Ga Ferromagnetic Shape Memory Alloys Research Articles

Acoustic emission during approaching the critical point on stress-temperature diagram of martensitic transformation in Ni48Fe20Co5Ga27 (at.%) single crystal

Recently, the existence of martensite-austenite critical point, CP, on the stress-strain curves in the single crystalline Ni–Fe(Co)-Ga ferromagnetic shape memory alloys has been discovered by disappearance of hysteresis. It was also argued that the shear modulus at the CP and in the postcritical phase should be zero. In the present work we continue the investigation of the above new features on the “compression stress-temperature” phase diagram of the martensitic transformation (MT) of Ni48Fe20Co5Ga27 (at.%) single crystals in the vicinity of the CP by acoustic emission (AE). It is found that the AE activity (amount of events and their amplitudes) drastically dropped down by approaching the CP and tended to stabilize in a postcritical region. The above decrease is mainly due to the decrease of the number of large avalanches, while the exponents of the probability distribution of amplitudes and energies remained approximately the same, illustrating no change in the mechanism of acoustic emission events. The coordinates of the CP were estimated to be (340 ± 50) MPa and (327 ± 40) K. The elastic modulus was reduced with increasing temperature from 20 GPa down to 2 GPa in the critical region, whereby a huge total deformation of about 13 % was achieved. The stress-induced MT also shifted as a function of temperature and its slope decreased by about a factor of two upon approaching the CP. All features of the critical state determined have an obvious practical interest, as well as present a solid basis for theoretical understanding of critical and postcritical states in solids.

Kinetics of Reordering in Quenched Ni2Mn0.8Cu0.2Ga Ferromagnetic Shape Memory Alloys

Quenched Ni2Mn1−xCuxGa ferromagnetic shape memory alloys undergo two consecutive post-quench ordering processes. The kinetics of order recovery has been analysed in detail for Ni2Mn0.8Cu0.2Ga, based on the calorimetric curves obtained during post-quench heating at constant rates. Isoconversional methods have been used to determine the activation energy, the pre-exponential factor, and the reaction model that best fits the two reordering processes. The kinetic analysis has been extended to samples quenched from different temperatures. The kinetic study shows that order improvement processes in quenched Ni2Mn0.8Cu0.2Ga alloys can be described by a first order reaction model, consistent with site-saturation nucleation and homogeneous diffusion-controlled growth, with apparent activation energies around 1.1 eV. The pre-exponential factors, especially those obtained for samples quenched from different temperatures, highlight the crucial role of the vacancies retained by high temperature quenching on the atomic reordering underlying the observed processes.

Role of Fe addition in Ni–Mn–Ga–Co–Cu–Fe ferromagnetic shape memory alloys for high-temperature magnetic actuation

The Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) are a group of active materials that undergo martensitic transformations (MTs) induced by temperature, stress and/or magnetic fields, resulting in large recoverable mechanical deformations. Their fast response and high energy density make them ideal candidates for implementation in sensors and actuators. Recently, the development of high-temperature FSMAs (HTFSMAs), working at temperatures over 373 K, has become an important task to meet the current requirements of modern technologies. The magnetic and magnetoelastic properties of FSMAs are very sensitive to the interactions between the magnetic moments of atoms that, in turn, depend on the atomic positions within the lattice. Here we investigate a series of the polycrystalline Ni45Mn25-xGa20Co5Cu5Fex (x = 0, 1, 2 and 5 at%) HTFSMAs. Their MT and Curie temperatures, the crystal structures of the martensitic and austenitic phases, the temperature evolutions of the lattice parameters of both phases and the atomic site occupancies have been studied by means of powder neutron diffraction measurements, complemented by standard characterization techniques. Based on the atomic site occupancies and additional measurements of the saturation magnetization, the influence of the structure and atomic ordering on the magnetism in these materials is analysed. The most promising candidate for high temperature magnetic actuation is the alloy with Fe 5 at%, following a combination of the high MT and Curie temperatures, around 370 K and 440 K, respectively, with a low tetragonality ratio, c/a ≈ 1.16, in the proximity of MT.

