Mn Ga Alloys Research Articles

Preparation of melt textured Ni–Mn–Ga

Ni–Mn–Ga alloys are the best-known group of magnetically active alloys, which show magnetic field-induced strain (MFIS) via twin boundary motion. This phenomenon is often referred to as the magnetic shape memory effect. In single crystals, strains of up to 10% have been achieved that way. Up to now, no systematic study of this effect in polycrystals is available. In order to obtain this effect also in polycrystals, stationary casting is employed to achieve coarse-grained, textured samples. Therefore, a hot ceramic mold is mounted on a cold copper plate. Thus, a unidirectional heat flow along the sample axis is realized. The direction of heat flow coincides with the growth direction, which is preferentially [1 0 0]. After solidification, the samples were annealed at high temperature (1000 °C) to ensure chemical homogeneity on a local scale. The samples were cut by spark erosion. In order to allow for a direct investigation of the resulting microstructure by electron backscatter diffraction (EBSD) measurements, an alloy composition with a martensitic transformation below room temperature was chosen (Ni 48Mn 30Ga 22). The transformation temperature was checked by differential scanning calometry (DSC). The orientation of the grains and the preferred growth direction was analyzed using EBSD.

Annealing influence on the atomic ordering and magnetic moment in a Ni–Mn–Ga alloy

We have studied an alloy of composition Ni 51Mn 28Ga 21 prepared by rapid quenching in the form of a ribbon, with transformation temperature T M=337 K below the magnetic-order temperature, T C=344 K. Annealing of the samples was performed at 600 K for different times. From magnetic characterization a clear increase of the saturation magnetization accompanied with an increase of T C (up to 20 K) and T M (about 10 K) has been observed. XMCD measurements of both as-quenched and annealed samples have revealed great changes for the Mn and also the existence of a strong Ni signal. These results point out a possible non-negligible role of Ni, through the polarization of the conduction band, into driving the interplay between annealing and the magnetic properties in these materials.

Ferromagnetic Shape Memory Materials: Underlying Physics and Practical Importance

The underlying physics of the giant magnetic or mechanical field-induced-strains in Ni–Mn–Ga alloys are briefly discussed. For the purpose of illustrating some fundamental aspects, experimental data relative to lattice instability and composition dependence of magnetization are presented alongside reported results. An implementation of Ni–Mn–Ga single crystal as a strain sensor is described.

The way composition affects martensitic transformation temperatures of Ni–Mn–Ga Heusler alloys

AbstractA systematic substitution of Ge, Si, C and Co for Ga in the non‐stoichiometric Ni–Mn–Ga alloys was performed. The relationship between the compositions of different elements including Ni, Mn, Ga, Ge, Si, C, Co, In and martensitic transformation temperature (Ms) was studied in detail for the present alloys together with data collected from a variety of sources. It is found that Ms is a sensitive parameter to the composition. The size factor and electron concentration are usually thought to be the way the composition influences Ms in the Ni–Mn–Ga alloys. Here, analyzing by linear regression, the electron density maybe a most appropriate parameter to describe the way the composition influences Ms when compared with size factor and electron concentration. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The phase diagrams of Ni–Mn–Ga alloys in the magnetic field

With the help of the Ginzburg–Landau theory the temperature–magnetic field phase diagrams of Heusler alloys Ni–Mn–Ga are theoretically investigated. The influence of the parameters of the magnetoelastic constants and elastic moduli on the phase diagrams are discussed. It is shown that with the determined combination of the phenomenological parameters, the critical magnetic field for the phase transition appears. The theoretically predicted value of the critical magnetic field is possible to be achieved in the experiment with the help of modern magnetic field sources.

Effect of atomic ordering on the phase transformations in Ni–Mn–Ga shape memory alloys

The influence of the L2 1 order degree in single crystalline Ni–Mn–Ga alloys is analysed in the present work. The alloys studied display a sequence of martensitic (MT) and intermartensitic (IMT) transformations on cooling, the reverse transformation to austenite, on heating, taking place in a single step. Quenching from different temperatures produces distinct effects on the MT and IMT: while the MT temperatures are not practically affected by the performed heat treatments, the IMT shifts towards lower temperatures after quenching from increasing temperatures. Such evolution can be related to changes in the L2 1 order degree, in the sense that ordering favours the occurrence of the IMT while it hardly affects the MT temperatures. The closeness of the free energies of the different martensitic structures and the differences between the MT and IMT entropy changes allow this behaviour to be explained. In turn, the Curie temperature increases with the L2 1 ordering for an alloy undergoing magnetic transition in martensite phase, no changes being detected if the Curie temperature is above the martensitic transformation.

