Ni Mn Ga Research Articles

Localization versus delocalization of d-states within the hbox {Ni}_{{2}}MnGa Heusler alloy

We present calculations based on density-functional theory with improved exchange-correlation approaches to investigate the electronic structure of hbox {Ni}_2MnGa magnetic shape memory alloy prototype. We study the effects of hybrid functionals as well as a Hubbard-like correction parameter U on the structural, electronic and magnetic properties of the alloy. We show that the previously successful application of U on Mn should be extended by including U on Ni to describe the d localized electrons more accurately and in better agreement with experiments. The bonding interactions within this intermetallic alloy are analysed including the role of non-transition metal. We found that the strongest and most stabilizing bond is formed between the Ga–Ni pairs due to the delocalized s–s and p–s orbital hybridization. Our findings suggest that minimization of the over-delocalization error introduced by standard semi-local exchange-correlation functionals leads to a better description of the hbox {Ni}_2MnGa alloy. Furthermore we propose that the experimental total magnetic moment of Ni–Mn–Ga alloys could be increased after carefully selected heat treatment procedures.

Mechanical response during bending of Ni–Mn–Ga-based melt-spun ribbons

In this paper, we report on mechanical properties observed during bending experiments conducted on quinary Ni–Mn–Ga–Co–Cu melt-spun ribbons. Depending on the ribbon’s side to which force is applied, different mechanical response is noted. Substantially larger mechanical instabilities are observed when force is applied to the “free side” than to the “wheel side” of ribbons. When force is applied to the latter, much lower force fluctuations are recorded and the amplitude of the force–displacement response remains within the experimental resolution limit. It is also shown that the character of the force–displacement curve changes upon cycling; mainly by decreasing the maximum force and mechanical hysteresis. These results are important for materials design and optimization of the magnetic field-induced bending effect recently shown in Ni–Mn–Ga-based melt-spun ribbons.

Synergistic enhancement of coalbed methane hydration process by magnetic field and NiMnGa micro-nano fluid

Coalbed methane is an important renewable energy source. Gas hydration technology is a new method for enhancing the utilization of coalbed methane and reducing environmental pollution. Long induction periods, sluggish formation rates, low hydrate yields, and difficulty removing heat during hydrate formation are all issues with gas hydration technology. In this paper, 3 wt% NiMnGa (NMG) phase-change micro/nanoparticles and 0.05% sodium dodecyl sulphate (SDS) were compounded, and gas hydration experiments were conducted under various initial pressures and gas sample conditions to investigate. The findings revealed that NMG has efficient mass transfer properties as well as phase-change heat absorption properties, which significantly improved the kinetic process of the gas hydrate by mass and heat transfer, shortened the induction time, increased gas consumption, and increased the gas consumption rate during the rapid hydrate growth period. When the initial pressure was 6.2 MPa, the induction time was reduced by 89.26%, 92.48%, and 95.64%, and the maximum gas consumption rate was increased by 238.18%, 175.55%, and 113.60%, respectively, when using different concentrations of methane in the NMG-SDS system compared to the pure SDS system. The NMG used in this paper showed potential for future use in mixed gas hydration technology.

Shape memory effect and superelasticity of Ni50Mn30Ga20 porous alloy prepared by imitation casting method

Microstructure, martensitic transformation and mechanical property of Ni50Mn30Ga20 porous alloy with a pore diameter of 350–500 μm fabricated by imitation casting method using NaAlO2 as a pore former were investigated. The influence of porous structure on the shape memory effect and superelasticity of Ni50Mn30Ga20 alloy was analyzed. The porous structure reduces the constraints of the grain boundaries on the phase transformation and martensitic rearrangement. The heat treatment process can effectively optimize the grain structure at the nodes, pillars and walls of porous structure. Porous Ni50Mn30Ga20 has completely different mechanical behaviors at different temperatures. The heat recovery strain of the martensite porous alloy is as high as 80% at the room temperature and a superelasticity of 50% was obtained in the austenite porous alloy at 160 °C. The introduction of porous structure provides a new idea for improving the comprehensive performance of Ni–Mn-Ga alloy.

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.

