Magnetic Field-induced Strain Research Articles

A kinetics model for martensite variants rearrangement in ferromagnetic shape memory alloys

A kinetics model describing macroscopic behavior of martensitic variants rearrangement in ferromagnetic shape memory alloys is proposed. The model is based on a tensor description of thermodynamic continuum mechanics taking into account magnetomechanical coupling on the rearrangement process. A generalized thermodynamic driving force which rearranges the martensitic variants is derived. The thermodynamic evolution equations of the kinetics are obtained by means of the dissipation, which is induced by the twin boundary motion during rearrangement process, reaching a maximum. The ability of the model to characterize the macroscopic stress-strain behavior and magnetic field-induced strain of a ferromagnetic shape memory alloy rod under both magnetic field and stress field is demonstrated. The application of the theoretical kinetics model shows the good agreement with experimental observation.

Size Effects on Magnetic Actuation in Ni‐Mn‐Ga Shape‐Memory Alloys

The off-stoichiometric Ni(2)MnGa Heusler alloy is a magnetic shape-memory alloy capable of reversible magnetic-field-induced strains (MFIS). These are generated by twin boundaries moving under the influence of an internal stress produced by a magnetic field through the magnetocrystalline anisotropy. While MFIS are very large (up to 10%) for monocrystalline Ni-Mn-Ga, they are near zero (<0.01%) in fine-grained polycrystals due to incompatibilities during twinning of neighboring grains and the resulting internal geometrical constraints. By growing the grains and/or shrinking the sample, the grain size becomes comparable to one or more characteristic sample sizes (film thickness, wire or strut diameter, ribbon width, particle diameter, etc), and the grains become surrounded by free space. This reduces the incompatibilities between neighboring grains and can favor twinning and thus increase the MFIS. This approach was validated recently with very large MFIS (0.2-8%) measured in Ni-Mn-Ga fibers and foams with bamboo grains with dimensions similar to the fiber or strut diameters and in thin plates where grain diameters are comparable to plate thickness. Here, we review processing, micro- and macrostructure, and magneto-mechanical properties of (i) Ni-Mn-Ga powders, fibers, ribbons and films with one or more small dimension, which are amenable to the growth of bamboo grains leading to large MFIS, and (ii) "constructs" from these structural elements (e.g., mats, laminates, textiles, foams and composites). Various strategies are proposed to accentuate this geometric effect which enables large MFIS in polycrystalline Ni-Mn-Ga by matching grain and sample sizes.

Effect of Volume Fraction of NiMnGa Powders on Magnetic-Field-Induced Strain in NiMnGa/Polymer Composites

Polymer composites inserted with high volume fraction of NiMnGa powders were fabricated and their magnetic-field-induced strain (MFIS) was investigated. It is found that MFIS of NiMnGa/polymer composites increases with the increase of volume fraction of NiMnGa powders in composites. Large MFIS of 950 ppm has been obtained along the orientation magnetic field while only 220 ppm was on the perpendicular orientation magnetic field.

Magnetic field-induced strain and magnetoelectric effects in sandwich composite of ferromagnetic shape memory Ni-Mn-Ga crystal and piezoelectric PVDF polymer

A sandwich composite consisting of one layer of ferromagnetic shape memory Ni-Mn-Ga crystal plate bonded between two layers of piezoelectric PVDF polymer film was fabricated, and its magnetic field-induced strain (MFIS) and magnetoelectric (ME) effects were investigated, together with a monolithic Ni-Mn-Ga crystal, as functions of magnetic fields and mechanical load. The load-free dc- and ac-MFISs were 0.35 and 0.05% in the composite, and 5.6 and 0.3% in the monolithic crystal, respectively. The relatively smaller load-free MFISs in the composite than the monolithic crystal resulted from the clamping of martensitic twin-boundary motion in the Ni-Mn-Ga plate by the PVDF films. The largest ME coefficient (α(E)) was 0.58 V/cm·Oe at a magnetic bias field (H(Bias)) of 8.35 kOe under load-free condition. The mechanism of the ME effect originated from the mechanically mediated MFIS effect in the Ni-Mn-Ga plate and piezoelectric effect in the PVDF films. The measured α(E)-H(Bias) responses under different loads showed good agreement with the model prediction.

