Search

NiMnGa Heusler alloy plays a key role as reference system for ferromagnetic shape memory alloys (FeSMAs) and their peculiar functional properties including large magnetic-field-induced strain, magnetocaloric and elastocaloric effects. Moreover, the microstructure of polycrystalline NiMnGa alloys has been investigated and optimized to improve the mechanical properties and to reduce their typical brittleness. For this reason, increasing interest has been devoted to different kinds of fabrication routes for this alloy, such as powder metallurgy processes. In the present study, a polycrystalline Ni50Mn30Ga20 (atomic %) alloy is produced by means of an unconventional sintering method that involves the canning of powders and the subsequent processing by hot rolling. This process was implemented according to the results obtained by means of the open die pressing (ODP) sintering of NiMnGa which was investigated in a previous work. The present study is aimed at developing an alternative and cost-effective sintering method for the consolidation of fully-dense NiMnGa samples. The process allowed reducing the intrinsic brittleness of the alloy. A heat treatment at 925 °C for 6 h was applied and allowed achieving a maximum adiabatic ΔT of +6.3 °C and - 4.5 °C with a strain of 4 % and a strain rate of 400 %/min in compression. The novel method led to very promising elastocaloric properties, making NiMnGa a suitable candidate for solid-state cooling and heating applications.

Shape-memory alloys set high demands on the production technologies being used. During cutting, continuous heat input and mechanical stress have an undesirable influence on the shape-memory effect. Pulsed electrochemical machining (PECM), which is based on anodic dissolution, enables force-free machining without thermomechanical influence on the edge-zone properties of the workpiece. Depending on the desired geometry, the development of a customized PECM fixture is necessary. The design of the fixtures is often based on the experiences of the designers and manufacturers, which often results in an estimation of the functionally critical dimensions. For this reason, the study focuses on a methodical approach for evaluating crucial fixture dimensions using knowledge of the specific material dissolution behavior linked with a numerical simulation model. It has been shown that the shape-memory alloy NiMnGa has a non-linear dissolution behavior in sodium nitrate. A reduction of stray currents up to 20% resulting from a lateral gap between the cathode and electrical insulation was demonstrated using numerical simulation. The study shows that a low cathode shaping height has the strongest influence on precise processing. Varying the process parameters allowed for the lateral gap to be adjusted between 0.15 and 0.25 mm. [ABSTRACT FROM AUTHOR]

To improve gas hydrate storage and transportation technology, ferromagnetic intermetallic compound NiMnGa particles with martensitic transformation endothermics were used to form micro/nanofluids with sodium dodecyl sulfate (SDS) to further strengthen the gas hydration process. In this work, the kinetic process of gas hydration in NiMnGa fluids with different concentrations (0, 0.1, 1, 2, and 3 wt.%) was investigated using a rotating magnetic field gas hydration separation experimental setup. The results show that the induction time of the 3 wt.% NiMnGa system was shortened by 98.3%, the gas consumption was increased by 50.5%, and the gas consumption rate was increased by 351.9% compared with the SDS system. Therefore, it is inferred from the mass transfer that NiMnGa micro/nanofluids can accelerate the formation of hydrates. [ABSTRACT FROM AUTHOR]

Shape-memory alloys set high demands on the production technologies being used. During cutting, continuous heat input and mechanical stress have an undesirable influence on the shape-memory effect. Pulsed electrochemical machining (PECM), which is based on anodic dissolution, enables force-free machining without thermomechanical influence on the edge-zone properties of the workpiece. Depending on the desired geometry, the development of a customized PECM fixture is necessary. The design of the fixtures is often based on the experiences of the designers and manufacturers, which often results in an estimation of the functionally critical dimensions. For this reason, the study focuses on a methodical approach for evaluating crucial fixture dimensions using knowledge of the specific material dissolution behavior linked with a numerical simulation model. It has been shown that the shape-memory alloy NiMnGa has a non-linear dissolution behavior in sodium nitrate. A reduction of stray currents up to 20% resulting from a lateral gap between the cathode and electrical insulation was demonstrated using numerical simulation. The study shows that a low cathode shaping height has the strongest influence on precise processing. Varying the process parameters allowed for the lateral gap to be adjusted between 0.15 and 0.25 mm.

