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Low twinning stress (TS) is a prerequisite for magnetic shape memory functionality in ferromagnetic martensites. We compare Ni50Mn28Ga22 (nominal at.%) single crystals from four different producers to reveal the effect of crystal quality on the TS. Near the reverse martensite transformation, the TS is generally low, about 1 MPa, regardless of mosaicity of up to 1.7° and chemical composition deviations of up to 2 at.% of Mn. Pure type I and type II twin boundaries occur in crystals with smooth chemical composition gradients. The corresponding temperature dependences of TS follow universal linear trends with the slopes of −0.045 MPa/K for type I twins and −0.001 MPa/K for type II twins, enabling a very low TS down to 2 K for the latter. An intermediate slope of −0.023 MPa/K appears for mixed type I/type II twin boundaries in a crystal with sharp local chemical deviations. We conclude that the crystal quality affects the magnitude of the TS indirectly, through its impact on the character of the twin boundaries. The effect is weak near the martensite transformation temperature and strong at low temperatures.

Magnetic shape memory alloy-based actuator (MSMA-BA) has the capability to generate the micro–nano-scale precision positioning, which has unparalleled virtue in contrast with the traditional motion driving mechanism. Nevertheless, the output displacement of the MSMA-BA exhibits complex hysteresis nonlinearity, which hinders its utility in the high-precision positioning field. In this article, an online Volterra series model based on wavelet neural network (VSM-WNN) is proposed to describe the rate-dependent hysteresis of the MSMA-BA. The kernel function of the VSM is extracted with a WNN, in which the feasibility of this extraction is proved in theory via Taylor expansion of the wavelet function. Finally, the validity of the proposed model is confirmed by means of experiments. Experimental results indicate that the VSM-WNN has a remarkable performance in describing the traits of hysteresis behavior of the MSMA-BA.

The magnetic shape memory alloy (MSMA) is a new family of smart materials, which exhibits great strain deformation and high energy density. Based on these properties, the MSMA has excellent potential to represent an available means for developing a novel generation of actuators in the micro-positioning application. However, the MSMA-based actuator suffers from the inherent hysteresis and it has become a bottleneck in the industrial application. A hybrid hysteresis model, which consists of a simple dynamic hysteresis operator (SDHO) and chaotic neural network (CNN), is proposed in this article. This developed model possesses a concise construction and distinguished generalization capability. By conducting comparative experiments, the proposed approach has a superior ability to predict the hysteresis behaviors under various input signals.

It is known from experimental findings that three-dimensional effects can have a strong influence on magnetic shape memory behavior. Such phenomena are, however, often neglected in MSMA constitutive models, as they only become meaningful under complex loading conditions. The extensions of our original modeling framework, cf. Bartel et al. (2020), to include 3D-effects is threefold: (i) vector-valued microstructural variables are now elements in R 3 , i.e. no longer parameterizable in polar coordinates, (ii) a third tetragonal martensite variant may form/vanish by switching from/back into both other variants, and (iii) a more general and robust algorithmic treatment is necessary. The latter includes the implementation of a staggered Augmented Lagrangian scheme to handle the now much larger and numerically more advanced sets of equality and inequality constraints. In this context, two extended model formulations are presented. The first considers a first-order, two-variant laminate approach (rank-one convexification), in which domain magnetizations, interface orientations etc. are now three-dimensional vectors. The second model is based on a convexification approach, for which the incorporation of the third martensitic variant is quite natural. Numerical examples are investigated to test the generalized modeling framework. Firstly, it is confirmed that both extended models recover the solution of the previously established two-dimensional model for a simple loading case. Secondly, response predictions for more complex loading scenarios (non-proportional bi-axial stresses, orthogonal magnetic field), motivated by experiments, are investigated. It is found that capturing the formation, elimination and mutual interaction of all martensitic variants as well as general three-dimensional magnetization vector orientations is of key importance under these conditions. The extended convexification model and modified algorithmic formulation are shown to reliably handle even such general cases.

Magnetic shape memory alloy (MSMA), which has emerged in recent years, has been proven to have great potential for electromagnetic actuators due to the long life cycle, fast response, and relatively large load capacity. Current research on such material mainly focuses on the fundamentals, thermal effect, composition, and magnetic induction strain, but issues involved in its application to the industry are rarely addressed in detail. This article first analyzed the principle of magnetically induced strain and the corresponding constitutive equations, and investigated the magnetomechanical characteristics of the MSMA sample through measurement carried out on a specially designed experimental setup. Taking advantage of good drive performance, MSMA electromagnetic actuator with spring recovery was designed in detail. In order to resolve the contradiction between compactness and high magnetic field strength, a novel configuration with double-layered coils was proposed to achieve the required magnetic flux density. Influences of input current, prepressure, and spring stiffness on the dynamic response characteristics of the actuator excited by step signal and square wave signal input were analyzed by simulation. Finally, the experimental platform is built to verify the feasibility of the configuration and dynamic characteristics of the actuator. The experimental results show that the designed actuator has good output performance with a response time of 25 ms, demonstrating the superiority of MSMA in microdisplacement actuator with high dynamics.

The exchange bias is of technological significance in magnetic recording and spintronic devices. Pursuing a large bias field is a long-term goal for the research field of magnetic shape memory alloys. In this work, a large bias field of 0.53 T is achieved in the Ni50Mn34In16− x Fe x ( x = 1, 3, 5) system by tuning the magnetic ground state (determined by the composition x ) and the magnetic-field history (determined by the magnetic field H FC during field cooling and the maximum field H Max during isothermal magnetization). The maximum volume fraction of the interfaces between the ferromagnetic clusters and antiferromagnetic matrix and the strong interfacial interaction are achieved by tuning the magnetic ground state and the magnetic-field history, which results in strong magnetic unidirectional anisotropy and the large exchange bias. Moreover, two guidelines were proposed to obtain the large bias field. Firstly, the composition with a magnetic ground state consisting of the dilute spin glass and the strong antiferromagnetic matrix is preferred to obtain a large bias field; secondly, tuning the magnetic-field history by enhancing H FC and reducing H Max is beneficial to achieving large exchange bias. Our work provides an effective way for designing magnetically inhomogeneous compounds with large exchange bias.

