Ni Mn Ga Shape Memory Alloys Research Articles

Manipulation of microstructure evolution and deformation behavior in Ni–Mn–Ga shape memory alloys with varied Ni/Ga under uniaxial cyclic compression

Manipulation of microstructure evolution and deformation behavior in Ni–Mn–Ga shape memory alloys with varied Ni/Ga under uniaxial cyclic compression

Simulation-Assisted Tool Design for Pulsed Electrochemical Machining of Magnetic Shape-Memory Alloys

Shape-memory alloys set high demands on the production technologies being used. During cutting, continuous heat input and mechanical stress have an undesirable influence on the shape-memory effect. Pulsed electrochemical machining (PECM), which is based on anodic dissolution, enables force-free machining without thermomechanical influence on the edge-zone properties of the workpiece. Depending on the desired geometry, the development of a customized PECM fixture is necessary. The design of the fixtures is often based on the experiences of the designers and manufacturers, which often results in an estimation of the functionally critical dimensions. For this reason, the study focuses on a methodical approach for evaluating crucial fixture dimensions using knowledge of the specific material dissolution behavior linked with a numerical simulation model. It has been shown that the shape-memory alloy NiMnGa has a non-linear dissolution behavior in sodium nitrate. A reduction of stray currents up to 20% resulting from a lateral gap between the cathode and electrical insulation was demonstrated using numerical simulation. The study shows that a low cathode shaping height has the strongest influence on precise processing. Varying the process parameters allowed for the lateral gap to be adjusted between 0.15 and 0.25 mm.

Stress-induced detwinning/reorientation of hierarchically twinned martensite and deformation micro-mechanism in Ni52Mn27Ga17Co4 shape memory alloys: Experimental and phase-field studies

Mechanical training via cyclic loading and unloading was widely used in NiMnGa shape memory alloys (SMAs) to create single or double variant states for reducing the resistance to variant reorientation and thus achieving a maximum shape memory strain. In this work, an effort was made to reveal the deformation mechanism of a polycrystalline Ni52Mn27Ga17Co4 high-temperature SMAs consisting of non-modulated (NM) martensite with uniform orientation and surrounding γ precipitates. The NM martensite microstructure was characterized as a self-accommodated hierarchical twinned-structure prior to deformation. Based on the interrupted in-situ EBSD measurements and phase-field simulations, it was demonstrated that the compressive loading resulted in the thickening of the stress-favored variants with the <001>NM direction perpendicular to compression axis at different dimensional scales. The path consisted of two subsequent processes–first detwinning of the nano-lamellae within one micro-variant through inter-lamellae boundary motion, and then reorientation of micro-variants through inter-plate boundary motion. The Schmid factor and preferred orientation of nano-lamellae (or strain accommodation) dominated the entire detwinning/reorientation process of NM martensite. As a result, the hierarchically twinned microstructure almost evolved into a single-variant state with the disappearance of packet boundaries. The ductile γ phase with network structure possessed more excellent strain-accommodated ability to the macroscopic deformation, hence significantly enhancing the mechanical properties including the ultimate compressive strength and elongation of NiMnGa SMAs.

Ferromagnetic shape memory alloy Ni52.5Mn22.5Ga25: Phase transformation at high pressure and effects on natural gas hydration

Ferromagnetic shape memory alloy NiMnGa was used to prepare the micro/nano fluid which has a potential role during the formation of natural gas hydrate (NGH). In the present work, phase transformations of Ni52.5Mn22.5Ga25 micro/nano particles under various pressures, and the crystal structure before and after participating in the natural gas hydration were investigated. The effect of the micro/nano fluid containing Ni52.5Mn22.5Ga25 particles on the formation heat of NGH was studied. It was worked out that Ni52.5Mn22.5Ga25 particles exhibit a high stability both in phase transformation below 15 MPa pressure and crystal structure after natural gas hydration. The fluid containing Ni52.5Mn22.5Ga25 particles can reduce the hydration reaction temperature and promote the formation of NGH.

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.