Switching the soft shearing mode orientation in Ni–Mn–Ga non-modulated martensite by Co and Cu doping

Resonant ultrasound spectroscopy measurements and density functional theory calculations were used to analyze the effect of Co and Cu doping (3–6 at. %) on elastic constants of non-modulated martensite of the Ni–Mn–Ga ferromagnetic shape memory alloy. Due to the doping, the studied alloys exhibited decreased tetragonal ratios c/a ≈ 1.14. Both the experiments and the calculations revealed that the lowering of the c/a ratio resulted in a change of the orientation of the softest shearing modes of the tetragonal lattice. The newly appearing softest shearing modes for the doped materials have approximately orientations and indicate a lattice instability directly related to the particularly low twinning stress for compound twins, needed for the magnetically induced reorientation.

Systematic Trends of Transformation Temperatures and Crystal Structure of Ni–Mn–Ga–Fe–Cu Alloys

Here we report a systematic research on effects of Fe and Cu upon properties relevant for the magnetic shape memory effect of Ni–Mn–Ga ferromagnetic shape memory alloys. Fe and Cu were identified as elements with potential synergism to increase the martensite transformation temperature of Ni–Mn–Ga magnetic shape memory (MSM) alloys. Eighteen Ni–Mn–Ga–Fe–Cu alloys with different systematic trends in substituting the ternary elements with Cu and Fe have been investigated. We found a method to describe the effectiveness of Ni, Mn, and Cu upon raising the martensitic transformation temperature, lowering the saturation magnetization, and varying the Curie temperature. We find the martensite transformation temperature most influenced by the Ni content, followed by Mn, with a smaller effect of Cu. The saturation magnetization decreases with similar coefficients for Mn and Cu alloying. The Curie temperature monotonously decreases with Mn, but not Cu. The 10M martensite structure is stable for the composition Ni46.5Mn25+XGa25−X−YFe3.5CuY with X and Y range of 0–5.7, and 0.8–3.0. Used in combination with the total e/a, the elemental e/a-ratio gives some insight into the complex behavior of quinary MSM alloys and is a useful method of analyzing MSM alloys for improved functional properties.

Crystallographic insights into diamond-shaped 7M martensite in Ni-Mn-Ga ferromagnetic shape-memory alloys.

For Heusler-type Ni-Mn-Ga ferromagnetic shape-memory alloys, the configuration of the martensite variants is a decisive factor in achieving a large magnetic shape-memory effect through field-induced variant reorientation. Based upon the spatially resolved electron backscatter diffraction technique, the microstructural evolution associated with the martensitic transformation from austenite to seven-layered modulated (7M) martensite was investigated on a polycrystalline Ni53Mn22Ga25 alloy. It was clearly shown that grain interior nucleation led to the formation of diamond-shaped 7M martensite within the parent austenite matrix. This diamond microstructure underwent further growth through an isotropic expansion with the coordinated outward movement of four side habit planes, followed by an anisotropic elongation with the forward extension of a type-I twin pair. A two-step growth model is proposed to describe the specific morphology and crystallography of 7M martensite. In addition, the habit planes were revealed to possess a stepped structure, with the {1 0 1}A plane as the terrace and the {0 1 0}A plane as the step. The characteristic combination of martensite variants and the underlying mechanism of self-accommodation in the martensitic transformation have been analysed in terms of the minimum total transformation strain, where the deformation gradient matrix was constructed according to the experimentally determined orientation relationship between the two phases. The present results may deepen the understanding of special martensite microstructures during the martensitic transformation in ferromagnetic shape-memory alloys.

Submicron pillars of ferromagnetic shape memory alloys: Thermomechanical behavior

Remarkable shape memory, superelasticity and rubber-like effects on the submicron scale have been disclosed in Ni–Fe(Co)–Ga and Ni–Mn–Ga ferromagnetic shape memory alloys. Arrays of pillars with the different cross-section and length have been prepared onto 001-oriented faces of the alloys single crystals and their thermomechanical behavior across the martensitic transformation was studied in the bending mode inside a scanning electron microscope. Recovered strains of up to 5% and 7% have been obtained as a result of shape memory and superelasticity effects, respectively. These findings are important for the development of novel micro/nanoelectromechanical systems to be controlled, contactless, by a magnetic field.

