Ni Mn Ga Single Crystal Research Articles

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.

In situ neutron diffraction study of twin reorientation and pseudoplastic strain in Ni–Mn–Ga single crystals

Twin variant reorientation in single-crystal Ni–Mn–Ga during quasi-static mechanical compression was studied using in situ neutron diffraction. The volume fraction of reoriented twin variants for different stress amplitudes were obtained from the changes in integrated intensities of high-order neutron diffraction peaks. It is shown that, during compressive loading, ∼85% of the twins were reoriented parallel to the loading direction resulting in a maximum pseudoplastic strain of ∼5.5%, which is in agreement with measured macroscopic strain.

Effects of surface damage on twinning stress and the stability of twin microstructures of magnetic shape memory alloys

Twinning is the primary deformation mechanism in magnetic shape memory alloys (MSMAs). Obstacles such as inclusions, precipitates and defects hinder or even prevent twin boundary motion in the bulk of Ni–Mn–Ga MSMA single crystals. Here, we study the effect of surface damage on the mechanical properties and twin structure of Ni–Mn–Ga single crystals. Any methods that produce defects may be considered for modifying the near-surface microstructure. In this study deformations were produced by grinding and mechanical polishing using abrasive particles. The amount of damage was characterized with X-ray diffraction: damage causes peak broadening. Deformation and damage localized near the surface increases the twinning stress. Surface damage stabilizes a densely twinned microstructure. The twins are thin but extend over the entire sample and allow a large strain to be accommodated at moderate stress. This effect is critical for preventing damage accumulation in high-cycle magnetomechanical actuation and for achieving high dynamic performance.

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.

Corrosion study of single crystal Ni–Mn–Ga alloy and Tb0.27Dy0.73Fe1.95 alloy for the design of new medical microdevices

Once placed in a magnetic field, smart magnetic materials (SMM) change their shape, which could be use for the development of smaller minimally invasive surgery devices activated by magnetic field. However, the potential degradation and release of cytotoxic ions by SMM corrosion has to be determined. This paper evaluates the corrosion resistance of two SMM: a single crystal Ni-Mn-Ga alloy and Tb(0.27)Dy(0.73)Fe(1.95) alloy. Ni-Mn-Ga alloy displayed a corrosion potential (E (corr)) of -0.58V/SCE and a corrosion current density (i (corr)) of 0.43 μA/cm(2). During the corrosion assay, Ni-Mn-Ga sample surface was partially protected; local pits were formed on 20% of the surface and nickel ions were mainly found in the electrolyte. Tb(0.27)Dy(0.73)Fe(1.95) alloy exhibited poor corrosion properties such as E (corr) of -0.87V/SCE and i (corr) of 5.90 μA/cm(2). During the corrosion test, this alloy was continuously degraded, its surface was impaired by pits and cracks extensively and a high amount of iron ions was measured in the electrolyte. These alloys exhibited low corrosion parameters and a selective degradation in the electrolyte. They could only be used for medical applications if they are coated with high strain biocompatible materials or embedded in composites to prevent direct contact with physiological fluids.

Acoustic energy absorption in Ni–Mn–Ga/polymer composites

Composites of Ni–Mn–Ga particles in a polyurethane (PU) matrix are known to show up to 70% energy absorption under quasi-static mechanical loading. Reported here are results showing that the large mechanical energy absorption in Ni–Mn–Ga/PU composites persists up to at least 1 kHz and exceeds that in each of these control samples of pure PU, as well as in Fe 2(Tb 1/3Dy 2/3) particles in epoxy, and single crystal Ni–Mn–Ga. The transmitted acoustic amplitude decreases linearly with increase in frequency, consistent with the theory, and at 1 kHz it is 0.7 V in the Ni–Mn–Ga single crystal, 0.65 V in pure PU, 0.45 in the Fe 2(Tb 1/3Dy 2/3)/PU composites and 0.25 V in Ni–Mn–Ga/PU composites.

