Magnetic Shape Memory Research Articles

Investigation of the inverse magnetocaloric effect with the fraction method

In this study, we examine Ni49Nb1Mn36In14 (nom. at.%) magnetic shape memory alloy (MSMA) to illustrate the inverse magnetocaloric effect (MCE) using the fraction method. The magnetic entropy change, , was calculated with both, the fraction method and the thermomagnetic Maxwell relation. Our results demonstrate that there exists a large magnetization difference between field-cooling and field-heating histories in Ni49Nb1Mn36In14 (nom. at.%) MSMA, which can be attributed to the pinning of lattice entropy and magnetic entropy, as it is well-known that the temperature and applied magnetic fields have an opposing effect on the total entropy change. In addition, we describe the inverse MCE and the contradictory roles on the total entropy change between the two stimuli via the fraction method.

Theoretical design of Ti2-based magnetic shape memory alloys from first-principles

Magnetic shape memory alloys (MSMAs) typically comprise transition metal and main group atoms. We developed a class of Ti2-based Heusler MSMAs formed by transition metal and metalloid elements using the first-principles calculation method based on density functional theory. For all the studied Ti2YZ (Y = Cr, Mn, Fe, and Co; Z = B, Si, Ge, As, Sb, and Te) Heusler alloys, the calculated formation energies in the austenitic phase show that, except for Ti2CrTe, all alloys have thermodynamic stability. Based on a comprehensive analysis of the total energy difference between austenite and martensite, the density of states (DOS), and the bulk mechanical properties, we predict that of all the Ti2-based alloys studied here, 17 alloys are likely to undergo a martensitic phase transition. The DOS of the cubic structures reveals that the substantial peak structures around the Fermi level, mainly formed by strong hybridization between the Ti atom and its neighbor Y and Z atoms, lead to cubic instability. Compared with the typical MSMA of Ni2MnGa, most Ti2YZ alloys possess a larger martensitic driving force, a higher martensitic transition temperature, and better ductility. Therefore, our study could inspire the development and application of new Ti2-based MSMAs.

Investigations of the Crystallographic Orientation on the Martensite Variant Reorientation of the Single-Crystal Ni-Mn-Ga Cube and Its Composites for Actuator Applications

High-speed actuators are greatly required in this decade due to the fast development of future technologies, such as Internet-of-Things (IoT) and robots. The ferromagnetic shape memory alloys (FSMAs), whose shape change could be driven by applying an external magnetic field, possess a rapid response. Hence, these materials are considered promising candidates for the applications of future technologies. Among the FSMAs, the Ni-Mn-Ga-based materials were chosen for their large shape deformation strain and appropriate phase transformation temperatures for near-room temperature applications. Nevertheless, it is widely known that both the intrinsic brittleness of the Ni-Mn-Ga alloy and the constraint of shape deformation strain due to the existence of grain boundaries in the polycrystal inhibit the applications. Therefore, various Ni-Mn-Ga-based composite materials were designed in this study, and their shape deformation behaviors induced by compressive or magnetic fields were examined by the in situ micro CT observations. In addition, the dependence of the martensite variant reorientation (MVR) on the crystallographic directions was also investigated. It was found that most of the MVRs are active within the magnetic field range applied in the regime of the <100>p, <110>p, and <111>p of the single-crystal {100}p Ni-Mn-Ga cubes.

Microstructural and crystallographic characteristics of martensite micro-variants in NiMnGa-based ferromagnetic alloy

A systematic investigation on microstructural and crystallographic characteristics of martensite micro-variants in Ni54Mn25Ga20.9Dy0.1 ferromagnetic shape memory alloy was performed. The alloys exhibited a dual-phase microstructure consisting of self-accommodated tetragonal martensite and hexagonal Dy-rich dispersive precipitates. In each colony, four distinct micro-variants with different crystallographic-orientations existed, where the adjacent variant pairs connected by crossing-interfaces possessed a non-twinned relationship and other pairs connected by extending-interfaces were twin-related to each other. The corresponding twinning planes and directions were {112} and 〈111〉, respectively. Nevertheless, the twinning relationships of these twin-related micro-variant pairs were not perfect, which contained a small rotation. Interface analysis demonstrated that these two types of interface planes coincide with the orientation relationship planes and twinning planes of their connecting micro-variants, respectively.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.