First principles calculations on martensitic transformation and phase instability of Ni–Mn–Ga high temperature shape memory alloys

Martensitic transformation and phase instability of Ni8+xMn4Ga4−x high-temperature shape memory alloys have been investigated by first principles calculations using the pseudopotentials plane wave method. The tetragonality (the ratio between the lattice parameter c and a) increases with the increasing of Ni content. The total energy difference between parent and martensite phases plays an important role on the martensitic transformation. The formation energy calculations indicate the instability of Ni–Mn–Ga alloy with extra high Ni content. The densities of states of the alloys are discussed.

Doping and disorder in theCo2MnAlandCo2MnGahalf-metallic Heusler alloys

We expand our study on the full-Heusler compounds [I. Galanakis \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 042502 (2006)] to cover also the case of doping and disorder in the case of Co$_2$MnAl and Co$_2$MnGa half-metallic Heusler alloys. These alloys present a region of very small minority density of states instead of a real gap. Electronic structure calculations reveal that doping with Fe and Cr in the case of Co$_2$MnAl retains the half-metallicity contrary to the Co$_2$MnGa compound. Cr impurities present an unusual behavior and the spin moment of the Cr impurity scales almost linearly with the concentration of Cr atoms contrary to the Co$_2$MnZ (Z= Si, Ge, Sn) where it was almost constant. Half-metallicity is no more preserved for both Co$_2$MnAl and Co$_2$MnGa alloys when disorder occurs and there is either excess of Mn or $sp$ atoms.

Magnetically controlled high damping in ferromagnetic Ni52Mn24Ga24 single crystal

An unusually high damping peak of 20% has been observed in Ni52Mn24Ga24 single crystals accompanying with martensitic transformation. The high damping capacity can be retained in the low-temperature martensite phase when a bias field of 0.4T was applied along the measurement direction. The magnetically controlled high damping and adjustable martensitic-austenitic transformation temperature indicate a great potential of using Ni–Mn–Ga alloys as high damping or energy absorbing materials in a wide temperature range.

Effect of aging on transformation temperatures and microstructure of an Fe-doped Ni–Mn–Ga ferromagnetic shape memory alloy

Effect of aging on transformation temperatures and microstructure of an Fe-doped Ni–Mn–Ga alloy has been studied by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). DSC results show the transformation temperatures can be obviously raised by aging. When the aging temperature increases, the transformation temperatures gradually decrease to the minimum values and then increase slightly upon further increasing the aging temperature. The defects introduced by aging should be responsible for the change of transformation temperatures. In the sample aged at 573 K for 30 min, a large number of stacking faults are observed by TEM.

Negative and positive magnetocaloric effect in Ni–Fe–Mn–Ga alloy

The phase transition process and magnetic entropy change Δ S of Ni 54.5 FeMn 20 Ga 24.5 alloy were studied. Substitution of Fe for Ni increases the Curie temperature and decreases the temperature of martensitic phase transition. The transition from ferromagnetic martensitic to ferrormagnetic austenitic state leads to an abrupt increase of magnetization below 0.5 T and an abrupt decrease of magnetization above 0.5 T. The sign of Δ S changes from positive to negative with increasing the applied field from 0.5 to 2 T. The maximal value of the positive magnetic entropy change Δ S is about 3.1 J/kg K for the applied field from 0 to 0.5 T. The increase of applied field from 1.5 T results in a negative Δ S . The peak of negative Δ S is - 2.1 J / kg K for a field change of 2 T.

