NiMnGa Magnetic Shape Memory Alloys Research Articles

Fe and Co selective substitution in Ni2MnGa: Effect of magnetism on relative phase stability

We studied the effect of selective substitution in Fe and Co 3d elements on the structural and magnetic phase transitions in the prototypical magnetic shape memory alloy Ni–Mn–Ga. We determined the phase diagram for each elemental doping substitution by means of calorimetry and ac susceptibility measurements. An effort was made to substitute each constituent element by the same amounts of doping and to study the role of parameters other than electronic concentration in controlling phase stability. Specifically, selective doping with atoms of similar atomic radii but different magnetic properties allowed us to investigate the role of magnetic interactions on the relative phase stability of the ternary compound. We determined the entropy change associated with the martensitic transition for each quaternary alloy to obtain further information on the effect of magnetism on the relative stability of the involved phases exhibited by these compounds.

Comparing properties of substrate-constrained and freestanding epitaxial Ni–Mn–Ga films

In order to use the magnetic shape memory alloy Ni–Mn–Ga with its high achievable strain of up to 10% for microactuators, freestanding epitaxial films are required. Here we show that these conditions can be fulfilled when using chromium as a sacrificial layer. The low misfit towards Ni–Mn–Ga enables epitaxial growth. Furthermore, Cr can afterwards be removed selectively by wet-chemical etching and during deposition no significant interdiffusion is observed. The structure, microstructure and magnetic properties of micrometer thick films are not affected by the etching process. Films are ferromagnetic at room temperature and we observe the coexistence of non-modulated (NM) and seven layered modulated (14 M) martensite. Tensile stress–strain measurement of a freestanding bridge reveals a broad strain plateau of 12% at a twinning stress of 25 MPa, indicating reorientation of NM variants.

An acoustic emission study of the effect of a magnetic field on the martensitic transition in Ni2MnGa

Acoustic emission measurements during the martensitic transition of a Ni–Mn–Ga magnetic shape-memory alloy reveal the intermittent and jerky character of the transition. The distribution of the amplitude of the acoustic emission events shows power law behavior, which reflects the absence of characteristic scales in the process. In this paper we show that the distribution is affected by an applied magnetic field, which proves that the transition dynamics is strongly influenced by magnetostructural coupling taking place at multiple length scales. The martensitic start temperature and the power law exponent of the amplitude distribution are measured in dependence of the applied field.

Epitaxial Ni–Mn–Ga films derived through high temperature in situ depositions

Epitaxial films of the magnetic shape memory alloy Ni–Mn–Ga have been deposited on(100) YSZ substrates in a previously unexplored high temperature depositionregime. The temperatures employed encouraged the development of the desiredmicrometer-size highly twinned martensitic grain structure, where the loss of manganeseand gallium at high temperatures was compensated for through enrichments tothe target material. The films show the desired shape memory alloy propertiesof having a well-defined reversible martensitic transformation, centered around85 °C, a room temperature martensitic phase with an in-plane saturation magnetization of64 emu g−1, and a magnetically induced reorientation of the martensite variants which occurs at a lowmagnetic field of 400 Oe. The films also exhibit a hysteresis loop in the magnetic transition,a feature attributed to the overlap of the magnetic and structural phase transitions. Theresults suggest that the described in situ route provides a superior pathway to theformation of Ni–Mn–Ga films.