Study on Powder Metallurgical Preparation of NiCoMnIn Alloy Foam

The magnetic shape-memory alloy NiCoMnIn shows, in monocrystalline form, a large reversible magnetic-field-induced strain (MFIS). But it is difficult to achieve the properties in polycrystalline NiCoMnIn alloys. The technique of powder metallurgical preparation of NiCoMnIn foam was studied to improve the properties of polycrystalline NiCoMnIn alloys in the present paper. We introduced a processing route including choosing appropriate space-holding fillers, sintering NiCoMnIn alloy and the filling agent with appropriate grain size. The sintering temperature and time and the optimum volume fraction of the filling agent were determined by analysis of the structure of sintered bulk foams.

Magnetic, mechanical and fatigue properties of a Ni45.4Mn29.1Ga21.6Fe3.9 single crystal

In this work the magnetic, mechanical and fatigue behaviour of a Ni45.4Mn29.1Ga21.6Fe3.9 single crystal are reported. The crystal shows a martensite temperature (TM) of 50 °C. A mechanically induced strain of 6% could be detected. The Ni–Mn–Ga–Fe alloy exhibited a magnetic field-induced strain (MFIS) of 4% in a magnetic field of 400 mT. After more than 60,000 magneto-mechanical cycles in a constant magnetic field of 1 T a MFIS of 4.5% was measured.

Magnetic-field-induced strains in polycrystalline Mn1-xCux(0.1≤x≤0.3) alloys

Magnetic-field-induced strain(MFISs) in polycrystalline Mn1-xCux(0.1≤x≤0.3, atomic fraction) alloys are studied by means of X-ray, photomicrograph, DSC, resistance strain gauge method at room temperature. The results show that Mn1-xCux alloys consist of fcc (γ) and fct (γ’) phases after a long period of homogenization because of fcc(γ)→fct(γ’) martensitic transformation in the cooling process. The volume proportion of γ’ phase increases with the increase of Mn content, so that the samples possess much better MFIS performance. At room temperature, the MFIS in Mn0.9Cu0.1 sample reaches 91 ppm in 0.9 T magnetic field.

An Interpretation of Martensitic Transformation in L1&lt;SUB&gt;2&lt;/SUB&gt;-Type Fe&lt;SUB&gt;3&lt;/SUB&gt;Pt from Its Electronic Structure

Partly ordered Fe 3 Pt is one of ferromagnetic shape memory alloys exhibiting a large magnetic field-induced strain in its martensite phase formed by a second-order-like martensitic transformation from the L1 2 -type structure to the so-called FCT martensite (L6 0 -type structure). We have investigated the origin of this transformation from their electronic structures calculated in the present study. 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.

Incommensurate and Commensurate Structural Modulation in Martensitic Phases of FSMA

Magnetic and structural properties in multifunctional FSMA (Ferromagnetic Shape Memory Alloys) belonging to Heusler family are frequently related to the occurrence of structural modulation in martensitic phases. The highest MFIS (Magnetic Field Induced Strain) effect has been observed in Ni-Mn-Ga alloys showing martensitic modulated structures. Depending on the composition, pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along specific crystallographic directions, accompanies the martensitic transformation. Over the years, different modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding superstructure (nM with n=3, 5, 6, 7, 12 etc). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure can be solved by applying superspace approach. The crystallographic representation of different modulated structures, obtained by structure refinement on powder diffraction data, suggests a unified description where every different “nM” periodicity can be straightforwardly represented. It will be presented an overview illustrating structural features of several displacive modulated martensitic lattices. For a specific Ni-Mn-Ga composition, the evolution of structural modulation upon temperature change will be illustrated.

Recent Developments in Ni-Mn-Ga Foam Research

Grain boundaries hinder twin boundary motion in magnetic shape-memory alloys and suppress magnetic-field-induced deformation in randomly textured polycrystalline material. The quest for high-quality single crystals and the associated costs are a major barrier for the commercialization of magnetic shape-memory alloys. Adding porosity to polycrystalline magnetic-shape memory alloys presents solutions for (i) the elimination of grain boundaries via the separation of neighboring grains by pores, and (ii) the reduction of production cost via replacing the directional solidification crystal growth process by conventional casting. Ni-Mn-Ga foams were produced with varying pore architecture and pore fractions. Thermo-magnetic training procedures were applied to improve magnetic-field-induced strain. The cyclic strain was measured in-situ while the sample was heated and cooled through the martensitic transformation. The magnetic field-induced strain amounts to several percent in the martensite phase, decreases continuously during the transformation upon heating, and vanishes in the austenite phase. Upon cooling, cyclic strain appears below the martensite start temperature and reaches a value larger than the initial strain in the martensite phase, thereby confirming a training effect. For Ni-Mn-Ga single crystals, external constraints imposed by gripping the crystal limit lifetime and/or magnetic-field-induced deformation. These constraints are relaxed for foams.

