Ni Mn Ga Research Articles

Facilitating martensitic reorientation via porous structure of Ti-doped Ni–Mn–Ga shape memory alloy

Abstract Porous Ni–Mn–Ga shape memory alloy with the pore size of 20–30 μm was fabricated by the powder metallurgy with the pore-forming agent of NaCl. The prepared alloy has a uniform pore distribution and a complete sintering neck, which reduces the number of grain boundaries. Pores constrain the transmission of stress, leading to stress concentration, which decreases the critical stress of martensitic twin variants reorientation (<10 MPa). Meanwhile, the strength of porous alloys can be tuned by the alloying of Ti. In addition, the porous Ni–Mn–Ga alloy obtained a lower critical stress for martensitic twin boundary motion after cyclic compression, which makes it suitable for devices that require energy absorption under low stress. The microstructure and mechanical properties of Ni–Mn–Ga porous alloy were analyzed, and the effects of pores on the Ni–Mn–Ga alloy were also discussed.

A point of view of the elastocaloric effect associated with 7M and 5M modulated martensite in Ni–Mn–Ga alloys

Solid-state refrigeration has emerged as the most promising alternative to conventional refrigeration technology. However, for this technology to be applicable, the caloric effects produced in the alloys must be highly reversible. In this context, we compare the elastocaloric effect of two Ni–Mn–Ga alloys with different types of modulated martensite. The elastocaloric effect, quantified as the isothermal entropy change (ΔSela), was investigated in Ni50Mn28Ga22 and Ni50Mn30Ga20 alloys with 5M and 7M modulated martensite, respectively. Maximum ΔSela values obtained were 1.91 J kg−1 K−1 during cooling and 1.83 J kg−1 K−1 during heating in martensite 5M and 0.19 J kg−1 K−1 during cooling and 0.26 J kg−1 K−1 during heating in martensite 7M, for a constant applied stress of 10 MPa. However, although the 7M modulated martensite exhibited a lower ΔSela, its reversibility was higher. Therefore, our results could be useful for selecting a good material to be used in solid-state refrigeration.

Manipulation of microstructure evolution and deformation behavior in Ni–Mn–Ga shape memory alloys with varied Ni/Ga under uniaxial cyclic compression

Manipulation of microstructure evolution and deformation behavior in Ni–Mn–Ga shape memory alloys with varied Ni/Ga under uniaxial cyclic compression

Spin reorientation in premartensite and austenite Ni–Mn–Ga

The premartensite state of Ni–Mn–Ga magnetic shape memory alloy, sometimes called the martensite precursor state, was studied by careful and detailed measurement of the evolution of magnetization curves of magnetically closed samples to evidence local symmetry breaking. During the heating cycle after the martensite transformation, the magnetization loop slowly transforms from a typical sigmoidal shape, corresponding to the magnetization along the easy axis, to a constricted loop indicative of magnetization along a harder magnetic axis. These changes are explained by a switching of the macroscopic magnetic easy axis from [100] to [110]. Above the premartensite transformation temperature, the magnetic easy axis slowly changes back to [100]. After cooling the sample, starting at the Curie temperature, the process reverses.

Martensitic transformation and inverse magnetocaloric effect in Ni-Mn-Ga-Co-Gd microwires

Ni–Mn-Ga-Co-Gd microwires were fabricated by the glass-coated melt spinning method, resulting in a bamboo-grained structure. In this paper, the magnetocaloric effect of Ni–Mn-Ga-Co-Gd microwires is mainly investigated, which is represented by the isothermal magnetic entropy change calculated using Maxwell equation. Specifically, Ni43Mn30Ga19.9Co7Gd0.1 microwires exhibit maximum entropy change of 11.09 J/kg·K under 5 T magnetic field, which is generally greater than that of the current Ni–Mn-Ga microwires. Additionally, Ni43Mn30Ga19.9Co7Gd0.1 microwires show the temperature corresponding to the greatest magnetic entropy change nearer to room temperature compared to other Ni–Mn-Ga microwires. Furthermore, the larger specific surface area of microwires will facilitate their application in the field of magnetic refrigeration.