Evolution of magnetic and structural phase transitions in solid solutions Ni2MnGa1-xGex

Structural and magnetic properties of the prominent magnetic shape memory alloy Ni2MnGa have been investigated in detail, as well as their evolution with tuning parameters – most frequently stoichiometry changes in the Ni-Mn-Ga system or substitution of one or more elements in the parent alloy by other d- and/or p-electron elements. Surprisingly, Ga-Ge substitution, although one of the most natural substitutions, has largely been neglected. We present our study on the preparation, phase and structural characterization, and magnetization properties of alloys within the Ni2MnGa1-xGex series. The isostructural substitution of Ga by Ge results in (i) a significant decrease of the melting point of the alloy, (ii) a contraction of the L21 crystal lattice following the Vegard’s law, (iii) a suppression of martensitic transformation of the lattice for x ≥ 0.25, and (iv) a substantial decrease of ferromagnetic Curie temperature with Ge content increasing above x = 0.4. Tentative signs of a possible martensitic transition are observed also for the other parent compound Ni2MnGe. The crucial role of sample annealing on its overall properties is demonstrated. The presented data are discussed in the frame of previous results on the parent alloy and other related doped systems.

The effects of homogenization time on the crystal structure and hardness of NiMnGaMo alloy

This study is based on X-ray diffraction (XRD) and DSC analysis, Scanning Electron Microscopy (SEM) and Optical Microscope observations and Vickers Micro Hardness Test results with structural effect and phase transformations on NiMnGa shape memory alloys with different composition. The addition of different elements to the shape-memory NiMnGa alloys and the heat treatment of the alloys are extremely useful for improving the shape memory, magnetic, mechanical, chemical and other physical properties. The NiMnGaMo alloys were successfully obtained using a vacuum induction melting furnace and then homogenized by heat treatment at 950 °C for 2, 24 and 96 h, respectively. After homogenization, all samples were cooled in ice water. Crystallographic results calculated from XRD data were evaluated by comparison with the data obtained by DSC analysis, Scanning Electron Microscope (SEM), Optical Microscope and Vickers Micro Hardness Test Device. The results show that the alloy with a higher Mo amount has a smaller crystallite size and the L21 lattice volume increases with heat treatment time. The change on microhardness values were discussed with the change on the obtained lattice constants.

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.

Twinning stress of type I and type II deformation twins

Recent reports on highly mobile type II twin boundaries challenge the established understanding of deformation twinning and motivate this study. We consider the motion of twin boundaries through the nucleation and growth of disconnection loops and develop a mechanism-based model for twin boundary motion in the framework of isotropic linear elasticity. While such mechanisms are well established for type I and compound twins, we demonstrate based on the elastic properties of crystals that type II twin boundaries propagate in a similar way. Nucleation of a type I twinning disconnection loop occurs in a discrete manner. In contrast, nucleation of a type II twinning disconnection loop occurs gradually with increasing Burgers vector. The gradual nucleation of a type II disconnection loop accounts for the higher mobility of type II twin boundaries compared with type I twin boundaries. We consider the homogeneous nucleation of a disconnection loop, which is adequate for twinning in shape memory alloys with a low-symmetry crystal lattice. For the magnetic shape memory alloy Ni–Mn-Ga, the model predicts twinning stresses of 0.33 MPa for type II twinning and 4.7 MPa for type I twinning. Over a wide temperature range, the twinning stress depends on temperature only through the temperature dependence of the elastic constants, in agreement with experimental results.

Efficiency of Energy Harvesting in Ni–Mn–Ga Shape Memory Alloys

Many researchers have reported on the voltage and power generated while energy harvesting using Ni–Mn–Ga shape memory alloys; few researchers report on the power conversion efficiency of energy harvesting. We measured the magneto-mechanical behavior and energy harvesting of Ni–Mn–Ga shape memory alloys to quantify the efficiency of energy harvesting using the inverse magneto-plastic effect. At low frequencies, less than 150 Hz, the power conversion efficiency is less than 0.1%. Power conversion efficiency increases with (i) increasing actuation frequency, (ii) increasing actuation stroke, and (iii) decreasing twinning stress. Extrapolating the results of low-frequency experiments to the kHz actuation regime yields a power conversion factor of about 20% for 3 kHz actuation frequency, 7% actuation strain, and 0.05 MPa twinning stress.