Enhanced cyclic stability and enlarged working temperature window of NiFeGa elastocaloric refrigerant via introducing strong texture and ductile interfacial precipitate

The poor mechanical properties will result in low fatigue life and limited working temperature window of elastocaloric effect (eCE) for ferromagnetic shape memory alloys with lower critical stress. In this work, a microstructure design strategy, i.e., simultaneously introducing strong texture and ductile interfacial γ-phase, was adopted to reduce the strain discontinuity near the grain boundary and enhance the grain boundary cohesion, further improving the mechanical properties and eCE performance. With this strategy, the compressive strength and strain of the prototype Ni54Fe19Ga27 alloy can exceed 1.5 GPa and 30.0% at room temperature, respectively. Furthermore, a wide working temperature window of 120 K with an adiabatic temperature change of −8.5 to −1.2 K and superior eCE fatigue life of 2 × 104 cycles with an adiabatic temperature change of ∼3 K have been achieved. This work is expected to provide an effective strategy to enhance the eCE performance of ferromagnetic shape memory alloys.

Impact of electron concentration on the transformation temperature and important physical properties of NiMnSnFe ferromagnetic shape memory alloys

Impact of electron concentration on the transformation temperature and important physical properties of NiMnSnFe ferromagnetic shape memory alloys

Theoretical investigation on electronic structure, magnetism and martensitic transformation in Ni2Mn1.5Sc0.5 and Mn2Ni1.5Sc0.5 all-d-metal magnetic shape memory alloys

Two new all-d-metal Heusler alloys Mn2Ni1.5Sc0.5 and Ni2Mn1.5Sc0.5 are proposed as candidates for magnetic shape memory alloys. In these alloys, the Sc atom prefers occupying the D (0.75, 0.75, 0.75) sites in the Heusler alloy lattice and forms d–d hybridization with its nearest neighbor atoms such as Ni. Martensitic transformation is possible in both Mn2Ni1.5Sc0.5 and Ni2Mn1.5Sc0.5. Differing from the “volume-conserving” character in classic Heusler alloys, a large volume shrinkage of 2.3% and 2.8% occurs in Mn2Ni1.5Sc0.5 and Ni2Mn1.5Sc0.5 during the martensitic transformation, respectively. The magnetization difference between the austenite and martensite is as large as 4.50 μB/f.u. for Mn2Ni1.5Sc0.5 and 4.49 μB/f.u. for Ni2Mn1.5Sc0.5, due to the ferromagnetic–antiferromagnetic magnetic transition coupling with the martensitic transformation. The parallel coupled Mn (B) and Mn (D) spin moments in the austenite become antiparallel in the martensite, which leads to the large decrease of magnetization after the martensitic transformation. The energy difference ΔEM between the martensite and austenite is −45.5 and −25.9 meV/f.u. for Mn2Ni1.5Sc0.5 and Ni2Mn1.5Sc0.5, respectively. The large ΔEM of Mn2Ni1.5Sc0.5 derives from the strong Jahn–Teller effect in its density of states structure and makes it a promising candidate for magnetic shape memory alloys.

Ferromagnetic shape memory alloy Ni52.5Mn22.5Ga25: Phase transformation at high pressure and effects on natural gas hydration

Ferromagnetic shape memory alloy NiMnGa was used to prepare the micro/nano fluid which has a potential role during the formation of natural gas hydrate (NGH). In the present work, phase transformations of Ni52.5Mn22.5Ga25 micro/nano particles under various pressures, and the crystal structure before and after participating in the natural gas hydration were investigated. The effect of the micro/nano fluid containing Ni52.5Mn22.5Ga25 particles on the formation heat of NGH was studied. It was worked out that Ni52.5Mn22.5Ga25 particles exhibit a high stability both in phase transformation below 15 MPa pressure and crystal structure after natural gas hydration. The fluid containing Ni52.5Mn22.5Ga25 particles can reduce the hydration reaction temperature and promote the formation of NGH.