Adaptive Phase or Variant Formation at the Austenite/Twinned Martensite Interface in Modulated Ni–Mn–Ga Martensite

AbstractThe crystal structure and transformation path from austenite to 10M martensite in Ni–Mn–Ga single crystal are examined employing high‐energy synchrotron radiation, scanning, and transmission electron microscopy. Using temperature gradient, an austenite/twinned martensite interface is stabilized revealing the crystal structure and microstructure of both phases and the transformation sequence across the interface. Depending on the distance from the interface, three distinct types of martensite crystal lattice, namely simple tetragonal and two monoclinic modulated ones, that is, 10M′ and 10M are confirmed. In situ measurements show that lattice mismatch formed at the habit plane is compensated by the formation of micro‐twinned and branched martensite along with an elastic change in lattice parameters. It is shown that the characteristics for periodic shuffling twin boundaries, such as modulation twins or inverted stacking faults, are installed in later stages of transformation. In other words, large microstructure elements that most efficiently accommodate the strain are installed first, and then smaller elements are operable. Overall, the experimental results show that the crystal lattice does not adapt modulated phase at the habit plane and cannot be built up from simple non‐modulated tetragonal blocks but rather a specific sequence of phase transformations using shear/shuffling deformation takes place.

Factors influencing the martensitic transformation in Ni–Mn–Z (Z = Ga, Sn) Heusler alloys

The martensitic transition temperature of the Ni–Mn–Z (Z = Ga, In, Sn or Sb) generally scales with the average valence electron per atom ( e/a ) ratio. The e/a ratio can be increased either by increasing Ni content in lieu of Mn or Z (Ni-rich) or by substituting Mn for Z (Mn-rich). While the Mn-rich alloys display the expected scaling behaviour of the martensitic transition temperature, the Ni-rich alloys, with the exception of Z = Ga alloys, do not exhibit any dependency on the e/a ratio. By carefully studying and comparing the structure, local structure and the physical properties of the Ni-rich and Mn-rich Ni–Mn–Ga and Ni–Mn–Sn alloys, the cause of the difference in behaviour is identified. The off-stoichiometry brought about by increasing the Ni or Mn content, results in the formation of a new local structural environment in addition to the Heusler-like environment. If either of the two local structural environments is martensitic, then the resulting Ni-rich or Mn-rich alloy also exhibits martensitic transformation.

Magnetic and transformation properties of Ni2MnGa combinatorically substituted with 5 at.% of transition elements from Cr to Cu – Experimental insight

Heusler Ni–Mn–Ga alloys with martensitic transformation being multiferroic materials exhibit magnetic shape memory (MSM) phenomena and moreover, these are touted for magneto– and multi–caloric effect (MCE). MCE benefits from high saturation magnetization and also large variability and sensitivity of transformation temperatures on alloying. In contrast, MSM phenomena are limited by low temperatures of both ferromagnetic and ferroelastic transformations. In an attempt to increase or modify the transformation temperatures and to understand the physical fundamentals beyond transformation variations and magnetic ordering, we experimentally investigate the magnetic and transformation properties in Ni2MnGa alloyed with transitional elements spanning from Cr to Cu. These elements are consecutively substituted on Ni, Mn, and Ga sites, to establish the effect of chemical substitution and elemental evolution. The elemental dependence of saturation magnetization, magnetocrystalline anisotropy, Curie temperature, and martensitic transformation temperatures are compared with (non–)stoichiometric Ni–Mn–Ga Heusler compounds. The observed behavior is complex and there are no clear trends of transformation temperatures as a function of the number of valence electrons or of non–bonding electrons per atom. The saturation magnetization at 10 K seems to follow the Slater–Pauling curve particularly for Ni substitution. Obtained results can serve as useful rectification for the theoretical prediction of new Ni–Mn–Ga–X Heusler alloys.