Underlying mechanism of dual hysteresis in NiMnGa single crystals

NiMnGa single crystals are compressed in the [001] orientation at room temperature. The stress–strain response exhibits multiple stages as the deformation proceeds. Initially, the response exhibits a stress drop in the stress–strain curve preceding a plateau stress. By contrast, the second transition, which occurs at five times the initial critical transformation stress, produces a work hardening like response. Complete pseudoelastic (PE) recovery ensues upon unloading with the reverse transformation exhibiting the same two-stages. Furthermore, for the dual hysteresis that results, the second hysteresis is nearly four times wider than the first. The underlying transformation path for each stage is ascertained from local strain analysis utilizing variable magnification in situ digital image correlation (DIC). We distinguish three different morphological transitions; band formation, phase front propagation, and heterogeneous growth. The morphologies can be attributed to austenite undergoing the successive transitions 10M→14M (modulated to modulated) and 10M→L10 (modulated to non-modulated). Differential critical stress and hysteresis levels are rationalized based on the initial modulated-to-modulated and successive modulated-to-non-modulated transition. The strain–temperature response is reported, as well, and exhibits a tiny thermal hysteresis (5°C), which is attributed to the modulated-to-modulated conversion.

Transformation twinning of Ni–Mn–Ga characterized with temperature-controlled atomic force microscopy

The magnetomechanical properties of ferromagnetic shape memory alloy Ni-Mn-Ga single crystals depend strongly on the twin microstructure, which can be modified through thermomagnetomechanical training. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) were used to characterize the evolution of twin microstructures during thermomechanical training of a Ni-Mn-Ga single crystal. Experiments were performed in the martensite phase at 25 degrees C and in the austenite phase at 55 degrees C. Two distinct twinning surface reliefs were observed at room temperature. At elevated temperature (55 degrees C), the surface relief of one twinning mode disappeared while the other relief remained unchanged. When cooled back to 25 degrees C, the twin surface relief recovered. The relief persisting at elevated temperature specifies the positions of twin boundaries that were present when the sample was polished prior to surface characterization. AFM and MFM following thermomechanical treatment provide a nondestructive method to identify the crystallographic orientation of each twin and of each twin boundary plane. Temperature dependent AFM and MFM experiments reveal the twinning history thereby establishing the technique as a unique predictive tool for revealing the path of the martensitic and reverse transformations of magnetic shape memory alloys.

Dc- and ac-magnetic field-induced strain effects in ferromagnetic shape memory composites of Ni–Mn–Ga single crystal and polyurethane polymer

Ferromagnetic shape memory composites of multilayer and sandwich types were fabricated by laminating Ni–Mn–Ga single-crystal plates with polyurethane (PU) polymer plates. The dc- and ac-magnetic field-induced strains (MFISs) in the composites were measured as functions of both magnetic field and mechanical load, and the results were compared with those of the single crystal. It was found that the load-free dc-MFISs were 5.6%, 1.5%, and 0.8%, while the load-free ac-MFISs were 0.3%, 0.8%, and 0.5% in the single crystal, multilayer composite, and sandwich composite, respectively. The relatively smaller load-free dc-MFISs and larger load-free ac-MFISs in the composites than the single crystal originated from the stress bias of the Ni–Mn–Ga plates by the PU plates in the composites. The dc-MFISs of all samples and the ac-MFISs of the composites decreased with the increase in mechanical load amplitude, while the ac-MFIS of the single crystal peaked at 1.6 MPa load.

Effects of surface roughness and training on the twinning stress of Ni–Mn–Ga single crystals

Spark eroding, which is commonly employed to cut samples out of magnetic shape-memory alloy single crystals, produces a rough surface layer. Directly after cutting, the single crystals exhibit a high twinning stress. After removal of a surface layer by electropolishing, the twinning stress reduces significantly and stress–strain curves become serrated. The reduction of twinning stress has previously been attributed to the removal of the defective surface layer. In this work, it is shown that different surface treatments in combination with repeated mechanical deformation experiments significantly reduce the twinning stress, regardless of whether or not electropolishing is used. The reduction of the twinning stress is due to softening that takes place as a mechanical training effect, which occurs with mechanical testing. In addition, the stress–strain curves of samples subjected to different surface treatments differed in so far as the curves of electropolished samples showed serrated flow, while the curves of unpolished samples and those of mechanically polished samples were smooth. Furthermore, the unpolished samples displayed significant hardening at higher strain. Following subsequent mechanical polishing, this hardening reduced to nearly zero, and the average twinning stress decreased another 30–50% to 1.6 MPa and below. For these samples, the twinning stress stayed at a very low level until twinning was complete.