We report on low twinning stress of 0.2 MPa for type 1 twin boundaries with rational (101) twinning plane and 0.1 MPa for type 2 twin boundaries with rational [(1) over bar 01] twinning direction, in Ni2MnGa0.8Fe0.2 seven-layer modulated martensite exhibiting about 12% stress-induced strain. The studied samples were free of compound twins with (100) and (110) twinning planes. The martensite lattice is monoclinic and shows a long-period commensurate modulation with wave vector q=(2/7)g(110), where g(110) is the reciprocal lattice vector. The investigated alloy has high potential for practical applications due to the low twinning stress and the elevated Curie point T-C=400 K. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.

Structural, elastic, electronic and magnetic properties of the Nickel-based magnetic shape memory alloys (MSMA) Ni2MnSb, Ni2MnSn and Ni2MnSb0.5Sn0.5, are investigated using the full-potential linearized plane wave plus local orbital method (FP-LAPW+lo). With Perdew-Burke-Ernzerhof (PBE) exchange-correlation, generalized gradient approximation (GGA) is used to describe the electronic exchange correlations energy. Equilibrium lattice constant, bulk modulus, and its pressure derivative are calculated and compared with available data. Using the total energy versus strain in the framework of the FP-LAPW+lo approach, we compute the elastic constants of the studied compounds in their austenite structure. Good agreement is found with other calculations both for Ni2MnSb and Ni2MnSn. Magnetic moments agree well with available results.

International audience; Thermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of seven-layered modulated (7 M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni50Mn30Ga20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7 M martensite variants with two mutually crossing systems of twin, i.e., type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10}7 M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e., {1 –2 –10}7 M as the twinning plane). A novel transformation orientation relationship between austenite and 7 M martensite in crossing twin is resolved to be {1 0 1}A//{1 –2 10}7 M and A//7 M, in contrast to that of self-accommodated case, i.e., {1 0 1}A//{1 –2 –10}7 M and A//7 M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed.

The present paper introduces a novel test procedure for the quasistatic magnetic characterization of FSMA (ferromagnetic shape memory alloy) components under mechanical loading for the use in actuator-level modelling. Its development was necessary due to certain requirements coming from the twin structure and shape of typical FSMA components which are not met by the presented test setups up to now. The measurements carried out in the developed test setup are verified by a reference sample and FE simulations. Error sources are discussed and the main systematic error is calculated. After correcting the remaining error, the acquire data correspond well to data from literature and show a physical consistency. The developed test procedure is hence able to measure effective but absolute magnetization properties of FSMA components under mechanical loading. The data directly and can be applied to any kind of actuator level modeling.

Twin boundary motion is the basic mechanism responsible for fast actuation of magnetic shape memory (MSM) alloys. Previous studies implied on a wide diversity of twin boundary velocities measured under similar conditions. Here, a data set of more than 400 velocity values is measured under conditions that are relevant for MSM applications and represents the dynamic behavior of 73 twin boundaries. Velocities are measured using an electro-mechanical setup that applies a controllable $$\upmu{\text{s}}$$ -scale force pulse and combines force measurements with high-speed imaging. The latter allows extracting velocities of individual twin boundaries, which are then correlated to the macroscopic stress in the sample. The appearance of type I twins is more prevalent than type II, resulting a much larger data set for type I twins. Statistical analysis of velocity points of type I twins reveals an increasing trend of the velocity with the macroscopic stress, as well as useful information on the probability distribution of velocity data. Velocities of type II twins exhibit large variability over a relatively narrow stress range. This can reason the large differences in maximal velocities reported previously for this twin type. The obtained statistical data can improve the design and modeling of MSM actuators.

This work investigates the effects of time dependency for isothermal sintering on additive manufactured Ni–Mn–Ga magnetic shape memory alloys. Binder jetting additive manufacturing was used to produce Ni–Mn–Ga parts from pre-alloyed powders. Additive manufacturing via the binder jetting technique produces parts with intrinsic porosities, based on the morphology of the source material. The Ni–Mn–Ga parts printed in this study using the binder jetting method possessed average densities of ~ 46% before sintering. These samples were sintered at 1353 K in increments of 10 h up to 50 h. Based on this temperature and time frame, (1) microstructural evolution, (2) crystallographic phase analysis, (3) transformation behaviors, and (4) thermal–physical properties were investigated. The additive manufactured Ni–Mn–Ga samples exhibited increases in densities, from ~ 74 to ~ 83% due to solid-state diffusion mechanisms. X-ray diffraction reveals that all of the additive manufactured samples have the 5 M martensitic phase at room temperature. Reversible martensitic transformation temperatures were recorded during heating and cooling cycles through differential scanning calorimetry, which indicate austenitic phase transformations occurring slightly above ambient temperatures. Additionally, analysis of the heating and cooling cycles prescribes that the entropy and Gibb’s energies decrease over the reversible martensitic transformations as sintering time increases. It is envisioned that this study will support a more synergistic manufacturing process between binder jetting additive manufacturing and post-heat treatment processes for Ni–Mn–Ga shape memory alloys.

In this contribution we present a constitutive modelling framework for magnetic shape memory alloys (MSMA) that builds on a global variational principle. The approach relies on concepts of energy relaxation and generalised notions of convexity to compute effective energy hulls to the non-convex energy landscape associated with the underlying multi-phase solid, from which the prediction of microstructure evolution results. In this sense it fundamentally distinguishes itself from MSMA models that essentially follow phenomenological concepts of classical plasticity (Kiefer and Lagoudas, 2005; 2009). The microstructure is not spatially resolved, but micro-scale quantities are taken into account in an effective sense by additional state variables—such as volume fractions or interface orientations—and appropriate mixture rules. The model allows all mechanisms central to MSMA behaviour—i.e. variant switching, magnetisation rotation away from easy axes, and magnetic domain evolution—to occur simultaneously. The authors have previously been able to demonstrate that such a modelling approach can quantitatively capture the key characteristics of single-crystalline MSMA response under standard loading scenarios (Kiefer et al., 2015). The modelling framework presented here is now further able to predict much more general response features, such as variant switching diagrams, magnetic field-biased pseudo-elasticity and the influence of specimen shape anisotropy. Moreover, the global variational framework is formulated in a manner that lends itself to finite element implementation. In this work, however, numerical examples are considered in which the nonlocal nature of the demagnetisation field is taken into account in an approximate sense through appropriate shape factors.

