Heusler Alloy Ni2MnGa Research Articles

A novel sintering method for polycrystalline NiMnGa production for elastocaloric applications

NiMnGa Heusler alloy plays a key role as reference system for ferromagnetic shape memory alloys (FeSMAs) and their peculiar functional properties including large magnetic-field-induced strain, magnetocaloric and elastocaloric effects. Moreover, the microstructure of polycrystalline NiMnGa alloys has been investigated and optimized to improve the mechanical properties and to reduce their typical brittleness. For this reason, increasing interest has been devoted to different kinds of fabrication routes for this alloy, such as powder metallurgy processes. In the present study, a polycrystalline Ni50Mn30Ga20 (atomic %) alloy is produced by means of an unconventional sintering method that involves the canning of powders and the subsequent processing by hot rolling. This process was implemented according to the results obtained by means of the open die pressing (ODP) sintering of NiMnGa which was investigated in a previous work. The present study is aimed at developing an alternative and cost-effective sintering method for the consolidation of fully-dense NiMnGa samples. The process allowed reducing the intrinsic brittleness of the alloy. A heat treatment at 925 °C for 6 h was applied and allowed achieving a maximum adiabatic ΔT of +6.3 °C and - 4.5 °C with a strain of 4 % and a strain rate of 400 %/min in compression. The novel method led to very promising elastocaloric properties, making NiMnGa a suitable candidate for solid-state cooling and heating applications.

Polymer‐based filaments with embedded magnetocaloric Ni‐Mn‐Ga Heusler alloy particles for additive manufacturing

AbstractOne important issue associated to magnetocaloric materials that hinders its technological application is the poor processability and structural integrity of those with the highest performance, usually intermetallics undergoing first‐order magnetic phase transitions. Additionally, the performance of these magnetocaloric materials highly depends on the structural stability of the magnetocaloric phase, which is, in many cases, very sensitive to temperature and mechanical processes. Additive manufacturing via the extrusion of polymer‐based composites is regarded as a promising way to overcome these issues. A recently presented manufacturing method of encapsulating functional fillers into polymer capsules has been used to produce a composite filament with a large load of magnetocaloric off‐stoichiometric Ni2MnGa Heusler alloy fillers with a uniform distribution throughout the polymer matrix as demonstrated by x‐ray tomography characterization. The incorporation of these metallic particles causes changes in the thermal behavior of the polymer as well as an increase in the flowability of the composite with respect to the polymer at the same temperature. The increased flowability of the composites found during manufacturing can be compensated by lowering the extrusion temperatures, making this technique even more convenient for preserving the filler properties, which is an important concern when additive manufacturing magnetocaloric materials. This is confirmed by the magnetic and magnetocaloric behavior of the composites, with responses proportional to the fraction of fillers.Highlights Ultrasonic‐atomization produces highly spherical Ni‐Mn‐Ga Heusler alloy particles. Ni‐Mn‐Ga filled polymer capsules allow a direct extrusion of composites for AM. X‐ray tomography shows uniform volumetric filler distribution within the filaments. Decreased viscosity of the matrix favors the lowering of the processing temperature. The low processing temperatures avoid altering the MCE of the alloy fillers.

Effect of Partial Substitution of Ga on the Structural and Magnetic Properties of the Ni–Mn–Ga Heusler Alloys

Effect of Partial Substitution of Ga on the Structural and Magnetic Properties of the Ni–Mn–Ga Heusler Alloys

Phase Field Simulations of Microstructures in Porous Ferromagnetic Shape Memory Alloy Ni2MnGa

The magnetic domain structures and martensite microstructures of porous Ni2MnGa Heusler alloys with various circle-shaped and ellipse-shaped pores were systematically studied by the phase field method. The magnetization curves and magnetic field-induced strains (MFIS) at the external field were determined. A mesoscopic mechanism was proposed for simulation to reveal the influence of the pores on the microstructures and the MFIS of porous magnetic shape memory alloy. The stress concentration effect and the recovery strain of the porous alloy are studied. The results indicate the MFIS value increases when ellipse-shaped pores elongate along the twin boundary. The effects of porosity and pore size on MFIS for porous Ni-Mn-Ga alloys with randomly distributed pores were also explored. The present study is of guiding significance for understanding the role played by pores on the MFIS and may provide a possible way to adjust the functional properties of ferromagnetic shape memory alloys.