Effect of Fe content on fracture behavior of Ni–Mn–Fe–Ga ferromagnetic shape memory alloys

The effect of Fe content on hardness and toughness of Ni48.7Mn30.1−xFexGa21.2 (x=0–11, number indicates at.%) alloys is investigated and the relationship between the fracture toughness and the fractography is discussed by observing the fracture morphology and analyzing the propagation of the crack. The micro-Vickers hardness results indicate that the hardness increases with increasing Fe content. The fracture toughness results indicate that Fe doping has improved the toughness of Ni–Mn–Ga alloys, and the toughness increases with increasing Fe content in Ni48.7Mn30.1−xFexGa21.2 (x=0–11) alloys. Fracture morphology shows typical intergranular fracture with rock candy pattern in the Ni48.7Mn30.1Ga21.2 (x=0) and river pattern cleavage fracture facets in the Ni48.7Mn19.1Fe11Ga21.2 (x=11) alloy. The fracture type changing from intergranular fracture to transgranular fracture is the main reason for the improved toughness of Ni–Mn–Ga alloys by addition of Fe.

Effect of pre-deformation on martensitic transformation behavior and the microstructure of a Ni–Mn–Ga alloy

The influence of pre-deformation on the martensitic transformation behavior and its microstructure has been put forward in a Ni 55Mn 20Ga 25 (at.%) ferromagnetic shape memory alloy. It is found that the reverse martensitic transformation temperatures ( A s and A f) increase linearly with increasing amount of deformation, while the martensitic transformation temperatures ( M s and M f) are not changed even when the deformation is up to 6%. However, the effect of the pre-deformation on the reverse martensitic transformation disappears in the following heating process. Transmission electron microscopy shows well-accommodated martensite variants in the undeformed specimen and interfacial boundaries are straight and well-defined. After deformation, the reorientation of martensite variants occurs at the expense of the unfavorable martensite variants, and the interfacial boundary changes from a sharp and straight morphology to a curved and irregular one. Meanwhile, some lattice defects are formed inside the variants even in slightly compressed samples. With increasing deformation, more lattice defects are generated inside the variants as well as in the interfacial boundaries between variants. Based on the analysis of the experimental results, the micromechanism of the effect of pre-deformation on martensitic transformation behavior has been clarified.

Structure of the layered martensitic phases of Ni–Mn–Ga alloys

The present work contains a high-resolution transmission electron microscopy study (including image simulations) of the layered martensitic structures formed in Ni–Mn–Ga alloys. The main purpose is to distinguish two structural models proposed in the literature. For the seven-layered martensite, the observations confirm its nano-twinned nature, which is inherent to the description as stacking of nearly close-packed planes derived from {1 1 0} aust with ( 5 2 ¯ ) sequence, i.e. the so-called 14M structure. The observed stacking sequence, however, is notably distorted (perfect ( 5 2 ¯ ) stacking is present only in small areas), although, it preserves a predominant periodicity of seven planes. Equivalent results have been obtained for the 10-layered structure and a less frequent 12-layered phase, for which ( 5 5 ¯ ) and ( 7 5 ¯ ) stacking sequences, respectively, have been predominantly observed. In case of the five-layered martensite, the image simulations and atom projections obtained from the two structural models fit well with the experimental images, which makes it very difficult to discern between them. Other ferromagnetic shape memory alloys present similar structures as the Ni–Mn–Ga system. Particularly, a “new” six-layered martensitic structure has been found in Ni–Fe–Ga alloys. The first observations reveal its nano-twinned nature as well, with a predominant ( 4 2 ¯ ) stacking sequence.

Martensitic transition and magnetic properties in Ni–Mn–X alloys

The structural and magnetic properties of Heusler alloy series Ni 50 Mn 50− x Sn x (numbers indicate at. %) and Ni 50 Mn 50− x In x have been studied. Magnetization and calorimetric measurements have been carried out in order to study the magnetic and structural transitions properties. The behaviour of the studied compounds is compared with that of the Ni–Mn–Ga alloy. Data for entropy and magnetization changes at the martensitic transition are reported. It is found that, as opposed to Ni–Mn–Ga, for Ni–Mn–In and Ni–Mn–Sn the magnetic moment in martensite is lower than in austenite.

