Reorientation Of Twin Variants Research Articles

Increase of twinning stress in single crystalline Ni-Mn-Ga micropillars

We have investigated the effect of characteristic size decrease on the magneto-mechanical properties of Ni-Mn-Ga single crystalline 10M martensite, using micropillars prepared by focused-ion-beam milling. Following the electro-chemical surface treatment, the twinning stress in a focused-ion-beam milled micropillar with a characteristic size of about 17 µm was reduced from 17 MPa to below 2 MPa, allowing the magnetically induced reorientation of twin variants. We found that twinning stress universally increases with the decrease of characteristic size. In combination with the data for bulk material, our results suggest a power law dependence of twinning stress on characteristic size, with an exponent of -0.5 for both Type I and Type II single twin boundaries. Extrapolation of the fit suggests the characteristic size limit for magnetically induced reorientation to be 30 µm and 2 µm for Type I and Type II twin boundaries, respectively.

Evolution of microstructure and crystallographic texture of Ni-Mn-Ga melt-spun ribbons exhibiting 1.15% magnetic field-induced strain

The microstructure and texture evolution of 10M Ni-Mn-Ga melt-spun ribbons were thoroughly evaluated by high-energy synchrotron radiation and electron backscatter diffraction. The as-spun ribbons were subjected to annealing treatment in order to tailor microstructure, atomic order degree, and crystallographic texture. The optimum annealing treatment at 1173 K for 72 h produced a homogenous <100> fiber texture and induced grain growth to the size that spans the entire ribbon thickness. This in turn reduced microstructural constraints for twin variant reorientation in the direction perpendicular to the ribbon surface. On the other hand, a homogenous radial microstructure ensured in-plane stress/strain compatibility giving rise to strain accommodation during variant reorientation. Particular attention was also given to the evaluation of atomic order, which to the largest extent controls the characteristic transformation temperatures. It also lowered the twinning stress to a level sufficiently low for martensitic variant reorientation under magnetic field. As a result, 1.15% magnetic field-induced strain without the aid of mechanical training in the self-accommodated state was achieved.

The Effect of Particle Shape on Magnetic Field-Induced Rubber-Like Behavior of Ni-Mn-Ga/Silicone Composites

The single crystalline of the prototype Ni-Mn-Ga ferromagnetic shape memory alloy exhibits a huge magnetic field-induced strain of 6% due to the re-orientation of twin variants. Therefore, it is promising as the magneto-driven actuator material. However, the magnetic field or mechanical stress applied along the orthogonal direction is necessary to restore the deformed bulk single crystal into the original shape. In order to avoid an application of the extra fields, the Ni-Mn-Ga particles/polymer composite is an alternative solution supported by the concept of the accumulated stress in a polymer matrix which drives a deformation recovery of the composite during switching off of the magnetic field. The objective of this study is to clarify the effect of embedded particle shape on the deformation recovery of Ni-Mn-Ga particles/silicone composites by using the finite element method (FEM) and to investigate the local stress distribution in a polymer matrix between either spherical or rectangular shape particles pairs, whereby to provide the guidelines for design/optimization of the magneto-strain-active Ni-Mn-Ga particles/polymer composites. The case studies of the simulation are divided into an isolated particle and a pair of particles, the particles being positioned either parallel or perpendicular to the applied magnetic field direction. Particularly, the simulations reveal that in case of 200 µm of the inter-particle distance in the pair of spherical particles aligned perpendicularly to the applied field, the polymer layer between particles generates the compressive recovery stress of -0.32 MPa, which is insufficient to restore the deformation of the embedded Ni-Mn-Ga particles during removal of the magnetic field. By contrast, the strain recovery effect can be achieved for the rectangular particle pairs, generating the stress concentration in a matrix of about -0.5MPa, in a similar condition and arrangement.

Thermal and structural properties of the martensitic transformations in Fe7Pd3 shape memory alloys: an ab initio-based molecular dynamics study

Ferromagnetic shape memory alloys, including the Fe7Pd3 system, constitute an upcoming class of functional materials, whose atomic-scale physical foundations are still insufficiently understood. The present work employs molecular dynamics simulations, based on ab initio derived embedded atom method potentials, to study martensitic transformations and twin variant reorientation. We address thermal and stress induced austenite-martensite transitions, twinning, as well as twin boundary mobility. While the predicted thermal properties are in accordance with experimental observations, we explore the detailed crystallography underlying transformation as well as twin boundary motion.

Magnetic shape memory effect in single crystalline Ni-Mn-Ga foil thinned down to 1μm

We report on observation of magnetic-field-induced twin variant reorientation in a thin foil with variable thickness, fabricated from a bulk five-layered Ni49.5Mn28Ga22.5 single crystal. Formation and motion of twinning interfaces in the foil were induced by magnetic fields of 0.6T. The characteristic “labyrinth” magnetic domain structure was fully retained down to 1 μm thickness. Dependence of the characteristic width of magnetic domains on the sample thickness followed a power law with the exponent of 2/3. The reported results indicate that micrometer-scale sized Ni-Mn-Ga devices, fabricated from a bulk, can be actuated by magnetic field.