Large magnetoelectric effect from mechanically mediated magnetic field-induced strain effect in Ni–Mn–Ga single crystal and piezoelectric effect in PVDF polymer

Abstract Magnetoelectric (ME) effect is observed in a bilayered composite fabricated by laminating one ferromagnetic shape memory Ni–Mn–Ga single-crystal plate with one piezoelectric PVDF polymer film. The reported ME effect originates from the mechanically mediated magnetic field-induced strain effect in the Ni–Mn–Ga plate and piezoelectric effect in the PVDF film. The induced ME voltage shows a good linear response to the applied ac magnetic field in the field range of 0–5 Oe. A large ME coefficient ( α E ) of 1.24 V/(cm Oe) is obtained at an optimal bias magnetic field of 5.1 kOe due to the reversible reorientation of martensitic twin variants in the Ni–Mn–Ga plate. Based on the combination of pseudo-piezomagnetism in the Ni–Mn–Ga plate and piezoelectricity in the PVDF film, the ME effect in the bilayered composite is predicted and found to agree with the experimental results.

Giant magnetic-field-induced strains in polycrystalline Ni–Mn–Ga foams

The magnetic shape-memory alloy Ni-Mn-Ga shows, in monocrystalline form, a reversible magnetic-field-induced strain (MFIS) up to 10%. This strain, which is produced by twin boundaries moving solely by internal stresses generated by magnetic anisotropy energy, can be used in actuators, sensors and energy-harvesting devices. Compared with monocrystalline Ni-Mn-Ga, fine-grained Ni-Mn-Ga is much easier to process but shows near-zero MFIS because twin boundary motion is inhibited by constraints imposed by grain boundaries. Recently, we showed that partial removal of these constraints, by introducing pores with sizes similar to grains, resulted in MFIS values of 0.12% in polycrystalline Ni-Mn-Ga foams, close to those of the best commercial magnetostrictive materials. Here, we demonstrate that introducing pores smaller than the grain size further reduces constraints and markedly increases MFIS to 2.0-8.7%. These strains, which remain stable over >200,000 cycles, are much larger than those of any polycrystalline, active material.

Large magnetic-field-induced strains in Ni–Mn–Ga nonmodulated martensite

Large magnetic field-induced strains of up to 0.17% for a stress-free Ni53.1Mn26.6Ga20.3 single crystal with nonmodulated martensite phase were generated in a rotating magnetic field. This magnetic-field-induced strain, which is ten times larger than values reported so far for nonmodulated martensites, evidences significant magnetic-field-induced twin boundary motion, which so far was thought to be impossible. This result reinforces the interest in nonmodulated martensites, which are formed as a ground state in the Heusler-type ferromagnetic shape memory alloys.

Martensitic transformation and magnetic field-induced strain in Fe–Mn–Ga shape memory alloy

Martensitic and magnetic properties of Fe–Mn–Ga single and polycrystalline alloys were investigated. It was found that Fe–Mn–Ga alloys exhibit martensitic transformation from the paramagnetic L21 Heusler parent phase to the ferromagnetic L10 martensite phase. The martensitic transformation temperatures increased by about 20 K by the application of a magnetic field of 7 T, and a metamagnetic phase transition was observed. In addition, a magnetic field-induced strain of 0.6% associated with magnetic field-induced forward transformation was confirmed in an Fe43Mn28Ga29 single crystal.

Stress-assisted large magnetic-field-induced strain in single-variant Co–Ni–Gaferromagnetic shape memory alloy

The magnetic anisotropy and the magnetic-field-induced strain (MFIS) in a single-variantCo47.5Ni22.5Ga30.0 ferromagnetic shape memory alloy (FSMA) have been investigated.From the magnetization curves for the single crystal, the hardc-axis was confirmed, and the uniaxial magnetic anisotropy constantKu at 300 K wasevaluated to be −1.07 × 106 erg cm−3 for thesingle-variant Co47.5Ni22.5Ga30.0 martensite phase. The magnitude of compressive shear stress for the variant rearrangementwas estimated to be 6.0–7.5 MPa from the stress–strain curves. An assisted stressτassist of 6.0 MPa was applied before applying a magnetic field, and then a magnetic stressτmag of 0.3 MPa was added. As a result, a large MFIS of about 7.6 % wasobtained at room temperature in the martensite phase of the single-variantCo47.5Ni22.5Ga30.0.