Determination of the compositions of NiMnGa magnetic shape memory alloys using hybrid evolutionary algorithms

Magnetic shape memory (MSM) alloys are a new class of actuator materials with high actuation frequency, energy density and strain. MSM effect occurs in alloys, which exhibit a martensitic transformation and are ferromagnetic. It involves, under effect of magnetic field, a high strain achieved via reorientation of twinned martensite plates. The major problem is that even a slight change in the alloy’s composition causes drastic changes in the martensitic transformation temperature (MTT) and MSM effect is only possible in the martensitic region. Therefore, it is crucial to be able to predict the MTT of any NiMnGa alloy. Artificial neural networks (ANN) with their learning and generalization ability may act as a suitable tool to predict the MTTs of NiMnGa alloys. ANNs are generally used when the problem cannot be explicitly described by an algorithm, a set of equations, or a set of rules. A genetic algorithm (GA) is a search technique used in computing to find true or approximate solutions to optimization and search problems. In a previous study, in order to predict the MTT, the performance of a multi-layer perceptron has been studied. Training and validation stages of the approach are performed by using data sets from many separate analysis results and our chemical analysis results were used for testing. In this paper, as an inverse design approach, we concentrated on finding the composition of any NiMnGa alloy by using the MTT as the input in our ANN model. To build an advanced solution, we used GA and ANN in a hybrid manner to obtain the composition values. In order to compare the performance of the candidate solutions obtained from alternative methods, the required fitness function for the MTT was determined by the ANN developed in the previous study. Solution quality was used as the crosscheck parameters for the comparison of results obtained from either method.

Single-crystal growth of NiMnGa magnetic shape memory alloys

The crystal growth of NiMnGa magnetic shape memory alloys was investigated by the optical zone melting method. Slightly macroscopic convex and planar solid–liquid interfaces were achieved by controlling the molten zone morphologies. Single crystals were successfully prepared in three typical series of NiMnGa magnetic shape memory alloys, including stoichiometric Ni 50Mn 25Ga 25, Ni-rich NiMnGa and Mn-rich NiMnGa alloys. Studies on the solute partition during crystal growth revealed the enrichment of Mn and deficiency of both Ni and Ga in front of the solid–liquid interface for both stoichiometric and Mn-rich NiMnGa alloys, while a slight enrichment of Ni for Ni-rich NiMnGa alloys. An initial transient stage was determined to be about 20 mm, and a uniform composition distribution existed along the axis of the crystals in the stable growth parts, which matches well with the proposed mathematic model.

Crystal and magnetic structure temperature evolution in Ni–Mn–Ga magnetic shape memory martensite

The current work presents the original results concerning the temperature evolution of the magnetic and crystal structure of Ni–Mn–Ga magnetic shape memory alloys within the entire temperature interval of the martensite phase. The atomic positions and the correct space group of the 5M martensite unit cell has been derived, which is crucial for the electronic structure calculations. Neutron diffraction studies of the Ni–Mn–Ga alloy, possessing a large magnetic shape memory effect (>5% shear strain), revealed that there are several magnetic transitions in the martensite phase due to the smooth strongly anisotropic temperature dependence of the martensite lattice parameters.

Development of a Quasi-static Model of NiMnGa Magnetic Shape Memory Alloy

A quasi-static model for NiMnGa magnetic shape memory alloy is formulated along the lines of the Brinson and Tanaka SMA constitutive models. Since the shape memory effect (SME) and pseudoelasticity exist in both NiTi and NiMnGa, constitutive models for SMAs offer a basis for ferromagnetic shape memory alloys (FSMA) modeling. Two types of quasi-static tests involving constant external magnetic field and constant stress are conducted to identify nine model parameters. These model parameters include free strain, Young's moduli, fundamental critical stresses, fundamental critical threshold fields, and stress-influence coefficients. The Young's moduli of the material in its field and stress preferred states are determined to be 450 and 820 MPa respectively, while the free strain is measured to be 6.5%. These test data are used to assemble a critical stress—magnetic field intensity profile that is useful for determining the model parameters and for predicting the various states of the material for a wide range of magnetic or mechanical loading conditions. Although all of the parameters can be obtained from constant magnetic field testing, useful insight into NiMnGa actuator behavior can be gained from constant axial stress tests. Once implemented, the analytical model shows good correlation with the test data, capturing both the magnetic shape memory effect and pseudoelasticity. Because the model is piecewise linear, it does not capture material behavior resulting from nonlinear effects such as magnetic saturation. Despite its inherent limitations, this model shows encouraging results, providing a solid basis for future modeling efforts.

