Ni Mn Ga Single Crystal Research Articles

Adaptive Phase or Variant Formation at the Austenite/Twinned Martensite Interface in Modulated Ni–Mn–Ga Martensite

AbstractThe crystal structure and transformation path from austenite to 10M martensite in Ni–Mn–Ga single crystal are examined employing high‐energy synchrotron radiation, scanning, and transmission electron microscopy. Using temperature gradient, an austenite/twinned martensite interface is stabilized revealing the crystal structure and microstructure of both phases and the transformation sequence across the interface. Depending on the distance from the interface, three distinct types of martensite crystal lattice, namely simple tetragonal and two monoclinic modulated ones, that is, 10M′ and 10M are confirmed. In situ measurements show that lattice mismatch formed at the habit plane is compensated by the formation of micro‐twinned and branched martensite along with an elastic change in lattice parameters. It is shown that the characteristics for periodic shuffling twin boundaries, such as modulation twins or inverted stacking faults, are installed in later stages of transformation. In other words, large microstructure elements that most efficiently accommodate the strain are installed first, and then smaller elements are operable. Overall, the experimental results show that the crystal lattice does not adapt modulated phase at the habit plane and cannot be built up from simple non‐modulated tetragonal blocks but rather a specific sequence of phase transformations using shear/shuffling deformation takes place.

Framework of magnetostrain responsive Ni–Mn–Ga microparticles driving magnetic field induced out-of-plane actuation of laminate composite

Ni–Mn–Ga single crystals (SC) exhibiting a giant magnetic field induced strain (MFIS), resulting from twin boundaries rearrangements, are excellent materials for novel actuators although enhanced brittleness and high costs are remaining the issues for applications. In polycrystalline state Ni–Mn–Ga alloys show small MFIS due to grain boundary constraints. By simple size reduction of the mentioned materials it is hardly possible to create quasi-two-dimensional MFIS actuators on the microscale with a pertinent out-of-plane performance. In pursuit of the trend for next generation materials and functions by design, in the present work we have developed a laminate composite as a prototype of microactuator with the out-of-plane stroke being driven by a framework of magnetostrain responsive Ni–Mn–Ga microparticles. The laminate consisted of the layer of crystallographically oriented Ni–Mn–Ga semi-free SC microparticles sandwiched between bonding polymer and Cu foils. Such design provided a particles isolation with a minimum constraint condition from the polymer. MFIS of the individual particles and the whole laminate composite was investigated by X-ray micro-CT 3D imaging. Both particles and laminate exhibited the same recoverable out-of-plane stroke produced by the particles´ MFIS of around 3% under 0.9 T. The developed microactuator design is promising for applications in the areas of micro-robotics, optical image stabilization in cameras, pumps for microfluidics etc.

Effect of Acoustic Oscillations on Non-Equilibrium State of Magnetic Domain Structure in Cubic Ni2MnGa Single Crystal.

Magnetic hysteresis is a manifestation of non-equilibrium state of magnetic domain walls trapped in local energy minima. Using two types of experiments we show that, after application of a magnetic field to a ferromagnet, acoustic oscillations excited in the latter can "equilibrate" metastable magnetic domain structure by triggering the motion of domain walls into more stable configurations. Single crystals of archetypal Ni2MnGa magnetic shape memory alloy in the cubic phase were used in the experiments. The magnetomechanical absorption of ultrasound versus strain amplitude was studied after step-like changes of a polarizing magnetic field. One-time hysteresis was observed in strain amplitude dependences of magnetomechanical internal friction after step-like variations of a polarizing field. We distinguish two ingredients of the strain amplitude hysteresis that are found in the ranges of linear and non-linear internal friction and show qualitatively different behavior for increasing and decreasing applied polarizing fields. The uncovered effect is interpreted in terms of three canonical magnetomechanical internal friction terms (microeddy, macroeddy and hysteretic) and attributed to "triggering" by acoustic oscillations of the irreversible motion of domain walls trapped in the metastable states. To confirm the suggested interpretation we determine the coercive field of magnetization hysteresis through the measurements of the reversible Villari effect. We show that the width of the hysteresis loops decreases when acoustic oscillations in the non-linear range of domain wall motion are excited in the crystal. The observed "equilibration" of the magnetic domain structure by acoustic oscillations is attributed to the periodic stress anisotropy field induced by oscillatory mechanical stress.