Frequency response of acoustic-assisted Ni–Mn–Ga ferromagnetic-shape-memory-alloy actuator

A prototype of Ni–Mn–Ga based ferromagnetic-shape-memory-alloy (FSMA) actuator was designed and built; an acoustic-assist technique was applied to the actuator to enhance its performance. A piezoelectric stack actuator was attached to the Ni–Mn–Ga sample to generate acoustic energy to enhance twin-boundary mobility and, hence, reduce the magnetic threshold field required for activating twin-boundary motion. The dynamic response of the acoustic-assist FSMA actuator was measured up to 1 kHz actuation. The acoustic assistance improves the actuator performance by increasing the reversible magnetic-field-induced strain (MFIS) by up to 100% (increase from 0.017 to 0.03 at 10 Hz) for drive frequencies below 150 Hz. For frequencies above 150 Hz, the acoustic-assist effect becomes negligible and the resonant characteristic of the actuator takes over the actuator response. Even though the acoustic assist does not improve the actuation at high frequencies, the MFIS output of 5% can be obtained at the resonant frequency of 450 Hz without acoustic assistance. The FSMA actuator is shown to be ideal for applications that require large strain at a specific high frequency

Giant magnetic-field-induced strain in NiMnGaSi magnetic shape memory alloy prepared by diffusion-reduction

NiMnGaSi alloys were prepared by diffusion-reduction reacting which is a new and simple method. The effect of small mount of Si addition on the martensitic transformation temperature (Tm) and strain was investigated. It was found that the addition of Si caused a stronger magnetic exchange interaction leading to higher Curie temperature and giant magnetic field-induced strain. The typical Ni43.03Mn32.07Ga18.48Si6.43 alloy shows high martensitic transformation temperature (Tm) of 300K and a 0.56% giant strain measure at a magnetic field of 0.7 T.

Application of MSM bulk material in a hybrid microactuator

Smart materials such as magnetic shape memory (MSM) metals allow to coming up with new actuators with a rather : simple design. NiMnGa MSM material was applied for creating a hybrid microactuator. Two MSM stripes are mounted in a row. Thin-film stators provide a magnetizing field causing a magnetic field induced strain (MFIS) in NiMnGa MSM stripes. However, the magnetic field is only used for elongating the stripes. The stripe subjected to strain is returning the other MSM stripe mechanically into its non-strained state and vice versa. Compressing them to their non-strained state is provided by mechanical means.

The magnetic field induced strain without prestress and with stress in a polycrystalline Mn–Fe–Cu antiferromagnetic alloy

In this paper magnetic field induced strain (MFIS) of an antiferromagnetic γMn–Fe(Cu) is investigated. It is discovered that there is a high reversible MFIS of 1.6% with positive strain parallel to applied magnetic field at 3.8 T and ultimate negative strain of −0.15% at 1 T in the same way parallel to applied magnetic field after a precompressive stress (strain). The qualitative analysis of positive strain without prestress and the negative strain with precompressive stress under the same condition of applied magnetic field is discussed.

Acoustic-assist effect on magnetic threshold field and twinning-yield stress of Ni–Mn–Ga single crystals

Ni–Mn–Ga based ferromagnetic shape memory alloys (FSMAs) have been shown to exhibit large magnetic-field-induced strain (MFIS) up to 10% for frequencies up to 1 kHz. However, a magnetic threshold field of 150–300 kA/m is typically required to achieve MFIS, which will require a large electromagnet. The application of a small acoustic signal to the FSMA crystal during MFIS has been shown to reduce the threshold field by up to 80 kA/m and increase the stress and strain outputs. We systematically study the threshold field and the twinning-yield stress of Ni–Mn–Ga single crystals with and without acoustic assist. The acoustic-assist technique helps to reduce the required threshold field and twinning-yield stress of the samples studied here by 60%. The effectiveness of the acoustic assistance in enhancing twin-boundary motion increases with increasing acoustic drive frequency up to a limiting frequency of 2–5 kHz. The magnitude of the acoustic-assist effect appears to be limited by the performance of the piezoelectric stack used in this experiment. The acoustic assistance is a result of a bipolar stress wave that enhances twin-boundary motion in either direction. For a given piezodrive frequency and displacement amplitude, the stress-wave theory provides an estimate of the expected acoustic stress-wave magnitude, and this acoustic stress value is consistent with the observed decrease in twinning-yield stress.