Modulated Martensite Research Articles

Understanding Motion of Twin Boundary—A Key to Magnetic Shape Memory Effect

High mobility or the ease of movement of twin boundary in magnetic field, necessary for magnetically induced reorientation (MIR) is studied. In modulated 10M martensite two types of a-c twin boundaries, Type I and Type II are mobile in magnetic field. Here, we focus on the behavior of a single twin boundary in magnetic field at different temperatures. Using Type II, twin boundary MIR resulting in 6% field-induced deformation can occur in the field ~10 kA/m. For Type I, in contrast, the field more than ten times larger is needed. The effect of demagnetization is evaluated. In addition, the temperature dependence of the field needed to initiate twin motion is flat for Type II compared with sharp increase for Type I. The explanations for this sharply differing behavior are proposed.

Magnetocaloric effect in Ni–Mn–In–Co microwires prepared by Taylor-Ulitovsky method

Ni–Mn–In–Co microwires with diameter of 30–100 μm are prepared by glass-coated metal filaments (Taylor–Ulitovsky) method. The effects of magnetic field on martensite transformation temperature in the as-prepared and annealed microwires are investigated using a physical property measurement system (PPMS). Magnetocaloric effect (MCE) attributed to field-induced austenite transformation in the as-prepared and annealed microwires is analyzed indirectly from the isothermal magnetization (M–B) curves. The as-prepared microwire has a 7-layer modulated martensite structure (7M) at room temperature. The changes of austenite starting temperature induced by an external magnetic field (δAs/δB) in the as-prepared and annealed microwires are −1.6 and −4 K/T, respectively. Inverse martensite to austenite transformation exists in annealed microwires when an external magnetic field is applied at temperatures near As. The entropy change (δS) obtained in the annealed microwires is 3.0 J/(kg·K), which is much larger than that in the as-prepared microwires 0.5 J/(kg·K). The large entropy change and low price make Ni–Mn–In–Co microwires a potential working material in magnetic refrigeration.

Over 7% magnetic field-induced strain in a Ni-Mn-Ga five-layered martensite

A Ni-Mn-Ga single crystal with a modulated five-layered martensite structure is reported, demonstrating giant magnetic field induced strain (MFIS) of 7.1% at room temperature and of 6% at temperatures close to the austenite transformation (TA = 71 °C). The room temperature MFIS clearly exceeds the best results of around 6% measured earlier in 10M martensites. The larger MFIS is connected to the huge (>1%) change in the lattice distortion of the 10M structure, obtained within a narrow temperature interval of 47 K, which has been previously observed only during intermartensitic transformation. The present material shall effectively reduce the size of magnetic shape memory actuators.

In-situ neutron diffraction study of martensitic variant redistribution in polycrystalline Ni-Mn-Ga alloy under cyclic thermo-mechanical treatment

The influences of uniaxial compressive stress on martensitic transformation were studied on a polycrystalline Ni-Mn-Ga bulk alloy prepared by directional solidification. Based upon the integrated in-situ neutron diffraction measurements, direct experimental evidence was obtained on the variant redistribution of seven-layered modulated (7M) martensite, triggered by external uniaxial compression during martensitic transformation. Large anisotropic lattice strain, induced by the cyclic thermo-mechanical treatment, has led to the microstructure modification by forming martensitic variants with a strong ⟨0 1 0⟩7M preferential orientation along the loading axis. As a result, the saturation of magnetization became easier to be reached.

Composition dependent phase stability of Ni–Mn–Ga alloys studied by ab initio calculations

Composition dependent phase stability of Ni–Mn–Ga magnetic shape memory alloys was studied by first-principles density-functional calculations. It is demonstrated that the three kinds of doping (Ni substitution for Mn, Ni for Ga, and Mn for Ga) destabilize all the three structures. However, Ni-doping relatively stabilizes the non-modulated martensite (NM) with simple tetragonal crystal structure, whereas proper Mn-doping relatively stabilizes the monoclinic seven-layered modulated (7M) martensite with monoclinic structure. Comparing the energy difference between the parent and the product phases, we found that martensitic transformation experiences much larger driving force than that of the intermartensitic transformation. Chemical bonds between Ni and Mn are observed not only with the stoichiometric Mn, but also with the excess ones. Bonds between Ni and Mn in modulated martensite is stronger than that of the non-modulated martensite, which is beneficial to the stability of the modulated martensite. The present result provides useful information for further development of magnetic shape memory alloys that is difficult to be obtained from experiments.

