Mn Ga Ferromagnetic Shape Memory Alloys Research Articles

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.

Influence of internal stress on magnetostrain effect in Ni–Mn–Ga/polymer composite

The outstanding role of internal stress in the silicone matrix driving a magnetostrain reversal of embedded particles of the Ni–Mn-Ga ferromagnetic shape memory alloy after removal of magnetic field, has been disclosed by finite element (FE) simulations and validated by experiment. For simulations, the three case studies have been considered: an isolated particle, and the particles pairs aligned parallel or perpendicularly to the applied magnetic field. The isolated particle provided a 0.4 ​MPa of the reverse stress accumulated in a matrix that was insufficient to recover the composite shape after the removal of magnetic field. Simulations revealed that the strong elastic inter-particle interactions are needed to enhance both the local effective stiffness of composite and the reverse stress. The case where the particles pairs with the optimized inter-particle distance are aligned perpendicularly to the applied magnetic field, is more favorable to obtain the largest magnetostrain recovery. Simulation results demonstrate a 1.8% of the compressive magnetostrain of the 30 ​vol%Ni–Mn-Ga/silicone composite under the field applied parallel to the particles chains, which is in agreement with the experiment. The criteria for a selection of matrix, facilitating a large reversible magnetostrain of composite, have been determined.

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.

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.

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.

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.

Influence of atomic ordering on elastocaloric and magnetocaloric effects of a Ni–Cu–Mn–Ga ferromagnetic shape memory alloy

The coexisting elastocaloric and magnetocaloric effects in ferromagnetic shape memory alloys have attracted much attention for the potential application in solid state refrigeration. Previous studies show that the L21 atomic ordering of Heusler ferromagnetic shape memory alloys plays important role on their magnetocaloric effect. However, no research work investigates the effect of atomic ordering on their elastocaloric effect yet. In this study, we investigated the influence of atomic ordering on the elastocaloric and magnetocaloric effects of a Ni51Cu4Mn20Ga25 ferromagnetic shape memory alloy. The alloy exhibits normal elastocaloric effect and normal magnetocaloric effect near room temperature. Moreover, we found that the enhancement of atomic order in this alloy can greatly increase the entropy change and refrigeration capacity of its elastocaloric and magnetocaloric effects. This is attributed to that the atomic ordering modifies the magnetic and martensitic transitions of the system.

Magneto-Structural Properties of Ni2MnGa Ferromagnetic Shape Memory Alloy in Magnetic Fields

The purpose of this review was to investigate the correlation between magnetism and crystallographic structures as it relates to the martensite transformation of Ni2MnGa type alloys, which undergo martensite transformation below the Curie temperature. In particular, this paper focused on the physical properties in magnetic fields. Recent researches show that the martensite starting temperature (martensite transformation temperature) TM and the martensite to austenite transformation temperature (reverse martensite temperature) TR of Fe, Cu, or Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys increase when compared to Ni2MnGa. These alloys show large field dependence of the martensite transformation temperature. The field dependence of the martensite transformation temperature, dTM/dB, is −4.2 K/T in Ni41Co9Mn32Ga18. The results of linear thermal strain and magnetization indicate that a magneto-structural transition occurred at TM and magnetic field influences the magnetism and also the crystal structures. Magnetocrystalline anisotropy was also determined and compared with other components of Ni2MnGa type shape memory alloys. In the last section, magnetic field-induced strain and magnetostriction was determined with some novel alloys.

Phase transformation, thermoelastic and magnetic properties of Ni50Mn30Ga20−xCux ferromagnetic shape memory alloys

Abstract Effect of addition of Cu on phase transformation temperatures, enthalpy and entropy changes, Curie temperature, magnetization saturation were investigated in Ni–Mn–Ga ferromagnetic shape memory alloy. The results show that the Ni 50 Mn 30 Ga 20− x Cu x alloys exhibit thermoelastic martensitic transformation. The martensitic and reverse martensitic transformation temperatures, enthalpy and entropy changes and thermal hysteresis increase with increase of Cu content. Martensite structure changes from 7 M with 0–0.5 at.% Cu to non-modulated T martensite when the content of Cu is more than 0.5 at.%. In addition, the Curie temperature almost remains unchanged at low-Cu content and subsequently decreases obviously. Magnetization saturation of alloys decrease with increasing Cu content since it is sensitive to ordered atomic arrangement.

ChemInform Abstract: Influence of Long‐Range Atomic Order on the Structural and Magnetic Properties of Ni—Mn—Ga Ferromagnetic Shape Memory Alloys

AbstractReview: 67 refs.

An x-ray absorption spectroscopy study of Ni–Mn–Ga shape memory alloys

The austenite to martensite phase transition in Ni–Mn–Ga ferromagnetic shape memory alloys was studied by extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectroscopy. The spectra at all the three elements’, namely, Mn, Ga and Ni, K-edges in several Ni–Mn–Ga samples (with both Ni and Mn excess) were analyzed at room temperature and low temperatures. The EXAFS analysis suggested a displacement of Mn and Ga atoms in opposite direction with respect to the Ni atoms when the compound transforms from the austenite phase to the martensite phase. The first coordination distances around the Mn and Ga atoms remained undisturbed on transition, while the second and subsequent shells showed dramatic changes indicating the presence of a modulated structure. The Mn rich compounds showed the presence of antisite disorder of Mn and Ga. The XANES results showed remarkable changes in the unoccupied partial density of states corresponding to Mn and Ni, while the electronic structure of Ga remained unperturbed across the martensite transition. The post-edge features in the Mn K-edge XANES spectra changed from a double peak like structure to a flat peak like structure upon phase transition. The study establishes strong correlation between the crystal structure and the unoccupied electronic structure in these shape memory alloys.