Coalbed methane is an important renewable energy source. Gas hydration technology is a new method for enhancing the utilization of coalbed methane and reducing environmental pollution. Long induction periods, sluggish formation rates, low hydrate yields, and difficulty removing heat during hydrate formation are all issues with gas hydration technology. In this paper, 3 wt% NiMnGa (NMG) phase-change micro/nanoparticles and 0.05% sodium dodecyl sulphate (SDS) were compounded, and gas hydration experiments were conducted under various initial pressures and gas sample conditions to investigate. The findings revealed that NMG has efficient mass transfer properties as well as phase-change heat absorption properties, which significantly improved the kinetic process of the gas hydrate by mass and heat transfer, shortened the induction time, increased gas consumption, and increased the gas consumption rate during the rapid hydrate growth period. When the initial pressure was 6.2 MPa, the induction time was reduced by 89.26%, 92.48%, and 95.64%, and the maximum gas consumption rate was increased by 238.18%, 175.55%, and 113.60%, respectively, when using different concentrations of methane in the NMG-SDS system compared to the pure SDS system. The NMG used in this paper showed potential for future use in mixed gas hydration technology.

• A crystal-plasticity based constitutive model is established in the framework of finite strain and irreversible thermodynamics. • Three types of martensite phases, 5M, 7M and NM are introduced and five different martensite transformation processes, the transformations between A and 5M, A and 7M, A and NM, 5M and 7M, 7M and NM phases are considered. • The proposed model can reasonably describe the deformation behaviors of single crystal NiMnGa ferromagnetic shape memory alloy at various temperatures. Existing experimental results have shown that multi-step martensite transformation can be induced by the applied stress in single crystal NiMnGa ferromagnetic shape memory alloys (FSMAs) and the transformation property depends on the ambient temperature strongly. In this work, a crystal-plasticity based constitutive model is established to quantify such a phenomenon. Based on the crystallographic symmetry, the lattice structures and eigenstrains of austenite (A) and three different martensite phases, i.e., the modulated martensite phases with a periodicity of 5 lattice cells (5M) and a periodicity of 7 lattice cells (7M), and a non-modulated (NM) martensite phase are analyzed. A three-dimensional (3D) single crystal constitutive model is constructed in the framework of finite strain by considering five different martensite transformation processes, i.e., the transformations between A and 5M, A and 7M, A and NM, 5M and 7M, and 7M and NM phases. Thermodynamically consistent kinetic equations for the five different martensite transformation processes are proposed based on the constructed Helmholtz free energy and Clausius's dissipative inequality. The capability of the proposed model to describe the tensile stress-induced multi-step martensite transformation of single crystal NiMnGa FSMA in [100] crystallographic orientation is verified by comparing the predictions with corresponding experimental results. Further, the proposed model is used to predict the deformations of the single crystal under the tension and compression in various crystallographic orientations. [ABSTRACT FROM AUTHOR]

Highlights • Ti fully dissolves in NiMnGa-based single phase structure. • Gd addition promotes dual-phase microstructure in the NiMnGa-based alloy. • Ti for Ga substitution leads to the decrease of the martensitic transition temperature. • Gd shifts the martensitic transition temperature to the room temperature regime. • Ti and Gd improved the mechanical properties of the NiMnGa-based alloy. Abstract The present study is focused on the influence of Ti and Gd doping at the Ga site on the microstructure, magnetic and mechanical properties of the polycrystalline Ni 50 Mn 25 Ga 20-x Z x (x = 0 or 5, Z = Gd, Ti) magnetic shape memory alloys. Microstructure investigations show that reference Ni 50 Mn 25 Ga 25 and Ti-doped alloys, both in the as-cast and annealed state, are single phase materials, whereas Gd-doped sample reveals dendritic dual-phase structure with substantial distinction between Gd-rich and Gd-poor regions. Thermomagnetic measurements expose reversible martensitic transition in the Ni 50 Mn 25 Ga 25 and Ni 50 Mn 25 Ga 20 Ti 5 alloys, where Ti addition to NiMnGa composition leads to the decrease of phase transformation temperature from T M = 193 K for reference sample to T M = 172.5 K for Ti-doped material. Furthermore, the Ni 50 Mn 25 Ga 20 Gd 5 alloy does not experience fully martensitic transition. Temperatures of magnetic transformation also varies with chemical composition and equals to 379 K, 318 K and 370 K for the annealed Ni 50 Mn 25 Ga 25 , Ni 50 Mn 25 Ga 20 Ti 5 and Ni 50 Mn 25 Ga 20 Gd 5 alloy, respectively. Mechanical properties investigation based on the nanoindentation measurements shows beneficial influence of doping elements on material hardness. In addition, planar distributions of hardness allow to deconvolute mechanical properties of each individual phase of the Ni 50 Mn 25 Ga 20 Gd 5 alloy. [ABSTRACT FROM AUTHOR]

A miniature-scale device for harvesting of vibrational energy is presented, which is based on the inverse magnetic shape memory effect caused by stress-induced reorientation of magnetic moments. The active material is a bulk NiMnGa sample of 1 mm × 2.5 mm × 4.5 mm. A new harvester design is developed consisting of a moving magnetic circuit of mass M mounted on a spring to convert external vibrations to the active material. Thus, the deformation of the NiMnGa sample can be adjusted by tailoring the design parameters (mass M, sample dimensions, spring constant, etc.) for a given vibration source. The design and performance of first prototypes are presented showing an output power of 1.5 μW during a single load cycle at an optimum biasing magnetic field of 0.35 T. [ABSTRACT FROM AUTHOR]

