Magnetic Field-induced Strain Research Articles

Magnetoelastic Anomalies Exhibited by Ni–Fe(Co)–Ga Polycrystalline Ferromagnetic Shape Memory Alloy

The martensitic transformation (MT) as well as the structure, magnetic and magnetostrain properties of a polycrystalline Ni50.1Fe18.6Ga27.2Co4.1 (at%) ferromagnetic shape memory alloy are investigated. The studied alloy exhibits a martensitic ferromagnetic phase near room temperature that shows a non-modulated tetragonal structure with a tetragonality ratio of c/a > 1. Strain measurements reveal a complex spontaneous change at MT amounting to about 0.15% that increases up to 0.22% at 5T of the applied magnetic field indicating orientation influence of the field on the martensitic variants growth. Magnetostrain measurements enable evaluation of the magnetic fieldinduced strain effect in the vicinity of MT. The sample shows moderate deformation achieving values of 70ppm related to the magnetization rotation and volume magnetostriction that does not saturate at applied magnetic field of 5T. [doi:10.2320/matertrans.M2013154]

Magnetic Field-Induced Transition in Co-Doped Ni<sub>41</sub>Co<sub>9</sub>Mn<sub>31.5</sub>Ga<sub>18.5</sub> Heusler Alloy

Thermal strain, magnetostriction and magnetization measurements of Ni41Co9Mn31.5Ga18.5 polycrystalline ferromagnetic shape memory alloy (FSMA) were performed across the martensitic transition temperature, TM, and the reverse martensitic transition temperature, TR ,a t atmospheric pressure. When cooling from the austenite phase, a steep decrease in thermal expansion due to the martensitic transition at TM was found. When heating from the martensitic phase, a steep increase in the thermal expansion due to the reverse martensitic transition at TR was observed. These transition temperatures decreased gradually with increasing magnetic field. The field dependence of the martensitic transition temperature, dTM/dB ,i s¹4.2K/T and that of the reverse martensitic transition temperature, dTR/dB ,i s¹7.9K/T. The metamagnetic transition appeared between 330 and 390K. The results of thermal strain and magnetization measurements indicate that a magneto-structural transition occurred at TM. The region above TM or TR is the ferromagnetic austenite phase and that below TM or TR is the paramagnetic or weak ferromagnetic martensitic phase. At constant temperature, a magnetic field-induced strain was observed with a value of 1.0 © 10 ¹3 , which indicates that this alloy is sensitive to magnetic fields. Strong magneto-structural coupling was revealed by the magnetic properties and phase transitions. [doi:10.2320/matertrans.M2012289]

Heat treatment induced martensitic accommodation and adaptive anisotropy in melt spun Ni55Mn22Ga23 (at. %) ribbons

The effect of annealing on melt spun Ni55Mn22Ga23 (at. %) ribbon has been addressed in terms of structural, thermal, thermomagnetic, and magnetic field induced strain (MFIS) behaviour. In comparison to as-spun ribbons, the samples annealed within the L21 domain at 1073 K for 30 h with subsequent furnace cooling, showed a rise in martensitic transformation temperature. This was endorsed from electron microscopy studies revealing change in morphology of martensite plate and stacking of dislocations in a preferential orientation. The mechanism is associated to martensitic accommodation which enhanced ferro-elastic magnetic coupling and also lowered magnetic coercivity. Such accommodations and adaptive anisotropy improved the MFIS behavior in the annealed sample.

Direct measurement of large reversible magnetic-field-induced strain in Ni–Co–Mn–In metamagnetic shape memory alloys

Direct measurements of reversible magnetic-field-induced strain (MFIS) on a single crystalline Ni45Co5Mn36.5In13.5 metamagnetic shape memory alloy were attained via magnetic-field-induced martensitic transformation under different stress levels and at various temperatures. This was achieved using a custom-designed micro-magneto-thermo-mechanical testing system capable of applying constant stress while measuring strain and magnetization simultaneously on the samples, which can fit into conventional superconducting magnets. MFIS levels are reported as a function of temperature, magnetic field and external bias stress. It was necessary to apply an external bias stress in these materials to detect a notable MFIS because a magnetic field does not favor a specific martensite variant resulting in no shape change even though magnetic field leads to reversible martensitic transformation. Fully recoverable transformation strains up to 3.10% were detected under repeated field applications in the presence of different compressive stress levels up to 125MPa. The bias stress opposes the field-induced martensite-to-austenite phase transformation and causes the critical field for the transformation to increase at a given temperature in accordance with the Clausius Clapeyron relationship. The effect of the bias stress on the kinetic arrest of austenite is also explored.