The transformation temperatures, magnetization behavior, shape memory behavior, and mechanical properties of polycrystalline Ni45Mn40Co5Sb10-xBx(at.%) (x = 0, 1, 2, 3, 4, 5) alloys were systematically investigated. It was revealed that substituting Sb with B drastically increases the transformation temperatures, while it decreases the saturation magnetization due to the alteration of electron concentration of the matrix and formation of Co-rich second phases. With the substitution of Sb with 5% B, martensite start temperature and activation energy were increased from 50 to 316.8 degrees C, and 185 to 722.6 kJ mol(-1), respectively. The thermal cycling under stress, superelasticity, and failure experiments showed that shape memory properties and strength were improved by the substitution of Sb with B. The shape memory effect with maximum recoverable strain of 1.6% was observed with in Ni45Mn40Co5Sb9B1, and perfect superelasticity was exhibited at 220 degrees C in Ni45Mn40Co5Sb8B2. It was concluded that NiMnCoSb alloys can be used as high-temperature magnetic shape memory alloys as they exhibit transformation temperatures above 100 degrees C and show promising shape memory and superelasticity behavior, and there is a magnetization difference between their transforming phases.

Heusler Ni-Mn-Sn-based alloys are good candidates for magnetic refrigeration. This application is based on cycling processes. In this work, thermal cycles (100) have been performed in three ribbons produced by melt-spinning to check the thermal stability and the magnetic response. After cycling, the temperatures were slowly shifted and the thermodynamic properties were reduced, the entropy changed at about 3–5%. Likewise, the thermomagnetic response remains similar. Thus, these candidates maintain enough thermal stability and magnetic response after cycling. Likewise, Cu addition shifts the structural transformation to higher temperatures, whereas the Curie temperature is always near 310 K. Regarding magnetic shape memory applications, the best candidate is the Ni49Mn36 Sn14Cu1 alloy This study was funded by University of Girona PONT2020-01 and Spanish Mineco MAT2016-75967-P projects

The synergistic control of resistance reduction and sealing poses challenges to enhancing the rapid dynamic response ability of servo hydraulic cylinders, the key to solving this problem is effectively controlling the sealing gap value. In this study, a micro-variation between the hydraulic cylinder and the piston based on the disadvantage of conventional seals, constant gap seals, and lip gap seals was constructed, MSMA assist support blocks were designed on the piston to form a gap seal strip, then, the sealing gap value could be changed by controlling the magnetic field intensity. Simultaneously, the effects of magnetic field strength, parts-manufacturing precision, temperature, and hysteresis on the micro-variation in the MSMA were analyzed, and effective solutions were proposed. Finally, experiments on the magnetic field, temperature, and hysteresis were conducted by the measurement system. The results showed that the variable value of the sealing gap with the MSMA is feasible under ideal conditions, and can effectively change the amount of MSMA expansion by controlling the magnetic field strength, temperature, preload, etc., and then change the amount of the sealing gap of the hydraulic cylinder. This is the key to achieving friction and sealing control, which plays a crucial and active role in improving the efficiency of hydraulic systems. However, the impact of hysteresis effects cannot be ignored, which will be the main problem to be solved in the future.

The magnetic field-induced stress and strain of magnetic shape memory alloy (MSMA) are fundamental to describe its magnetomechanical behavior. In order to investigate the operating characteristics of MSMA-based actuator, this paper presents a 3-D quasi-static modeling approach of magnetomechanical behavior of MSMA with the improved constitutive equation by introducing the activation field strength. The relationships between three critical parameters of the activation field strength, the maximum of strain, as well as the saturation field strength, and the pre-compressing force are obtained by fitting the data collected from the experiment. Two kinds of test specimen in different geometric dimensions are studied by the numerical modeling method. Considering that the key material parameters vary dramatically depending on the size and composition, verifications through experiments show that this numerical modeling and simulation can accurately describe the force and deformation characteristics of MSMA. The modeling analysis and simulation results are significant to design of the actuator based on MSMA to predict the actuator performance under the given working conditions.

The electronic structure, magnetism and martensitic transformation in Co-V based Heusler alloys Co2V1.5Z0.5 (Z = Al, Ga, In) were investigated theoretically. In these alloys, the tetragonal martensitic phase has a higher stability compared with the cubic austenite. This can be explained based on their electronic structures. Here the energy difference ΔEM between the martensite and austenite is large in Co2V1.5Al0.5 and Co2V1.5Ga0.5, suggesting that they are possible candidates for magnetic shape memory alloys (MSMAs). This also agrees with the existing experimental observation in Co-V-Ga. In Co2V1.5Z0.5 a ferromagnetic to paramagnetic transition occurs together with the martensitic transition. The spin polarization ratio also decreases drastically from close to 100% in austenite to almost zero in martensite. All this makes Co2V1.5Z0.5 alloys a cross-over point of MSMA and spintronics, which can be controlled by magnetic/temperature/stress multi-field. In Co2V1.5Ga0.5 alloy, calculations suggest a degree of Co-V disorder in the experimental sample, which may introduce a weak ferromagnetic state in the martensite and lower the phase transition driving force ΔEM. This can explain the small saturation magnetization of the martensite reported in literature. To improve the properties of Co-V-Ga alloys, adjusting their atomic ordering can be of great importance.