High efficiency direct magnetocaloric effect in Heusler Ni2MnGa microwire at low magnetic fields

A perspective magnetocaloric Heusler alloy Ni2MnGa in the unique form of a microwire is studied in this work. Magnetic and calorimetric analyses are compared, providing insight into the operating conditions of magnetic refrigeration, which is the essential interconnection of material sciences and device engineering. Presented results describe the highly efficient magnetocaloric effect of Ni2MnGa at low magnetic fields up to 0.3 T. A direct thermometry method, by micro-thermocouple, is employed to directly measure the temperature change of the single piece of a microwire; weighing only 2.36 mg. The mentioned method allows studying the magnetocaloric effect even on the micro-samples in operating conditions. It later allows finding the best conditions, e.g. the temperature range and the amplitude and frequency of the magnetic field change for studied materials.

Selective incorporation of Fe and Co into the Ni2MnGa (001) surfaces: a DFT analysis

By DFT calculations, we investigated the Co and Fe incorporation into the Ni2MnGa Heusler alloy (001) surface. Calculations consider the Ga-, Ni- and MnGa-terminated surfaces. First-principles thermodynamics was used to determine the stability of the structures. Fe and Co provide selective incorporation into different sites that depend on the growth conditions. For Fe incorporation, four surfaces are stable. High-selectivity was noticed, at Ga-rich and Mn-rich conditions Fe incorporates into Ni sites. From Mn-rich to Mn-poor at Ga-poor conditions, the Mn by Fe substitution is the most stable. At Mn-poor conditions and Ga-intermediate conditions, Fe atoms take on Ga sites. In the case of Co, our stability analysis shows four stable surfaces as well, with similar behavior to the Fe incorporation, since for the region from Mn-rich to Mn-poor at Ga-poor conditions the substitution of Mn by Co is stable; at Ga-rich conditions and from Mn-rich to Mn-intermediated conditions, Co atoms occupy the Ni sites just as in the Fe case. However, in contrast to the Fe case, the Co by Ga substitution is not favorable. Instead, the ideal surfaces appear as the stable structures. Magnetic properties are investigated, finding in all cases a ferromagnetic coupling between the impurities and surface.

Characterizing Changes in Grain Growth, Mechanical Properties, and Transformation Properties in Differently Sintered and Annealed Binder-Jet 3D Printed 14M Ni–Mn–Ga Magnetic Shape Memory Alloys

Ni–Mn–Ga Heusler alloys are multifunctional materials that demonstrate macroscopic strain under an externally applied magnetic field through the motion of martensite twin boundaries within the microstructure. This study sought to comprehensively characterize the microstructural, mechanical, thermal, and magnetic properties near the solidus in binder-jet 3D printed 14M Ni50Mn30Ga20. Neutron diffraction data were analyzed to identify the martensite modulation and observe the grain size evolution in samples sintered at temperatures of 1080 °C and 1090 °C. Large clusters of high neutron-count pixels in samples sintered at 1090 °C were identified, suggesting Bragg diffraction of large grains (near doubling in size) compared to 1080 °C sintered samples. The grain size was confirmed through quantitative stereology of polished surfaces for differently sintered and heat-treated samples. Nanoindentation testing revealed a greater resistance to plasticity and a larger elastic modulus in 1090 °C sintered samples (relative density ~95%) compared to the samples sintered at 1080 °C (relative density ~80%). Martensitic transformation temperatures were lower for samples sintered at 1090 °C than 1080 °C, though a further heat treatment step could be added to tailor the transformation temperature. Microstructurally, twin variants ≤10 μm in width were observed and the presence of magnetic anisotropy was confirmed through magnetic force microscopy. This study indicates that a 10 °C sintering temperature difference can largely affect the microstructure and mechanical properties (including elastic modulus and hardness) while still allowing for the presence of magnetic twin variants in the resulting modulated martensite.