Key Properties of NiMnGa Based Single Crystals Grown with the SLARE Technique

AbstractMagnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of NiMnGa based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of NiMnGa, NiMnGaFe, and NiMnGaCo of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. NiMnGa alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as NiMnGa with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.

Processing Routes Toward Textured Polycrystals in Ferromagnetic Shape Memory Alloys

AbstractNi–Mn–Ga single crystals can produce large strain in moderate magnetic fields. It is the scope of this article to demonstrate, that also polycrystalline materials can show strain in a magnetic field, so called MFIS (magnetic field‐induced strain). In order to design functional polycrystalline materials the interactions of twin boundaries and grain boundaries have to be understood. Therefore, different ways of introducing a texture into Ni–Mn–Ga polycrystals are presented. The different kinds of texture and the consequences for the corresponding materials are discussed. Moreover, thermal, magnetic, and/or mechanical training concepts are presented and their working principle is explained. Several possibilities of evaluating the MFIS capability of the resulting samples are displayed. Finally methods of increasing the strain further are discussed.

Magnetostriction in the vicinity of structural transitions in Ni2MnGa

We report parallel and perpendicular magnetostriction measurements performed in polycrystalline Ni2MnGa to shed light on the magnetoelastic coupling, responsible for the ordinary magnetostriction, and giant magnetic field-induced strains (MFIS). Parallel magnetostriction is negative while its perpendicular component switches from positive to negative at the martensitic transformation. The saturation magnetostriction is negative, takes moderate values and has a large increase in absolute value at both pre- and martensitic temperatures, arising from the lattice softening at the transitions. The reported values are those of the true magnetostriction and can be a starting point for further studies of MFIS.

A large magnetic-field-induced strain in Ni–Fe–Mn–Ga–Co ferromagnetic shape memory alloy

Abstract To obtain a large strain of Ni–Fe–Ga–Co ferromagnetic shape memory alloys, the effects of Mn on the magnetocrystalline anisotropy constant Ku, and the magnetic-field-induced strain (MFIS) under static stresses were investigated. The value of Ku was increased by the addition of Mn, and Ni49.5Fe14.5Mn4.0Ga26.0Co6.0 alloy was accompanied by a relatively large value of −1.5 × 105 J/m3 at room temperature. From the stress–strain curves of this alloy, the twinning stress is estimated to be 8–10 MPa. Consequently, the Ni49.5Fe14.5Mn4.0Ga26.0Co6.0 alloy provides a large MFIS of about 11.3% at room temperature under a static compressive stress of about 8 MPa.

A variational approach towards the modeling of magnetic field-induced strains in magnetic shape memory alloys

In this paper, the magneto-mechanical responses of a magnetic shape memory alloy (MSMA) sample are modeled through the variational approach. Motivated by the experimental results, a constitutive form for the effective magnetization vector is proposed. A (pseudo-)total energy functional for the whole magneto-mechanical system is then formulated, which takes into account the energy dissipations during the variant transformation process. By calculating the variations of the total energy functional with respect to the independent variables, the governing equation system for this model is derived, which is composed of the mechanical equilibrium equations, the magnetic field equations and some evolution laws for the internal variables. Under some simplifications, this model is used to study the magneto-mechanical responses of a MSMA sample under different magnetic and mechanical loading patterns. It can be seen that the results obtained in the model can capture the characteristic features of the field–strain, field–magnetization and stress–strain curves measured in the experiments.

Geometric factors on magnetically driven actuation behaviour for polycrystalline Ni–Mn–Ga and its composites

Abstract The thermo-mechanical behaviour of polycrystalline Ni–Mn–Ga depends on microstructure and several geometric sample properties like grain diameter and sample size. The interdependence of microstructure and shape is more pronounced in plate-shaped specimens. A significant increase of the magnetic field induced strain (MFIS) may be expected for coarse grained specimen because of fewer grain boundaries which are hindering twin boundary movement. It was found that larger grains do not result in higher reorientation strain in every case because the elastically stored energy is distributed on less grain boundaries. Thus, in this case the instability of grain boundaries against cracks is larger than in the case of fine grained samples. The findings were used to design new composites of Ni–Mn–Ga and polyurethane which show up to 1% MFIS and are stable over one million cycles of strain. The particles in this composite are still oriented in a {1 1 0} fibre texture and the volume fraction of polyurethane is up to 10%.