Magnetic shape memory alloys (MSMAs) have attracted great interests due to their multifunctional properties associated with external-field-induced martensitic transition, such as magnetostrain, magnetoresistance, magnetocaloric effect and elastocaloric effect. Search for MSMAs displaying a unique combination of strong metamagnetic transition and stable, large caloric effect is nowadays very active. Here we report the simultaneous achievement of a strong metamagnetic transition as well as a large magnetic-field-induced entropy change of 17.4 J kg−1 K−1 under 2 T and a stable, large elastocaloric effect with adiabatic temperature change of ∼5.0 K during more than 2100 mechanical cycles, in a Ni-Fe-Co-Mn-Sn MSMA. Such combination in a single material has never been reported in the literature. Furthermore, we demonstrate that fully reversible metamagnetic transition and consequently large, reversible multicaloric effect can be achieved by increasing magnetic field from 0 T to 2 T at zero stress and decreasing field from 2 T to 0 T at a uniaxial stress of 23.5 MPa. This dual-stimulus magnetic-mechanical multicaloric cooling cycle also provides an opportunity for achieving reversible magnetoresistance and magnetostrain under the coupling of a low magnetic field and small stress, which is beneficial for practical applications. The high cyclic stability of large caloric effects is ascribed to the good geometric compatibility at the interface between austenite and martensite as revealed by employing multiscale characterization approaches, including in-situ synchrotron high-energy X-ray diffraction and transmission electron microscopy. This study has great implications on developing high-performance multifunctional alloys with magnetostructural transition for actuation, magnetic recording and solid-state refrigeration applications.

Recent reports on highly mobile type II twin boundaries challenge the established understanding of deformation twinning and motivate this study. We consider the motion of twin boundaries through the nucleation and growth of disconnection loops and develop a mechanism-based model for twin boundary motion in the framework of isotropic linear elasticity. While such mechanisms are well established for type I and compound twins, we demonstrate based on the elastic properties of crystals that type II twin boundaries propagate in a similar way. Nucleation of a type I twinning disconnection loop occurs in a discrete manner. In contrast, nucleation of a type II twinning disconnection loop occurs gradually with increasing Burgers vector. The gradual nucleation of a type II disconnection loop accounts for the higher mobility of type II twin boundaries compared with type I twin boundaries. We consider the homogeneous nucleation of a disconnection loop, which is adequate for twinning in shape memory alloys with a low-symmetry crystal lattice. For the magnetic shape memory alloy Ni–Mn-Ga, the model predicts twinning stresses of 0.33 MPa for type II twinning and 4.7 MPa for type I twinning. Over a wide temperature range, the twinning stress depends on temperature only through the temperature dependence of the elastic constants, in agreement with experimental results.

Significance Magnetic shape memory (MSM) refers to a change in shape and/or size of a magnetic material upon applying a magnetic field. There are 2 types of MSM effects; the first one occurs in a twinned magnetically ordered material, in which the crystallographic axes are irreversibly reoriented by the applied magnetic field. In the second type, the applied field drives a magnetoelastic phase transition. In certain iron tellurides Fe 1+ y Te, both types of MSM occur. Notably, the first antiferromagnetic compound found to display an MSM effect is a parent material to the well-studied high-T c cuprate superconductor La 2−x Sr x CuO 4 . Observation of MSM effects in 2 known material families related to high-T c superconductors points to a prominent role of electron–phonon coupling arising through the spin–orbit interactions.

The use of magnetic shape memory alloys (MSMAs) in manufacturing industry has increased significantly in recent years. This is mainly due to their great interest in their potential applications in smart devices, because of the reversible distortions suffered. The well-known example of these combinations is the Heusler type. A review is given of experimental works concerning the examination of magnetic field, structural phase transitions, and the magnetocaloric impact in Heusler Ni-Mn-X (X = In, Sn, Sb) and Ni-Co-Mn-Y (Y = In, Sn, Sb) alloys. This type of compounds has excellent properties, for example, the presence of coupled magnetostructural (coïncident magnetic and martensitic transitions) and metamagnetostructural phase transitions (coïncident metamagnetic (ferromagnetic-antiferromagnetic) and martensitic transitions), the magnetocaloric impact (MC), and the large magnetoresistance change (MR). The conceivable difficulties and remaining problems are briefly discussed This work was funded by the“Taishan Scholar” Project of Shandong Province and Key Basic Research Project of Shandong Natural Science Foundation of China (No. ZR2017ZB0422)

Small samples of a Ni-Mn-Ga single crystal of three different geometries were subjected to bending by applying a rotating magnetic field. The magneto-mechanical behavior of the sample in cantilever was analyzed and special attention was given to elongations and curvature along the deformation process. A sequence of 3000 images was made using a high-resolution camera and the data was analyzed using a code in Matlab. Furthermore, the geometric analysis showed that, when the magnetic field is equal to cero, the sample do not recover its original shape totally and the presence of a pseudo-elastic behavior was observed. Analysis and interpretation of the data allows the presentation of some hypotheses concerning to the crystalline structure and the role of dislocations, represented by a dislocation density, in the martensite phase of these materials. These hypotheses are discussed more formally in the second part of this paper. Some experiments are proposed that would give the opportunity to a wider theoretical knowledge of MSMM.

Bruno, Nickolaus/0000-0002-9464-8902; Karaman, Ibrahim/0000-0001-6461-4958 WOS: 000457815700009 We report on the magnetic, magnetocaloric properties, and hysteresis of Ni45Co5Mn38Sn12 and Ni45Co5Mn38.5Sn11.5 Heusler polycrystalline alloys under several magnetic field values. Both alloys have a moderate magnetization difference ( approximate to 70 emu/gr) between austenite and martensite phase with a moderate thermal hysteresis ( approximate to 8 K) around the martensitic transition. Magnetic field dependent measurements reveal that the application of a magnetic field stabilizes the austenite phase, which has higher magnetization. Both alloys have substantial inverse magnetic entropy changes and adiabatic temperature changes. In addition, we investigate the accessibility of the calculated entropy change that is quantified by means of the minimum magnetic field required to start and to complete the magnetostructural phase transition. U.S. National Science Foundation, Division of Materials Research, Metals and Metallic Nanostructures Program under Materials World Network Initiative [1108396]; Scientific Research Projects of Ankara University (BAP)Ankara University [15H0443007] This work was partially supported by the U.S. National Science Foundation, Division of Materials Research, Metals and Metallic Nanostructures Program, Grant No. 1108396, under the umbrella of the Materials World Network Initiative. BE acknowledges the financial support by Scientific Research Projects of Ankara University (BAP) under grant no. 15H0443007.