Modeling of the Magnetization and Magnetocaloric Effect in Ni2MnGa Heusler Alloy with the Effective Field Theory

Modeling of the Magnetization and Magnetocaloric Effect in Ni2MnGa Heusler Alloy with the Effective Field Theory

Ab-Initio Study of Tuning the Electronic and Magnetic Properties of Ni2mnga Heusler Alloy by Co and Mn Compound Doping

Ab-Initio Study of Tuning the Electronic and Magnetic Properties of Ni2mnga Heusler Alloy by Co and Mn Compound Doping

The effects of Cu-substitution and high-pressure synthesis on phase transitions in Ni2MnGa Heusler alloys

The magnetic, structural, and thermal behaviors of the Cu-doped Heusler alloy Ni2Mn1−xCuxGa (0 ⩽ x ⩽ 0.4) were studied as a function of concentration x. As the Cu concentration increased, the structural transition temperatures increased, whereas the chemical order-disorder transitions and melting points decreased. The experimental results from temperature dependent X-ray diffraction reveal different crystal structures of the martensite phase at low temperatures for samples with different x, but all the samples ultimately crystallized in the L21 cubic crystal structure upon heating above their respective structural transitions. The experimental data were used to construct a comprehensive magnetic and structural phase diagram as a function of x from below their respective structural transition temperatures to their melting temperatures. The XRD analysis shows that the observed volume reduction is associated with the increasing structural transition temperature. Therefore, one of the samples was annealed under high pressure to permanently reduce its volume, and the correlation between the increasing structural transition temperatures and volumes was confirmed.

The effect of uniaxial stress on magneto-electronic properties and band Jahn–Teller distortion of Ni2MnGa Heusler alloy: an ab initio study

ABSTRACT In the present work, we employed the DFT (Density Functional Theory) to investigate the electronic and magnetic properties of Ni2MnGa Heusler alloy. We focused on the structural transition between tetragonal and cubic structures related to the band Jahn Teller effect. The structural electronic and magnetic properties are calculated for cubic and tetragonally distorted structures. According to the optimisation of total energy versus volume, the tetragonal phase is the ground state for Ni2MnGa alloy 0 K. The band structure and density of state calculations show the ferromagnetic (FM) behaviour, with a total magnetic moment of 4.053 µB and 19.21% of spin polarisation at the Fermi level. Ni2MnGa is a cubic compound above 276 K under a thermal effect, where it gets its shape memory behaviour. Applying uniaxial stress according to the z axis, the studied alloy undergoes a phase transition to the parent phase (cubic) at a pressure around 2.8 GPa. Ni2MnGa alloy has the double possibility of phase transition under thermal effect and uniaxial stress.

A Ternary Map of Ni–Mn–Ga Heusler Alloys from Ab Initio Calculations

In the search for new magnetic functional materials, non-stoichiometric compounds remain a relatively unexplored territory. While experimentalists create new compositions looking for improved functional properties, their work is not guided by systematic theoretical predictions. Being designed for perfect periodic crystals, the majority of first-principles approaches struggle with the concept of a non-stoichiometric system. In this work, we attempt a systematic computational study of magnetic and structural properties of Ni–Mn–Ga, mapped onto ternary composition diagrams. Compositional stability was examined using the convex hull analysis. We show that the cubic austenite has its stability region close to the stoichiometric Ni2MnGa, in agreement with experimental data, while the tetragonal martensite spreads its stability over a wider range of Mn and Ni contents. The unstable compositions in both austenite and martensite states are located in the Ga-rich corner of the ternary diagram. We note that simultaneous stability of the austenite and martensite should be considered for potentially stable compounds suitable for synthesis. The majority of compounds are predicted to be ferrimagnetically ordered in both austenitic and martensitic states. The methodology used in this work is computationally tractable, yet it delivers some predictive power. For experimentalists who plan to synthesize stable Ni–Mn–Ga compounds with ferromagnetic order, we narrow the target compositional range substantially.