• Phase transformation of Ni 52.5 Mn 22.5 Ga 25 particle keeps stable below 15 MPa. • Structure of Ni 52.5 Mn 22.5 Ga 25 particles keep stable during the gas hydration. • NiMnGa-SDS systems reduce the reaction temperature and time of NGH. Ferromagnetic shape memory alloy NiMnGa was used to prepare the micro/nano fluid which has a potential role during the formation of natural gas hydrate (NGH). In the present work, phase transformations of Ni 52.5 Mn 22.5 Ga 25 micro/nano particles under various pressures, and the crystal structure before and after participating in the natural gas hydration were investigated. The effect of the micro/nano fluid containing Ni 52.5 Mn 22.5 Ga 25 particles on the formation heat of NGH was studied. It was worked out that Ni 52.5 Mn 22.5 Ga 25 particles exhibit a high stability both in phase transformation below 15 MPa pressure and crystal structure after natural gas hydration. The fluid containing Ni 52.5 Mn 22.5 Ga 25 particles can reduce the hydration reaction temperature and promote the formation of NGH. [ABSTRACT FROM AUTHOR]

Using surface acoustic waves (SAW) the elastic properties of 500 nm thick epitaxial off-stoichiometric Ni-Mn-Ga films prepared by sputtering on MgO substrate were determined in a broad temperature range both in austenite and martensite and across the martensitic transformation. At room temperature the films were in non-modulated martensite with tweed-like microstructure. Structural and magnetic studies proved that the martensitic structure can be considered homogenous on the scale of surface waves owing to the fine twinning. Measured angular dependence of SAW velocities indicated that the averaged in-plane tetragonal structure can be used for the calculation of elastic coefficients in martensite. The phase velocity of SAW showed an anomaly around the phase transition temperature due to changes of the shear elastic coefficients, c ' and c 44 . During cooling there was a gradual softening of c ' in austenite. The softening turned to stiffening at the onset of the martensitic transformation while c 44 sharply decreased. The shear coefficients for martensite and paramagnetic austenite of the film are comparable to Ni-Mn-Ga bulk single crystal. No significant effect of magnetic field on elastic coefficients was detected in agreement with the bulk. [ABSTRACT FROM AUTHOR]

In this article, a previously developed constitutive model for ferromagnetic shape memory alloys is phenomenologically enhanced using experimental observations. A modified phase diagram along with a method for calibration of the required model parameters is further presented. The model is implemented into a user material subroutine to equip commercial finite element software ABAQUS with the capability of simulating magneto-mechanical behaviors of ferromagnetic shape memory alloys. A combined convergence scheme is employed to solve the implicit equations. The proposed model together with the presented numerical solution is shown to be able to study shape memory effect and pseudoelasticity at different constant magnetic fields. The simulated magnetic loading/unloading cycles at different constant stresses are found to be well-fitted to the experimental findings. As a practical application of the ferromagnetic shape memory alloy coupled magneto-mechanical response, a spring actuator (a bias spring serially connected to one ferromagnetic shape memory alloy element) is investigated, and the numerical predictions are shown to be in a good agreement with available experimental results. As a novel case, geometrically graded NiMnGa elements are also introduced and are simulated with the use of this approach. [ABSTRACT FROM AUTHOR]

Mechanical anisotropy in an austenitic ferromagnetic shape memory alloy (SMA), Ni50Mn26.25Ga23.75, is investigated along (010), (¯120), (¯121), (231¯) and (232) using nanoindentation. While (010) exhibits the highest reduced modulus, Er, and hardness, H, (232) shows the lowest amongst the grain orientations examined in this study. The significant elastic anisotropy measured is attributed to differences in planar packing density and number of in-plane Ni-Mn and Ni-Ga bonds, whereas the plastic anisotropy is due to the differences in the onset of slip, which is rationalized by recourse to Schmid factor calculations. This would help determine the grain orientations in austenitic NiMnGa which exhibit better mechanical properties for SMA applications such as improving vibration damping characteristics of the alloy. [ABSTRACT FROM AUTHOR]

Interaction of conjugate twin boundaries under applied stress was studied using electron backscatter diffraction in the scanning electron microscope. Compression and unloading resulted in a recoverable strain. EBSD maps and BSE images reveal a branched needle microstructure inside and outside the central triangle domain. Type I twin boundaries are mostly observed inside the central domain while some type II twin boundaries can be predominantly detected outside. Moreover, a characteristic branching of modulation domains close to the intersection of conjugate boundaries is observed. A mechanism of twin annihilation based on the rigid body rotation is proposed. The shape and distribution of stress-induced twins is explained by minimizing of elastic energy. [ABSTRACT FROM AUTHOR]

