Ni Mn Ga Research Articles

Switching the soft shearing mode orientation in Ni–Mn–Ga non-modulated martensite by Co and Cu doping

Resonant ultrasound spectroscopy measurements and density functional theory calculations were used to analyze the effect of Co and Cu doping (3–6 at. %) on elastic constants of non-modulated martensite of the Ni–Mn–Ga ferromagnetic shape memory alloy. Due to the doping, the studied alloys exhibited decreased tetragonal ratios c/a ≈ 1.14. Both the experiments and the calculations revealed that the lowering of the c/a ratio resulted in a change of the orientation of the softest shearing modes of the tetragonal lattice. The newly appearing softest shearing modes for the doped materials have approximately orientations and indicate a lattice instability directly related to the particularly low twinning stress for compound twins, needed for the magnetically induced reorientation.

Systematic Trends of Transformation Temperatures and Crystal Structure of Ni–Mn–Ga–Fe–Cu Alloys

Here we report a systematic research on effects of Fe and Cu upon properties relevant for the magnetic shape memory effect of Ni–Mn–Ga ferromagnetic shape memory alloys. Fe and Cu were identified as elements with potential synergism to increase the martensite transformation temperature of Ni–Mn–Ga magnetic shape memory (MSM) alloys. Eighteen Ni–Mn–Ga–Fe–Cu alloys with different systematic trends in substituting the ternary elements with Cu and Fe have been investigated. We found a method to describe the effectiveness of Ni, Mn, and Cu upon raising the martensitic transformation temperature, lowering the saturation magnetization, and varying the Curie temperature. We find the martensite transformation temperature most influenced by the Ni content, followed by Mn, with a smaller effect of Cu. The saturation magnetization decreases with similar coefficients for Mn and Cu alloying. The Curie temperature monotonously decreases with Mn, but not Cu. The 10M martensite structure is stable for the composition Ni46.5Mn25+XGa25−X−YFe3.5CuY with X and Y range of 0–5.7, and 0.8–3.0. Used in combination with the total e/a, the elemental e/a-ratio gives some insight into the complex behavior of quinary MSM alloys and is a useful method of analyzing MSM alloys for improved functional properties.

Over 2% magnetic-field-induced strain in a polycrystalline Ni50Mn28.5Ga21.5 alloy prepared by directional solidification

Ni–Mn–Ga single crystal alloys can exhibit giant magnetic shape memory effect through variant reorientation induced by the magnetic field. However, such effect is greatly weakened in polycrystalline alloys, due to the orientation differences of martensite variants and the constraints of grain boundaries. Here, to improve the magnetic shape memory effect, a polycrystalline Ni50Mn28.5Ga21.5 alloy with coarse columnar shaped grains and strong <0 0 1>A texture is prepared by directional solidification. With the aids of mechanical training, the twinning stress of five-layered modulated (5 M) martensite in the directionally solidified Ni50Mn28.5Ga21.5 alloy is successfully lowered to ~0.9 MPa. A giant magnetic field induced strain up to ~2.1% is achieved under the magnetic field of 1 T, being much higher than those reported previously in polycrystalline alloys. In addition, a reversible magnetostrain of ~0.4% is also attained without the assistance of an external stress or a magnetic field. It is demonstrated that the microstructure control through directional solidification as well as mechanical training could be an effective solution to enhance the magnetic field induced output strain in polycrystalline Ni–Mn-Ga alloys.

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.

Variant reorientation in a single-crystalline Ni–Mn-Ga sample induced by a quasi-static rotating magnetic field: Mechanism analyses based on configurational forces

In this paper, we introduce a modeling work on the variant reorientation in a single-crystalline Ni–Mn-Ga sample induced by a quasi-static rotating magnetic field. First, some constitutive assumptions are proposed for the variant state distribution, the effective magnetization and the elastic energy density of Ni–Mn-Ga alloys. Then, the total energy functional for a Ni–Mn-Ga sample subject to coupled magneto-mechanical loads is formulated. Through a variational approach, the governing equation system of the model can be formulated, which is composed of the magnetic and mechanical field equations, the evolution laws of some internal variables and the twin interface movement criteria. An efficient numerical scheme is adopted to solve the governing system. With the obtained numerical solutions, we first analyze the evolution properties of the configurational force on the twin interfaces. Especially, the effects of the application directions of magnetic fields on the configurational force are investigated. The influences of the different energy terms on the configurational force are also analyzed. Based on these results, we further conduct numerical simulations on the magneto-mechanical behavior of the Ni–Mn-Ga sample subject to a quasi-static rotating magnetic field. The predicted global responses of the Ni–Mn-Ga sample show good consistency with the experimental result. The configurations of the sample and the distributions of some important physical quantities in the sample during the different loading stages can also be described. Furthermore, by considering the properties of the configurational forces, the underlying mechanisms responsible for the rotating field-induced variant reorientations can be revealed.