Magnetoplasticity

Abstract Magnetocrystalline anisotropy and twinned microstructure are at the origin of the appreciable magnetic-field-induced strains found in some ferromagnetic thermoelastic martensites. The change of magnetization across the twinning plane leads to a force on the twinning dislocations when a magnetic field is applied. The sensitivity of this basic microscopic mechanism not only to crystallographic and magnetic properties of the material, but also to microstructural details, leads to a large variance in the macroscopically attainable magnetic-field-induced strains, while the maximum strain is fixed for any given martensite structure. The magnetic-field-induced deformation of Ni–Mn–Ga single crystals was studied in uniaxial (“static”) and rotating (“dynamic”) magnetic fields. The spatial and orientational distribution of martensite domains, twin thickness and mutual interactions between twinning dislocations all contribute to the macroscopic result.

Unraveling magneto-structural coupling of Ni2MnGa alloy under the application of stress and magnetic field using in situ polarized neutron diffraction

The magneto-structural evolution of a Ni2MnGa single crystal was investigated by in situ polarized neutron diffraction under both magnetic field and mechanical loading. The ability to separate the nuclear and magnetic scatterings by polarized neutrons enables simultaneous observations of the twin reorientation (through twin boundary motion) and magnetic moment configuration. It is found that under a 1.0 T saturating magnetic field, twin reorientation can be activated by compressive stress and is accompanied by a realignment of the magnetic moment, which follows the easy axis of the twin variant. On the other hand, compressive stress as small as 1.0 MPa can freeze the twin reorientation. The evolution of the magnetic scattering amplitudes suggests that the change in magnetization was mainly achieved through the magnetic domain wall motion, while the twin boundary motion was blocked.

Ambient effects on the output strain of Ni–Mn-Ga single crystal magnetic shape memory alloy

Abstract 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.

Large Non-ergodic Magnetoelastic Damping in Ni–Mn–Ga Austenite

Resonant ultrasound spectroscopy was used to analyze magnetoelastic damping in Ni–Mn–Ga single crystals in austenite and premartensite phases. Crystals with different treatment were studied, exhibiting different density of antiphase boundaries (APBs), as confirmed by magnetic force microscopy, and different magnetic behavior. For a quenched single crystal with high density of APBs, extremely strong damping was observed in a broad temperature range in the austenite phase. It was shown that this damping is history-dependent, i.e., non-ergodic, appearing only during heating runs preceded by a premartensite → austenite transition. We suggest that this non-ergodicity results from the pinning of the fine magnetic domain structure on APBs and other defects.

Hierarchical twin microstructure in modulated 10M Ni–Mn-Ga single crystals. An analysis including shuffling of atomic layers

The trained and self-accommodated martensite microstructures of 10M Ni–Mn-Ga single crystals were studied by back scattered electron imaging and electron backscatter diffraction in a scanning electron microscope. These data were then compared with theoretical calculations using a long periodic commensurate modulated monoclinic lattice and Gautam and Howe (G-H) model in order to establish twin boundary (TB) hierarchy according to their surface energy. The whole spectrum of TBs including type I, type II, and compound twins were detected and theoretically predicted. Using the G-H model and long periodic structure along with a small displacement of atoms connected with lattice modulation allowed to calculate all twin elements including their surface energy. These data are then confronted with theoretical predictions of classical continuum mechanics and minimum shear approaches, which disregard the lattice modulation. As it is shown including the modulation modifies the twin systems (twinning plane and twinning direction) for some TBs. Moreover, it strongly affects the interfacial energy which allows to rank TBs in the 10M Ni–Mn-Ga system. Large differences in surface energy between different TBs are associated with atomic interface configurations and de-shuffling of atoms to form a coherent twin plane. Therefore, unlike the classical geometrical concepts, the G-H model allows to perform not only a quantitative but also qualitative analysis of all possible twin boundaries. As a result of new approach, a model that involves interfacial energy, shuffling of atoms and homogenous shear type deformation for TB determination is presented. In addition, irrational or step-like planes not only for type II but also for type I twin boundaries is predicted. Furthermore, a description of twin formation with respect to two different reference systems is completed, i.e. parent-based and monoclinic ones. In this way, a hierarchical twin microstructure of 10M martensite is established.

Effect of Stress-Induced Martensite Aging on Martensite Variant Reorientation Strain in NiMnGa Single Crystals

This work presents the effect of stress-induced martensite aging (SIM-aging) on the functional properties of Ni53Mn25Ga22 (at.%) single crystals, oriented along the [001]L21-direction. The most effective regime of SIM-aging occurred under a compressive stress of 175 MPa along the [110]L21||[100]L10-direction at 423 K, for 2 h, which induced the high-temperature two-way shape memory effect with a reversible tensile strain of +9.0%, causing the jumping of the sample during cooling/heating in free state, and a rubber-like behavior with a giant reversible compression strain up to − 13.2% caused by L10-martensite variant reorientation.

