Mn Ga Alloys Research Articles

Twin relationships of 5M modulated martensite in Ni–Mn–Ga alloy

For Ni–Mn–Ga ferromagnetic shape memory alloys, the characteristic features of modulated martensite (including the number/shape of constituent variants, the inter-variant orientation relationship and the geometrical distribution of variant interfaces) determine the attainability of the shape memory effect. In the present work, a comprehensive microstructural and crystallographic investigation has been conducted on a bulk polycrystalline Ni 50Mn 28Ga 22 alloy. As a first attempt, the orientation measurements by electron backscatter diffraction (EBSD) – using the precise information on the commensurate 5M modulated monoclinic superstructure (instead of the conventionally simplified non-modulated tetragonal structure) – were successfully performed to identify the crystallographic orientations on an individual basis. Consequently, the morphology of modulated martensite, the orientation relationships between adjacent variants and the characters of twin interfaces were unambiguously determined. With the thus-obtained full-featured image on the configuration of martensitic variants, the possibility of microstructural modification by proper mechanical “training” was further discussed. This new effort makes it feasible to explore the crystallographic/microstructural correlations in modulated martensite with high statistical reliability, which in turn provides useful guidance for optimizing the microstructure and shape memory performance.

Microstructure and properties of Ni–Mn–Ga alloys produced by rapid solidification and pulsed electric current sintering

Ni–Mn–Ga alloys were compacted using pulsed electric current sintering (PECS) at 850–875 °C (50 MPa, 8 min) of flake-like powders made from the rapidly quenched melt-spun ribbons. Two kinds of ribbons were used: one made with a relatively slow wheel speed (6 m/s; average grain size ∼14 μm), and another with a faster wheel speed (23 m/s; average grain size ∼5 μm). Both sets of flake-like powders consisted of a mixture of non-modulated martensite (NM) and seven-layered modulated martensite (7M) structure. The amount of NM was greater in the slower speed material, while the other one exhibited mostly the 7M structure. These crystal structures were inherited by the sintered samples. In the compacts having the NM structure the multi-step martensitic reaction overlapped with the magnetic transition, and the Curie temperatures during heating and cooling differed from each other. In the compacts having mainly 7M structure the Curie point was about 100 °C and the martensitic transition took place in the paramagnetic state, while the intermartensitic one occurred in the region of 60–85 °C. This material demonstrated good magnetic properties and saturation magnetization, at best ∼50 emu/g. Mechanical properties of the compacts were good, and comparable to those of the polycrystalline Ni–Mn–Ga samples in compression.

Magnetic moment and chemical order in off-stoichiometric Ni–Mn–Ga ferromagnetic shape memory alloys

Recent studies have shown that the total magnetic moment in off-stoichiometric Ni–Mn–Ga alloys depends not only on electronic concentration but also on the degree of chemical order in the alloy. We have performed neutron diffraction experiments and magnetization measurements for determining the preferential atomic order and saturation moment in off-stoichiometric compounds (44–52 at.% Ni), having excess Mn and deficient in Ga. These alloys include isoelectronic alloys with different magnetic moments and were chosen in an effort to study the impact of chemical order on the magnetic moment distribution. In this work, we present an improved model of magnetic interaction between Mn atoms, which carry most of the localized magnetic moment of the alloys. The Mn atoms at Ga sites, which are nearest neighbors to properly sited Mn, couple antiferromagnetically to the dominant moment. In contrast, Mn atoms at Ga sites, which are nearest neighbors to Mn at Ni sites, couple ferromagnetically. Mn at Ni sites is always antiferromagnetic (AF). The new model is supported by the exchange variation with the Mn–Mn distance and demonstrates excellent agreement between experimental and calculated magnetic moments. The proposed model is shown to better explain the observed experimental results as compared to the rigid band model and previous localized moment models that assumed AF coupling for all off-site Mn atoms.