Optimizing the Caloric Properties of Cu-Doped Ni-Mn-Ga Alloys.

With the purpose to optimize the functional properties of Heusler alloys for their use in solid-state refrigeration, the characteristics of the martensitic and magnetic transitions undergone by Ni50Mn25−xGa25Cux (x = 3–11) alloys have been studied. The results reveal that, for a Cu content of x = 5.5–7.5, a magnetostructural transition between paramagnetic austenite and ferromagnetic martensite takes place. In such a case, magnetic field and stress act in the same sense, lowering the critical combined fields to induce the transformation; moreover, magnetocaloric and elastocaloric effects are both direct, suggesting the use of combined fields to improve the overall refrigeration capacity of the alloy. Within this range of compositions, the measured transformation entropy is increased owing to the magnetic contribution to entropy, showing a maximum at composition x = 6, in which the magnetization jump at the transformation is the largest of the set. At the same time, the temperature hysteresis of the transformation displays a minimum at x = 6, attributed to the optimal lattice compatibility between austenite and martensite. We show that, among this system, the optimal caloric performance is found for the x = 6 composition, which displays high isothermal entropy changes (−36 J·kg−1·K−1 under 5 T and −8.5 J·kg−1·K−1 under 50 MPa), suitable working temperature (300 K), and low thermal hysteresis (3 K).

Sintering effects on additive manufactured Ni–Mn–Ga shape memory alloys: a microstructure and thermal analysis

This work investigates the effects of time dependency for isothermal sintering on additive manufactured Ni–Mn–Ga magnetic shape memory alloys. Binder jetting additive manufacturing was used to produce Ni–Mn–Ga parts from pre-alloyed powders. Additive manufacturing via the binder jetting technique produces parts with intrinsic porosities, based on the morphology of the source material. The Ni–Mn–Ga parts printed in this study using the binder jetting method possessed average densities of ~ 46% before sintering. These samples were sintered at 1353 K in increments of 10 h up to 50 h. Based on this temperature and time frame, (1) microstructural evolution, (2) crystallographic phase analysis, (3) transformation behaviors, and (4) thermal–physical properties were investigated. The additive manufactured Ni–Mn–Ga samples exhibited increases in densities, from ~ 74 to ~ 83% due to solid-state diffusion mechanisms. X-ray diffraction reveals that all of the additive manufactured samples have the 5 M martensitic phase at room temperature. Reversible martensitic transformation temperatures were recorded during heating and cooling cycles through differential scanning calorimetry, which indicate austenitic phase transformations occurring slightly above ambient temperatures. Additionally, analysis of the heating and cooling cycles prescribes that the entropy and Gibb’s energies decrease over the reversible martensitic transformations as sintering time increases. It is envisioned that this study will support a more synergistic manufacturing process between binder jetting additive manufacturing and post-heat treatment processes for Ni–Mn–Ga shape memory alloys.

An evaluation of the Mn–Ga system: Phase diagram, crystal structure, magnetism, and thermodynamic properties

The Ni–Mn-Ga alloy system is attractive due to its functional properties with potentials in various applications. However, the fundamental alloy thermodynamics of the binary Mn–Ga system is still lack of investigation. Therefore, a comprehensive evaluation of the Mn–Ga is performed in this work. Different versions of Mn–Ga phase diagrams available in the literature are reviewed. A new version of the Mn–Ga phase diagram is recommended along with possible invariant reactions. The crystal structure, magnetic transition, and thermochemical properties of intermetallic compounds are reviewed by considering available experimental and modeling such as ab initio calculations. In fact, more experimental information on the Mn-rich side is required in order to perform CALPHAD thermodynamic modeling for a reliable database. Further experiments are recommended to study the high-temperature phase equilibria between liquid, (γMn), (δMn) and Mn2Ga(h), phase reactions between Mn8Ga5 and Mn7Ga6, and invariant reactions involving the MnGa phase. Nevertheless, the summarized information on phase equilibria, phase diagram, crystallography, magnetic transition temperature, magnetic moment, heat capacity, and enthalpy of formation can support the future thermodynamic investigation of the Mn–Ga system, which is critical for the materials design and discovery of Ni–Mn-Ga alloys.