Meta-magnetic shape memory alloys (MMSMAs) display an abrupt change in magnetization across the martensitic transformation. Simultaneous detection and statistical analysis of the emitted acoustic and magnetic noises, accompanied with martensitic transformations are important for better understanding of the magneto-structural coupling, and for utilization of well-characterized acoustic and magnetic signals for practical applications. In the present work, characteristics of thermal, acoustic and magnetic noises, detected during martensitic transformation in the Ni45Co5Mn36.6In13.4 MMSMA single crystals, were investigated for the first time. Statistical analysis of the thermal spikes, appeared during the calorimetric measurements at low cooling rates (typically below 1 K/min) showed that the energy distributions followed a power law with e = 2.0 ± 0.3 exponent, which is in good agreement with the exponents of energy distribution of acoustic and magnetic emission events (e = 1.7 ± 0.1 and e = 1.7 ± 0.2, respectively). The above exponents were independent of the external magnetic field between B = 0 and 230 mT. Interestingly, the acoustic and magnetic exponents during cooling and heating were the same in contrast to the expected asymmetry based on the fact that the transformation demonstrates an isothermal character during cooling and an athermal one for heating. Upon interrupting cooling, and dwelling at a temperature above the martensite finish temperature, practically no acoustic and magnetic noises were detected, while upon continued cooling the noise appeared again, with the same critical exponents. This indicates that the main discontinuous events, leading to noise emissions, should be the same before and after dwelling as well as during cooling and heating. In magnetic noise measurements, under zero external field, the magnetic noise packets contained voltage peaks in the detector coils in both (positive and negative) directions. Under small external magnetic fields (between 20 and 230 mT), the activity of magnetic signals increased and the magnetic peaks, during both heating and cooling, appeared more and more unidirectional (i.e. peaks only in one direction were observed) which can be attributed to the increased magnetization of the austenite.

To obtain a material exhibiting magnetically induced reorientation (MIR), one of the magnetic shape memory effects, a well-developed method for single crystal growth is crucial. A [ 100 ] oriented Ni2MnGa single crystal exhibiting MIR was successfully prepared using the floating-zone technique with a high speed of 80 mm/h. The comparison with a sample grown by Bridgman method and a reference sample grown in AdaptaMat Ltd. by directional solidification is provided based on elemental homogeneity, crystallographic quality by x-ray diffraction, transformation behaviour and magnetic properties. All these methods indicated that the single crystal grown by floating zone with high speed is comparable or superior to the samples prepared by other methods. Importantly, the magnetization measurement revealed that the single crystal prepared this way exhibits MIR down to 10 K.

The crystallization mechanism of Ni-Mn-Ga thin films and the reason for the difference of crystallization temperatures of Ni-Mn-based magnetic shape memory alloys (MSMAs) are unknown. Here, the crystallization kinetics of Ni53Mn28Ga19 magnetic shape memory alloy thin films have been determined by non-isothermal and isothermal differential scanning calorimetry (DSC). The reason for the difference of crystallization temperatures of Ni-Mn-based MSMAs was studied by the first-principles methods. The crystal structure of annealed Ni53Mn28Ga19 thin films are 7 M martensite. In non-isothermal DSC, crystallization peak temperatures are 588.2 K, 593.7 K, 601.3 K, 604.6 K and 608.2 K at different heating rates. The apparent activation energy calculated by Kissinger’s method is 148.3 kJ/mol. For isothermal crystallization, the Avrami exponent of Ni53Mn28Ga19 thin films is approximately 1.6. The local Avrami exponents n(x) which range from 1.1 to 2.8 imply that the crystallization mechanism of Ni53Mn28Ga19 thin films changes from one-dimensional diffusion-controlled growth to two-dimensional and three-dimensional diffusion-controlled growth. Moreover, it is found that crystallization peak temperatures of Ni-Mn-based MSMAs increase with their increasing vacancy formation energy with Ni vacancy.

Multiferroic phase-transforming alloys demonstrate intriguing multicaloric effects, but they are intrinsically brittle and their elastocaloric effect shows poor cyclability, which remains a major challenge for exploiting more efficient multicaloric refrigeration. Here, by employing a novel strategy of strengthening grain boundary, the cyclability of elastocaloric effect in the prototype Ni-Mn-based multiferroic phase-transforming alloys is strikingly enhanced by two orders of magnitude. Ultrahigh cyclability of a large elastocaloric effect is achieved. This not only paves the way for exploiting multicaloric effects for more efficient cooling, but also provides a strategy for overcoming the cyclability issues in the ubiquitous brittle intermetallic phase-transforming materials. The cyclability of elastocaloric effect in the prototype Ni-Mn-based multiferroic phase-transforming alloys has been strikingly enhanced by two orders of magnitude through employing the strategy of strengthening grain boundary.

Ni-29Mn-21.5Ga (at. %) wires are fabricated via a combination of (i) extrusion of liquid inks containing a binder, solvents, and elemental Ni, Mn, and Ga powders and (ii) heat treatments to remove the polymer binder and to interdiffuse and sinter the powders. To study the microstructural evolution, sintering mechanisms, and grain growth in these wires, both ex situ metallography and in situ X-Ray tomography were conducted while sintering at 800–1050 °C for up to 4 h. After debinding, Ga-rich regions melt and induce transient liquid phase sintering of the surrounding Ni and Mn powders, resulting in localized swelling of the wires and an increase in the wire porosity. After solidification of the melt and diffusion of the Ga into the Ni and Mn powders, solid-state sintering occurs. The interdiffusion of Ni, Mn, and Ga during solid-state sintering improves sintering compared to fully pre-alloyed powders. At the end of the 4 h sintering period, chemically homogenized, oligocrystalline wires with bamboo-like grains are observed with porosities ranging from 30 to 57%. Furthermore, significant grain growth occurs in wires sintered at 1000 and 1050 °C (11–35 μm vs. 1–10 μm initial powder size). The results from this study enable tailoring the porosity and grain size of printed Ni-Mn-Ga wires and 3D-printed micro-architectures and may be used to enhance their magnetic shape-memory and magnetocaloric effects in future work.