Reversible Magnetic Field Induced Strain in Ni2MnGa‐Polymer‐Composites

AbstractComposite materials consisting of magnetic shape memory alloy particles and a polymer matrix combine the advantages of both material classes: the high achievable magnetic field induced strain (MFIS) of 6% of Ni‐Mn‐Ga with a ductile matrix. Engineering the particle‐matrix interface as well as matching stiffness of polymer matrix is of importance for achieving high reversible MFIS to use this material as actuator or damper. We investigated those properties for Ni50.9Mn27.1Ga22.0 and Ni50.3Mn24.6Ga25.1 polymer composites. Particles were produced by gently crushing melt‐extracted and subsequently annealed fibres. At room temperature, the Ni50.9Mn27.1Ga22.0 particles exhibit a 5M martensitic structure, while the Ni50.3Mn24.6Ga25.1 particles are austenitic. These particles were embedded into the polymer, either a stiff epoxy resin or a soft polyurethane. In response to an external applied magnetic field, the particles tend to relocate within the polyurethane due to its very low Young's modulus and magnetostatic interaction between particles. Slightly stiffer polymer matrices are advantageous for achieving controllable MFIS. In Ni50.9Mn27.1Ga22.0 epoxy composites, a MFIS of 0.1% was observed and was resettable by rotating the magnetic field by 90°. Furthermore, single fibre pull‐out tests indicated significant improvements of the interfacial properties when using silane coupling agent treated fibres.

Stable magnetic-field-induced strain above 1% in polycrystalline Ni–Mn–Ga

Polycrystalline Ni–Mn–Ga samples were directionally solidified and trained in order to lower the twinning stress. Polyurethane was used to fill the cracks. Magnetic-field-induced strain (MFIS) was measured in a rotating magnetic field over more than 10 6 cycles. The operating temperature was varied between 0 and 45 °C. With increasing temperature, the MFIS increased from 0.6% to 1.2%. After cooling to room temperature, an enhanced MFIS of 1.0% was retained and maintained over another 10 6 cycles.

Stress Wave Propagation in Ni–Mn–Ga Ferromagnetic Shape Memory Alloys

Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) are a prominent candidate for actuation application due to their large magnetic field induced strain (MFIS). MFIS in these alloys are mainly due to the twin variant reorientation. This can be achieved only above the magnetic threshold, which requires a large electromagnet. The higher threshold magnetic field for magnetic actuation can be reduced by the assistance of stress waves. The present work is focused on the stress wave propagation in Ni–Mn–Ga alloys. It has been observed that the stress amplitude is high in the frequency range 2 to 5 kHz and is found to be dependent on the input voltage supplied to the actuator which gives out stress wave. The advantage is that, in this frequency range, the threshold field for magnetic actuation can be reduced. It is noted that the stress amplitude is higher in the Ni–Mn–Ga polycrystal (456 mV) than the Ni–Mn–Ga single crystal (385 mV) for the 30 V input peak to peak voltage (Vpp). The finding of this work supports the Ni–Mn–Ga alloy polycrystals for actuation applications with the assistance of stress waves.

Instability of the Parent Phase in Nearly Ordered Fe<sub>3</sub>Pt Invar Alloys

The martensitic transformation in a nearly ordered Fe3Pt is weak first order, and the martensite phase shows a large magnetic field-induced strain of several percent. We have investigated the origin of this transformation from its electronic structure. A characteristic feature in the electronic structure is the existence of a relatively high peak in the density of states of the minority spin band just below the Fermi energy. This peak splits into two peaks by tetragonal distortion, and one of them shifts to lower energy by the distortion, suggesting that the band Jahn-Teller effect is the main cause for the transformation.

Phase-field simulations of magnetic field-induced strain in Ni2MnGa ferromagnetic shape memory alloy

The magnetization and magnetic field-induced strain behavior of the ferromagnetic shape memory alloy, Ni2MnGa, under constant compressive stress were studied using the phase-field method. Based on the evolving magnetic domain and martensitic structures, we analyzed the cycling effect, magnetization hysteresis, strain recoveries, and coupling between the domain wall and martensite twin boundaries. We compared the switching behavior of single variant and multivariant martensite structures. We observed three types of magnetic field-induced strain mechanisms, depending on the magnitude of the applied compressive stress. The study revealed that the martensite microstructure of the magnetic shape memory alloy plays an important role in magnetization and strain evolution during loading and unloading of an external magnetic field under different stress conditions. The results are compared with existing experimental observations.