The Effect of Local Arrangement of Excess Mn on Phase Stability in Ni–Mn–Ga Martensite: An Ab Initio Study

The effect of excess Mn on the stability of modulation and elastic properties was investigated using ab initio electronic structure calculations in Ni–Mn–Ga magnetic shape memory alloy. We used the structure of four layered modulated martensite known as 4O to describe modulation of the martensitic lattice. We found that elastic properties of stoichiometric 4O martensite are very similar to elastic properties of 10M martensite reported in previous calculations. The modulated structure becomes less stable than nonmodulated martensite above 3 at.% of excess Mn which corresponds very well to experimental observation at low temperature. Elastic properties of NM martensite are not significantly affected by Mn content nor local arrangements of Mn-excess atoms. We also found that Mn-excess atoms prefer occupation of distant positions for low Mn-excess composition. The occupation of closest position is preferred for alloys with higher content of Mn.

Crossing twin of Ni–Mn–Ga 7M martensite induced by thermo-mechanical treatment

Thermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of seven-layered modulated (7 M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni50Mn30Ga20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7 M martensite variants with two mutually crossing systems of twin, i.e., type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10}7 M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e., {1 –2 –10}7 M as the twinning plane). A novel transformation orientation relationship between austenite and 7 M martensite in crossing twin is resolved to be {1 0 1}A//{1 –2 10}7 M and <1 0 –1>A//<10 10 1>7 M, in contrast to that of self-accommodated case, i.e., {1 0 1}A//{1 –2 –10}7 M and <1 0 –1>A//<–10 –10 1>7 M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed.

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.

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.

Combinatorial synthesis of Ni–Mn–Ga-(Fe,Co,Cu) high temperature ferromagnetic shape memory alloys thin films

High temperature ferromagnetic shape memory alloys (HT-FSMAs) in form of a thin film materials library of Ni–Mn–Ga, alloyed with Fe, Co and Cu has been fabricated by co-sputtering and characterized by high-throughput screening techniques. The weak dependence of martensitic transformation temperature and tetragonal ratio of the martensitic lattice on the doping composition and their irregular variation are ascribed to the competitive influence of the alloying elements Fe, Co and Cu.

Highly mobile twin boundaries in seven-layer modulated Ni–Mn–Ga–Fe martensite

We report on low twinning stress of 0.2 MPa for type 1 twin boundaries with rational (101) twinning plane and 0.1 MPa for type 2 twin boundaries with rational [1¯01] twinning direction, in Ni2MnGa0.8Fe0.2 seven-layer modulated martensite exhibiting about 12% stress-induced strain. The studied samples were free of compound twins with (100) and (110) twinning planes. The martensite lattice is monoclinic and shows a long-period commensurate modulation with wave vector q=(2/7)g110, where g110 is the reciprocal lattice vector. The investigated alloy has high potential for practical applications due to the low twinning stress and the elevated Curie point TC=400 K.

Numerical Modeling of Magnetomechanical Characteristics of Ni–Mn–Ga Magnetic Shape Memory Alloy

The magnetic field-induced stress and strain of magnetic shape memory alloy (MSMA) are fundamental to describe its magnetomechanical behavior. In order to investigate the operating characteristics of MSMA-based actuator, this paper presents a 3-D quasi-static modeling approach of magnetomechanical behavior of MSMA with the improved constitutive equation by introducing the activation field strength. The relationships between three critical parameters of the activation field strength, the maximum of strain, as well as the saturation field strength, and the pre-compressing force are obtained by fitting the data collected from the experiment. Two kinds of test specimen in different geometric dimensions are studied by the numerical modeling method. Considering that the key material parameters vary dramatically depending on the size and composition, verifications through experiments show that this numerical modeling and simulation can accurately describe the force and deformation characteristics of MSMA. The modeling analysis and simulation results are significant to design of the actuator based on MSMA to predict the actuator performance under the given working conditions.

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.

Influence of internal stress on magnetostrain effect in Ni–Mn–Ga/polymer composite

The outstanding role of internal stress in the silicone matrix driving a magnetostrain reversal of embedded particles of the Ni–Mn-Ga ferromagnetic shape memory alloy after removal of magnetic field, has been disclosed by finite element (FE) simulations and validated by experiment. For simulations, the three case studies have been considered: an isolated particle, and the particles pairs aligned parallel or perpendicularly to the applied magnetic field. The isolated particle provided a 0.4 ​MPa of the reverse stress accumulated in a matrix that was insufficient to recover the composite shape after the removal of magnetic field. Simulations revealed that the strong elastic inter-particle interactions are needed to enhance both the local effective stiffness of composite and the reverse stress. The case where the particles pairs with the optimized inter-particle distance are aligned perpendicularly to the applied magnetic field, is more favorable to obtain the largest magnetostrain recovery. Simulation results demonstrate a 1.8% of the compressive magnetostrain of the 30 ​vol%Ni–Mn-Ga/silicone composite under the field applied parallel to the particles chains, which is in agreement with the experiment. The criteria for a selection of matrix, facilitating a large reversible magnetostrain of composite, have been determined.