On the stress-assisted magnetic-field-induced phase transformation in Ni2MnGa ferromagnetic shape memory alloys

The effect of magnetic field on the martensitic phase transformation in Ni2MnGa single crystals was investigated under compression. Reversible and one-way stress-assisted field-induced phase transformations were observed under low field magnitudes. The total work output levels achieved during reversible stress-assisted field-induced phase transformation are similar to that attained using field-induced martensite reorientation in NiMnGa magnetic shape memory alloys (MSMAs). However, the actuation stress levels are an order of magnitude higher. Possible magneto-microstructural mechanisms and necessary magnetic and mechanical conditions to accomplish field-induced phase transformation are discussed. A thermodynamical description is introduced to understand magnetic energy contributions to trigger the phase transformation. Materials design and selection guidelines are proposed to search for this new mechanism in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation. The present work output levels achieved in the Ni2MnGa MSMA and the possibility of further increase place MSMAs above many currently available high frequency active materials.

Energy harvesting using martensite variant reorientation mechanism in a NiMnGa magnetic shape memory alloy

Magnetic shape memory alloys demonstrate significant potential for harvesting waste mechanical energy utilizing the Villari effect. In this study, a few milliwatts of power output are achieved taking advantage of martensite variant reorientation mechanism in Ni51.1Mn24Ga24.9 single crystals under slowly fluctuating loads (10Hz) without optimization in the power conversion unit. Effects of applied strain range, bias magnetic field, and loading frequency on the voltage output are revealed. Anticipated power outputs under moderate frequencies are predicted showing that the power outputs higher than 1W are feasible.

Structure and properties of Ni–Mn–Ga magnetic shape memory alloys containing minor addition of terbium

Effect of terbium addition in the range of 0–2 at% on the structure, phase constitution and hardness of the Ni 49Mn 29Ga 22 alloy was studied. The Tb addition substantially refined the grain size, which dropped from 200 to 400 μm, for the Tb-free alloy, down to 30–50 μm for the 2 at% Tb material. The terbium exhibited negligible solubility in the matrix phase and formed grain boundary layer. The mean composition of the boundary layer was: Tb, 16 at%; Ni, 55 at%; Mn, 7 at%; and Ga, 22 at%. The phase analysis revealed the presence of the following major phases in the alloys: Ni 2MnGa and Ni 3Ga. All the alloys exhibited martensitic structure at room temperature. The Tb addition did not affect the Curie temperature, however, the A s temperature (martensite-to-austenite transformation starting temperature) and M s temperature (martensite-to-austenite starting temperature) grew slightly for low Tb concentrations and subsequently decreased for the higher Tb contents. The Tb-containing alloys exhibited increased hardness, by about 40%, which was apparently caused by the grain refinement. No significant effect of the Tb addition on the magnetic shape memory effect was observed.

A quasi-static model for NiMnGa magnetic shape memory alloy

A quasi-static model for NiMnGa magnetic shape memory alloy (MSMA) is formulated inparallel to the Brinson and Tanaka thermal SMA constitutive models. Since the shapememory effect (SME) and pseudoelasticity exist in both NiTi and NiMnGa, constitutivemodels for SMAs can serve as a basis for MSMA behavioral modeling. The simplified,linear, quasi-static model for NiMnGa was characterized by nine material parametersidentified by conducting a series of uniaxial compression tests in a constant fieldenvironment. These model parameters include free strain, Young’s modulus, fundamentalcritical stresses, fundamental threshold fields, and stress-influence coefficients. The Young’smoduli of the material in both its field and stress preferred configurations were determinedto be 450 MPa and 820 MPa respectively, while the free strain was measured to be 5.8%.These test data were used to assemble a critical stress profile that is useful for determiningmodel parameters and for understanding the dependence of critical stresses onmagnetic fields. Once implemented, the analytical model shows good correlationwith test data for all modes of NiMnGa quasi-static behavior, capturing both themagnetic shape memory effect and pseudoelasticity. Furthermore, the model isalso capable of predicting partial pseudoelasticity, minor hysteretic loops andstress–strain behaviors. To correct for the effects of magnetic saturation, a series ofstress influence functions were developed from the critical stress profile. Althoughrequiring further refinement, the model’s results are encouraging, indicating thatthe model is a useful analytical tool for predicting NiMnGa actuator behavior.