Magnetostriction of ferromagnetic shape memory alloy Ni2.27Mn0.73Ga studied in magnetic fields up to 10 T

Magnetostriction of a non-modulated martensite was measured for Ni2.27Mn0.73Ga ferromagnetic shape memory alloy in steady magnetic fields μ0H up to 10 T at temperatures below room temperature and in μ0H ≤ 1 T at temperatures above room temperature. Linearity of the longitudinal magnetostriction λ|| observed at high fields μ0H > 6 T implies that the magnetostriction is associated with a 3d electron band. The anisotropic magnetostriction obtained in a magnetic field μ0H = 1 T at T = 4.2 K is ∼16 ppm and decreases down to 3 ppm with the increase in temperature. The volume magnetostriction is positive but very small (∼0.6 ppm) under these conditions. In the region of the magnetostructural phase transition the longitudinal and volume magnetostriction showed a complex sigh-changing behavior which depends on the strength of applied magnetic field. Thermal expansion coefficient α of Ni2.27Mn0.73Ga in the martensitic state was measured to be α ∼13 × 10−6 K−1 which is comparable with the thermal expansion coefficient of a non-stoichiometric Ni2.19Mn0.81Ga single crystal.

Magnetic phases in thin films of Ni–Fe(Co)–Ga ferromagnetic shape memory alloy

1 µm-thick polycrystalline thin films of the ferromagnetic shape memory alloy, Ni49.7Fe17.4Co4.2Ga28.7 (at.%), were sputter-deposited on MgO(0 0 1) substrates heated at 773 K. The films’ structure, magnetic properties and transformation behavior have been characterized. They exhibit both forward and reverse martensitic transformations at 218 K and 232 K, respectively. Ferromagnetic resonance and x-ray diffraction measurements discerned three ferromagnetic phases: cubic L21-ordered austenite, tetragonal martensite and disordered cubic γ-phase. The latter phase showed a unique spin-waves resonance spectrum, enabling estimation of the value of the exchange stiffness constant equal to A = 0.7 × 10−6 erg cm−1.

Effect of martensitic structure on the magnetic field controlled damping effect in a Ni–Fe–Mn–Ga ferromagnetic shape memory alloy

We investigated the effect of martensitic structure on the magnetic field controlled damping effect in a Ni53Fe2Mn20Ga25 polycrystalline ferromagnetic shape memory alloy, which undergoes the structure transitions in the sequence of parent phase → seven-layer modulated (14M) martensite → non-modulated tetragonal martensite. We found that large increase in damping capacity by applying magnetic field appears in the 14M martensite, but insignificant increase is observed in the non-modulated tetragonal martensite. This demonstrates that the magnetic field controlled damping effect is strongly dependent on the martensitic structure. It is proposed that magnetic field controlled damping effect relies on the twinning stress and magnetic stress of martensite. Large magnetic field controlled damping effect is expected to appear in the martensitic structure with large magnetocrystalline anisotropy and small twin shear.

Specific Changes in the Magnetoresistance of Ni–Fe–Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions

In this paper, we investigate the influence of Cu, Co, and Al substitutions on the transport properties, and in particular, the magnetoresistive effect in Ni–Fe–Ga ferromagnetic shape memory alloys (FSMAs) prepared as ribbons by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetometry, and magnetoresistive characterizations were performed. In the range of the martensitic transformation (MT), different FSMA compositions show a rich spectrum of different behaviors. For one of the compositions (Ni52Fe20Co2Ga23Al3), the magnetoresistance (MR) showed a local minimum or, on the contrary, a local maxima of reduced amplitude on cooling, in the range of the MT, depending on the performed thermal treatments. In the same composition, by replacing one Al atom with a Co one, no local extremes are seen, the alloy having a concomitant magneto-structural transition. When the magnetic field was varied, the MR showed a nonmonotonic variation in the martensite phase for some compounds, possibly due to the dynamics of the martensite variants realignment. From the studied compositions, the highest MR found on the MT of −9% for 5 T is for Ni50Fe20Ga27Cu3.