An electrochemical synthesis route for NiMnGa alloys is presented. Thin films of NiMnGa were fabricated by single step electrodeposition from aqueous electrolytes using direct current over a range of current densities. By electrolyte tuning, homogeneous films with high Ga and Mn content could be achieved at current densities as high as −400 mA cm −2 . Detailed compositional analysis of the alloys showed that growth was homogeneous and oxygen content was minimized. Films plated at very low current densities were found to be nanocrystalline/amorphous. In order to obtain fully crystalline samples, thermal annealing was carried out. Mechanical characterization was assessed by nanoindentation, and the effect of Ga content on mechanical properties was investigated. [ABSTRACT FROM AUTHOR]

The present investigation addresses the magnetocaloric behaviour in a series of Ni 77− x Mn x Ga 23 ( x =23, 24, 25, 27 and 29) rapidly solidified alloys prepared in the form of ribbons by melt spinning technique. The approach of the study is to identify the off-stoichiometric composition wherein room temperature magneto-structural transformation is achieved. The alloy chemistry was tailored through Mn incorporation for Ni such that the magnetic and structural transitions were at close proximity to achieve highest entropy value of Δ S equal to 8.51 J Kg −1 K −1 for #Mn 24 ribbon measured at an applied field of 3 T. When such transitions are more staggered as in #Mn 29 the entropy value of ribbon reduced to as low as 1.61 J Kg −1 K −1 . Near room temperature transformations in #Mn 24 ribbon have been observed through calorimetric and thermomagnetic evaluation. Reverse martensitic transformation (martensite→autstenite) temperature indicates not only distinct change in the saturation flux density but also an inter-martensitic phase. Microstructural analysis of #Mn 24 alloy ribbon revealed structural ordering with the existence of plate morphology evidenced for martensitic phase. [ABSTRACT FROM AUTHOR]

Magnetic domains structure of a Ni–Mn–Ga single crystal was analyzed using a magneto-optical indicator film (MOIF), magnetic force microscopy (MFM), and scanning electron microscopy (SEM) (Lorenz contrast). Thanks to magnetically induced reorientation, the different configurations of magnetic domains can be obtained in the same place of the crystal. MOIF gives the overall domain configuration, while MFM shows the details of the structure revealing the domain branching. The magnetic domain structure in the vicinity of the mobile single twin boundary of Type II was observed by SEM and related to the complex twin structure of the martensite obtained from the calculation. [ABSTRACT FROM AUTHOR]

Abstract: Mechanical anisotropy in an austenitic ferromagnetic shape memory alloy (SMA), Ni50Mn26.25Ga23.75, is investigated along (010), (120), (121), (231) and (232) using nanoindentation. While (010) exhibits the highest reduced modulus, Er, and hardness, H, (232) shows the lowest amongst the grain orientations examined in this study. The significant elastic anisotropy measured is attributed to differences in planar packing density and number of in-plane Ni–Mn and Ni–Ga bonds, whereas the plastic anisotropy is due to the differences in the onset of slip, which is rationalized by recourse to Schmid factor calculations. This would help determine the grain orientations in austenitic NiMnGa which exhibit better mechanical properties for SMA applications such as improving vibration damping characteristics of the alloy.

Abstract: We have prepared composite nanowires consisting of In2O3 and NiMnGa, in which the In2O3 core was coated with a continuous layer of NiMnGa via pulsed laser deposition technique. We have verified the tubular structure of NiMnGa shell by means of transmission electron microscopy (TEM). X-ray photoelectron spectroscopy revealed the presence of Ni, Mn, and Ga elements and TEM-energy-dispersive X-ray spectroscopy indicated that these elements resided mainly in the shell region. Since X-ray diffraction patterns and lattice-resolved TEM images showed no crystalline NiMnGa phase, with scanning electron microscopy revealing that the shell is sufficiently thick (~55nm), we suggest that the shell layer is amorphous. The ferromagnetic behavior, in terms of coercive field, remanent magnetization, and saturation magnetization in the hysteresis loop, was considerably enhanced by coating the In2O3 core with a NiMnGa shell layer, at the measurement temperature of 300K as well as 5K. We have carried out a comparison study for revealing the mechanisms, by which the ferromagnetism was enhanced by the shell-coating. Although the annealing-induced changes of In2O3 core cannot be ruled out, the presence of NiMnGa phase played a decisive role in enhancing the ferromagnetic behavior of composite nanowires. [Copyright &y& Elsevier]