Bulk electronic structure of Mn2NiGa using hard x-ray photoelectron spectroscopy and density functional theory

Mn2NiGa is a potential magnetic shape memory alloy with austenite to the martensite phase transition. Here, we have investigated the bulk electronic structure of Mn2NiGa in the austenite and martensite phases studying its valence band using hard x-ray photoelectron spectroscopy (HAXPES). In the austenite phase, we observe a wide (≈10 eV) valence band (VB) spectrum with several prominent features. In order to explain the HAXPES VB spectra, we have compared our experimental VB spectra with the theoretical VB calculated using the partial density of states from our existing density functional theory (DFT) calculations. The shape of the experimental VB and energy positions of all features are in excellent agreement with the calculated VB and we find that the former is dominated by Ni 3d as well as 4s states of Mn, Ni, and Ga. An important observation is that experimental VB combined with the DFT-based VB calculation establishes the prevalence of the anti-site disorder in Mn2NiGa. Compared to the austenite phase, in the martensite phase, the VB shows a marginal decrease in the density of states around −0.5 eV below the Fermi level (E F ), and the main peak is slightly shifted towards E F . These experimental observations have been explained by considering the tetragonally distorted structure with the anti-site disorder in the martensite phase.

Growth Twins and Premartensite Microstructure in Epitaxial Ni-Mn-Ga Films

Magnetic shape memory alloys have been examined intensively due to their multifunctionality and multitude of physical phenomena. For both areas, epitaxial films are promising since the absence of grain boundaries is beneficial for applications in microsystems and they also allow to understand the influence of a reduced dimension on the physical effects. Despite many efforts on epitaxial films, two particular aspects remain open. First, it is not clear how to keep epitaxial growth up to high film thickness, which is required for most microsystems. Second, it is unknown how the microstructure of premartensite, a precursor state during the martensitic transformation, manifests in films and differs from that in bulk.Here, we focus on micrometer-thick austenitic Ni-Mn-Ga films and explain two distinct microstructural features by combining high-resolution electron microscopy and X-ray diffraction methods. First, we identify pyramid-shaped defects, which originate from {1 1 1} growth twinning and cause the breakdown of epitaxial growth. We show that a sufficiently thick Cr buffer layer prevents this breakdown and allows epitaxial growth up to a thickness of at least 4 µm. Second, premartensite exhibits a hierarchical microstructure in epitaxial films. The reduced dimension of films results in variant selection and regions with distinct premartensite variants, unlike its microstructure in bulk.

Mechanical and Wear Properties of CoNiAlSiSb and CoNiAlSiIn Ferromagnetic Shape Memory Alloys: An Experimental Assessment

CoNiAl-based ferromagnetic shape memory alloys (FSMAs) are used in various engineering fields but still, need to be improved for tribological applications. In the present study, the dry sliding wear behavior of CoNiAlSiSb and CoNiAlSiIn FSMAs was investigated as they were articulated against an alumina abrasive ball using a ball-on-disk tribometer. The experiments were carried out at a load of 20 N, a sliding velocity of 20 mm/s, and a sliding distance of 250 m. The worn surfaces were assessed using a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). The mechanical properties of the CoNiAl-based FSMAs were investigated using the nanoindentation technique. The results showed that as compared to CoNiAlSiSb, CoNiAlSiIn FSMA showed a 42% increase in Young’s modulus and a 10% increase in microhardness. The mean coefficient of friction (COF) of CoNiAlSiIn (0.56) was observed to be slightly lower than that of CoNiAlSiSb (0.58). The higher hardness and elastic modulus of CoNiAlSiIn than CoNiAlSiIn caused only a 7% increase in wear resistance. The operative wear mechanisms were abrasion, ad-hesion, plastic deformation, and micro crack-induced delamination. In conclusion, even though the difference in the tribological performance of the two FSMA surfaces was fairly small, CoNiAlSiIn exhibited better results and thereby would be preferable in possible tri-bological applications.