Ab initio study of the composite phase diagram of Ni–Mn–Ga shape memory alloys

The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.

Peek into the World of Materials Using Thermopower and Xafs as Investigative Probes

Over the last few decades, there has been growing interest for developing technologies aimed at providing cleaner and more sustainable energy sources. Great efforts are directed towards synthesis of newer functional materials and tailoring the existing ones with an aim to optimize their usability. As materials are being developed with various complexities in their physical properties and forms like single crystals, thin-films, nanostructure and composites, measurement of their basic physical properties is also getting equally challenging. This review deals with a brief summary of our efforts in developing the basic understanding of some functional materials, using experimental tools that are best known to us, viz. measurement of Seebeck coefficient and X-ray absorption fine structure spectroscopy (XAFS). In particular, we discuss the results of our investigation of magnetic shape memory alloy Ni2MnGa and multiferroic CdCr 2 Se 4 .

Nano-Localized Thermal Analysis and Mapping of Surface and Sub-Surface Thermal Properties Using Scanning Thermal Microscopy (SThM).

Determining and acting on thermo-physical properties at the nanoscale is essential for understanding/managing heat distribution in micro/nanostructured materials and miniaturized devices. Adequate thermal nano-characterization techniques are required to address thermal issues compromising device performance. Scanning thermal microscopy (SThM) is a probing and acting technique based on atomic force microscopy using a nano-probe designed to act as a thermometer and resistive heater, achieving high spatial resolution. Enabling direct observation and mapping of thermal properties such as thermal conductivity, SThM is becoming a powerful tool with a critical role in several fields, from material science to device thermal management. We present an overview of the different thermal probes, followed by the contribution of SThM in three currently significant research topics. First, in thermal conductivity contrast studies of graphene monolayers deposited on different substrates, SThM proves itself a reliable technique to clarify the intriguing thermal properties of graphene, which is considered an important contributor to improve the performance of downscaled devices and materials. Second, SThM's ability to perform sub-surface imaging is highlighted by thermal conductivity contrast analysis of polymeric composites. Finally, an approach to induce and study local structural transitions in ferromagnetic shape memory alloy Ni-Mn-Ga thin films using localized nano-thermal analysis is presented.

Direct observation of fast-moving twin boundaries in magnetic shape memory alloy Ni–Mn–Ga 5 M martensite

We report a direct observation of fast Type II twin boundary motion under a pulsed magnetic field excitation in Ni–Mn–Ga 5M martensite, utilizing high-speed camera technique. Our results demonstrate saturation velocities up to ~39m/s measured along [100] direction for individual boundaries moving at RT, showing an order of magnitude larger values as compared to the earlier studies. Actuation response times of ~1μs were observed using laser vibrometry technique. The reported results show a minor role of magneto-static forces and emphasize the role of inertia and internal friction in twin boundary dynamics at the measured speeds.

Latent heat contribution to the direct magnetocaloric effect in Ni–Mn–Ga shape memory alloys with coupled martensitic and magnetic transformations

We report the direct magnetocaloric response of materials that present a second-order phase transition in the temperature range where a first-order structural transition also occurs. In particular, the influence of the latent heat on the field-induced adiabatic temperature change has been analyzed in a Ni–Mn–Ga alloy with coupled martensitic and magnetic transformations. It is found that discrepancies around 20% arise depending on whether the latent heat is taken into account or not. From the observed results, a general expression for the indirect determination of the adiabatic temperature change, that takes into account the contributions of both the martensitic and magnetic transformations, is proposed and experimentally confirmed. The observed key role of the latent heat allows us to understand why materials with first-order transformations do not present adiabatic temperature changes as higher as those which would correspond to materials undergoing second-order transformations with similar isothermal entropy change.

Self-patterning of epitaxial Ni–Mn–Ga/MgO(001) thin films

In this work, a detailed report on the growth of elongated bar-like crystallites with a definite crystallographic orientation, forming a self-organized patterned surface layer, with a thickness of about 100 nm, onto 900 nm thick ferromagnetic shape memory alloy Ni–Mn–Ga films epitaxially grown on MgO(001) is presented. The phenomenon appears above a threshold of lattice mismatch (≈2%) between the film and substrate. The structural, magnetic and transformation properties of such films are described and analyzed. Particularly, the different magnetic states of the two film layers are discerned by ferromagnetic resonance. The feasibility of creating a ferromagnetic shape memory alloy with a nanoscale self-patterned surface, during the very same process of film deposition, can be interesting for potential applications where control of the surface architecture would be needed.