Martensitic and magnetic transitions in Ni2+xMnGa1−x ferromagnetic shape memory alloys

This study experimentally investigates a series of phase transitions of ferromagnetic shape memory alloys, Ni2+xMnGa1−x, in the composition range 0.02 ≤ x ≤ 0.27. An experimental phase diagram of these alloys was constructed by the use of electrical resistivity, thermomagnetization, and thermoanalysis measurements, and compared with both the Ni-rich Ni2+xMn1−xGa and Mn-rich Ni2Mn1+yGa1−y alloy systems which have been extensively studied as proto-materials for ferromagnetic Heusler alloys. The comparison makes it clear that there are remarkable composition dependencies for the following characteristic temperatures. All three systems have the common property that Curie temperature decreases and martensitic transition temperature increases with increasing x till the both temperatures coincide each other. As for the magnetostructural coupling temperature, however, the Ni-rich system has a wide temperature range, and the Mn-rich system does not show signs of the magnetostructural coupling, whereas the magnetostructural coupling was observed in Ni2+xMnGa1−x alloys in the range 0.10 ≤x≤ 0.12 which has similar characteristics between the former two systems. The premartensitic transition temperature was also observed in Ni2+xMnGa1−x alloys; it reached the ambient temperature at x = ~0.04, the highest of the three alloy systems. In addition, an intermartensitic transition appeared in Ni2+xMnGa1−x alloys in the composition range 0.06 ≤ x ≤ 0.10, which showed a drastic change especially in the electrical resistivity. These characteristic temperatures are also compared in terms of the number of valence electrons among the Ni–Mn–Ga Heusler alloy systems, and the differences are clarified and discussed in detail.

Structure, magnetic and magnetocaloric properties of Ni2MnGa Heusler alloy nanowires

Heusler alloys are favorable candidates for fabricating functional devices and sensors due to their characteristic structural and magnetic properties, which vary at different length scales. In this paper, for the sake of expanding the fields of application and systematically studying the mechanisms at the nanoscale, Ni2MnGa Heusler alloy nanowires were fabricated via the electrospinning method followed by optimized heat treatments. While the nanowires exhibited ferromagnetic-paramagnetic transition near room temperature, anomalies in magnetic behaviors were observed by power fitting of χ−1-T curves disobeying Curie-Weiss law. The Ni2MnGa nanowires exhibited maximum isothermal entropy change of 0.35 J/kgK accompanied by extremely wide working temperature region (150 K), with a peak temperature (305 K) suitable for room temperature applications. Arrott plots and quantitative analysis of field/temperature dependence of magnetic entropy change determined second order magnetic phase transition in the nanowires. Construction of phenomenological universal curves addressed demagnetizing effect on magnetocaloric response, which was compared to the multiphase composition obtained by structural analysis. These novel findings in Ni-Mn-Ga nanowires enrich our knowledge of structure and magnetism of Heusler alloys at the nanoscale and could be taken as reference facilitating future relevant research.

First-principles study of Pt doping effects on Ni2MnGa and Ni2FeGa ferromagnetic shape memory alloys

The influence of Pt doping on the lattice structure, magnetism, elastic moduli, and martensite transition of Ni2MnGa and Ni2FeGa Heusler alloys was studied using first-principles calculations. We investigated Pt substitution at all three different sites of Ni, Mn (Fe), and Ga with different concentrations. Partial Pt substitution at the Ni and Ga sites appeared to strengthen the ferromagnetism of these two alloys, whereas increasing the Pt content at the Mn (Fe) site weakened the total magnetic moment significantly. Pt doping at the three sites was conducive to austenite-martensite transformation, and the martensitic transition temperature TM increased with increasing Pt concentration, especially for the replacement of Ga by Pt. Most Pt-doped Ni2MnGa and Ni2FeGa alloys follow the general rule regarding the tetragonal shear modulus C′ and TM: a softening of C′ corresponds to an increase of TM. Unlike the case of Pt occupation at the Ga site, substantial Pt substitution at the Ni and Mn (Fe) sites led to a remarkable decrease in the inherent brittleness of Ni2MnGa and Ni2FeGa and thus promoted their technological application.