Anisotropy of the electrical transport properties in a Ni2MnGa single crystal: Experiment and theory

Electrical transport properties in ferromagnetic shape memory Ni–Mn–Ga single crystal have been investigated both in experiment and theory by analyzing electrical resistivity along different crystallographic directions during heating. The experimental results show a clear first-order martensitic transformation and a large anisotropic resistivity (AR) of 23.7% at the tetragonal martensitic phase. The theoretical conductivity (σ=1/ρ), estimated using first-principles calculations combined with classical Boltzman transport theory, proves essential crystallographic anisotropic resistivity (AR=31%) in the martensitic phase and agrees well with experimental results. The AR in the martensitic phase is reveled to mainly originate from the splitting of the minority-spin Ni 3d and Ga 4p states near the Fermi level and hence reconstruction of the minority-spin Fermi surface upon martensitic transformation.

Twin microstructure dependent mechanical response in Ni–Mn–Ga single crystals

Ni–Mn–Ga single crystals with a twinning stress of about 0.1 MPa were studied. They showed a tendency to stay in a single variant state and to retain only one or very few twin boundaries during martensite reorientation induced by an external stress or magnetic field. This makes the crystals problematic for application in a magnetic actuator. To solve the issue, we introduced many parallel twin boundaries into the crystals by bending. However, this twin microstructure was not stable under cycling load. Additionally, it exhibited a twinning stress of 0.8 MPa—about ten times higher than a crystal with a single boundary.

Magnetomechanical characterization and unified energy model for the quasistatic behaviorof ferromagnetic shape memory Ni–Mn–Ga**This work was performed by the authors at The Ohio State University.

This paper presents an overview of the characterization and modeling of single crystalferromagnetic shape memory Ni–Mn–Ga. A continuum thermodynamics model is presentedwhich describes the magnetomechanical characterization of single crystal Ni–Mn–Ga for thefollowing behavior: (i) sensing effect; (ii) actuation effect; (iii) blocked force (stressgeneration). The thermodynamic potentials, namely the magnetic Gibbs energy and theGibbs energy, are obtained from the Helmholtz energy in order to arrive at the set ofrequired independent and dependent variables; the potentials include magnetic energyconsisting of Zeeman, magnetostatic and anisotropy components, and mechanical energyconsisting of elastic and twinning components. Mechanical dissipation and themicrostructure of Ni–Mn–Ga are incorporated in the continuum model through theinternal state variables volume fraction, domain fraction, and magnetization rotationangle. The constitutive response of the material is obtained by restricting theprocess through the second law of thermodynamics. The model requires only sevenparameters identified from two simple experiments. Several interesting characteristics ofNi–Mn–Ga are examined in concert with the magnetomechanical characterization.

Effect of Mn concentration on the phase transformation in Ni–Mn–Ga single crystal

Aim of this paper is to study the transformation temperature of Ni–Mn–Ga single crystals by using differential scanning calorimetry. It shows that martensitic temperature abruptly increase with increasing Mn concentration whereas Curie transition to decrease. The calculated value of electron-to-atom ratio ( e/ a) reveal that it increases with increased doping of Mn in place of Ga at fixed Ni content. The crystal structures at room temperature were studied using XRD pattern. Thermo-magnetic behavior of the Ni–Mn–Ga alloy shows the ferromagnetic nature and its behaviour is slightly losing due to antiferromagnetic ordering of excess Mn atoms. Microstructure from SEM image confirm that the alloys are martensitic plates. The martensitic plates contain fewer stripes like magnetic domains.

Ultrahigh anisotropic damping in ferromagnetic shape memory Ni–Mn–Ga single crystal

Damping behavior with an oscillating bending force applied to each of the three different crystallographic planes a– b, a– c, and c– a is measured in a tetragonal ferromagnetic shape memory Ni 50Mn 29Ga 21 single crystal at different combinations of frequency and temperature by a dynamic mechanical analysis method. The results show a frequency-independent anisotropic damping behavior in the martensitic phase with a loss factor of 0.03, a high loss factor of 0.15, and an ultrahigh loss factor of 0.26 for the a– b, a– c, and c– a crystallographic planes, respectively. The observed anisotropic damping behavior is found to originate from the reorientation of different numbers of martensitic twin variants upon bending. It is also adjustable by an applied magnetic field via the control of the martensitic twin-variant bending states.