The conventional exchange bias and spontaneous exchange bias are two interesting but fundamentally different effects in Mn rich Ni-Mn-Z magnetic shape memory alloys. Previous investigations show that these two phenomena appear separately. Here, we report that the conventional and spontaneous exchange bias phenomena coexist in a Ni50Mn34In13Fe3 alloy and the superposition of them increases the total exchange bias of the alloy. Our results suggest that the canonical spin glass and super spin glass may coexist and are embedded in the antiferromagnetic matrix of the martensite, through properly tuning the temperature-field history, which further results in the superposition of conventional and spontaneous exchange bias for this alloy. The interfacial interaction between the canonical spin glass and the antiferromagnetic matrix forms during field cooling process, which results in the conventional exchange bias. Moreover, the application of large magnetic field during the isothermal magnetization process changes the superspin glass into a superferromagnetic state, which interacts with the antiferromagnetic matrix at the interface, causing the spontaneous exchange bias. The superposition of exchange bias provides an effective way to achieve large exchange bias in materials.

In this paper, the hydraulic switching valve is designed and its dynamic performance is investigated through proposing a fast response actuator with magnetic shape memory alloy (MSMA) to drive the valve. MSMA actuator with spring return is designed and a double-layered coil is constructed to achieve compactness of electromagnetic case. The dynamic characteristics of the MSMA actuator are analyzed and the step response characteristics is tested. Hydraulic switching valve with MSMA actuator is designed with poppet type. Pressure and velocity field in the flow channel under different valve opening and different inlet and outlet pressure differences are analyzed in COMSOL Multiphysics software. The dynamics of the valve poppet during opening and closing process is modeled mathematically, and simulation analysis are conducted in AMESim software to analyze the response of valve under step and square wave signals. The step response of output flow rate and pressure-flow characteristic under different operating conditions are obtained through experiment. The results show that the MSMA based valve can achieve fast response with opening time of 5 ms at the pressure difference of 1 MPa, providing a theoretical support for the development of hydraulic switching valve with high performance actuator driven by MSMA.

The local actuation of a magnetic shape memory (MSM) element as used in an MSM micropump is considered. This paper presents the difference between an electromagnetic driver and a driver that uses a rotating permanent magnet. For the magnetic field energy of the permanent magnetic drive, the element takes in a significant stray field. In a particular case, energy reduction was 12.7 mJ. For an electromagnetic drive with an identical size of the MSM element, the total magnetic field energy created by the system was 2.28 mJ. Attempts to experimentally nucleate twins in an MSM element by energizing an electromagnetic drive failed even though the local magnetic field exceeded the magnetic switching field. The energy variation is an order of magnitude smaller for the electromagnetic drive, and it does not generate the necessary driving force. It was assumed in previous work that the so-called magnetic switching field presents a sufficient requirement to nucleate a twin and, thus, to locally actuate an MSM element. Here, we show that the total magnetic field energy available to the MSM element presents another requirement.

This paper presents a lumped element model (LEM) to describe the coupled dynamic properties of thermomagnetic generators (TMGs) based on magnetic shape memory alloy (MSMA) films. The TMG generators make use of the concept of resonant self-actuation of a freely movable cantilever, caused by a large abrupt temperature-dependent change of magnetization and rapid heat transfer inherent to the MSMA films. The LEM is validated for the case of a Ni-Mn-Ga film with Curie temperature TC of 375 K. For a heat source temperature of 443 K, the maximum power generated is 3.1 µW corresponding to a power density with respect to the active material’s volume of 80 mW/cm3. Corresponding LEM simulations allow for a detailed study of the time-resolved temperature change of the MSMA film, the change of magnetic field at the position of the film and of the corresponding film magnetization. Resonant self-actuation is observed at 114 Hz, while rapid temperature changes of about 10 K occur within 1 ms during mechanical contact between heat source and Ni-Mn-Ga film. The LEM is used to estimate the effect of decreasing TC on the lower limit of heat source temperature in order to predict possible routes towards waste heat recovery near room temperature.

Physical review materials 5(11), 113607 (2021). doi:10.1103/PhysRevMaterials.5.113607, The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transition and stabilization of the PM phase over a very wide temperature range from 300 to ���5K in a magnetic shape memory alloy (MSMA), Ni$_{50}$Mn$_{34}$In$_{16}$, using magnetic susceptibility, synchrotron x-ray powder diffraction (SXRPD) studies, and first-principles calculations. The ac-susceptibility studies show a highly skewed and smeared peak around 300 K without any further transition up to the lowest temperature of our measurement (5 K) for ���5% Al substitution. The temperature evolution of the SXRPD patterns confirms the appearance of the PM phase related satellite peaks at T���300K without any splitting of the main austenite (220) peak showing preserved cubic symmetry. This is in marked contrast to the temperature evolution of the SXRPD patterns of the martensite phase of the Al free as well as ���3% Al substituted compositions where the austenite (220) peak shows a clear splitting due to Bain distortion signalling symmetry breaking transition. Our theoretical calculations support the experimental findings and reveal that the substitution at the In site by a smaller size atom, like Al, can stabilize the PM phase with preserved cubic symmetry. Our results demonstrate that Al-substituted Ni-Mn-In MSMAs provide an ideal platform for investigating the physics of various phenomena related to the PM state., Published by APS, College Park, MD