Soft Phonon Modes in Ni2MnGa and Ni2MnAl Heusler Alloys

The phonon spectra of Ni2MnGa and Ni2MnAl Heusler alloys are investigated using first-principle methods. The effect the supercell elongation of the considered alloys has on the features of phonon dispersion curves is studied. It is shown that cell elongation affects the behavior of soft transverse acoustic mode ТА2 in both Ni2MnGa and Ni2MnAl.

Time dependent nanomechanical properties of NiMnGa Heusler alloy

In this work nano-indentation has been adopted as a technique to measure the Time dependent deformation (TDD) and other related mechanical properties of NiMnGa Heusler alloys. The measurement of TDD utilizes two different techniques namely: CRL (Constant Rate of Loading) and CSR (Constant Strain Rate) modes. Tests to measure the behavioral dependence on the holding time were also done. The loading rates for CRL technique were varied from 0.1 mN/s to 1 mN/s. Strain rates for CSR mode were varied from 0.05 to 2. The tests were done at room temperature on Ni50Mn28Ga22 alloy which was prepared by TIG arc melting. Hardness and strain rate, both are inversely dependent on depth of indentation and time. The amount of plastic work was calculated and its dependence on loading rate, holding time and strain rate has been studied. The parabolic dependence of the fraction of plastic work to the time of holding was obtained by least squares method. The amount of recovery also declined as the loading rate and strain rates were increased. Microstructural studies were also performed to observe the changes that are inflicted underneath the surface during indentation. The microstructure was initially a twinned martensitic structure having two different variants which developed into a single martensitic variant under the influence of load. Nano twins were also observed in the microstructure.

Anisotropy of the Thermal Expansion of a Polycrystalline Ni–Mn–Ga Alloy Subjected to Plastic Deformation by Forging

The formation of a sharp crystallographic texture in a Ni–Mn–Ga Heusler alloy by the multiple isothermal forging has been studied. An analysis of the thermal expansion near the martensitic transformation temperatures in the as-cast and forged states of the alloy shows that the thermomechanical treatment leads to an increase in the anisotropy of the sample geometric size changing during the phase transformation, which favors an increase in the functional characteristics of the alloy. The structural studies show that the alloy structure after multiple isothermal forging has the bimodal distribution of grain sizes. The formation of the bimodal structure by forging is assumed to make it possible to enhance the stability of the functional properties of the alloy during repeated cycles of the phase transformation.

On entropy determination from magnetic and calorimetric experiments in conventional giant magnetocaloric materials

In this work, we discuss measurement protocols for the determination of the magnetic entropy change associated with first-order magneto-structural transitions from both magnetization and calorimetric experiments. The Cu-doped Ni2MnGa Heusler alloy with a first-order magneto-structural phase transition is used as a case study to illustrate how commonly-used magnetization measurement protocols result in spurious entropy evaluations. Two magnetization measurement protocols which allow for the accurate assessment of the magnetic entropy change across first-order magneto-structural transitions are presented. In addition, calorimetric measurements were performed to validate the results from the magnetization measurements. Self-consistent results between the magnetization and calorimetric measurements were obtained when the non-equilibrium thermodynamic state was carefully handled. Such methods could be applicable to other systems displaying giant magnetocaloric effects caused by first-order phase transitions with magnetic and thermal hysteresis.

Magnetocaloric effect and magnetoelastic properties of NiMnGa and NiMnSn Heusler alloy thin films

Series of NiMnSn and NiMnGa Heusler alloys thin films of different compositions were obtained by DC magnetron sputtering in order to achieve strong magnetocaloric effect in the temperature range close to the room temperature. The maximal magnetocaloric effect was found in the temperature range 320–350 K, with the maximal entropy change -ΔS ranges from 1.4×104 erg/gK to 2.7×104 erg/gK. Strain modulated ferromagnetic resonance technique was used to determine magnetoelastic properties of the films. Magnetoelastic constants of an order of 106 erg/cm3 were found.