Identification of Crystal Structure and Crystallographic Features of NiMnGa Thin Films by Combination of X-Ray Diffraction (XRD) and Electron Backscatter Diffraction (EBSD)

In this work, NiMnGa thin film composed of non-modulated martensite (NM) and seven-layered modulated martensite (7M) was produced. The crystal structure and lattice constants were determined by X-ray diffractometer (XRD). The preferred crystallographic orientation of martensite was determined using the four-circle XRD. SEM/EBSD was employed to verify the crystal structure of the martensite and to reveal its crystallographic features correlated with the microstructure. According to the XRD patterns, the crystal structure of NM and 7M was determined as tetragonal and monoclinic crystal structure, respectively. Pole figures measured by four-circle diffractometer revealed that the NM martensite possesses (004)NM and (220)NM preferred plane texture close to the substrate surface, whereas the 7M martensite has (2 0 20)7M, (2 0 )7M and (040)7M preferred plane texture close to the substrate surface. SEM/EBSD analysis shows that the surface layer of the film is mainly composed of NM martensite that is organized in variant groups. In each variant group, all the martensite plates consist of paired lamellar (112)NM compound twins and there are eight orientation variants in each variant group.

Crystallographic insights into the intermartensitic transformation in Ni–Mn–Ga alloys

Ferromagnetic shape memory alloys such as Ni–Mn–Ga usually undergo intermartensitic transformation upon cooling or heating. Here, an attempt is made to explore the microstructural features and orientation relationships associated with the intermartensitic transformation from seven-layered modulated (7M) martensite to non-modulated (NM) martensite in a polycrystalline Ni53Mn22Ga25 alloy. Based on electron backscatter diffraction analysis, it is demonstrated that the intermartensitic transformation proceeds in an in-plate manner (through atomic reshuffling and lattice distortion) with specific orientation relationships between the two martensitic phases, i.e. (001)7M//(112)NM and [100]7M//[111¯]NM as well as (001)7M//(112)NM and [1¯00]7M//[111¯]NM, accompanied by the thickening of martensite plates and the surface relief effect. Consequently, one 7M variant (plate) evolves into one NM plate consisting of two twin-related fine variants, and four differently oriented 7M variants in one variant group result in a total of eight NM variants.

Magneto-optical spectroscopy of ferromagnetic shape-memory Ni-Mn-Ga alloy

Magneto-optical properties of single crystal of Ni50.1Mn28.4Ga21.5 magnetic shape memory alloy in martensite and austenite phase were systematically studied. Crystal orientation was approximately along {100} planes of parent cubic austenite. At room temperature, the sample was in modulated 10M martensite phase and transformed to cubic austenite at 323 K. Spectral dependence of polar magneto-optical Kerr effect was obtained by generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.2 to 4 eV, and from room temperature to temperature above the Curie point. The Kerr rotation spectra exhibit prominent features typical for complexes containing Mn atoms. Significant spectral changes during transformation to austenite can be explained by different optical properties caused by changes in density of states near the Fermi energy.

Effect of partial substitution of Fe by Mn in Ni55Fe19Ga26 on its microstructure and magnetic properties

Abstract Ni–Fe–Ga-based Ferromagnetic Shape Memory Alloys (FSMAs) show considerable formability because of the presence of a disordered FCC γ-phase, but they lack in magnetocaloric property. Addition of Mn has been explored as a way to improve their magnetocaloric property. The current study presents a detailed structural and magnetization analyses of a two-phase ternary Ni55Fe19Ga26 alloy and its quaternary counterparts obtained by partial replacement of Fe by Mn, Ni55Fe19−xMnxGa26 (x = 2.5, 2.75, 3, 5, 10). Characterization of these alloys has been carried out using Optical and Scanning Electron Microscopy, Electron Probe Microanalysis, X-ray (XRD) and Neutron Diffraction (ND), Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC) and DC magnetization measurement. Ni55Fe19Ga26 alloy shows predominantly non-modulated (NM) internally-twinned martensite, with traces of a modulated 14M martensite and the parent L21 phase along with the FCC γ-phase. Quaternary addition of Mn in partial replacement of Fe stabilizes14M martensite, drastically reduces the amount of γ-phase, keeps the martensitic transition temperatures unchanged, but raises TC considerably. Magnetocaloric effect improves significantly with increasing Mn-content and a maximum value of −19.8 J/kg K for ΔSM has been observed at 9 T for the alloy containing 10 at.% Mn.