Antiphase boundaries, magnetic domains, and magnetic vortices in Ni–Mn–Ga single crystals

The interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni–Mn–Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments. The APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ~1 K/min resulted in a low density (<1/µm), air quenching in a medium density (≈8/µm), and water quenching in a high density (≈15/µm) of APBs. Abundant pinning of domain walls on APBs was observed, which resulted in close correspondence between the magnetic domain walls and antiphase boundaries (APBs), magnetic domain memory, and finer domain patterns for the high APB density. For low APB density, a labyrinth domain structure was established between domain walls pinned on APBs. For low and medium density of APBs the magnetization was oriented mostly parallel to the out-of-plane easy magnetization axis. For high APB density the magnetization switched to the in-plane orientation, indicating that the effective magnetic anisotropy decreased and became lower than the stray field energy. In addition, magnetic vortices appeared. The novel functionalities based on a combination of MIR and interactions of the magnetic structure with APBs are feasible since the MIR was observed even for the highest density of APBs.

A simple method to characterize high rate twin boundary kinetics in Ni-Mn-Ga

Experimental characterization of twin boundary kinetics is essential to systematically test and reproduce the actuation properties of Magnetic Shape Memory (MSM) elements at high rates. Here, we present a simple, nondestructive, experimental method to quantify the dynamic response of an MSM crystal and extract the major material properties that govern its kinetics. The tested sample is subjected to a mechanical pulse that is produced by a simple off-the-shelf solenoid. The mechanical pulse leads to actuation of the tested MSM Ni–Mn–Ga single crystal within 10 ms, during which the twin boundary velocity varies between zero and 2 m/s. The displacement and force in the MSM crystal are measured simultaneously using an optical sensor and a miniature force sensor, respectively. The data captured during a single loading experiment allow plotting a dynamic stress-strain curve as well as a kinetic relation that characterizes the macroscopic response of the crystal. In particular, the obtained kinetic relation enables the extraction of the transition driving force between slow (thermally activated) and fast (athermal) twin boundary motions. This transition driving force is a key material property that governs fast actuation capabilities of MSM elements. The macroscopic behavior of the sample is correlated with the motion of individual twin boundaries within the crystal by adding high speed microscopy to the experimental setup. This allows simultaneous high-rate tracking of individual twinning interfaces in Ni–Mn–Ga.

Modelling the stress-induced multi-step martensite transformation of single crystal NiMnGa ferromagnetic shape memory alloys

Existing experimental results have shown that multi-step martensite transformation can be induced by the applied stress in single crystal NiMnGa ferromagnetic shape memory alloys (FSMAs) and the transformation property depends on the ambient temperature strongly. In this work, a crystal-plasticity based constitutive model is established to quantify such a phenomenon. Based on the crystallographic symmetry, the lattice structures and eigenstrains of austenite (A) and three different martensite phases, i.e., the modulated martensite phases with a periodicity of 5 lattice cells (5M) and a periodicity of 7 lattice cells (7M), and a non-modulated (NM) martensite phase are analyzed. A three-dimensional (3D) single crystal constitutive model is constructed in the framework of finite strain by considering five different martensite transformation processes, i.e., the transformations between A and 5M, A and 7M, A and NM, 5M and 7M, and 7M and NM phases. Thermodynamically consistent kinetic equations for the five different martensite transformation processes are proposed based on the constructed Helmholtz free energy and Clausius's dissipative inequality. The capability of the proposed model to describe the tensile stress-induced multi-step martensite transformation of single crystal NiMnGa FSMA in [100] crystallographic orientation is verified by comparing the predictions with corresponding experimental results. Further, the proposed model is used to predict the deformations of the single crystal under the tension and compression in various crystallographic orientations.

Magnetic noises emitted during martensitic transformation in Ni49Fe18Ga27Co6 single crystalline shape memory alloys

Magnetic noises during martensitic transformations in single crystalline Ni49Fe18Ga27Co6 (at. %) alloys were measured. The critical exponents of the acoustic and magnetic noises were slightly different from each other in contrast to those obtained in Ni2MnGa single crystals, measured so far. The presence of nano-sized γ-particles increased the noise activity and resulted in bipolar magnetic noise, in contrast to mostly unipolar signals observed in the homogeneous sample or in Ni2MnGa. Application of a moderate external magnetic field (20.9 mT) decreased the bipolar nature of the noise, which was qualitatively explained by the magnetoelastic coupling and by the more ordered magnetic domain structure. Nano-sized precipitates resulted in more fine steps of the jerky motion of the interface and caused barriers against of the easy development of the magnetic structure.