Incommensurateness in nanotwinning models of modulated martensites

Martensites, which are the product of diffusionless phase transitions, can form phases with small periodic modulations of crystalline atomic lattice positions. Experiments have shown both that the modulation function is smooth and that when more than one modulation is present they can be incommensurate. These observations have appeared to be inconsistent with nanotwinning models. Here the authors show theoretically that in the case of the ferromagnetic shape memory alloy NiMnGa, the presence of two competing mechanisms favoring different commensurate periodic structures, a result of the interaction between different minima in the energy landscape, can lead to effective smooth incommensurate modulations consistent with nanotwinning models.

Micro-processing of NiMnGa shape memory alloy by using a nanosecond fiber laser

The interest on Ferromagnetic Shape Memory Alloys (FSMAs), such as NiMnGa, is growing up, thanks to their functional properties to be employed in a new class of micro-devices. The most evident critical issue, limiting the use of these systems in the production of industrial devices, is the brittleness of the bulk material; its workability by using convectional processing methods is very limited. Thus, alternative processing methods, including laser processing, are encouraged for the manufacture of FSMAs based new devices. In this work, the effect of the nanosecond laser microprocessing on Ni45Mn33Ga22 [at%] has been studied. Linear grooves were realized by a nanosecond 30W fiber laser; the machined surfaces were analyzed with scanning electron microscopy, coupled with energetic dispersion spectroscopy for the composition analysis. The morphology of the grooves was affected by the laser scanning velocity and the number of laser pulses while the measured material removal rate appeared to be influenced mainly by the number of laser pulses. Compositional modification, associated to the loss of Ga content, was detected only for the lower scanning velocity, because of the high fluence. On the contrary, by increasing the velocity up to 1000mm/s no Ga loss can be seen, making possible the laser processing of this functional alloy without its chemical modification. The use of short pulses allowed also to reduce the amount of recast material and the compositional change with respect to long pulses. Finally, the calorimetric analysis indicated that laser nanosecond microprocessing could affect the functional properties of this alloy: a larger decrease of the characteristic temperatures of the martensitic transformation was observed in correspondence of the low scanning velocity.

Martensitic transformation in shape memory crystal with defects: Monte Carlo simulations and Landau theory

The efficiency of complex (microscopic and macroscopic) theoretical studies of the influence of crystal defects on the transformational properties of shape memory alloys was demonstrated. To this end, the numerical Monte Carlo simulations were carried out for the microscopic modeling of the influence of vacancies on the martensitic transformation (MT) of a Ni–Mn–Ga shape memory alloy and the dependence of MT temperature on concentration of the vacancies was determined. Applicability of the Monte Carlo method to the description of martensite stabilization was demonstrated. Landau theory of MT in the real (with defects) alloy specimen was then used for the evaluation of mechanical stress and internal pressure, which the vacancies create in the alloy. Time dependencies of the mechanical stress and the internal pressure were found from experimental data on the martensite aging. Concentration of the vacancies that provides the experimentally observed time dependence of MT temperature was estimated.

Micromechanical modeling of stress-induced strain in polycrystalline Ni–Mn–Ga by directional solidification

Polycrystalline ferromagnetic shape memory alloy Ni–Mn–Ga produced by directional solidification possess unique properties. Its compressive stress–strain behaviors in loading–unloading cycle show nonlinear and anisotropic. Based on the self-consistent theory and thermodynamics principle, a micromechanical constitutive model of polycrystalline Ni–Mn–Ga by directional solidification is developed considering the generating mechanism of the macroscopic strain and anisotropy. Then, the stress induced strains at different angles to solidification direction are calculated, and the results agree well with the experimental data. The predictive curves of martensite Young’s modulus and macro reorientation strain in different directions are investigated. It may provide theoretical guidance for the design and use of ferromagnetic shape memory alloy.