Ni–Mn–Ga high-temperature shape memory alloys

The relationship among the composition, structure and martensitic transformation temperatures have been studied in detail for Ni 50− x Mn 25Ga 25− x (at.%) alloys by optical and transmission election microscopy, X-ray diffraction and differential scanning calorimeter measurements. The martensitic transformation temperatures almost linearly increase with increasing Ni content up to 670 K showing their potentials as high-temperature shape memory alloys. The results show that the tetragonality ratio ( c/ a) increases with the Ni substitution for Ga, but the unit-cell volume reduces simultaneously. The martensitic transformation characteristics of Ni 54Mn 25Ga 21 alloy with single martensite phase show excellent stability during thermal cycles or aging. The polycrystalline Ni 54Mn 25Ga 21 alloy exhibited a shape-memory effect of 4.2% and compressive strain larger than 10% by using grain refining technique. Addition of Fe as a fourth element is an effective method to improve the mechanical properties, especially plasticity of polycrystalline Ni–Mn–Ga alloy due to the precipitation of γ-phase. A compressive strain of 30% and a shape-memory effect of 2.8% are obtained for Ni 56Mn 17Fe 8Ga 19 alloy.

Effect of intermartensitic reaction on the co-occurrence of the magnetic and structural transition in Ni–Mn–Ga alloys

The different magnetic transition temperatures during heating and cooling in certain Ni–Mn–Ga alloys have recently gained interest, since they may be beneficial for the giant magneto-calorimetric effect. Khovailo et al. and Chernenko et al. have suggested that this dissimilarity is connected to the different Curie points of the cubic parent phase and the non-layered tetragonal martensite as the magnetic transition is delayed due to the structural transformation. The present work extends this approach to the alloys exhibiting the intermartensitic transformation with the seven-layered orthorhombic martensite (7M) in the co-occurrent magnetic and structural transition region. We confirm the previous results that the Curie point of the tetragonal martensite is higher than the one of the parent cubic phase. It is also confirmed that in certain alloys the magnetic transition occurs in the 7M structure and, consequently, the magnetic transition temperature should be the Curie point of the 7M martensite. We verify that in the same alloy the Curie point of the 7M martensite is lower than the one of the tetragonal martensite.

Low-temperature behaviour of Ni–Fe–Ga shape-memory alloys

Three polycrystalline alloys with compositions Ni 53+ x Fe 20− x Ga 27 ( x = 0.5, 1, 2 at %) have been studied, paying particular attention to the low-temperature behaviour, between the martensite finish and liquid nitrogen temperature. In this respect, the dynamic mechanical analysis has proven to be a powerful tool in detecting structural and microstructural changes. The studied alloys display, on cooling in the above-mentioned temperature domain, a broad internal friction peak accompanied by a marked increase in the elastic modulus slope; on further cooling an additional modulus drop and concurrent internal friction jump take place. The internal friction and elastic modulus evolution is not strictly reversible on heating. Basing in the earlier results obtained for Ni–Mn–Ga alloys, the internal friction peak is attributed to the oscillating movement of twin boundaries in the layered martensites, while the lowest temperature anomaly suggests the occurrence of slight changes in the mixture of modulated martensite structures formed on cooling.

Magnetization curves for tetragonal martensites: Experiments and modeling

In the absence of twin boundary motion, the magnetization curves of tetragonal martensites depend on the relative orientation of crystal axes, the shape anisotropy, and the applied field. Two tetragonal martensites with the lattice parameters ratio c∕a<1 (easy axis) and c∕a>1 (easy plane) exist in Ni–Mn–Ga alloys. The magnetization curves of rods and plates of both types of martensite, chosen for their immobile twin boundaries, have been measured at room temperature. The M(H) curves are well described by a theory including magnetocrystalline anisotropy and magnetostatic energy. The results can indicate the variant distribution in each of the samples.

Experimental characterization of the sensor effect in ferromagnetic shape memory Ni–Mn–Ga

The characterization of a commercial Ni–Mn–Ga alloy for use as a deformation sensor is addressed. The experimental determination of flux density as a function of strain loading and unloading at various fixed magnetic fields gives the bias field needed for maximum recoverable flux density change. This bias field is shown to mark the transition from irreversible (quasiplastic) to reversible (pseudoelastic) stress-strain behavior. A reversible flux density change of 145mT is observed over a range of 5.8% strain and 4.4MPa stress at a bias field of 368kA∕m. The alloy investigated shows potential as a high-compliance, high-displacement deformation sensor.