Determination of the orientation relationship between austenite and incommensurate 7M modulated martensite in Ni–Mn–Ga alloys

For Ni–Mn–Ga ferromagnetic shape memory alloys, a large magnetic-field-induced strain could be reached through the reorientation of martensitic variants in the martensite state. Owing to the collective and displacive nature of the austenite to martensite transformation, a certain orientation relationship (OR) between the parent and the product phase is required to minimize the transformation strain and the strain energy generated, which brings about self-accommodating groups of martensitic variants with specific orientation correlations. In this work, the microstructural and crystallographic characteristics of martensitic variants in a polycrystalline Ni 50Mn 30Ga 20 alloy were investigated by electron backscatter diffraction analysis. With accurate orientation measurement on inherited martensitic variants, the local orientations of parent austenite grains were predicted using four classical OR for the martensitic transformation. Furthermore, a specific OR, namely the Pitsch relation with (1 0 1) A//(1 2 ¯ 10 ¯ ) 7M and [1 0 1 ¯ ] A//[ 10 ¯ 10 ¯ 1] 7M, was unambiguously determined by considering the magnitude of discontinuity between the lattices of the product and parent phases and the structural modulation of the incommensurate 7M modulated martensite. The present procedure to determine the OR, without recourse to the presence of retained austenite, is in general applicable to a variety of materials with modulated superstructure for insight into their martensitic transformation processes.

Effect of Annealing on Transformation Temperature and Magnetization in the Ni–Mn–Ga Alloy

This article covers the detailed study on the preparation and characterization of off-stoichiometric ferromagnetic Ni–Mn–Ga shape memory alloy. It has been found that annealing the polycrystalline Ni–Mn–Ga alloy at 700°C for 20 hours under argon atmosphere reduces the range of transformation temperature. The differential scanning calorimetric measurement reveals that the alloy has the martensitic transformation temperature at room temperature and it is less than the critical temperature. Magnetic measurement has been performed using vibrating sample magnetometer. The observed narrow transformation hysteresis around 5°C during cooling and heating process suggests that annealing modifies the microstructure, transformation temperature, and the magnetic properties of the polycrystalline Ni–Mn–Ga alloy.

Phase transition of Ni–Mn–Ga alloy powders prepared by vibration ball milling

This study investigated the phase transformation of the flaky shaped Ni–Mn–Ga powder particles with thickness around 1 μm prepared by vibration ball milling and post-annealing. The SEM, XRD, DSC and ac magnetic susceptibility measurement techniques were used to characterize the Ni–Mn–Ga powders. The structural transition of Heusler → disordered fcc occurred in the powders prepared by vibration ball milling (high milling energy) for 4 h, which was different from the structural transition of Heusler → disordered fct of the powders fabricated by planetary ball milling (low milling energy) for 4 h. The two different structures after ball milling should be due to the larger lattice distortion occurred in the vibration ball milling process than in the planetary ball milling process. The structural transition of disordered fcc → disordered bcc took place at ∼320 °C during heating the as-milled Ni–Mn–Ga powders, which was attributed to the elimination of lattice distortion caused by ball milling. The activation energy for this transition was 209 ± 8 kJ/mol. The Ni–Mn–Ga powder annealed at 800 °C mainly contained Heusler austenite phase at room temperature and showed a low volume of martensitic transformation upon cooling. The inhibition of martensitic transformation might be attributed to the reduction of grain size in the annealed Ni–Mn–Ga particles.

Corrosion study of single crystal Ni–Mn–Ga alloy and Tb0.27Dy0.73Fe1.95 alloy for the design of new medical microdevices