Giant two-way shape memory effect in high-temperature Ni–Mn–Ga single crystal

A perfect two-way shape memory effect (TWSME) with reversible strain of about 9% has been found in the high-temperature Ni57.5Mn22.5Ga20.0 single crystal transforming into 2M non-modulated martensitic phase. Two thermal/mechanical treatment routes were utilized in the experimental procedures. The outstanding high-temperature TWSME is observed as a result of tensile stress-strain cycling along 〈100〉 axis involving huge superelastic strains due to stress-induced martensitic transformation. It is argued that the TWSME is related to the anisotropic internal stresses produced by a network of lattice defects. The defects are generated in the course of oriented growth/shrinkage of the dominating martensitic variant in the tensile sample.

Production and characterization of large single crystals made of ferromagnetic shape memory alloys Ni–Mn–Ga

Ferromagnetic shape memory alloys (MSMA) are well-known smart materials for actuation applications, due to their large shape change in applied magnetic fields. Off-stoichiometric Ni–Mn–Ga single crystals of MSMA exhibit a magnetic-field induced strain of about 10%. In the past decade a lot of effort has been put into understanding and optimizing the properties of Ni-Mn-Ga alloys in terms of the magnetic shape memory effect, starting with the production of single crystals and going all the way to heat treatment and magneto-mechanical training.Here we focus on and present the production and characterization of single crystal Ni–Mn–Ga alloys with off-stoichiometric composition. The molten material was cast into preheated ceramic shells and directionally solidified in a Bridgman-type process. Cylindrical single crystal bars of diameters up to 48 mm and a length of 110 mm were obtained by using suitable grain selectors. Only small misorientations between the single crystal c-axis and the bar axis were found in these bars. The crystals were characterized in terms of composition, segregation, crystal orientation and microstructural features by means of SEM/EDS, EBSD and light microscopy.

In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition of Ni2MnGa Single Crystal under High Magnetic Field

The defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni2MnGa single crystal under a high magnetic field of 50 KOe applied along the \( \left[ {1\bar{1}0} \right] \) crystallographic direction of the Heusler phase were studied by the in-situ high-energy X-ray diffuse-scattering experiments on the high energy synchrotron beam line 11-ID-C of APS and thermomagnetization measurements. Our experiments show that a magnetic field of 50 KOe applied along the \( \left[ {1\bar{1}0} \right] \) direction of the parent Heusler phase can promote the premartensitic transition of Ni2MnGa single crystal, but puts off martensite transition and the reverse transition. The premartensitic transition temperature (TPM) increases from 233 to 250 K (−40 to −23 °C). The martensite transition start temperature (Ms) decreases from 175 to 172 K (−98 to −101 °C), while the reverse transition start temperature (As) increases from 186 to 189 K (−87 to −84 °C). The high magnetic field leads to a rapid rearrangement of martensite variants below the martensite transition finish temperature (Mf). The real transition process of Ni2MnGa single crystal under the high magnetic field was in-situ traced.

Nonlinear magneto-mechanical finite element analysis of Ni–Mn–Ga single crystals

This paper presents a three-dimensional (3D) nonlinear quasi-static isothermalfinite element formulation for single crystals of Ni–Mn–Ga ferromagnetic shapememory alloy (FSMA). This formulation is directly coupled (strong coupling)between the magnetic vector potential and mechanical displacement. As a result thisformulation is applicable to general structural configurations, where the mechanical andmagnetic behaviour of the entire structural and magnetic system can be computedsimultaneously. This finite element formulation has the potential for applications thatrequire accurate prediction of the magnetic field distribution and strain response ofNi–Mn–Ga single crystals. One of the major advantages of this finite elementformulation is the ability to predict the behaviour of the material for generalized orcomplicated shapes and geometries where analytical methods may not be adequate.The two-dimensional nonlinear finite element analysis computer software MAGSTRAN(magnetic structural analysis) is developed based on this formulation. Numerical results forselected examples are validated against experimental data available in the literature. It isdemonstrated that there is reasonable correlation between MAGSTRAN results andexperimental data available in the literature.

Direction of modulation during twin boundary motion

To study the change of modulation direction during deformation, two NiMnGa single crystals , one with a five-layered modulated tetragonal structure (5 M) and the other with a seven-layered modulated orthorhombic structure (7 M), were chosen. Synchrotron diffraction experiments show that the modulation takes place on the {1 1 0) plane along the 〈1 1 0] directions. During deformation the c -axis orientation changes by twinning, and with this the direction of modulation obeying the twin relation also changes.