Cast and Rolling Textures of NiMnGa Alloys

AbstractThe texture of two polycrystalline NiMnGa magnetic shape memory alloys fabricated by directional solidification and hot rolling has been measured with high-energy synchrotron radiation and neutron diffraction. At room temperature the alloys used are composed of a modulated (7M) and non-modulated (NM) martensitic structure. The texture of the directionally solidified alloy for all phases is a pronounced fibre texture with <100> preferentially aligned along the growth direction. In the directionally solidified case a variant selection took place with [001] dominating. The texture of the hot rolled alloy shows a weak recrystallization texture with {111} and <112> aligned parallel to the rolling plane and rolling direction, respectively. The texture results are discussed with respect to material, processing and phase transformations including variant selection.

Crystal structure and macrotwin interface of five-layered martensite in Ni–Mn–Ga magnetic shape memory alloy

The crystal structure of the five-layered (5M) martensite and the martensite to martensite interface in the alloys Ni 48.9Mn 30.8Ga 20.3 and Ni 49.5Mn 28.6Ga 21.9 (numbers indicate at.%) were investigated by the X-ray diffraction as well as by the conventional and by the high resolution transmission electron microscopy (HRTEM). The martensite to martensite interface arises from meeting of growth fronts of two microtwin sequences with the twin planes (1 1 0) and ( 1 1 ¯ 0 ) , respectively. The interface was found to consist of two different configurations, the crossing and the step type. The HRTEM images reveal that the martensitic structure is not a perfect five-layered structure, but there are also other periodic structures (seven-layered and ten-layered), together with aperiodic structure and the irregular plane faults.

Magnetic domain evolution with applied field in a Ni–Mn–Ga magnetic shape memory alloy

Abstract The evolution of a magnetic domain pattern with an applied field in two perpendicular directions was studied in a Ni–Mn–Ga magnetic shape memory alloy by means of optical and scanning electron microscopy (SEM). The optical contrast of magnetic domains arises from surface undulation. This surface relief is coherent at the (1 0 1) twin boundary. However, the surface relief causes the trace of the (0 1 1) twin boundary to follow a zigzag pattern. These optical observations were confirmed by SEM Type II magnetic contrast.

The effect of doped elements on the martensitic transformation in Ni–Mn–Ga magneticshape memory alloy

Ni–Mn–Ga alloy is a new actuator material due to the fact that its shape memoryeffect can be controlled by magnetic field in addition to the conventional controlsby temperature and stress. However, the alloy shows relatively low martensitictransformation and Curie temperatures. In this paper, we report the results ofadding small amounts of Fe, Co and Tb to NiMnGa alloys. The effect of smalladditions of these doped elements on the martensitic transformation temperatureis remarkable, but the Heusler structure of the alloys remains unchanged. ForNi50Mn27Ga23−xFex (x = 0,1,2) with partial replacement of Ga by Fe, the martensitic transformationtemperatures increase with increase of the Fe content, and so does the Curietemperature. This phenomenon of increasing both the martensitic transformationtemperatures and the Curie temperature was found for the first time. ForNi47Mn31X1Ga21 (X = Fe,Co), Fe and Co substitution for Mn, Fe increases the martensitic transformationtemperature but decreases the Curie temperature, while Co has the opposite effect. ForNi48Mn33Ga18Tb1, the addition of the rare earth element Tb decreases the martensitic transformationtemperature and the Curie temperature remarkably. Therefore, the transformationtemperatures of the alloys can be improved by these doping methods.