Once placed in a magnetic field, smart magnetic materials (SMM) change their shape, which could be use for the development of smaller minimally invasive surgery devices activated by magnetic field. However, the potential degradation and release of cytotoxic ions by SMM corrosion has to be determined. This paper evaluates the corrosion resistance of two SMM: a single crystal Ni-Mn-Ga alloy and Tb(0.27)Dy(0.73)Fe(1.95) alloy. Ni-Mn-Ga alloy displayed a corrosion potential (E (corr)) of -0.58V/SCE and a corrosion current density (i (corr)) of 0.43 μA/cm(2). During the corrosion assay, Ni-Mn-Ga sample surface was partially protected; local pits were formed on 20% of the surface and nickel ions were mainly found in the electrolyte. Tb(0.27)Dy(0.73)Fe(1.95) alloy exhibited poor corrosion properties such as E (corr) of -0.87V/SCE and i (corr) of 5.90 μA/cm(2). During the corrosion test, this alloy was continuously degraded, its surface was impaired by pits and cracks extensively and a high amount of iron ions was measured in the electrolyte. These alloys exhibited low corrosion parameters and a selective degradation in the electrolyte. They could only be used for medical applications if they are coated with high strain biocompatible materials or embedded in composites to prevent direct contact with physiological fluids.

Effect of pore architecture on magnetic-field-induced strain in polycrystalline Ni–Mn–Ga

Monocrystalline Ni–Mn–Ga alloys show magnetic-field-induced strains (MFIS) of up to 10% as a result of reversible twinning; by contrast, polycrystalline Ni–Mn–Ga shows near-zero MFIS due to strain incompatibilities at grain boundaries inhibiting twinning. Recently, we showed that porous polycrystalline Ni–Mn–Ga exhibits a small, but non-zero, MFIS value of 0.12% due to reduction of these incompatibilities by the porosity. Here, we study the effect of pore architecture on MFIS for polycrystalline Ni–Mn–Ga foams. Foams with a combination of large (∼550 μm) and small (∼80 μm) pores are fabricated by the replication method and exhibit thinner nodes and struts compared to foam containing only large (∼430 μm) pores. When magnetically cycled, both types of foams exhibit repeatable MFIS of 0.24–0.28% without bias stress. As the cycle number increases from a few tens to a few thousands, the MFIS drops due to damage accumulation. The rate of MFIS decrease is lower in the dual-pore foam, as expected from reduced constraints on the twin boundary motion, since twins span the whole width of the thinner nodes and struts.

Martensitic transformation, mechanical property and magnetic-field-induced strain of Ni–Mn–Ga alloy fabricated by spark plasma sintering

Martensitic transformation, mechanical property and magnetic-field-induced strain of Ni–Mn–Ga alloys obtained using the spark plasma sintering method have been investigated. The results show that the martensitic transformation of the sintered specimen is basically similar to that of conventional bulk alloys. The ductility of sintered Ni–Mn–Ga alloys is significantly enhanced compared to the specimen obtained by conventional melting technique. The highest fracture strain so far is reported. Moreover, magnetic-filed-induced strain in sintered Ni–Mn–Ga alloys has been studied for the first time.

Effect of Fe substitution on the phase stability and magnetic properties of Mn-rich Ni–Mn–Ga ferromagnetic shape memory alloys

The influence of Fe additions on the martensitic transformation and magnetic properties of Mn-rich Ni–Mn–Ga alloys was investigated by substituting either 1 at% Fe for each atomic species or by substituting Ni with varying amounts of Fe. The magnetic structure of the alloys was studied using 57Fe Mössbauer spectroscopy. Mössbauer spectra revealed typical paramagnetic features in Mn-rich Ni–Mn–Ga–Fe alloys owing to the preferential site occupancy of Fe atoms at Ni sites. The evolution of the magnetic properties and phase stability has been correlated with the chemical and atomic ordering in these alloys.

Magnetic hysteresis and refrigeration capacity of Ni–Mn–Ga alloys near Martensitic transformation

This paper studies the magnetic hysteresis and refrigeration capacity of Ni-Mn-Ga alloys in detail during heating and cooling isothermal magnetisation processes. The Ni-Mn-Ga alloys show larger magnetic hysteresis when they transform from austenite to martensite, but smaller magnetic hysteresis when they transform from martensite to austenite. This behaviour is independent of either the pure Ni-Mn-Ga alloys or the alloys doped with other elements. Because of the existence of the magnetic hysteresis, the relation between the magnetic entropy change and refrigeration capacity is not simply linear. For practical consideration, magnetocaloric effect of Ni-Mn-Ga alloys should be investigated both on cooling and heating processes.

