with the corresponding changes in the PIMA and mechanical properties. Magnetic properties of these samples were studied by performing hysteresis experiments at room temperature. It was found that partial substitution of cobalt with copper resulted in samples with lower magnetic and PIMA properties but better mechanical property, however, in case of substituting aluminum, it resulted in improvement of the ferromagnetic property but destroyed the PIMA effect altogether. In order to understand these properties changes, the electron density of each of the samples was calculated and compared. These studies should help in deciding on use of new alloys with optimization of PIMA and mechanical property for desired and customized engineering applications. Image 1 • Effect of copper doping on photo induced micro actuation property of Co-Ni-Al FSMA. • Effect of copper doping on mechanical properties of Co-Ni-Al FSMA. • Microstructural evolution of copper doped Co-Ni-Al FSMA due to doping at different sites. • Dependence of PIMA effect on shape memory effect, magnetic properties and structure of copper doped Co-Ni-Al FSMA. [ABSTRACT FROM AUTHOR]

The present work focused on the microstructural, thermal, electrical, and damping characterization of NiMnGa samples produced through a powder pressing and a sintering process; the effect of sintering times and of the starting powder size were evaluated. Moreover, an observation of the evolution of martensitic transformation typical of NiMnGa ferromagnetic shape memory alloy was conducted in comparison with the cast material behavior and in correlation with the material densification. The optimum powder size and sintering time for the process, i.e., 50 µm or lower and 72 h, were identified considering the investigated physical properties of the sintered samples in comparison to the cast material. The corresponding sample showed the best compromise between density, thermal and electrical properties, and damping and functional behaviour. In general, the outcomes of this study could be the basis of a useful tool for production processes that include a sintering step as well as being a starting point for the evaluation of an alternative low cost fabrication method of this alloy. [ABSTRACT FROM AUTHOR]

Abstract: Martensitic transformation, mechanical property and magnetic-field-induced strain of Ni–Mn–Ga alloys obtained using the spark plasma sintering method have been investigated. The results show that the martensitic transformation of the sintered specimen is basically similar to that of conventional bulk alloys. The ductility of sintered Ni–Mn–Ga alloys is significantly enhanced compared to the specimen obtained by conventional melting technique. The highest fracture strain so far is reported. Moreover, magnetic-filed-induced strain in sintered Ni–Mn–Ga alloys has been studied for the first time. [Copyright &y& Elsevier]

Abstract: A new method for propulsion using a Ni 2 MnGa ferromagnetic shape memory flapper is introduced. We optically examine the magnetic field induced strain of pure shear by means of a state of the art generator that provides alternating magnetic fields of 7000Oe at frequencies of up to 100Hz. Preliminary measurements show local shear deformation of about 5%, which open new frontiers in propulsion mechanisms. [Copyright &y& Elsevier]

Abstract: A Ni56Mn27Ga17 ferromagnetic shape memory alloy thin film was deposited onto a thermal substrate of 673K by using radio frequency (RF) magnetron sputtering technique. The crystallographic structure and microstructures of the thin film are investigated by X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The results show the thin film is in martensitic state with a non-modulated tetragonal structure at room temperature. The martensite variants in the thin film are twinned relationship with (212) I type twin. The nanotwinned structures for non-modulated martensite variants are revealed at atomic level. [Copyright &y& Elsevier]

Ni 50 Mn 34.7 In 15.3 alloys based on differently sized particles were prepared by spark plasma sintering. Compared with the arc-melted alloy, the compressive strength and fracture strain of the Ni 50 Mn 34.7 In 15.3 alloys were tremendously increased via SPS sintering. Simultaneously, their magnetocaloric effects were enhanced by increase the size of particles for SPS sintering. The compressive strength and fracture strain of the sintered alloy were better than 1050 MPa and 12.5%, respectively. A large adiabatic temperature change up to −3.2 K was achieved under a 1.5 T magnetic field. The present work offers an approach to simultaneously improve the mechanical properties and magnetocaloric effects of Ni-Mn-In alloys. Image 1 • The mechanical properties and magnetocaloric effects of Ni 50 Mn 34.7 In 15.3 alloys were simultaneously enhanced. • The magnetocaloric effects and mechanical properties of the Ni 50 Mn 34.7 In 15.3 alloys were dependent on the particle size. • The compressive strength and fracture strain of the Ni 50 Mn 34.7 In 15.3 alloy were better than 1050 MPa and 12.5%, respectively. • The adiabatic temperature change Δ T ad up to −3.2 K were achieved under a 1.5 T magnetic field. [ABSTRACT FROM AUTHOR]

Damping materials have attracted much attention for wide potential applications in the industry. Previous research shows that annealing treatment is an effective and costless way of improving the functional properties of conventional shape memory alloys. However, there are few investigations concerning the annealing effect of the ambient-temperature damping behavior. In this paper, we present the influence of annealing treatment on the martensitic transformation and damping behaviors of Ni 55 − x Cu x Mn 25 Ga 20 (x = 0, 2, 4, 6) alloys within the ambient-temperature range. With increasing annealing time, the martensitic transformation temperature and the temperature span of martensitic transformation decrease. Moreover, annealing treatment greatly enhances the twin boundary damping peak of martensite. The X-ray diffraction (XRD) measurement demonstrates that annealing can improve the degree of L2 1 atomic order, which relieves the pinning effects for the twin boundary motion and thus leads to the enhancement of the twin boundary damping of these alloys. [ABSTRACT FROM AUTHOR]

With the purpose to optimize the functional properties of Heusler alloys for their use in solid-state refrigeration, the characteristics of the martensitic and magnetic transitions undergone by Ni50Mn25−xGa25Cux (x = 3–11) alloys have been studied. The results reveal that, for a Cu content of x = 5.5–7.5, a magnetostructural transition between paramagnetic austenite and ferromagnetic martensite takes place. In such a case, magnetic field and stress act in the same sense, lowering the critical combined fields to induce the transformation; moreover, magnetocaloric and elastocaloric effects are both direct, suggesting the use of combined fields to improve the overall refrigeration capacity of the alloy. Within this range of compositions, the measured transformation entropy is increased owing to the magnetic contribution to entropy, showing a maximum at composition x = 6, in which the magnetization jump at the transformation is the largest of the set. At the same time, the temperature hysteresis of the transformation displays a minimum at x = 6, attributed to the optimal lattice compatibility between austenite and martensite. We show that, among this system, the optimal caloric performance is found for the x = 6 composition, which displays high isothermal entropy changes (−36 J·kg−1·K−1 under 5 T and −8.5 J·kg−1·K−1 under 50 MPa), suitable working temperature (300 K), and low thermal hysteresis (3 K). [ABSTRACT FROM AUTHOR]