Coexistence of five domains at single propagating interface in single-crystal Ni[sbnd]Mn[sbnd]Ga shape memory alloy

Coexistence of both austenite and martensite during phase transformation is a common feature of all Shape Memory Alloys (SMAs). The martensite has different variants featuring characteristic deformations rotationally linked to each other due to the symmetries of the austenite parent phase, and the martensite variants can form twins with different mean characteristic deformations. Multiple-domain microstructures (consisting of austenite, martensite twins and individual martensite variants) evolve collectively within an SMA sample during the phase transformation, contributing thus to the material's macroscopic response (e.g., its global deformation). Particularly, the multiple domains can exist at the diffuse austenite-martensite interface nucleating and propagating in a single crystal in certain conditions. This implies an energy barrier for this interfacial structure, influencing the interface kinetics and the driving force (energy dissipation) of the phase transformation. In this paper, we report an experimentally observed interface consisting of five domains (austenite, one martensite variant and three twins) in a Ni–Mn–Ga single-crystal initially consisting of one martensite variant and subjected to a uniaxial thermal gradient. The compatibility analysis (performed from the characteristic strains of the three martensite variants having approximately a tetragonal symmetry) reveals that the five-domain interface is not a perfectly compatible pattern like the basic habit plane (consisting of only one twin compatible with austenite). However, its level of non-compatibility is similar to that of the quite common X-interface (four-domain coexistence) which is observed in many SMAs. Further, the significant effects of the thermal loading path and the material initial state (the initial martensite variant) on the domain pattern formation are demonstrated and analyzed. The experimental observation and the theoretical analysis of the domain patterns can provide hints to better understand diffuse interface kinetics and phase transformation hysteresis.

Effect of Partial Substitution of Ga on the Structural and Magnetic Properties of the Ni–Mn–Ga Heusler Alloys

Effect of Partial Substitution of Ga on the Structural and Magnetic Properties of the Ni–Mn–Ga Heusler Alloys

Influence of Si on the Structure and Martensitic Transformation in Deformed Ni–Mn–Ga Alloys

Abstract—The studying the effect of multiaxial isothermal forging on the microstructure and martensitic transformation in the Ni58Mn18Ga24 alloy is presented. Forging was carried out in two stages: stage 1 − forging at 700°C (4 passes, true degree of deformation e ≈ 1.64), stage 2 − forging at 500°C (1 pass, e ≈ 0.24). Forging led to the transformation of the original equiaxed grain structure. As a result of the 1st stage of processing, no new grains are formed. The new recrystallized grains are observed only after the 2nd stage of deformation, the proportion of which is very small. Apparently, the mechanism of fragmentation of the grain structure at the first stage is not triggered due to insufficient defect density at a deformation of 700°C. The characteristic temperatures of martensitic transformation are shifted to the low temperature region. The anharmonic change in the sample length is observed in the region of martensitic transformation for the both treated states. In general, this indicates a low level of defect density and internal stresses in the sample.

Effect of Ga Partial Substitution on the Structural and Magnetic Properties of Heusler Ni–Mn–Ga Alloys

Abstract—The effect of doping with Cu and Zn atoms on the magnetic and structural properties of the cubic and tetragonal phase of the Heusler Ni2Mn1.125Ga0.875 alloy has been investigated in the framework of density functional theory. Partial substitution of Ga atoms by Cu and Zn atoms leads to a decrease in the energy difference between the ferromagnetic and ferrimagnetic states of the cubic phase and an increase in the energy barrier between the cubic and tetragonal phase. The latter observation indirectly points to the increase in the structural transition temperature observed experimentally. Within the framework of calculations of exchange interaction parameters from first principles and Monte Carlo modeling of temperature dependences of magnetization, a phase T–x diagram is constructed. It is shown that the Curie temperature of the tetragonal phase decreases significantly compared to the Curie temperature of the cubic phase with increasing Cu and Zn content due to an increase in the inter-sublattice antiferromagnetic interaction between Mn atoms.