In this study, the thermal and microstructural properties of the first and second types of NiMnGa alloys were compared by using XRD and DSC measurements. According to the XRD measurements, these alloys exhibits both a martensite phase with tetragonal structure and a γ phase. The results show that the second type of NiMnGa alloy has a higher intensity of the main peak, with indices of (222), than the first type of FSMA has. This means that the crystal structure of the second type of alloy is more ordered. Similarly, the DSC curves of the two types of alloys show some differences. When heating and cooling, there are two peaks in the DSC curve of the first type of alloy, while the second type of alloy has only one peak. For the first type of alloy, the initial peak is an indicator of martensitic transformation and the other peak is that of Curie temperature. For the second type of alloy, the major peak shows the martensitic transformation temperature. The highest enthalpy change and activation energy are seen for the second type of alloy. [ABSTRACT FROM AUTHOR]

The phase transition, Curie temperature and magnetic properties of NiMnGa alloys represented by S1, S2, S3, and S4 in different atomic percentage were investigated from TG/DTA and PPMS measurements. It was found that the electron concentration ( e/ a) ratio of S1 and S4 as well as S2 and S3 are equal to each other. The transformation temperatures and magnetization values are similar for these alloys having the same e/ a. Results show that room temperature magnetization depends on valence electron concentration of alloys and saturation of alloys increases with the decrease of e/ a ratio. The measured coercivity values for all alloys were above 125 Oe and they had a long saturation field, i.e., above 10000 Oe, by exhibiting, the characteristics of hard magnetic materials. [ABSTRACT FROM AUTHOR]

Abstract: Oxidation behavior of NiMnGa alloy has been investigated under isothermal temperature by thermo gravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy equipped with an energy dispersive X-ray (SEM–EDX) spectroscope systems. The Ni–28.5Mn–20.5Ga alloy (composition in atomic percent) was exposed to oxygen atmosphere isothermally, i.e., between 600°C and 1000°C, for 1h. A gravimetric method was used to determine the oxidation kinetics; weight gain per unit area as a function of time. It was determined that the oxidation constant increases significantly with isothermal temperature. Activation energy of the oxidation was found to be 152kJ/mol. X-ray diffraction patterns of the heat-treated samples contain oxide peaks, mainly belonging to Mn3O4. X-ray analyses demonstrate that the amount of the oxide increases with isothermal temperature while that of martensite phase decreases. The scanning electron microscopy equipped with an energy dispersive X-ray (SEM–EDX) spectroscope analysis also gives the same result. According to magnetic measurements, the saturation of NiMnGa alloys decreases with rising isothermal oxidation temperature. [Copyright &y& Elsevier]

Abstract: We have studied the role that vacancy type defects play in the martensitic transformation of Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in five ternary alloys. Three of them transform to modulated and two to non-modulated martensitic phases. With these five samples we cover a large range in composition. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures and up to a maximum temperature of 600°C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in non-modulated samples. However, the response in the modulated samples is quite different. The results are discussed in term of different type of positron trapping defects and their evolution with the annealing temperature. The present work shows a correlation between vacancy concentration and martensitic transformation temperature of ferromagnetic shape memory alloys. [Copyright &y& Elsevier]

Distinctive features of plastic deformation of a magnetic NiMnGa alloy are studied by the method of local dosed deformation. It is found that the martensite phase of an NiMnGa single crystals is susceptible to mechanical twinning. Here twins represent thin lenses formed away from the imprint of the indenter. [ABSTRACT FROM AUTHOR]

Abstract: Ni–Mn–Ga magnetic shape memory alloy films have been prepared by the DC magnetron sputtering technique. As-deposited films show a quasi-amorphous structure that crystallizes at ~500K. Crystallization study using Kissinger''s analysis reveals a relatively low activation energy indicating partial crystallinity in the films. In situ X-ray diffraction studies show reversible martensite phase transformations, and phase segregation to non-transforming L12 precipitates at higher temperatures. It was observed that the phase segregation can be suppressed by low temperature heat treatment. [Copyright &y& Elsevier]