Phase transition of Ni55−xCoxMn20Ga25 (8.5 ≤ x ≤ 11.0) alloys with different compositions and magnetic fields

Studies on Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) have been quite active topics in recent years. Unlike previous reports where the amount of Co doping was generally less than 8 at%, in this work Ni55−xCoxMn20Ga25 (8.5 ≤ x ≤ 11.0) alloys were studied with high Co doping. Unusual effects of both composition and magnetic field on phase transitions were observed. With the increase in substitution of Co for Ni, the magnetic transition temperatures increase gradually but the martensitic transformation temperature decreases quite sharply. In particular, the average decrease in the martensitic transformation temperatures is up to 100 K which is nearly twice that in the case of Co content of less than 8 at%. Further, under an applied magnetic field ranging from 0.03 to 0.60 T, abnormal stabilization of a martensite phase with lower magnetization was monitored. Magnetic entropy change of 1.6 J·kg−1·K−1 was induced at the martensitic transformation of Ni46.5Co8.5Mn20Ga25 alloy by an applied field of 1 T. The magnetic contributions, including the magnetocrystalline anisotropy and the Zeeman energy, to the thermodynamics of the martensitic transformation are considered to understand the observed unusual phenomena. This work results in new insights into the understanding of Co-doped Ni–Mn-based ferromagnetic shape memory alloys.

Influence of Strain Rate on Stress–Strain Response of Ni–Mn–Ga Ferromagnetic Shape Memory Alloy Single Crystals

Stress–strain behaviors of Ni–Mn–Ga single crystals were characterized by measurement of stress-induced martensite reorientation under constant magnetic fields. For this purpose, a new setup was designed and manufactured. This setup was supplemented to Santam STM-50 compression testing machine to carry out compression tests, while the specimen was subjected to a constant magnetic field. All the experiments were performed at room temperature in which the sample was at martensite phase. The results indicate that strain rate has no significant effect on stress–strain response of the specimen for strain rates less than 2 × 10−2 s−1 at complete and incomplete loading–unloading cycles.

High-frequency performance of ferromagnetic shape memory alloys

We conduct systematic dynamic experiments on the martensite reorientation in different samples of the single crystal Ni–Mn–Ga Ferromagnetic Shape Memory Alloy (FSMA) driven by a high-frequency magnetic field and a compressive stress. It is found that the output reversible strain strongly depends on the loading frequency, with the maximum output strain up to 6 % at the resonance frequency; and this resonance frequency can be changed by modifying the setting of the compressive stress (the pre-stress level). That provides an alternative way to control/design the system’s optimal working frequency range, besides modifying the spring stiffness and sample geometry. On the other hand, temperature rise accompanies the high-frequency field-induced strain because of the energy dissipation of the martensite twinning and eddy current, which depend on both the frequency and the sample geometry. With these results, some guidelines for improving the FSMA engineering designs are given and some challenging issues for further theoretical study such as the magneto–thermal–mechanical coupling are pointed out.

Crystallographic correlation between 5M and 7M martensite in an Ni–Mn–Ga alloy

In Ni–Mn–Ga ferromagnetic shape memory alloys, a structural transformation from one type of martensite to another is frequently observed upon cooling or heating. In this work, the microstructural features associated with the transformation from 5M to 7M martensite in an Ni50Mn26Ga22Cu2 alloy were studied. On the basis of the crystallographic orientation determination and an examination of the microstructure by means of the electron backscatter diffraction technique, the 5M to 7M transformation was found to be accompanied by the thickening of martensite plates. The two kinds of martensite (5M and 7M) possess a specific orientation relationship with (001)5M//(001)7M and [100]5M//[100]7M. Through further lattice distortion, four types of 5M variant can evolve into four 7M martensite variants in one variant colony. The present study is expected to provide a deep insight into the crystallographic correlation between 5M and 7M martensite in Ni–Mn–Ga alloys.