Effect of Cu Doping on Phase Transition, Thermal Strain, and Thermal Expansion Property in Polycrystalline Ni50Mn23−xCuxGa27 Alloys

The influence of Cu doping on phase transition, thermal strain, and thermal expansion property of polycrystalline Ni50Mn23−xCuxGa27(x = 1, 3, 5, 6 and 7) alloys is investigated. The results show that with increasing Cu concentration, martensitic transformation gradually gets close to Curie transition. A magnetostructural transition (MST) is observed in samples with x = 6 and 7. Such MST can enhance magnitude of transformation strain. Besides, Ni50Mn23−xCuxGa27 alloys possess an isotropic, recoverable, and thermally stable thermal strain. Excitedly, Cu doping can effectively tune the phase transition, thermal strain, and thermal expansion properties of Ni50Mn23−xCuxGa27 alloys. Adjustable coefficients of thermal expansion from positive to negative are obtained by Cu‐doped Ni50Mn23−xCuxGa27 alloys. The average linear expansion coefficient (α) between −140.44 and −207.70 ppm K−1 is observed in samples with x = 6 and 7 for a narrow temperature span of 11–14 K. When the temperature span is about 72 K (x = 6) and 73 K (x = 7), which is the largest temperature span observed in Ni–Mn–Ga‐based alloys recently, the α values decrease to about −36 ppm K−1. These findings are beneficial for manipulating the thermal expansion property of Ni–Mn–Ga–Cu alloys and their multifunctional applications.

Anharmonic modulation and its interplay with twinned microstructure in Ni–Mn–Ga–(Fe) 10M martensite

Anharmonic modulation and its interplay with twinned microstructure in Ni–Mn–Ga–(Fe) 10M martensite

Lamb waves analysis in high anisotropy alloys material: Propagation and infinite zero group velocity resonances

In the present paper, the dispersion curve of guided modes in a cubic Ni–Mn–Ga thin plate is studied. Exceptional dispersion behavior is reported, with multiple crossings between antisymmetric (An) and symmetric (Sn) Lamb modes. In addition, several dispersion branches contain one or more zero group velocity (ZGV) points. The number of ZGV points per symmetric mode, Sn and per antisymmetric mode An, turns out be 3n-2.

Giant magnetoelectric effect of Ni–Mn-Ga/piezopolymer composites tailored by a martensitic transformation

Polymer-based magnetoelectric (ME) composites, composed of magnetostrictive and piezoelectric phases, in addition to exhibiting an effective coupling between the magnetic and electric orders of the matter, also offer important advantages for the Internet-of-Things, digitalization, and 4.0 revolution environments: light weight, flexibility, wearability, environmental friendliness, printability, and biocompatibility. Nevertheless, their successful implementation on applications such as sensors, actuators, energy harvesters, biomedical devices and spintronics strongly depends on the increase of its ME voltage response.This work explores, both experimentally and theoretically, the magnetostrictive Ni–Mn-Ga's martensite ‒ austenite phase transformation to shift/tailor/design the ME resonance peak exhibited by Ni–Mn-Ga/P(VDF-TrFE) piezoelectric composites. In the austenite phase, the determined peak value of the ME coefficient of 18.1 V cm−1 Oe−1 was much higher than the value of 6.05 V cm−1 Oe−1 obtained for the martensite phase, whereas the estimated theoretically value of magnetoelastic constant in the austenite is much smaller. The ways to further increase the magnetically induced ME response are also outlined.

Framework of magnetostrain responsive Ni–Mn–Ga microparticles driving magnetic field induced out-of-plane actuation of laminate composite

Ni–Mn–Ga single crystals (SC) exhibiting a giant magnetic field induced strain (MFIS), resulting from twin boundaries rearrangements, are excellent materials for novel actuators although enhanced brittleness and high costs are remaining the issues for applications. In polycrystalline state Ni–Mn–Ga alloys show small MFIS due to grain boundary constraints. By simple size reduction of the mentioned materials it is hardly possible to create quasi-two-dimensional MFIS actuators on the microscale with a pertinent out-of-plane performance. In pursuit of the trend for next generation materials and functions by design, in the present work we have developed a laminate composite as a prototype of microactuator with the out-of-plane stroke being driven by a framework of magnetostrain responsive Ni–Mn–Ga microparticles. The laminate consisted of the layer of crystallographically oriented Ni–Mn–Ga semi-free SC microparticles sandwiched between bonding polymer and Cu foils. Such design provided a particles isolation with a minimum constraint condition from the polymer. MFIS of the individual particles and the whole laminate composite was investigated by X-ray micro-CT 3D imaging. Both particles and laminate exhibited the same recoverable out-of-plane stroke produced by the particles´ MFIS of around 3% under 0.9 T. The developed microactuator design is promising for applications in the areas of micro-robotics, optical image stabilization in cameras, pumps for microfluidics etc.