NiMnGa based ferromagnetic alloys are ideal candidates for applications such as actuators, magnetic refrigerators or magnetostrictive transducers due to their attractive properties such as magnetic field induced shape memory effect and large magnetocaloric effect. The properties of these alloys (e.g., the martensitic transformation temperature T) sensitively depend on the composition. Understanding the composition dependence of these properties so as to design the alloy as desired is one of the main research topics in this area. In recent years, we have investigated the composition dependent elastic modulus and phase stability of NiMnGa-based alloys by using a first-principles method, in hope of clarifying their connection to the properties of these alloys. In this article, we review the main results of our investigations. We show that the tetragonal shear modulus C′ is a better predictor of the composition dependent T than the number of valence electrons per atom ( e/ a) since the general T∼ C′ correlation works for some of the alloys for which the T∼ e/ a correlation fails, although there exist several cases for which both the general T∼ C′ and T∼ e/ a correlations break down. Employing the experimentally determined modulation function, the complex 5-layer modulated (5M) structure of the martensite of NiMnGa and the Al-doping effect on it are studied. We find that the shuffle and shear of the 5M structure are linearly coupled. The relative stability of the austenite and the martensites is examined by comparing their total energies. The non-modulated martensite β″′ with the tetragonality of the unit cell c/ a>1 is shown to be globally stable whereas the 5M martensite with c/ a<1 is metastable. The critical Al atomic fraction over which the martensitic transformation between the 5M martensite and austenite cannot occur is predicted to be 0.26, in reasonable agreement with experimental findings. [ABSTRACT FROM AUTHOR]

This paper reports on a novel approach of using magnetic shape memory (MSM) alloys as smart materials in mechanical sensing. The presented concept enables the measurement of tensile strain. The underlying sensor mechanism is based on the reorientation of the martensite variants of the tetragonal unit cell of the MSM alloy (MSMA) which can be induced by either a magnetic field or a mechanical stress. In the novel concept, a magnetic bias field exerts a restoring force opposing the externally applied mechanical stress. The magnetic stray field at the rim of an MSMA sample, measured using a conventional Hall sensor, is used as the sensor signal. The operating principle was experimentally verified using a custom-made tensile test setup. A linear dependence of the magnetic flux density of the stray field on the mechanical strain was observed. [ABSTRACT FROM PUBLISHER]

Abstract: The investigation addresses the effect of Mn incorporation for Ni on the properties of a series of Ni77−x Mn x Ga23 (x=22–29; at%) ferromagnetic shape memory alloys prepared in the form of ribbons by a melt spinning technique. Phase transformation studies in these ribbons by differential scanning calorimetry revealed that austenitic start and martensitic start temperatures decreased with the increase in Mn content. The Curie temperature (T C) of these alloys determined from thermal variation of magnetisations was found to rise with increasing Mn content. The martensitic transformation temperatures were above T C in low Mn containing (x=22 and 23) alloys. Morphology observed through transmission electron microscopy manifested complex martensitic features in the alloy with x=22 while x=29 had an austenitic phase. The alloys with intermediate Mn content (x=24, 25) had overlapping magnetic and martensitic transformations close to room temperature. The thermal lag between austenitic and martensitic characteristic temperatures in these alloys has been corroborated to their structural state. X-ray diffraction indicated a predominant martensite phase and austenite phase in low and high Mn containing alloys respectively. In-situ diffraction studies during thermal cycle indicate martensite–austenite transformations. [Copyright &y& Elsevier]

The change in microstructure during training of a Ni50Mn29Ga21 bicrystal was investigated by electron backscatter diffraction in a scanning electron microscope. The martensitic variants form macro and micro twins which are twin related with each other. The twin plane is of type. To achieve magnetic field induced strain in Ni–Mn–Ga alloys, a training process was applied. This process consists of successively compressing the sample along two axes. As a result the twinning stress is reduced and the twinning strain is maximized. [Copyright &y& Elsevier]

The texture in a polycrystalline Ni50Mn29Ga21 magnetic shape-memory alloy deformed by high-pressure torsion (HPT) at 1173K was investigated with high-energy synchrotron radiation. HPT changed the extrusion texture to a predominant slightly rotated cube {001} 〈100〉 and a weak F {110}〈100〉 component. During HPT the grain structure coarsened. Phase transformations during cooling from the deformation temperature led to coarse and fine sets of martensitic twin variants. The results are discussed with respect to deformation mode, grain growth and phase transformations. [Copyright &y& Elsevier]

Abstract: Magnetic shape memory (MSM) alloys are a new class of actuator materials with high actuation frequency, energy density and strain. MSM effect occurs in alloys, which exhibit a martensitic transformation and are ferromagnetic. It involves, under effect of magnetic field, a high strain achieved via reorientation of twinned martensite plates. The major problem is that even a slight change in the alloy’s composition causes drastic changes in the martensitic transformation temperature (MTT) and MSM effect is only possible in the martensitic region. Therefore, it is crucial to be able to predict the MTT of any NiMnGa alloy. Artificial neural networks (ANN) with their learning and generalization ability may act as a suitable tool to predict the MTTs of NiMnGa alloys. ANNs are generally used when the problem cannot be explicitly described by an algorithm, a set of equations, or a set of rules. A genetic algorithm (GA) is a search technique used in computing to find true or approximate solutions to optimization and search problems. In a previous study, in order to predict the MTT, the performance of a multi-layer perceptron has been studied. Training and validation stages of the approach are performed by using data sets from many separate analysis results and our chemical analysis results were used for testing. In this paper, as an inverse design approach, we concentrated on finding the composition of any NiMnGa alloy by using the MTT as the input in our ANN model. To build an advanced solution, we used GA and ANN in a hybrid manner to obtain the composition values. In order to compare the performance of the candidate solutions obtained from alternative methods, the required fitness function for the MTT was determined by the ANN developed in the previous study. Solution quality was used as the crosscheck parameters for the comparison of results obtained from either method. [Copyright &y& Elsevier]