Studies on ferromagnetic shape memory Ni–Mn–Ga alloys with Fe and rare-earths additives

The effect of Fe and rare-earths (Tb&Dy) addition on the martensitic transformation, magnetic and mechanical properties of the Ni 50Mn 30Ga 20 (at%) ferromagnetic shape memory alloy was investigated. The results show that the rare-earths exhibit negligible solubility in the Ni–Mn–Ga–Fe matrix phase and formed grain boundary phase. The grain boundary precipitate enhances the compressive ductility of the alloys. The martensite transformation temperature was found to be increased by the addition of rare-earths, while, Fe increased the Curie temperature of the Ni–Mn–Ga alloy. However, with increase in Fe content, an additional (Ni,Fe) 3(MnGa) precipitate is formed, leading to decrease in martensite transformation temperature and compressive ductility.

Change in microstructure during training of a Ni 50Mn 29Ga 21 bicrystal

The change in microstructure during training of a Ni 50 Mn 29 Ga 21 bicrystal was investigated by electron backscatter diffraction in a scanning electron microscope. The martensitic variants form macro and micro twins which are twin related with each other. The twin plane is of type. To achieve magnetic field induced strain in Ni–Mn–Ga alloys, a training process was applied. This process consists of successively compressing the sample along two axes. As a result the twinning stress is reduced and the twinning strain is maximized.

New approach to twin interfaces of modulated martensite

In Ni–Mn–Ga ferromagnetic shape memory alloys, the crystallographic nature of martensitic variant interfaces is one of the key factors governing the variant reorientation through field-induced interface motion and hence the shape memory performance. So far, the crystal structure studies of these materials – conducted by means of transmission electron microscopy – have suffered from uncertainties in determining the number of unit cells of modulated superstructure, and consequently improper interpretations of orientation correlations of martensitic variants. In this paper a new approach is presented for comprehensive analysis of crystallographic and morphological information of modulated martensite, using automated electron backscatter diffraction. As a first attempt, it has been applied for the unambiguous determination of the orientation relationships of adjacent martensitic variants and their twin interface characters in an incommensurate 7M modulated Ni–Mn–Ga alloy, from which a clear and full-featured image of the crystallographic nature of constituent twin interfaces is built up. Certainly, this new approach will make it feasible not only to generalize the statistical analysis of martensitic variant distributions for various materials with modulated superstructure, but also to give insight into the crystallographic characteristics of martensitic variant interfaces and the variant reorientation mechanism of new advanced materials for interface engineering.

Thermoelastic behaviour of martensitic alloy in the vicinity of critical point in the stress–temperature phase diagram

The theoretical phase diagram of the shape memory alloy, which exhibits the first-order martensitic phase transition of the cubic–tetragonal type, has been considered. The thermoelastic behaviour of the ultra-soft Ni–Mn–Ga alloy in the vicinity of the endpoint of the phase transitions line has been modelled. To this end, the strain–temperature and stress–strain dependencies have been computed with the account of the temperature dependence of the elastic modulus of the alloy. Two important features of thermoelastic behaviour of the alloy have been disclosed: (1) even in the case of complete stress-induced martensitic transformation (MT), the MT strain determined from the length of the plateaus at the stress–strain curves is smaller than the ‘spontaneous’ tetragonal distortion of the crystal lattice, which arises on cooling of the alloy and (2) the stress–strain loops may include the plateau-like segment even at temperatures above the critical temperature, which corresponds to the endpoint of the stress–strain phase diagram. These features render the observation of the endpoint of phase transitions line impossible with the help of the stress–strain tests and make preferable the direct structural studies of MTs in the stressed single-crystalline specimens.

Stress- and magnetic field-induced entropy changes in Fe-doped Ni–Mn–Ga shape-memory alloys

Isothermal stress- and magnetic field-induced entropy changes in a Fe-doped Ni–Mn–Ga alloy have been measured in the limits of low applied stress and magnetic field. We have obtained that in this limit while elastocaloric is conventional, giving rise to an increase of entropy when a stress is applied, magnetocaloric effect is inverse, which means that entropy decreases by application of an applied magnetic field. This inverse effect is a consequence of the magnetostructural coupling driven by the martensitic transition.