Mapping the magnetocaloric effect at the microscale on a ferromagnetic shape memory alloy with infrared thermography

An innovative study of the magnetocaloric effect (MCE) was performed by mapping the effect based on direct measurements of the temperature change during magnetic field cycles with microscopic resolution (85 μm) on a Co-doped Ni–Mn–Ga bulk sample using infrared thermography on the whole sample. The MCE maps were constructed for different sample temperatures (T sample), cycling both on heating (from 272.8 K up to T sample, with T sample ⩽ 327.0 K) and on cooling (from 340.0 K down to T sample, with T sample ⩾ 266.8 K), cycling a 1.2 T magnetic field at each T sample value. The MCE maps were calculated to evaluate the amplitude of the effect at the microscale for all T sample values. This allows to analyze the contribution of each micrometric portion of the sample to the spatially heterogeneous behavior that was found. Significant differences of the MCE on heating and cooling are present associated to inhomogeneity dynamics, mostly near the structural transformation. The amplitude of the MCE and its inhomogeneity are both much more pronounced on the heating process. On the cooling process the effect behaves quite homogeneously since the structural transformation already occurred during the cooling to reach T sample. The behavior of the MCE at selected map coordinates was scrutinized, revealing significant differences amongst sample locations. Moreover, the extreme amplitudes of MCE registered for diverse micro-regions occur at different temperatures, suggesting that the structural transformation occurs at varying temperatures and with different magnitudes. The study innovates by constructing MCE maps to evaluate minority behaviors in the MCE in contrast with the average behavior of the effect. This study displays the capability to discriminate the behavior of the transformation at the microscale.

Transformation strain and volume effect associated with martensitic transformation in polycrystalline Ni49+x+yMn23-xGa28-y Heusler alloys

Combined thermal strain and magnetization measurements were performed in polycrystalline alloys Ni49.31Mn22.76Ga27.93(Ni49), Ni53.00Mn21.04Ga25.96(Ni53) and Ni55.62Mn17.52Ga26.86(Ni55.5) to understand the influence of magnetic field (B) on transformation strain (Δλ) and relative volume change (ω) at martensitic transformation (MT). It is found that during cooling and heating both Ni49 and Ni53 samples undergo a MT while the Ni55.5 sample experiences a coupled magnetostructural transformation (MST). Such a MST can enhance the Δλ and ω whereas thermal hysteresis weakens them, which results in a non-monotonic variation in Δλ: 0.211% (Ni49) → 0.166% (Ni53) → 0.185% (Ni55.5) under B = 0 T. Similar tendency was observed in ω. Besides, the values of Δλ and ω increase with increasing field, partially due to the contribution of magnetic field-induced strain. The maximum value ω = 0.674% was obtained in Ni49 sample under 3 T, indicating that an extra volume space of ∼1% should be needed to avoid the destruction of refrigeration chamber. These findings provide key information for future use of Ni–Mn-Ga alloys in refrigeration engineering field.

Correlation between Acoustic Emission and Stress Evolution during Single Twin Boundary Motion in Ni–Mn–Ga Magnetic Shape Memory Single Crystal

Stress evolution and related acoustic emission (AE) in modulated 10Mmartensite are simultaneously studied during compression forced motion of Type I and Type II single twin boundary (TB) resulting in microstructure reorientation and large deformation. The single TB of both types is formed in the same sample and traveled the same volume. The single boundary exhibits random jerky movement and for Type I the stress fluctuation can be directly correlated to AE signal. The AE during the single TB motion is an avalanche‐like process surmounting the obstacles – crystal defects. The amplitudes of AE are approximately five times higher for Type I compared to Type II. The energy amplitudes of AE events follow the universal power law with different coefficients for Type I and Type II TB. The peak rate well correlates with stress fluctuation during TB motion. Large differences observed for the same sample indicate a different mechanism of pinning and ultimately a different mechanism of Type I and Type II TB motion.