Microstructure evolution and kinetic laws for the motion of multiple twin boundaries in Ni–Mn–Ga

The motion of individual twin boundaries in ferromagnetic Ni–Mn–Ga alloys is the basic mechanism by which large reversible strains are generated. The resulting mechanical response makes these alloys potential candidates for magneto-mechanical actuation applications. In this work, we characterize the relations between the magnetically induced strain response of Ni–Mn–Ga single crystals and their internal twin boundary arrangements and kinetics. The macroscopic response is measured under constant magnetic fields, while the kinetics of individual twin boundaries is measured by the pulsed magnetic field method. By testing two different crystals with different twinning microstructures, we show that the main characteristics of the macroscopic response can be controlled by the number of mobile twin boundaries. In addition, we show that the magnitudes of controlling kinetic parameters of different twin boundaries vary with their location within the sample.

Rapid floating zone growth of Ni2MnGa single crystals exhibiting magnetic shape memory functionality

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.

Elastic and anelastic phenomena related to eddy currents in cubic Ni2MnGa

We study temperature, magnetic field and strain amplitude dependences of the Young's modulus and ultrasonic absorption in the cubic phase of stoichiometric Ni2MnGa single crystal, using longitudinal oscillations (90kHz) along [100] direction. We show that the additional softening of the Young's modulus below the Curie temperature, usually considered as elastic premartensitic phenomenon, originates instead from conventional microeddy current relaxation associated with individual domain wall motion. In general, existing controversial experimentally observed damping and “elastic” softening behaviours are controlled by a combination of micro- and macroeddy current relaxations, operating at different spatial scales and over distinct frequency ranges.

Using Kerr Microscopy for Direct Observation of Magnetic Domains in Ni–Mn–Ga Magnetic Shape Memory Alloy

Giant magnetic-field-induced strain up to 12% in Ni–Mn–Ga single crystal is due to magnetically-induced reorientation of the twinned martensitic low symmetry crystal structure. An important interplay is arising from the interaction between ferroelastic domains (twin domains) and magnetic domains. Here, we studied the magnetic domains in an Ni50Mn28Ga22 single crystal exhibiting 6% strain by using a magneto-optical indicator film and for the first time also directly by the Kerr magneto-optical microscopy. To compare the magnetization behavior of bulk and surface, we measured the magnetization loops by the Kerr effect (MOKE) and by the vibrating sample magnetometry. We demonstrate that using a particular arrangement of the Kerr microscopy enables visualizing the magnetic domains structure and its evolution in the magnetic field.

The Effect of Heat Treatment on the Shape Memory Effect and Superelasticity in NiMnGa Single Crystals

It was shown in Ni53Mn25Ga22 single crystals that the annealing at 1273 K for 1 h followed by slow cooling, at which the (L21+γ)-structure is formed, provides the increase of crystals plasticity as compared with the brittle initial L21-single crystals. The increase of critical stresses by more than 2 times (up to σ &gt; 150 MPa), at which the cooling/heating cycles under stress are accompanied by sample destruction, is observed. Moreover, the value of thermal hysteresis is increased by 1.5 times and the perfect curve of superelasticity is observed at T=423 K.

Magnetic Torque in Single Crystal Ni–Mn–Ga

Magnetic shape memory alloys deform in an external magnetic field in two distinct ways: by axial straining—known as magnetic-field-induced strain—and by bending when exposed to torque. Here, we examine the magnetic torque that a magnetic field exerts on a long Ni–Mn–Ga rod. A single crystal specimen of Ni–Mn–Ga was constrained with respect to bending and subjected to an external magnetic field. The torque required to rotate the specimen in the field was measured as a function of the orientation of the sample with the external magnetic field, strain, and the magnitude of the external magnetic field. The torque was analyzed based on the changes in the free energy with the angle between the field and the sample. The contributions of magnetocrystalline anisotropy and shape anisotropy to the Zeeman energy determine the net torque. The torque is large when magneotcrystalline and shape anisotropies act synergistically and small when these anisotropies act antagonistically.

Magnetocaloric effect in Ni2MnGa single crystal in the vicinity of the martensitic phase transition

The magnetocaloric effect in the vicinity of the martensitic transformation for a single crystalline alloy with a composition close to the stoichiometric Ni2MnGa has been determined indirectly by M(T,H) magnetization measurements. It has an inverse character. The magnetocaloric parameters, i.e., the magnetic entropy change, refrigeration capacity and various hysteretic effects have been calculated from the M(T,H) dependences. Besides the martensitic transition a weak entirely separated intermartensitic transition was observed. These two successive magneto-structural transformations give contributions to the observed magnetocaloric effect. Unusual dependence of entropy change as a function of magnetic field has been explained as arising because of two different mechanisms. Additionally, to confirm that studied martensitic transformation is a first order phase transition electrical resistivity and thermoelectric power measurements have been performed.