Effect of magnetic field on thermal strain, thermal expansion and negative thermal expansion property in a polycrystalline Ni55Mn18Ga27 alloy

The magnetic field effect on phase transition and thermal expansion properties were investigated in Ni55Mn18Ga27 alloy. The results show that the alloy has an excellent isotropy, reproducibility and thermostability in thermal strain and exhibits a large negative thermal expansion (NTE) effect in the direct and reverse martensitic transformations (MTs). For the direct MT, the average linear expansion coefficient α is about −225.21 ppm/K and the corresponding temperature span is around 8.2 K. Additionally, the sensitivities of the transformation strain and peak temperatures to field are about 88.5 ppm/T and 1.31 K/T, respectively. The rates of α and temperature spans of NTE to magnetic field are about 8.88 ppm/K T and 0.82 K/T, indicating that the magnetic field can improve the NTE property of the Ni–Mn-Ga alloy. Finally, the NTE mechanism has also been discussed. Our findings are beneficial to improving the thermal expansion and NTE properties of Ni–Mn-Ga alloys.

Experimental Observations versus First‐Principles Calculations for Ni–Mn–Ga Ferromagnetic Shape Memory Alloys: A Review

This review summarizes recent advances in first‐principles simulations of the structural, mechanical, and magnetomechanical properties of Ni–Mn–Ga‐based ferromagnetic shape memory alloys from the perspective of experimental characterization. It focuses on calculations that can capture the main features of the low‐temperature phases, particularly the appearance of spatial modulations, the occurrence of intermartensitic transitions, and the high mobility of the twin boundaries, which allows for magnetically induced reorientation. Although the calculations can only be used to interpret some of these features and are not yet ready to be used as a tool for designing new alloys with the required properties, the gap between experimental observations and simulations is narrowing. When experiments and theory are discussed together, new challenges arise for both. In addition, new results on first‐principles‐determined elastic constants of the 10M martensite of Ni–Mn–Ga are presented and discussed with respect to the high mobility of the twin boundaries.

Observation of magnetic domain evolution in constrained epitaxial Ni–Mn–Ga thin films on MgO(0 0 1) substrate

• The evolutions of the magnetic domain of a typical epitaxial Ni–Mn–Ga thin film were investigated through wide-field magneto-optical Kerr-microscopy. • The abrupt magnetization changes in the hysteresis loops can be observed in the Type-Y area with the applied magentic field parallel to the twin interfaces. • The abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the magnetically induced variant reorientation. Epitaxial Ni–Mn–Ga thin films have promising application potential in micro-electro-mechanical sensing and actuation systems. To date, large abrupt magnetization changes have been observed in some epitaxial Ni–Mn–Ga thin films, but their origin - either from magnetically induced martensite variant reorientation (MIR) or magnetic domain evolution - has been discussed controversially. In the present work, we investigated the evolutions of the magnetic domain and microstructure of a typical epitaxial Ni–Mn–Ga thin film through wide-field magneto-optical Kerr-microscopy. It is demonstrated that the abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the MIR.

Structure and magnetic properties of Ni–Mn–Ga shape memory alloys

In this paper, we present structure, structural phase transformation, magnetic phase transition, and magnetocaloric effect of Ni50Mn50-x Ga x (x = 17, 18, 19, 20, and 21) shape memory alloys. X-ray diffraction patterns display nano-crystalline phases in the alloys. The samples are soft magnetic material with very narrow magnetic hysteresis. The martensitic-austenite transformation temperature increases from 219 K (for x = 17) to 322 K (for x = 21) with increasing Ga-concentration. Ga also increases the Curie phase transition temperature and the saturation magnetisation of the alloy. The external magnetic field also clearly affects the structural phase transition of the alloy. The magnitude of the martensitic-austenitic phase transition decreases with the increase of the applied magnetic field. Both the normal and invert magnetocaloric effects coexist in the alloy. Under the magnetic field change of 10 kOe, the maximum magnetic entropy change, ∣ΔS m ∣max, for the Ni50Mn30Ga20 ribbon is 0.95 J.kg−1.K−1 for the normal magnetocaloric effect.

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.

Transformation behavior of two Ni–Mn–Ga alloys

Abstract The transformation behavior of Ni50.5Mn30.4Ga19.1 and Ni53.7Mn26.4Ga19.9 was studied with ac magnetic susceptibility, differential scanning calorimetry, in-situ optical microscopy and X-ray diffractometry. The liquidus, solidus and L21⟶B2’ temperatures were detected for the alloys. The transformation enthalpies of melting, solidification and martensitic/reverse reactions were determined. In Ni53.7Mn26.4Ga19.9 the magnetic transition was clearly below the parent to martensite phase transformation and did not have hysteresis. The transformation sequence is suggested as Pparam ⟶Tparam ⟶ Tferrom with reverse reactions in respective order. In Ni50.5Mn30.4Ga19.1 the temperatures of the magnetic transition and structural phase transformations are close to each other and the Curie temperature shows a hysteresis of 9 K during cooling-heating cycle. Based on the combined measurements, the transformation sequences are assumed to be during cooling Pparam ⟶ 7Mparam ⟶ 7Mferrom ⟶ Tferrom and Tferrom ⟶ Tparam ⟶ 7Mparam ⟶ Pparam during heating. Consequently, the hysteresis in the magnetic transition is explained with the intermartensitic reaction and the difference of the Curie points of 7M and T phases.