Design and Development of Ti-Ni, Ni-Mn-Ga and Cu-Al-Ni-based Alloys with High and Low Temperature Shape Memory Effects.

In recent years, multicomponent alloys with shape memory effects (SMEs), based on the ordered intermetallic compounds B2–TiNi, L21–Ni2MnGa, B2– and D03–Cu–Me (Me = Al, Ni, Zn), which represent a special important class of intelligent materials, have been of great interest. However, only a small number of known alloys with SMEs were found to have thermoelastic martensitic transformations (TMTs) at high temperatures. It is also found that most of the materials with TMTs and related SMEs do not have the necessary ductility and this is currently one of the main restrictions of their wide practical application. The aim of the present work is to design and develop multicomponent alloys with TMTs together with ways to improve their strength and ductile properties, using doping and advanced methods of thermal and thermomechanical treatments. The structure, phase composition, and TMTs were investigated by transmission- and scanning electron microscopy, as well as by neutron-, electron- and X-ray diffraction. Temperature measurements of the electrical resistance, magnetic susceptibility, as well as tests of the tensile mechanical properties and special characteristics of SMEs were also used. Temperature–concentration dependences for TMTs in the binary and ternary alloys of a number of quasi-binary systems were determined and discussed. It is shown that the ductility and strength of alloys required for the realization of SMEs can be achieved through optimal alloying, which excludes decomposition in the temperature range of SMEs’ usage, as well as via various treatments that ensure the formation of their fine- (FG) and ultra-fine-grained (UFG) structure.

Crystallographic insights into diamond-shaped 7M martensite in Ni-Mn-Ga ferromagnetic shape-memory alloys.

For Heusler-type Ni-Mn-Ga ferromagnetic shape-memory alloys, the configuration of the martensite variants is a decisive factor in achieving a large magnetic shape-memory effect through field-induced variant reorientation. Based upon the spatially resolved electron backscatter diffraction technique, the microstructural evolution associated with the martensitic transformation from austenite to seven-layered modulated (7M) martensite was investigated on a polycrystalline Ni53Mn22Ga25 alloy. It was clearly shown that grain interior nucleation led to the formation of diamond-shaped 7M martensite within the parent austenite matrix. This diamond microstructure underwent further growth through an isotropic expansion with the coordinated outward movement of four side habit planes, followed by an anisotropic elongation with the forward extension of a type-I twin pair. A two-step growth model is proposed to describe the specific morphology and crystallography of 7M martensite. In addition, the habit planes were revealed to possess a stepped structure, with the {1 0 1}A plane as the terrace and the {0 1 0}A plane as the step. The characteristic combination of martensite variants and the underlying mechanism of self-accommodation in the martensitic transformation have been analysed in terms of the minimum total transformation strain, where the deformation gradient matrix was constructed according to the experimentally determined orientation relationship between the two phases. The present results may deepen the understanding of special martensite microstructures during the martensitic transformation in ferromagnetic shape-memory alloys.

Tailoring the structural and magnetic properties of epitaxially growth Ni–Mn–Ga magnetic films by Fe addition

ABSTRACTNi–Mn–Ga is one of the major magnetic materials in Heusler family due to its interesting magnetic and structural properties. In this study, the epitaxial 200 nm thin films are prepared by magnetron co-sputtering from a NiMnGa alloy target and an elemental Fe target on MgO substrates. The epitaxial growth mechanism is pursued and controlled by ex-situ X-ray refractometry (XRR) and X-ray diffraction (XRD) measurements. On account of analysing the physical parameters, temperature-dependent X-ray diffraction (XRD), transmission electron (TEM), scanning electron microscopy (SEM), temperature dependence of resistivity (ρ(T)), atomic force microscopy (AFM) and magnetisation (MOKE) measurements are performed systematically for different Fe contents. Structural and magnetic measurements show that the NiMnGa parent thin film transitions could be adjusted from martensitic to austenitic phases by adding Fe element from high temperature to the vicinity of room temperature. Measurements are carried out to get a further structural characterisation of 14M martensite which was found to consist of nano-twinned structures. The epitaxial growth and the stable coexisting austenite and martensitic phases for the thin film of the NiMnGa chemical composition are shown near room temperature. Due to this feature, magnetic thin films could increase the likelihood of possible room temperature using thin film applications.