Abstract: The Ni49.8Mn28.5Ga21.7 powders of micro-scale irregular equiaxial particles are prepared by ball milling method, and characterized by XRD, DSC and SEM techniques. The powders are found to contain disordered fct structure. Upon heating to high temperatures, the crystal structure of the as-milled powder is found to evolve from disordered fct to disordered bcc and then to a Heusler-type structure sequentially. The critical temperature for the transition from the bcc phase to the Heusler phase is 360 °C. This phase transition process is also a disorder–order transition. An atomic ordering model similar to the grain nucleation and growth is established to explain the annealing temperature dependence of the phase transformation temperature. [Copyright &y& Elsevier]

A novel concept to use a stress induced phase transformation in NiMnGa for solid state refrigeration is investigated. The stress needed to induce a phase transformation was measured at temperatures from 24 to 60°C. The stresses were sufficiently low enough such that an acoustic wave can in theory induce the phase transformation. DSC measurements were done on the NiMnGa material to measure the latent heat that can be absorbed and release. Theoretical calculations of the coefficient of performance for a heat pump using this stress induced thermal phenomena showed a possible efficiency of 67%. A solid state heat pump using a piezoelectric or magnetostrictive transducer to generate the acoustic pressure wave and a fluid medium suspended with NiMnGa nanoparticles is suggested as a possible implementation of the concept. [ABSTRACT FROM AUTHOR]

Ni56Mn21Co4Ga19 alloys have been successfully hot-rolled to smooth plates of 0.5mm thickness, and the mechanical and shape memory characteristics were investigated by tensile tests. The results show that the tensile stress and strain of dual-phases Ni56Mn21 Co4Ga19 alloy are 491MPa and 8.17%, respectively. Its martensitic transformation starting temperature is 412°C, and the recoverable strain due to shape memory effect is 2.1% under a residual strain of 4.3%. [Copyright &y& Elsevier]

Abstract: The evolution of the degree of L21 long-range atomic order and its influence on the martensitic transformation and Curie temperatures of a near-stoichiometric Ni2MnGa shape memory alloy were studied during several thermal treatments. Both temperatures depend on the quenching temperature and increase on subsequent heating treatments. This increase is concurrent with the presence of an exothermic peak in differential scanning calorimetry measurements. Neutron diffraction experiments were carried out in order to correlate the evolution of the transformation temperatures with the evolution of the L21 order degree, which was estimated through the intensity of the (111) and (200) reflections. The results confirm that the quenched-in degree of long-range atomic order depends on the quenching temperature and prove that the evolution of the transformation temperatures is due to the evolution of the atomic order. [Copyright &y& Elsevier]

A quasi-static model for NiMnGa magnetic shape memory alloy is formulated along the lines of the Brinson and Tanaka SMA constitutive models. Since the shape memory effect (SME) and pseudoelasticity exist in both NiTi and NiMnGa, constitutive models for SMAs offer a basis for ferromagnetic shape memory alloys (FSMA) modeling. Two types of quasi-static tests involving constant external magnetic field and constant stress are conducted to identify nine model parameters. These model parameters include free strain, Young's moduli, fundamental critical stresses, fundamental critical threshold fields, and stress-influence coefficients. The Young's moduli of the material in its field and stress preferred states are determined to be 450 and 820 MPa respectively, while the free strain is measured to be 6.5%. These test data are used to assemble a critical stress-magnetic field intensity profile that is useful for determining the model parameters and for predicting the various states of the material for a wide range of magnetic or mechanical loading conditions. Although all of the parameters can be obtained from constant magnetic field testing, useful insight into NiMnGa actuator behavior can be gained from constant axial stress tests. Once implemented, the analytical model shows good correlation with the test data, capturing both the magnetic shape memory effect and pseudoelasticity. Because the model is piecewise linear, it does not capture material behavior resulting from nonlinear effects such as magnetic saturation. Despite its inherent limitations, this model shows encouraging results, providing a solid basis for future modeling efforts. [ABSTRACT FROM AUTHOR]