The superelastic properties and stress-induced martensite (SIM) stabilization have been studied in a shape memory Ni51.1Fe16.4Ga26.3Co6.2 single crystal. The single crystal, characterized by a thermally induced forward martensitic transformation temperature around 56 °C in the initial state, has been submitted to compression mechanical testing at different temperatures well above, near and below the martensitic transformation (MT). After each mechanical test, the characteristic MT temperatures and the transformation enthalpy have been monitored by means of differential scanning calorimetry. At temperatures below MT, the stress–strain (σ–ε) curves show a large strain, around 6.0%, resulting from the detwinning process in the martensitic microstructure, which remains accumulated after unloading in the detwinned state of the sample as a typical behavior of the shape memory alloys (SMAs). After just two “σ–ε + heating” cycles the accumulation of strain was not observed any more indicating the formation of a two-way shape memory effect which consists in a spontaneous recovery of the aforementioned detwinned state of the sample during its cooling across the forward MT. Whereas the thermally induced shape recovery in conventional SMAs occurs at the fixed value of the reverse MT temperature, the heating DSC curves of the mechanically deformed martensite in the present work show a burst-like calorimetric peak at the reverse MT arising at temperatures essentially higher than the thermally activated one. This behavior is the result of the SIM stabilization effect. After a short thermal aging in the stress-free state, this effect almost disappears, showing a slight impact on the MT characteristic temperatures and the enthalpy. At temperatures higher than the transformation one, the SIM is not stabilized, as the mechanically induced martensite fully retransforms into austenite after the unloading. From the σ–ε curves, the critical stress, σc, as well as the values of Young’s moduli of martensite and austenite are determined showing linear dependences on the temperature with a slope of 3.6 MPa/°C. This research has been carried out with the financial support of the Spanish Ministry of Science, Innovation and Universities (project RTI2018-094683-B-C53-54) and Basque Government Department of Education (project IT1245-19) and in the framework of INNOSMAD ID546749 Project, PROGRAMMA DI COOPERAZIONE INTERREG V-A ITALIA SVIZZERA CCI 2014TC16RFCB035. st

Magnetic shape memory alloys (MSMAs) are an interesting class of smart materials characterized by undergoing macroscopic deformations upon the application of a pertinent stimulus: temperature, stress and/or external magnetic fields. Since the deformation is rapid and contactless, these materials are being extensively investigated for a plethora of applications, such as sensors and actuators for the medical, automotive and space industries, energy harvesting and damping devices, among others. These materials also exhibit a giant magnetocaloric effect, whereby they are very promising for magnetic refrigeration. The applications in which they can be used are extremely dependent on the material properties, which are, in turn, greatly conditioned by the structure, atomic ordering and magnetism of a material. Particularly, exploring the material structure is essential in order to push forward the current application limitations of the MSMAs. Among the wide range of available characterization tools, neutron scattering techniques stand out in acquiring advanced knowledge about the structure and magnetism of these alloys. Throughout this manuscript, a comprehensive review about the characterization of MSMAs using neutron techniques is presented. Several elastic neutron scattering techniques will be explained and exemplified, covering neutron imaging techniques—such as radiography, tomography and texture diffractometry; diffraction techniques—magnetic (polarized neutron) diffraction, powder neutron diffraction and single crystal neutron diffraction, reflectometry and small angle neutron scattering. This will be complemented with a few examples where inelastic neutron scattering has been employed to obtain information about the phonon dispersion in MSMAs. This work has been carried out with the financial support of the Spanish Ministry of Science, Innovation and Universities (project RTI2018-094683-B-C53-54) and Basque Government Department of Education (project IT1245-19). N.A.R.-L. wants to thank the Basque Government (Department of Education) for providing funding under the specific investigation PFPI grant.

In this letter we report the results of entropy variations in random anisotropy magnets composed of TbxY1−xAl2, with x=0.15, 0.20, 0.25, 0.35, 0.40, and 0.50. We discovered large entropy variation associated with the spin glass to paramagnetic transition. Both temperature transition and entropy changes were studied at different temperatures and with different compositions. Our conclusion is that these materials are suitable candidates for use as magnetic refrigerants in a temperature range below 40 K.

For magnetic shape memory (MSM) alloys, a magnetic field stimulates a shape change. We use the shape change to build devices such as micro-actuators, sensors, and microfluidic pumps. Currently, (as a novel technology,) devices suffer from some material and magnetic driver shortcomings. Here we address the issues related to operating temperature, repeatability, failure, and magnetic driver development. To increase the operating temperature of the MSM material, we alloyed Fe and Cu to Ni-Mn-Ga. We showed that the element-specific contribution to the valence electron density as parameter systematically determines the effect of each element on the variation of the martensite transformation temperature of the 10M phase. To stabilize the material, we developed a micro-shotpeening process that adds stresses to the material surface, thereby inducing a fine twin microstructure. The treatment allowed nearly full magnetic-field-induced strain, and extended fatigue life of the material from only one thousand cycles in the electropolished state to more than one million cycles in the peened state. We measured the effect of the peening process on material actuation when in MSM pump configuration. In the polished state, the deformation was stochastic, with a sharp-featured, faceted shrinkage. In the treated state, the deformation was smooth and repeatably swept along the surface akin to a wave. To actuate the MSM micropump without electromotor, we developed a linear electromagnetic actuation device and evaluated its effectiveness in the switching mechanism of the material. By compressing the magnetic field between opposing coils, we generated a strong magnetic field, which caused a localized region to switch at selected poles. In the next iteration of the drive, we inserted the MSM sample between two linear pole arrangements of high pitch density to approximate a moving vertical field. The incremental stepping of the vertical field between poles caused translation of the switched region. The results of this dissertation demonstrate the suitability of MSM alloys for high-precision, persistent, and reliable actuators such as micropumps.