Variation of Electrical Resistivity During Magnetic Field-Induced Martensitic Transformation in Vanadium added NiMnSnB alloys

In this study, the structural and electrical properties of Ni49-xVxMn37Sn12B2 (x = 0, 1, 2, and 3) ferromagnetic shape memory alloys were investigated. According to XRD analyzes at room temperature, the x=0 sample was in the martensite phase, the x=1 and 2 samples were in the mixture phase, and the x=3 sample was in the austenite phase. The resistivity analyses depend on temperature showed that all samples exhibited martensitic transformation and the phase transformation temperature decreased with V doping. Magnetoresistance (MR) values were calculated using ρ-T curves performed under 0 T and 1 T magnetic fields. The observed negative MR is consistent with Kataoka's s-d model. As-Af interval was determined and M-H measurements were made at constant temperatures determined in this interval. The results were attributed to the magnetic field-induced phase transformation (MFIPT). In order to examine the effects of MFIPT on the electrical resistivity, the resistivity depend on magnetic field was measured using the same thermal process. The overlapping of the curves in the high magnetic field revealed that the resistivity decreased due to the MFIPT as well as the MR.

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.

Molecular dynamics simulation of phase transformation and wear behavior of Ni35Al30Co35 high temperature shape memory alloy

Ni–Al–Co alloys as one of the ferromagnetic high-temperature shape memory alloys (HTSMA) are attractive in aerospace and defense applications, due to their high phase transition temperature range (400–700K), high recoverable strain and good machinability. However, the underlying physics for the phase transformation behavior of β single-phase and β+γ two-phase Ni–Al–Co ternary alloys remain unclear. In this work, molecular dynamics simulation of phase transition, pseudoelasticity (PE) and wear behavior of Ni35Al30Co35 alloy were analyzed using the EAM potential. It shows that Ni35Al30Co35 alloy has a high martensite start temperature of 825K and an austenite start temperature of 631K, respectively. After five thermal cycles, martensite and austenite start and finish temperatures are stable around 500K and 700K. The initial structure of Ni35Al30Co35 alloy at 1000K is identified as BCC (β phase) and FCC (γ phase) structure, transforming to L10 structure through martensitic transformation. Furthermore, PE of Ni35Al30Co35 alloy exhibits about a 9.1% recoverable strain under <001> oriented uniaxial tension tests and about a 6.6% recoverable strain under <001> oriented uniaxial compression tests at a temperature of 1200K, showing the potential used in extreme environment. Moreover, spherical indenters with a radius of 15 Å were used to investigate the wear behavior of Ni35Al30Co35 alloy under different temperature. It was found that the PE effect affects the frictional behavior during nano-scratching. Combining the effect of high temperature softening and phase transition PE, the wear rate increases nearly by 5.3% when the temperature rises from 300K to 1000K. Furthermore, the friction force curve obtained is consistent with the friction force curve obtained by simulation at 300K.

Effect of Selective Laser Melting Parameters on Physicomechanical Properties of Ferromagnetic Shape Memory Ni&lt;sub&gt;36&lt;/sub&gt;Al&lt;sub&gt;27&lt;/sub&gt;Co&lt;sub&gt;37&lt;/sub&gt; Alloy

In this work Ni36Al27Co37 ferromagnetic shape memory alloy was studied. Selective laser melting (SLM) was carried out at the modes providing the volumetric energy density (VED) of 70-200 J/mm3. It was shown that in order to obtain dense samples, VED should be at least 90 J/mm3 for this alloy. The maximum density was achieved at 150 J/mm3.With an increase in VED, the hardness nonlinearly decreases from 447 HV to 413 HV. The samples without visible defects showed a mean conventional tensile failure stress of 479 MPa and a residual deformation of about 0.04%, which confirms their brittleness when not heat-treated.Investigation into crystallographic structure of Ni36Al27Co37 SLM-samples showed that the laser power, scanning speed and hatch distance effects should not be considered separately but in complex. It was found that the most promising approach to obtain anisotropy in this alloy is a simultaneous decrease in the hatch distance and an increase in the scanning speed. In this case, the crystallites aspect ratio achieves values of up to 12 that makes the alloy prospective to show high magnetically induced strain.

A micromechanical constitutive model for porous ferromagnetic shape memory alloys under magneto-mechanical coupling

A micromechanical constitutive model for porous ferromagnetic shape memory alloys under magneto-mechanical coupling

Effect of Acoustic Oscillations on Non-Equilibrium State of Magnetic Domain Structure in Cubic Ni2MnGa Single Crystal.