Microstructure of as-cast single and twin roller melt-spun Ni2MnGa ribbons

Stoichiometric Ni2MnGa alloys are processed by two rapid solidification techniques – single-roller (SR) and twin-roller (TR) melt spinning – and the resulting microstructures and magnetic properties determined. Samples processed at tangential wheel speeds of 10m/s (V10) and 15m/s (V15) are studied in the as-cast condition to analyze the influence of the production methods on the microstructure. Important aspects like the resulting phases, their crystallographic texture, magnetic properties, martensitic transformation temperatures and Curie temperatures are compared. In addition, the magnetization mechanism involving twin boundary motion is explored. Our results indicate that the TR method provides lower cooling rates, thicker samples, higher internal stresses and larger MnS precipitates. However, the quenching rate is mainly determined by the tangential wheel velocity. TR samples also exhibit [100] texture normal to the ribbon plane but in a lesser extent than SR ribbons. Martensitic transformation temperatures are higher in samples V15 (~150K) than in V10 (~100K), with no clear difference between the SR and TR modes. This behavior is explained by considering distinct degrees of disorder in the L21 austenite phase resulting from quenching. The hysteresis of the transformation, defined as the difference Af−MS, takes similar values in the four samples analyzed. Pre-martensitic transformation temperatures are also slightly higher in samples V15, (230±3) K, than in samples V10, (222±3) K, as the magnitude of the Hopkinson effect, in good agreement with a higher residual stress level in TR ribbons. In the martensitic state, all ribbons exhibit hysteresis loops characteristic of a magnetization mechanism involving twin boundary motion. The switching magnetic fields for the onset of Type I twin boundary motion result between 220mT and 365mT, values equivalent to twinning stresses of about 1MPa. It is concluded that both procedures, SR and TR melt spinning, provide microstructures favoring magnetic field induced twin variant reorientation.

Origin of steps in magnetization loops of martensitic Ni-Mn-Ga films on MgO(001)

We study the temperature dependent magnetization properties of (010)-oriented Ni-Mn-Ga epitaxial films on MgO(001) substrates. In the martensitic phase, we observe pronounced abrupt slope changes in the magnetization loops for all studied samples. Our experimental findings are discussed in conjunction with the micromagnetic simulations, revealing that the characteristic magnetization behavior is governed solely by the magnetization switching within the specific martensitic variant pattern, and no reorientation of twin variants is involved in the process. Our study emphasizes the important role of the magnetostatic interactions in the magnetization behavior of magnetic shape memory alloy thin films.

Directin situmeasurement of coupled magnetostructural evolution in a ferromagnetic shape memory alloy and its theoretical modeling

In this study, ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next generation smart devices due to their exceptionally large magnetic-field-induced strains and fast response times. During application of magnetic fields in FSMAs, as is common in several magnetoelastic smart materials, there occurs simultaneous rotation of magnetic moments and reorientation of twin variants, resolving which, although critical for design of new materials and devices, has been difficult to achieve quantitatively with current characterization methods. At the same time, theoretical modeling of these phenomena also faced limitations due to uncertainties in values of physical properties such as magnetocrystalline anisotropy energy (MCA), especially for off-stoichiometric FSMA compositions. Here, in situ polarized neutron diffraction is used to measure directly the extents of both magnetic moments rotation and crystallographic twin-reorientation in an FSMA single crystal during the application of magnetic fields. Additionally, high-resolution neutron scattering measurements and first-principles calculations based on fully relativistic density functional theory are used to determine accurately the MCA for the compositionally disordered alloy of Ni2Mn1.14Ga0.86. The results from these state-of-the-art experiments and calculations are self-consistently described within a phenomenological framework, which provides quantitative insights into the energetics of magnetostructural coupling in FSMAs. Based onmore » the current model, the energy for magnetoelastic twin boundaries propagation for the studied alloy is estimated to be ~150kJ/m3.« less

Achieving giant magnetically induced reorientation of martensitic variants in magnetic shape-memory Ni-Mn-Ga Films by microstructure engineering.

Giant magnetically induced twin variant reorientation, comparable in intensity with bulk single crystals, is obtained in epitaxial magnetic shape-memory thin films. It is found to be tunable in intensity and spatial response by the fine control of microstructural patterns at the nanoscopic and microscopic scales. A thorough experimental study (including electron holography) allows a multiscale comprehension of the phenomenon.