Crystal Structures of Modulated Martensitic Phases of FSM Heusler Alloys

Multifunctional ferromagnetic shape memory Heusler alloys are frequently characterized by structural modulation in martensitic phases. In particular, modulated martensitic phases, showing the higher magnetic field induced strain (MFIS) performance, are the most promising candidates for technological applications. Depending on the composition, as well as pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along defined crystallographic directions, accompanies the martensitic transformation. Over the years, different Ni-Mn-Ga modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding ideal nM superstructure (where n indicates the number of basic unit cells constituting the superlattices). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure has been fully determined by applying superspace formalism. The results, obtained by structure refinements on powder diffraction data, suggest a unified crystallographic description of the modulated martensitic structures, here presented, where every different “nM” periodicity can be straightforwardly represented.

Magnetic Anisotropy of Ferromagnetic Martensites

The analytic survey of experimental and theoretical studies of the magnetic anisotropy of ferromagnetic shape memory alloys (FSMAs) is presented. The interdependence between the magnetic anisotropy of FSMAs, their lattice parameters, microstructure, and magnetostrain properties is considered. The temperature dependencies of the magnetocrystalline anisotropy energy density (MAED) and magnetically induced mechanical stress are described in the framework of magnetoelastic model based on Landau theory of phase transitions. The magnetic anisotropy of thin martensitic platelets/films and wires is considered. The effect of compensation of magnetocrystalline anisotropy by the magnetostatic one is studied. The reduction of MAED as a result of internal twinning of single crystal is discussed. The possibility of observation of reversible magnetic-fieldinduced strain in the twinned FSMAs with reduced MAED is demonstrated.

Large and highly reversible magnetic field-induced strains in textured Co1−xNixMnSi alloys at room temperature

The metamagnetic transition and magnetoelastic properties are investigated in transition-metal-based Co1−xNixMnSi (0 < x < 0.03) alloys. Large low-field magnetostrains are observed in the Ni-doped CoMnSi alloys. Compared with some other metamagnetic materials with large magnetic field-induced strains, the present alloys show highly reversible magnetoelastic behaviour and have almost constant strain outputs around room temperature. These observations may shed light on developing transition-metal-based magnetostrictive materials.

Influence of constraints and twinning stress on magnetic field-induced strain of magnetic shape-memory alloys

Magnetic-field-induced strain of hard 14M and soft 10M martensitic samples of Ni–Mn–Ga single crystals was measured and optically observed in a rotating magnetic field in one- and two-end-constrained conditions. The soft sample showed smooth, continuous deformation of 0.6% when two-end-constrained and 2.7% when one-end-constrained. The hard sample produced no strain when two-end-constrained, and discontinuous deformation of 1.8% when one-end-constrained. Thus, the constraints reduce, and even block, twin boundary motion. This finding must be considered in experiment and applications.

Effect of pore architecture on magnetic-field-induced strain in polycrystalline Ni–Mn–Ga

Monocrystalline Ni–Mn–Ga alloys show magnetic-field-induced strains (MFIS) of up to 10% as a result of reversible twinning; by contrast, polycrystalline Ni–Mn–Ga shows near-zero MFIS due to strain incompatibilities at grain boundaries inhibiting twinning. Recently, we showed that porous polycrystalline Ni–Mn–Ga exhibits a small, but non-zero, MFIS value of 0.12% due to reduction of these incompatibilities by the porosity. Here, we study the effect of pore architecture on MFIS for polycrystalline Ni–Mn–Ga foams. Foams with a combination of large (∼550 μm) and small (∼80 μm) pores are fabricated by the replication method and exhibit thinner nodes and struts compared to foam containing only large (∼430 μm) pores. When magnetically cycled, both types of foams exhibit repeatable MFIS of 0.24–0.28% without bias stress. As the cycle number increases from a few tens to a few thousands, the MFIS drops due to damage accumulation. The rate of MFIS decrease is lower in the dual-pore foam, as expected from reduced constraints on the twin boundary motion, since twins span the whole width of the thinner nodes and struts.