Влияние дислокационных и двойниковых структур на механические характеристики сплавов Ni–Mn–Ga на ультразвуковых частотах

Magnetic shape memory alloys are a specific subtype of shape memory materials. The magnetic deformation phenomenon causes the high research interest in these alloys. Thus, in one of the most promising alloys based on Ni–Mn–Ga, using a magnetic field, it is possible to achieve changes in a single crystal size by up to 10 % due to the reorientation of the magnetic field in magnetic domains. The high magnetic deformation is directly related to the high mobility of twin boundaries separating two domains. In this work, the authors used a composite piezoelectric oscillator at a frequency of about 100 kHz to determine the influence of such defects as dislocations and twin boundaries on the mechanical characteristics of Ni49Mn30Ga21. The authors investigated the features of temperature dependences of internal friction in the samples before and after deformation and provided the amplitude dependences of these characteristics. In the studied single-crystal martensitic phase, the transition from the tetragonal phase to the orthorhombic phase was detected at 235 K. In the Ni–Mn–Ga tetragonal phase, the formation of new defects contributes to the more pronounced and early onset of amplitude-dependent internal friction. At lower loads, the successive stages occur associated with the processes of dislocations and twin boundaries movements inside the Cottrell clouds, dislocations and twin boundaries movement outside the Cottrell clouds, and supposedly, the slowdown of dislocations and twin boundaries movement due to their interaction. As well as internal friction, the authors studied the change in Young’s modulus. Its decrease at all temperatures is most pronounced in the samples with the defective structures. The study identified that in the orthorhombic phase, it is possible to observe the internal friction dependence on the deformation amplitude at a lower load due to an increase in the twin boundaries mobility with increasing temperature.

Numerical simulations on the high-frequency dynamic mechanical response of single-crystalline Ni–Mn–Ga samples

ABSTRACT In this paper, an efficient numerical scheme is proposed to simulate the high-frequency dynamic mechanical response of single-crystalline Ni–Mn–Ga samples. First, based on a three-dimensional setting, the governing equation system for a single-crystalline Ni–Mn–Ga sample is set up, which is composed of the equation of motion and the twin interface movement criteria. In order to solve the governing system, an iterative numerical algorithm is proposed, in which the finite element method and the NewMark-β method are applied. To demonstrate the efficiency of this numerical scheme, the response of a single-crystalline Ni–Mn–Ga sample under some typical loading conditions are studied. In the numerical simulations, a specific discretisation of the sample is adopted, such that the possible positions of the twin interfaces in the sample can be naturally inherited. With the obtained numerical results, the mechanical behaviours of the sample under the different loading conditions are predicted, which show good consistency with the experimental results. The distribution of variant state and the propagation of twin interfaces in the sample can also be simulated. The numerical scheme proposed in the current work is helpful for the design of novel sensors or actuators based on single-crystalline Ni–Mn–Ga alloys.

Self‐Patterning of Multifunctional Heusler Membranes by Dewetting

AbstractNi‐Mn‐based Heusler alloys are an emerging class of materials which enable actuation by (magnetic) shape memory effects, magnetocaloric cooling, and thermomagnetic energy harvesting. Multifunctional materials have a particular advantage for miniaturization since their functionality is already built within the material. However, often complex microtechnological processing is required to bring these materials into shape. Here, self‐organized formation of single crystalline membranes having arrays of rectangular holes with high aspect ratio is demonstrated. Dewetting avoids the need for complicated processing and allows to prepare freestanding Ni–Mn–Ga–Co membranes. These membranes are martensitic and magnetic, and their functional properties are not disturbed by self‐patterning. Feature sizes of these membranes can be tailored by film thickness and heat treatment, and the tendencies can be explained with dewetting. As an outlook, the advantages of these multifunctional membranes for magnetocaloric and thermomagnetic microsystems are sketched.