Evolution of magnetic domain structure of martensite in Ni—Mn—Ga films under the interplay of the temperature and magnetic field

Ferromagnetic shape memory Ni—Mn—Ga films with 7M modulated structure were prepared on MgO (001) substrates by magnetron sputtering. Magnetization process with a typical two-hysteresis loop indicates the occurrence of the reversible magnetic field-induced reorientation. Magnetic domain structure and twin structure of the film were controlled by the interplay of the magnetic and temperature field. With cooling under an out-of-plane magnetic field, the evolution of magnetic domain structure reveals that martensitic transformation could be divided into two periods: nucleation and growth. With an in-plane magnetic field applied to a thermomagnetic-treated film, the evolution of magnetic domain structure gives evidence of a reorientation of twin variants of martensite. A microstructural model is described to define the twin structure and to produce the magnetic domain structure at the beginning of martensitic transformation; based on this model, the relationship between the twin structure and the magnetic domain structure for the treated film under an in-plane field is also described.

Nanoscale surface properties of a Ni–Mn–Ga 10M magnetic shape memory alloy

Highly mobile twin boundaries allow straining of the 10M Ni–Mn–Ga martensite single crystals with low as 0.15 MPa uniaxial stresses. This is clearly lower than what is possible to magnetically generate in this structure. Therefore, this material can be used for magnetically controlled actuation. In cyclic deformation occurring by twin variant reorientation, fatigue life up to 2 × 10 9 cycles has been demonstrated without decrease of the magnetic-field-induced strain or without failure of the specimen. However, the actuation behavior can be strongly impaired by unfavorable twin variant configuration or surface constraints which decrease the twin mobility and increase the twinning stress. In the present work, the surface properties of the 10M Ni–Mn–Ga are studied in details. The existence of an ultra-thin surface layer having chemical composition different from the bulk crystal is established with the XPS analysis. The influence of this layer on the nano-mechanical properties of the surface is investigated using instrumented nanoindentation both on electropolished crystals and crystals where surface deformation has been introduced by mechanical polishing. The results show that the surface layer appears both in the electropolished and mechanically polished surface. However, the nano-mechanical properties of the surface are distinctly different in the two cases. In the case of electropolished surface essentially elastic loading occurs first followed by a notable pop-in of the indenter, while in the case of mechanically polished surface minor or no pop-in is observed. This behavior is attributed to the differences of the initiation of plastic deformation in the two types of surfaces. The contribution of the layer to the performance and actuation properties of the material is discussed. © 2012 Elsevier B.V. All rights reserved.

Transformation of dislocations during twin variant reorientation in Ni–Mn–Ga martensite structures

The transformation of perfect dislocation configurations during twin variant reorientation in Ni–Mn–Ga martensites was analyzed by means of the correspondence matrix method. It was found that the dislocations can change the length of the Burgers vectors remarkably, and hence increase the self-energy by several percent of their initial value. It is suggested that this may affect the mechanical condition of operation of the mechanism responsible for twin variant reorientation.

Influence of grain size and training temperature on strain of polycrystalline Ni50Mn29Ga21 samples

The alloy Ni-Mn-Ga aroused great interest for application as a magnetic shape memory (MSM) material. This effect is caused by reorientation of twin variants by an external magnetic field. So far, most of the experiments were concentrated on single crystals. But, the MSM effect can also be realised in polycrystals which can be prepared much more efficiently. Here, polycrystalline samples were prepared by directional solidification with a <100> fibre texture of the high temperature cubic austenitic phase parallel to the heat flow. Afterwards, a heat treatment was applied for chemical homogenisation and stress relaxation in the austenitic state. Then the samples were heated up to the austenitic state and cooled down under load. The microstructure was analysed by Electron Back Scatter Diffraction (EBSD) before and after that treatment. Mechanical training at room temperature and 40°C was tracked by recording stress-strain curves. By increasing the number of training cycles the strain also increases. The influence of different training temperatures was investigated on samples with different grain sizes.

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.

Long-Term Cyclic Loading of 10M Ni-Mn-Ga Alloys

Martensitic 10M Ni-Mn-Ga single crystals are usually applied in the magnetomechanical actuators. Their behavior in the long-term cycling and the factors influencing upon are thus important to know. There are several publications concerning this. However, consistent statistical data is still missing to large extent. In this report we review and analyze the data available in the literature. In conclusion it can be stated that 10M Ni-Mn-Ga single crystals survive well in long-term actuation (millions of cycles) when the frequency of twin variant reorientation is not exceeding 250 Hz, and the strain used in actuation is below 3 %. There are several factors influencing the long-term behavior and these are discussed in more details in this work.