Temperature dependence of the giant magnetostrain in a NiMnGa magnetic shape memory alloy

The temperature dependence of the magnetostrain was investigated in the Ni50Mn27.5Ga22.5 magnetic shape memory alloy with a five-layer martensitic (5M) structure in the temperature range from 110Kto300K. A temperature threshold at 166K was found for the magnetostrain. A giant magnetostrain of 6.3% was achieved above the temperature, while no magnetostrain was monitored below the temperature. No intermartensitic transformation was detected around the temperature threshold. The lattice parameter a slightly increases, c largely decreases, and the tetragonality (a∕c−1) drastically increases with decreasing the temperature. The increase of the tetragonality is thought to be related to the temperature threshold of the magnetostrain by inducing a change of the electronic structure, twin structure, or the type of the variant with the same 5M martensitic structure below the temperature threshold. The interpretation is reasonably understood by the fact that only few samples with the same 5M martensitic structure exhibit a giant magnetostrain.

Martensitic Transformation and Magnetic-Field-Induced Strain in Magnetic Shape Memory Alloy NiMnGa Melt-Spun Ribbon

A non-stoichiometric polycrystalline Ni50Mn27Ga23 magnetic shape memory alloy was prepared by melt-spinning technology. The effects of melt-spinning on the martensitic transformation and magnetic-field-induced strain (MFIS) of the melt-spun ribbon were investigated. The experimental results show that the melt-spun ribbon undergoes the thermal-elastic martensitic transformation and exhibits the thermo-elastic shape memory effect. But the martensitic transformation temperature decreases and Curie temperature remains unchanged. A particular internal stress induced by melt-spinning made a texture structure in the melt-spun ribbon, which made the melt-spun ribbon obtain larger transition-induced strain and MFIS. The internal stress was released under cycling of magnetic field. This resulted in a decrease of MFIS of the melt-spun ribbon.

Effect of Fe addition on transformation temperatures and hardness of NiMnGa magnetic shape memory alloys