Effect of crystal quality on twinning stress in Ni–Mn–Ga magnetic shape memory alloys

Low twinning stress (TS) is a prerequisite for magnetic shape memory functionality in ferromagnetic martensites. We compare Ni 50 Mn 28 Ga 22 (nominal at.%) single crystals from four different producers to reveal the effect of crystal quality on the TS. Near the reverse martensite transformation, the TS is generally low, about 1 MPa, regardless of mosaicity of up to 1.7° and chemical composition deviations of up to 2 at.% of Mn. Pure type I and type II twin boundaries occur in crystals with smooth chemical composition gradients. The corresponding temperature dependences of TS follow universal linear trends with the slopes of −0.045 MPa/K for type I twins and −0.001 MPa/K for type II twins, enabling a very low TS down to 2 K for the latter. An intermediate slope of −0.023 MPa/K appears for mixed type I/type II twin boundaries in a crystal with sharp local chemical deviations. We conclude that the crystal quality affects the magnitude of the TS indirectly, through its impact on the character of the twin boundaries. The effect is weak near the martensite transformation temperature and strong at low temperatures.

Hysteretic structural changes within five-layered modulated 10M martensites of Ni–Mn–Ga(–Fe)

Hysteretic structural changes within five-layered modulated 10M martensites of Ni–Mn–Ga(–Fe)

Intermartensitic transformation between modulated structures in Heusler Ni–Mn–Ga(–Fe) single crystals

Intermartensitic transformation between modulated structures in Heusler Ni–Mn–Ga(–Fe) single crystals

Ordering mechanism and kinetics in Ni2Mn1−xCuxGa ferromagnetic shape memory alloys

The effect of atomic order on the martensitic and magnetic transformations undergone by Ni2Mn1−xCuxGa ferromagnetic shape memory alloys has been investigated. Different degrees of order have been induced by performing selected thermal treatments, and it has been found that, although both the structural (martensitic) and magnetic transformation temperatures increase with the improvement of the atomic order, each of these temperatures evolve with different kinetics. Reinforcing this result, during post–quench heating of Ni2Mn1−xCuxGa alloys two consecutive DSC exothermic peaks are observed, while the ternary Ni–Mn–Ga, Co–doped Ni–Mn–Ga and even Cu–doped Ni–Mn–Ga alloy when Cu replaces Ni show a single peak. The kinetics of the post–quench ordering processes have been studied using the Kissinger’s method, and two different activation energies have been obtained: 1.16 eV for the process that is common to all alloys and 1.35 eV for the one that is only observed in Ni2Mn1−xCuxGa alloys. The former, responsible for the change in Curie temperature, is attributed to the improvement of L21 order mainly due to Mn diffusion; for the second, which underlies the rise of martensitic transformation temperatures, diffusion of Cu atoms, misplaced in the Ni sublattice after quench, towards their most favourable sites in the Mn sublattice is proposed as the responsible mechanism.

Surface Energy upon the Martensitic Phase Transition in Microsized Samples of Ni–Mn–Ga–Cu Heusler Alloy

Surface Energy upon the Martensitic Phase Transition in Microsized Samples of Ni–Mn–Ga–Cu Heusler Alloy

Traveling surface undulation on a Ni–Mn–Ga single crystal element

Active materials couple a stimulus (electrical, magnetic, and thermal) with a mechanical response. Typical materials such as piezoelectrics strain as bulk materials to the stimuli. Here we consider an undulation created by heterogeneous deformation within a magnetic shape memory alloy (MSM) transducer. We study the mechanical response of an MSM element vs two surface treatments: a polished state with minimal surface stresses, and a micropeened state with compressive surface stress. The polished element had a sharp-featured, faceted trough shape. The micropeened element had a smooth trough shape and an additional crest. The undulation was created by a rotating localized magnetic field, which caused heterogeneous variation of the twin-microstructure. For the polished and micropeened elements, the twin-microstructures were coarse and fine, respectively. For the polished element, the undulation moved by the nucleation of a few twin boundaries, which traveled along the entire element. For the micropeened sample, the twin boundaries moved back and forth over a short distance, thereby creating a dense twin lamellar, which formed the trough. The motion of the lamellar approximated the single thick twin while allowing additional degrees of freedom due to increased mobile interface density and different initial conditions of domain volume fraction. The dense twin microstructure also smoothed the magnetic flux pattern. The undulation amplitude was about 40 μm for the sample in both treatments.