Biocompatibility of Single Crystalline Ferromagnetic Shape Memory Films for Cell Actuation

Ferromagnetic shape memory alloys (FSMAs) have received great attention recently as an exciting class of smart functional materials. They exhibit large reversible strains of several percent at moderate stresses due to an external magnetic field induced reorientation of twin variants in the martensitic phase. External controllability at constant temperatures and sufficiently high strains thus make them excellent candidates for biomedical actuation devices, such as surgical implant materials, applying for bone prostheses or drug delivery systems. In comparison to conventional shape memory alloys, FSMA bears the significant potential for miniaturized devices for single cell actuation which is capable of yielding magnetically controllable shear strains and/or volume dilations of several percent, thus perfectly matching the requirements of cell investigations. However, the biocompatibility of this material must first be well confirmed as it has not been done yet. Thus, our work focuses on the interaction of fibroblast cells with single crystalline Fe70Pd30 FSMA films on MgO substrates. Additionally, corrosion resistance of the films was obtained employing simulated body fluid (SBF) tests. Calcium-phosphate aggregates with granular microstructure were detected on the film surface after soaking in SBF. Cell viability and biocompatibility tests with NIH 3T3 cells revealed that the cells adhered and proliferated on the surface of the FSMA, whereas cells were smaller compared to cells on culture dish surfaces. Biocompatible polymer coatings on the Fe70Pd30 film can be employed to improve cell proliferation and cell-substrate interactions.

Simulation of an Improved Microactuator with Discrete MSM Elements

Magnetic Shape Memory (MSM) alloys are a new class of “smart” materials. In the martensite state, they exhibit a reversible strain due to a reorientation of twin variants, based on twin boundary motion driven by an external magnetic field occurring in the martensite state. This effect allows for the development of linear microactuators. This work presents the simulation results for the fabrication of a microactuator based on an MSM alloy with an optimized design. A stator element consists of a NiFe45/55 flux guide, two poles, and double-layer Cu coils wound around each pole for generating the magnetic field. The MSM material applied is NiMnGa. The integrated microactuator is subjected to dynamic simulation, using a “checkerboard” pattern to locally switch the magnetic properties when the relative permeability µr is changed. The model is described with the Ansys Parametric Design Language (APDL). Design, modeling, and simulation of the magnetic system including MSM material, are conducted by Finite Element Method (FEM) analysis using the software tool ANSYS™.

Preferential reorientation of twin variants in Ni–Mn–Ga single crystal

The evidence for the preferential orientation of Ni–Mn–Ga martensitic variants by compression or magnetic field is presented. The evolution of martensitic twinning configuration in Ni 50Mn 27.5Ga 22.5 single crystal was optically observed initially under the applied compressive stress and then followed by the magnetic field. The magnetic field induced strain (MFIS) was measured at different conditions. The MFIS was not observed in the original three-variant self-accommodated state. One percent MFIS was obtained after compression under 5 MPa, attributed to a disappearance of the self-accommodated structure replaced by a two-variant structure. During magnetization, the favorable variants expand at the expense of others. After several cycles of compression and successive magnetization, a near single variant state and MFIS of 5.5% were achieved in the rectangular single crystal. Forty-five degree macroscopic twin bands were observed during the MFIS testing on (0 1 0) surface of an oriented rectangular sample with {1 0 0} faces.

Performance of magnetically controlled shape memory alloys and actuators

Magnetically controlled shape memory (MSM) alloys develop large strains when exposed to a magnetic field. Strains are based on reorientation of twin variants in the martensite phase. Magnetic-field-induced reorientation of variants is possible if magnetocrystalline anisotropy energy of the material is sufficiently high and twinning stress is low enough. Among several ferromagnetic shape memory alloys only few of them exhibit so low twinning stress that conversion of twin variants can be controlled by the magnetic field. Such alloys are, e.g., Ni-Mn-Ga, Co-Ni-Ga, Fe-Pd and Fe-Pt. MSM effect was demonstrated in a non-stoichiometric Ni 2 MnGa alloy in 1996, and still the best performance is achieved in these alloys. Magnetic-field-induced strains of 10 % are reached in an orthorhombic phase of this alloy system. In a tetragonal phase free strain is 6 % and the reversible strain against a load of 1.5 MPa is even 5.5 percent. AdaptaMat produces single crystalline Ni-Mn-Ga whose twinning stress is as low as 0.7 MPa. In this material even 70 % of magnetic field energy is converted to mechanical work. Lowest operating temperature of the MSM effect in Ni-Mn-Ga was measured 120 K and the highest temperature is 330 K so far. Several kinds of actuator and sensor devices have been made, e.g., pneumatic valves, positioning devices and vibrators. MSM actuators work at high frequencies (over 2.5 kHz). After 200 million strokes actuators do not show any decrease of the magnetic-field-induced strains. MSM materials are a new way of producing motion and force in mechanical engineering. They are faster than hydraulic and pneumatic devices and over 100 times faster than shape memory alloys. MSM actuators' power output is higher than that of piezoelectric and magnetostrictive devices, voice coil transducers and solenoids.