In recent years, ferromagnetic shape memory alloys (FSMAs) have been widely investigated in many countries [1–4]. Of the developed FSMAs, NiMnGa alloys have been found to exhibit nearly 10% magnetic-fieldinduced strain which is the maximum obtained MFIS in these FSMAs [5]. Based on NiMnGa alloys, researchers have developed new ferromagnetic shape memory alloys, i.e., NiMnFeGa alloys [6, 7]. The structure, shape memory effect, and magnetic properties of Fe-doped NiMnGa alloys have been studied. It is assumed that the addition of Fe element enhances the alloy plasticity without sacrificing its magnetic and thermoelastic properties [6]. However, little work has been devoted to studying transformation behavior and the mechanical properties of polycrystalline Fe-doped NiMnGa alloys. The aim of the present letter is to study the effect of additions of Fe on transformation behavior and the hardness of NiMnGa alloys. A range of alloys with the nominal composition of Ni48.7Mn30.1−x Fex Ga21.2 (at%) (x = 0, 2, 5, 8, 11), each weighing about 80 g, were prepared. The alloys were fabricated by arc melting under an argon atmosphere using appropriate quantities of nickel (99.99% purity), manganese (99.9% purity), iron (99.9% purity) and gallium (99.99% purity). The ingots obtained were sealed in a quartz tube under a vacuum, followed by annealing at 1123 K for 24 hrs and quenched into ice water for homogeneity. Rectangular samples 4×4×6 mm were cut from the post-treated ingots for hardness measurements. Some 10 mg samples were taken from the ingots for DSC measurements. The transformation temperatures were determined by differential scanning calorimetry using a TA-2920. The heating and cooling rates were 10 K/min. The crystal structure at room temperature was confirmed by powder X-ray diffraction using D/MAX-RA X-ray diffractometer with Cu Kα radiation. Micro Vickers hardness (HV) of eight points was measured using a Akashi HM-102 Vickers Hardness Tester. The applied load and time were 200 g and 15 s, respectively. The HV values were averaged excluding the highest and lowest values [8]. Table I summarizes the transformation temperatures measured by DSC analysis in Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen, at room temperature, that the parent phase and martensite coexist in the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, whereas in the other three alloys only the parent phase exists. The results are in agreement with X-ray diffraction results in Fig. 1. X-ray patterns depict that the cubic parent phase and tetragonal martensite coexist when x = 0, 2, while only the cubic parent phase exists when x = 5, 8, 11. As shown in Table I, the thermal hysteresis of these five alloys are −1.38, −3.30, 0.46, 2.60 and 2.20 ◦C, respectively. For the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, the thermal hysteresis is negative. Hence Fe-doped NiMnGa alloys exhibit very narrow thermal hysteresis which means that the chemical energy of the phase transformation is very small. The narrow thermal hysteresis also shows that the driving force for inducing the martensitic transformation in Fe-doped NiMnGa alloy is very small. Wayman [9] divided thermal transformations into two types: the first is Af > As > Ms > Mf while the second is Af > Ms > As > Mf. From Table I it can be seen that the first two alloys belong to the second type of thermal transformation, while the other three alloys belong to the first type of thermal transformation. In addition, only single forward and reverse transformation are determined by DSC curves. Fig. 2 depicts the phase transformation temperatures vs. Fe content for Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen that with increase of Fe content all the transformation temperatures, Ms (martensitic transformation start temperature), Mf (martensitic transformation finish temperature), As (reverse martensitic transformation start temperature), and A f (reverse martensitic transformation finish temperature), decrease. The relationship between Ms temperature and Fe content is nearly linear, varing from 26.40 ◦C for Ni48.7Mn30.1Ga21.2 without Fe to −55.00 ◦C for Ni48.7Mn19.1Fe11Ga21.2 which has the maximum Fe content. Fig. 3 exhibits the relationship between transformation temperatures and valence electron numbers per atom of Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11) and the shape of the curves is similar to Fig. 2. The tendency is clear that all transformation temperatures decrease with increase of electronic concentration in these five alloys, and the increasing rate is also basically consistent with the change of the electronic concentration. Here, it is assumed that the number of valence electrons per atom for Ni, Mn, Fe, and Ga atoms is 10(3d84s2),7(3d54s2),8(3d64s2) and 3(3s24p1), respectively. It is believed that two factors, the conduction electronic concentrations [10] and the size factor [11],

Solid–liquid interface morphology and crystal growth of NiMnGa magnetic shape memory alloys

Zone-melting directional solidification was applied to prepare oriented or single crystals of NiMnGa magnetic shape memory alloys. The concave height of the quenched solid–liquid interface of the NiMnGa crystals was investigated. It was found that by increasing the temperature gradient, the crystal growth velocity, and the zone-melting length, the concave height of the solid–liquid interface could be reduced. Flat and convex interfaces were obtained by raising the temperature gradient. The stoichiometric Ni 2MnGa, Ni-rich NiMnGa single crystals and Mn-rich NiMnGa oriented crystals were successfully prepared with flat or convex solid–liquid interfaces. 〈1 0 0〉 of the high temperature austenite was detected to be the preferred orientation of the NiMnGa crystal growth. The composition distribution along the axis of the NiMnGa oriented or single crystals was uniform. The martensitic transformation temperature variation along the axis of the NiMnGa single crystals or oriented crystals is less than 10 °C, much better compared with those prepared by the Bridgman method. The function of the concave height of the solid–liquid interface is given as a function of the temperature gradient, the crystal growth velocity, the diameter of the crystals and the thermal physical parameters. The theoretical analysis results of the concave height are consistent with the experimental ones.