With proper magneto-mechanical driving forces (e.g., a high-frequency magnetic field plus a mechanical force), Magnetic Shape Memory Alloy (MSMA) can provide a large cyclic deformation (strain oscillation amplitude up to 6%), which makes it a good candidate for high-frequency large-stroke actuators. Moreover, as a kind of shape memory alloys, MSMA’s magneto-mechanical coupling behaviors are very sensitive to temperature, which allows researchers/engineers to modify the strain oscillation amplitude for a wider range of applications by controlling the working temperature. In this paper, we report systematic experiments on the thermal-ambient-dependent oscillation amplitude of the magnetic-field-induced martensite reorientation—utilizing compressed air (with controlled airflow velocity) passing through the surface of the MSMA specimen (Ni–Mn-Ga single crystal) to control the heat transfer between the ambient and the MSMA specimen during the high-frequency magneto-mechanical loading. It is found that the extremely weak or extremely strong ambient heat transfer can only have small strain oscillation amplitude while the maximum strain amplitude can be achieved only at a mild heat-transfer condition (i.e., non-monotonic dependence of the strain amplitude on the ambient heat transfer). It is also demonstrated that the strain amplitude is closely related to the working temperature, satisfying the balance between the heat generation (from the dissipative strain oscillation of the martensite reorientation) and the heat transfer to ambient (due to the temperature difference between the MSMA specimen and the ambient). With such understanding and constraints, three different schemes for controlling the strain amplitude by the thermal method are proposed and tested for their robustness/reliability. These results not only provide some guidelines/principles for designing MSMA actuators with flexible strain amplitude, but also demonstrate the delicate dynamics of the thermo-magneto-mechanical coupling that demand further theoretical/modelling study.

Magnetic shape memory alloys (MSMAs) have drawn significant research attention as potential high actuation energy multi-functional materials. Such a dissipative material system can be considered as a solid continuum interacting with a magnetic field. A continuum-based phenomenological model provides a magneto-mechanical system of equations that simulates and predicts primary MSMA behaviours. In this work, we investigate the local symmetries of the MSMA system equations through the Lie group analysis. Symmetry breaking due to stable-unstable transition is analysed. The conservation laws are derived, and their physical meaning is scrutinized.

This work presents the results of research on a Co49Ni21Ga30 magnetic shape memory single crystal. Based on a literature review, it was identified that analyses of phase transformations have been limited to specific heating and cooling rates, which could lead to an incomplete description of the resulting phenomena. Differential scanning calorimetry (DSC) performed with different heating/cooling rates enabled the precise determination of enthalpy values, which deviate from literature values. Weak and previously unnoticed thermal phenomena at temperatures below 190 K were also observed. Their presence was confirmed by low-temperature in situ transmission electron microscopy (TEM). Through DSC measurements and TEM observations, a model of the discovered phenomenon was proposed, which may have an impact on a better understanding of the physics of magnetic shape memory materials.

WOS: 000519988900002 Martensitic transformations are strain driven displacive transitions governing the mechanical and physical properties in intermetallic materials. This is the case in Ni2MnGa, where the martensite transition is at the heart of the striking magnetic shape memory and magnetocaloric properties. Interestingly, the martensitic transformation is preceded by a premartensite phase, and the role of this precursor and its influence on the martensitic transition and properties is still a matter of debate. In this work we report on the influence of Co doping (Ni50-xCoxMn25Ga25 with x = 3 and 5) on the martensitic transformation path in stoichiometric Ni2MnGa by neutron diffraction. The use of the superspace formalism to describe the crystal structure of the modulated martensitic phases, joined with a group theoretical analysis, allows unfolding the different distortions featuring the structural transitions. Finally, a general Landau thermodynamic potential of the martensitic transformation, based on the symmetry analysis, is outlined. The combined use of phenomenological and crystallographic studies highlights the close relationship between the lattice distortions at the core of the Ni2MnGa physical properties and, more in general, on the properties of the martensitic transformations in the Ni-Mn based Heusler systems. Departments of Excellence' program of the Italian Ministry for Education, University and Research (MIUR, 2018-2022) The authors acknowledge the Science and Technology Facilities Council for providing neutron beamtime on the WISH beamline. F.O. acknowledges Dr. Giovanni Romanelli for useful discussion. This work has benefited from the COMPHUB Initiative, funded by the `Departments of Excellence' program of the Italian Ministry for Education, University and Research (MIUR, 2018-2022).

The most extensively studied Heusler alloys are those based on the Ni-Mn-Ga system. However, to overcome the high cost of Gallium and the usually low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, two alloys (Mn50Ni35.5In14.5 and Ni50Mn35In15) have been obtained by melt spinning. We outline their structural and thermal behaviour. Mn50Ni35.5In14.5 alloy has the transformation above room temperature whereas Ni50Mn35In15 does not have this transformation in the temperature range here analyzed.

The effect of Zn doping on Ni-Mn-Ga magnetic shape memory alloy was studied by the first-principles calculations using exact muffin-tin orbital method in combination with the coherent-potential approximation and projector augmented-wave method. Trends in martensitic transformation temperature T M and Curie temperature T C were predicted from calculated energy differences between austenite and nonmodulated martensite, ΔE A−NM, and energy differences between paramagnetic and ferromagnetic state, ΔE PM−FM. Doping upon the Ga-sublattice results in stabilization of martensitic phase which indicates the increase in T M . T C is affected only weakly or slightly decreases, because ΔE PM−FM of martensite does not change significantly with doping. The substitution of Mn atoms by Zn causes the decrease in both T M and T C . Comparing to Cu-doped Ni-Mn-Ga alloys, we predict that doping with Zn results in smaller decrease in T C but also in smaller increase in T M . Moreover, Cu doping upon the Ga-sublattice strongly decreases the magnetic anisotropy energy of martensite, whereas such strong effect was not observed for Zn doping. Based on the calculations of Zn-doped Ni-Mn-Ga alloys we suggest that simultaneous doping with Zn and an element increasing T C can result in significant increase in both transformation temperatures without strong decrease of magnetic anisotropy.

Concentrated on additive manufacturing, this chapter discusses materials research in the Manufacturing 4.0 context. In addition to providing a brief review on stimuli-responsive materials and their applications under Manufacturing 4.0, this chapter also looks ahead and discusses in detail the recent developments in additive manufacturing of machines, particularly machines based on the special characteristics of certain stimuli-responsive materials, such as thermally activated shape memory alloys, magnetic shape memory alloys and magnetocaloric materials. Furthermore, this chapter emphasizes possible future directions for the additive manufacturing of metallic materials in general.