Magnetic hysteresis is a manifestation of non-equilibrium state of magnetic domain walls trapped in local energy minima. Using two types of experiments we show that, after application of a magnetic field to a ferromagnet, acoustic oscillations excited in the latter can "equilibrate" metastable magnetic domain structure by triggering the motion of domain walls into more stable configurations. Single crystals of archetypal Ni2MnGa magnetic shape memory alloy in the cubic phase were used in the experiments. The magnetomechanical absorption of ultrasound versus strain amplitude was studied after step-like changes of a polarizing magnetic field. One-time hysteresis was observed in strain amplitude dependences of magnetomechanical internal friction after step-like variations of a polarizing field. We distinguish two ingredients of the strain amplitude hysteresis that are found in the ranges of linear and non-linear internal friction and show qualitatively different behavior for increasing and decreasing applied polarizing fields. The uncovered effect is interpreted in terms of three canonical magnetomechanical internal friction terms (microeddy, macroeddy and hysteretic) and attributed to "triggering" by acoustic oscillations of the irreversible motion of domain walls trapped in the metastable states. To confirm the suggested interpretation we determine the coercive field of magnetization hysteresis through the measurements of the reversible Villari effect. We show that the width of the hysteresis loops decreases when acoustic oscillations in the non-linear range of domain wall motion are excited in the crystal. The observed "equilibration" of the magnetic domain structure by acoustic oscillations is attributed to the periodic stress anisotropy field induced by oscillatory mechanical stress.

Intrinsic instability to martensite phases in ferromagnetic shape memory alloy Ni2MnGa : Quasiparticle self-consistent GW investigation

We investigated the electronic structure of a ferromagnetic shape memory alloy ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ utilizing an advanced approach, quasiparticle self-consistent $GW$, which takes account of electron localization effects without empirical parameters. The Ni ${e}_{g}$ orbitals in the cubic phase, which lead to martensite phase transition, were found to locate on the Fermi level, implying a clear definitive origin of band Jahn-Teller (JT) effect in comparison with the results obtained by the density functional approach of generalized gradient approximation. From the analysis of generalized susceptibility in the cubic phase, the instabilities responsible for the modulated structures of 10M, 14M, and 6M were found to be an intrinsic property in the electronic states. These states may stabilize the modulated one, accompanied by tetragonal local JT distortions. Their property of Fermi surface nesting sensitively depends on a subtle change in the magnetic moment, corresponding to the experimental fact that the modulated structure appears depending on temperature and the composition of the magnetic element. The secondary nesting vector along [110] direction was discussed in relation to a modulation alignment of the nanotwin boundary.

Structure-property control of polycrystalline Ni-Mn-Ga by moderate Co-doping

It is well-known that the Co-doping of ferromagnetic shape memory alloys (FSMAs) is a crucial tool to control their multifunctional properties. The present work investigates the use of small quantities of Co to fine-tune the transformation, structure, microstructure, mechanical and magnetic properties of the polycrystalline Ni49.8Mn28.5Ga21.7 (at%) alloy, as the representative of the most elaborated magnetostrain-active Ni-Mn-Ga FSMAs. A series of seven alloys, undoped and doped with 0.5, 1, 1.5, 2, 3, and 4 at% Co, was fabricated using a standard induction melting technique with subsequent heat treatment. Alongside establishing the evolution of the basic characteristics as a function of the Co content in this series, an unusual effect of Co-doping on the grain microstructure was found. A Co-doping up to 1.5 at% instead of Mn significantly increased the average grain size while importantly maintaining the 10 M martensitic crystal structure. At Co concentrations of 1–1.5 at%, a microstructure with an average grain size of about 2.00 mm was formed with a twin structure, enabling the experimental observation of magnetic-field-induced twin variant rearrangement. At higher levels of Co-doping, the grain size was essentially reduced, and the crystal structure of the martensitic phase became 2 M martensite. The decreasing grain size and changing crystal structure are attributed to the progress of γ-phase precipitates. Alongside the academic aspect, the results of the present work point to the commercial advantage of fabricating 10 M Co-doped Ni-Mn-Ga actuating elements made from large grains of polycrystalline ingots obtained by a standard melting facility instead of grown single crystals.