As cast ingot of the polycrystalline shape memory Ni53·6Mn27·1Ga19·3 alloy was composed of columnar and equiaxed grains. Dilatation characteristics of the samples with columnar grains were studied in the temperature range of 25–370°C and compared with the dilatation characteristics of the samples with equiaxed grains. The results show that shape memory effect strongly depends on the microstructure. Preferred orientation of crystallites in the columnar structure determines its memory strain. The alloy with columnar structure was also examined after predeformation in compression. Two kinds of strains were found in the predeformed samples. Dilatation characteristics of the non-deformed and predeformed martensite show anomalous behaviour. [ABSTRACT FROM AUTHOR]

Abstract: The relationship among the composition, structure and martensitic transformation temperatures have been studied in detail for Ni50−x Mn25Ga25−x (at.%) alloys by optical and transmission election microscopy, X-ray diffraction and differential scanning calorimeter measurements. The martensitic transformation temperatures almost linearly increase with increasing Ni content up to 670K showing their potentials as high-temperature shape memory alloys. The results show that the tetragonality ratio (c/a) increases with the Ni substitution for Ga, but the unit-cell volume reduces simultaneously. The martensitic transformation characteristics of Ni54Mn25Ga21 alloy with single martensite phase show excellent stability during thermal cycles or aging. The polycrystalline Ni54Mn25Ga21 alloy exhibited a shape-memory effect of 4.2% and compressive strain larger than 10% by using grain refining technique. Addition of Fe as a fourth element is an effective method to improve the mechanical properties, especially plasticity of polycrystalline Ni–Mn–Ga alloy due to the precipitation of γ-phase. A compressive strain of 30% and a shape-memory effect of 2.8% are obtained for Ni56Mn17Fe8Ga19 alloy. [Copyright &y& Elsevier]

Abstract: Ni50Mn30Ga20 films of 13μm thickness were fabricated by DC magnetron sputtering on unheated glass substrates. The As-deposited films are partially crystalline and crystallize during rapid annealing. The successive appearance of ferromagnetic and shape memory properties was observed as the annealing temperature was increased. Ferromagnetic properties evolved after annealing at 400°C for 0.5h, while thermal annealing of at least 600°C for 0.5h led to polycrystalline films that transformed reversibly and martensitically as shown by structural analysis and differential scanning calorimetry and confirmed by mechanical spectroscopy data. Magnetic measurements also revealed the influence of the post deposition annealing on the ferromagnetic hysteresis. Transition temperatures and reaction enthalpies of the martensitic phase transformation were strongly influenced by the temperature of the rapid annealing process. X-ray absorption fine-structure (XAFS) spectroscopy proved these changes to be related to the change in the chemical order. It is proposed that the annealing data reflect the evolution of the crystalline state. Ferromagnetic order is established already in nano-grained samples whereas the shape memory effect is only observed above a critical grain size. [Copyright &y& Elsevier]

Abstract: The martensitic transformation strain up to 0.6% has been obtained in Ni51Mn24Ga25 single crystalline sample, which is attributed to the effect of preferential orientation of the martensite variants, resulted from the internal stress introduced by the directional solidification. An enhanced transformation strain up to 1.1 % is observed with a bias magnetic field of 1.2T. There is a large difference of MFISmax (λ max) from about 600 to 1300ppm in the temperature range from 192 to 236K in martensite phase. Based on the shear actuation model, the magnetocrystalline anisotropy energy and the energy for twin variants reorientation as a function of the temperature have been discussed. We proposed that the rate of the decrement of shear modulus (C′) with respect to the temperature is much larger than that of the magnetocrystalline anisotropy (K) as a result of the constraint λ max = K/C′γ t, which has been considered as the reason for the increasing MFISmax with increasing temperature. [Copyright &y& Elsevier]

Magnetic shape memory (MSM) effect is a new invention in actuator materials field, allowing even 50 times greater strains than in previous magnetically controlled materials (magnetostrictive). In MSM materials the magnetic field moves microscopic parts of the material (so-called twins) that leads to a net shape change of the material. In this study, the effect of alloy composition and heat treatment on the microstructure, local composition, and thermal and dilatometric properties of Ni2MnGa alloys were investigated. The results showed that as solidified, off-stoichiometric, alloys had three distinct microstructural features—a Heusler phase, a Mn-rich phase and a eutectic or eutectoid region. Various heat treatment procedures were applied to successfully remove the last phase. Heat treatment was also essential for the production of a distinct martensite transformation in DSC and dilatometry traces and a magnetic shape memory effect. Composition variations from Bridgman growth were large enough that a shift in martensite start temperature might occur in some parts of the alloy, based on literature data for the dependence of martensite start temperature on composition. [Copyright &y& Elsevier]

It is shown that the mechanical and shape memory characteristics of the polycrystalline Ni54Mn25Ga21 high temperature shape memory alloys can be improved by grain refinement. The maximum shape memory strain is 4.2%, and the compressive plasticity is higher than 10%. [Copyright &y& Elsevier]