Fatigue life and fracture mechanics of unconstrained Ni–Mn–Ga single crystals in a rotating magnetic field

Fracture resistance and long fatigue life are required to commercialize Ni–Mn–Ga ferromagnetic shape memory alloys. While Ni–Mn–Ga achieves high fatigue life at low strains, there is little data on the fatigue life of samples that are actuated near the theoretical strain limit. Furthermore, constraints imposed by clamping samples impact their magneto-mechanical and fatigue properties. Here, 10M Ni–Mn–Ga single crystals were exposed to a rotating magnetic field while holding them with rubber O-rings so they were minimally constrained. Prior to testing, sample surfaces were prepared to various levels of finish. Fatigue life varied with some samples reaching more than 800,000 cyclic loads before fracturing and some samples failing after only 4000 cyclic loads. Long fatigue life and fracture resistance are achieved when avoiding crystal defects and surface imperfections. Short fatigue life was caused by high surface roughness and stress concentration at defects and notches.

Constitutive modeling of Ni–Mn–Ga ferromagnetic shape memory alloys under biaxial compression

Ferromagnetic shape memory alloys are a type of shape memory alloys that exhibit inelastic strains when subjected to magnetic fields. So far, several constitutive models have been proposed to predict ferromagnetic shape memory alloys’ behaviors under the application of a magnetic field and a compressive stress. In this article, an available constitutive model in a continuum framework is used to investigate the behaviors of Ni–Mn–Ga under biaxial compressive stresses. Since the required model parameters in the original approach are not unique for all loading conditions, the model is modified so that a unique set of model parameters suffices to study different loading conditions. Loading history is also considered in an improved way, and two-dimensional phase diagram is generalized to three-dimensional phase diagram, called reorientation surface in this work, to directly enter the effects of loading history on reorientation start conditions. The orthogonality property of a surface and its gradient vector are used to obtain the general conditions which must be satisfied to ensure continuation of reorientations.

Temperature rise of high-frequency martensite reorientation via Type II twin boundary motion in NiMnGa Ferromagnetic Shape Memory Alloy

We measured the temperature rise of the high-frequency magnetic-field-induced martensite reorientation in Ni–Mn–Ga Ferromagnetic Shape Memory Alloy and observed the morphology of the associated twin boundaries. It was found that this temperature rise is mainly caused by low-friction Type II twin boundary motion, even though high-friction Type I twin boundary motion is dominant in quasi-static stress-induced martensite reorientation. A simple formula considering the frictional twin boundary motion and heat transfer is derived to describe this temperature rise.

Spectroscopic evidence of band Jahn-Teller distortion upon martensitic phase transition in Heusler-type Ni-Fe(Co)-Ga ferromagnetic shape-memory alloy films

The temperature evolution of the electronic structure of a Ni-Fe(Co)-Ga/MgO(100), Heusler-type, ferromagnetic shape-memory alloy thin film has been followed by a bulk-sensitive hard x-ray photoelectron spectroscopy, element-selective soft x-ray magnetic circular dichroism, and first-principles calculation. The reversible changes of the electronic states near the Fermi energy show a hysteresis associated with the martensitic phase transition (MPT), where the pseudogap opens on cooling and closes again on warming. In addition, the Ni $3d$ spin magnetic moment increases by approximately two times across the MPT, whereas the change of Fe $3d$ moment is moderate. By comparing the experimental results with the calculated spin-resolved density of states, we conclude that the band Jahn-Teller effect of Ni $3d$ and Fe $3d$ orbitals is responsible for MPT.

Characterisation and modelling of vacancy dynamics in Ni–Mn–Ga ferromagnetic shape memory alloys

The dynamics of vacancies in Ni–Mn–Ga shape memory alloys has been studied by positron annihilation lifetime spectroscopy. The temperature evolution of the vacancy concentration for three different Ni–Mn–Ga samples, two polycrystalline and one monocrystalline, have been determined. The formation and migration energies and the entropic factors are quite similar in all cases, but vary slightly according to composition. However, the number of jumps a vacancy has to overtake to reach a sink is five times higher in the single crystal. This is an expected result, due to the role that surfaces and grain boundaries should play in balancing the vacancy concentration. In all cases, the initial vacancy concentration for the samples quenched from 1173K lies between 1000ppm and 2000ppm. A phenomenological model able to explain the dynamics of vacancies has been developed in terms of the previous parameters. The model can reproduce the vacancy dynamics for any different kind of thermal history and can be easily extended to other alloys.