Thermal and microstructural evolution under ageing of several high-temperature Ni–Mn–Ga alloys

The thermal and microstructural evolution under ageing at temperatures up to 770 K has been studied on several polycrystalline high-temperature Ni–Mn–Ga shape memory alloys, all of them with an excess of Ni and higher e/a ratio as compared to the stoichiometric Ni 2MnGa. Alloys with a high content of Ni exhibit a relatively poor thermal stability, caused either by chemical decomposition or by fast precipitation of Ni-rich phases, which can promote loosing most of the transformation even after few cycles in the DSC. Two types of precipitates have been observed: lath-like precipitates with compositions not very far from the ones of the matrix and ellipsoidal L1 2 (γ′) precipitates with chemical composition rather different to the matrix. On the other hand, Mn-rich alloys with Ni close to 50 at% do not show precipitates even after 7 × 10 7 s (27 months) at 770 K then keeping the martensitic transformation similar to non-aged samples. Therefore, the thermal stability of some compositions of the Ni–Mn–Ga system is comparable to the most stable high-temperature shape memory alloys. Some features of the thermal and microstructural evolution under ageing as well as strategies to optimize these alloys are also discussed in terms of the Ni–Mn–Ga equilibrium phase diagram.

Effect of Mn concentration on the phase transformation in Ni–Mn–Ga single crystal

Aim of this paper is to study the transformation temperature of Ni–Mn–Ga single crystals by using differential scanning calorimetry. It shows that martensitic temperature abruptly increase with increasing Mn concentration whereas Curie transition to decrease. The calculated value of electron-to-atom ratio ( e/ a) reveal that it increases with increased doping of Mn in place of Ga at fixed Ni content. The crystal structures at room temperature were studied using XRD pattern. Thermo-magnetic behavior of the Ni–Mn–Ga alloy shows the ferromagnetic nature and its behaviour is slightly losing due to antiferromagnetic ordering of excess Mn atoms. Microstructure from SEM image confirm that the alloys are martensitic plates. The martensitic plates contain fewer stripes like magnetic domains.

Ageing effects on structural and magnetic transformations in a Ni–Co–Mn–Ga alloy

Partial substitution of Ni by Co in Mn-rich Ni–Mn–Ga alloys has been found to modify the magnetic ordering of the phases, improving in this way the possibility to obtain large magnetization difference between austenite and martensite, an essential requirement to induce the martensitic transformation by application of a magnetic field. Particularly, Ni50−xCoxMnyGa50−y alloys undergo, for Co content below x=9, structural transformation between ferromagnetic austenite and paramagnetic martensite, thus leading to enhanced magnetization difference values.The martensitic transformation temperatures as well as the martensite and austenite Curie temperatures depend on composition, but significant changes can be brought about by selected thermal treatments. In this work, the composition is chosen as Ni42Co8Mn32Ga18 in order to obtain concurrent martensitic transformation and austenite Curie temperature, and the effect of quench and subsequent ageing on the structural and magnetic transitions is studied. Aside from the monotonic transformation temperatures change, which is mostly attributed to atomic ordering taking place upon post-quench ageing, the results show the effect of the relative position of the structural and magnetic ordering reactions on the transformation entropy change.

The modeling of phase diagrams and premartensitic effects in Heusler Ni–Mn–Ga alloy by Monte Carlo method

In this work the coupled magnetostructural and premartensitic phase transitions in Heusler Ni–Mn–Ga alloy have been modeled by Monte Carlo method using Ising and Blume–Emery–Griffiths models. The phase diagrams in coordinates (Temperature–Magnetoelastic constant) were obtained. For the first time it is shown that the magnetoelastic interaction account in austenitic and martensitic phases allows to describe the coupled magnetostructural phase transition in Ni-Mn-Ga alloys. The area of the premartensitic phase transition in the temperature–magnetoelastic constant diagram has been found.