Microstructural characterisation and properties of quaternary Ni50Mn29Ga20Gd1 ferromagnetic shape memory alloy

A Heusler Ni50Mn29Ga20Gd1 ferromagnetic shape memory alloy is obtained by substituting 1 at-%Gd for Ga in a ternary Ni50Mn29Ga21 alloy. The microstructure, phase transformation, magnetic and mechanical properties of Ni50Mn29Ga20Gd1 alloy are investigated. It is shown that the Ni50Mn29Ga20Gd1 alloy has the best overall mechanical properties among the Ni50Mn29Ga21-xGdx (x = 0, 0·1, 0·5, 1, 2, 5) alloys. Compression tests show that a compressive strength of 1124·42 MPa with a compressive strain up to 16·25% can be achieved in this alloy. The mechanism of the improved mechanical properties is also discussed. The results demonstrate that the grains are refined obviously by the addition of 1 at-% Gd. The microstructure of the Ni50Mn29Ga20Gd1 alloy consists of the matrix and the hexagonal Gd(Ni, Mn)4Ga phase distributing mainly along the grain boundaries. Martensitic structure changes from five-layered martensite with 0 at-%Gd to seven-layered modulated martensite with 1 at-%Gd. One-step thermoelastic martensitic transformation occurs in this quaternary alloy. The martensitic transformation temperatures of the Ni50Mn29Ga20Gd1 alloy increase and its Curie temperature keeps unchanged compared with that of ternary Ni50Mn29Ga21 alloy. Coupling of the magnetic and structural transitions is observed in the Ni50Mn29Ga20Gd1 alloy.

Structural and magnetic characterization of the intermartensitic phase transition in NiMnSn Heusler alloy ribbons

Phase transitions and structural and magnetic properties of rapidly solidified Ni50Mn38Sn12 alloy ribbons have been studied. Ribbon samples crystallize as a single-phase, ten-layered modulated (10M) monoclinic martensite with a columnar-grain microstructure and a magnetic transition temperature of 308 K. By decreasing the temperature, martensite undergoes an intermartensitic phase transition around 195 K. Above room temperature, the high temperature martensite transforms into austenite. Below 100 K, magnetization hysteresis loops shift along the negative H-axis direction, confirming the occurrence of an exchange bias effect. On heating, the thermal dependence of the coercive field HC shows a continuous increase, reaching a maximum value of 1017 Oe around 50 K. Above this temperature, HC declines to zero around 195 K. But above this temperature, it increases again up to 20 Oe falling to zero close to 308 K. The coercivity values measured in both temperature intervals suggest a significant difference in the magnetocrystalline anisotropy of the two martensite phases.

Differently mobile twin boundaries and magnetic shape memory effect in 10 M martensite of Ni–Mn–Ga

We investigated twin boundaries with sharply different mobility or twinning stress in five-layered modulated (10M) martensite of Ni–Mn–Ga exhibiting magnetic shape memory effect or magnetically induced reorientation. Different mobility is ascribed to the different microstructures of the macrotwin planar interface. In monoclinic approximation the mobile boundaries are of Type I and Type II with complex microstructure of adjoining variants. These boundaries respond differently to magnetic field. For Type II boundary the reorientation takes place at very low field 250Oe.

Composition-dependent ground state of martensite in Ni–Mn–Ga alloys

For Ni–Mn–Ga alloys, giant magnetic-field-induced strains may be achieved in a modulated martensitic state, offering attractive chances for academic and practical exploration. However, the metastability of modulated martensite imposes a severe constraint on the capacity of these alloys as promising materials for sensors and actuators. In the present work, we conduct both experimental examinations and ab initio calculations to seek potential remedies of this critical problem through composition tuning. Results show that, for Group II alloys having modulated martensite at reasonable temperatures, the increase in Ni addition results in an enhanced tendency to the formation of non-modulated (NM) martensite, whereas the proper Mn addition leads to the stabilization of seven-layered modulated (7M) martensite, which serves as the structural ground state of martensite. By correlating the microstructural evolutions with the two-stage phase transformation (i.e. austenite→7M martensite→NM martensite), it is demonstrated that the 7M martensite possesses lower energy barriers in terms of the lattice distortion of parent austenite and the interfacial energy of martensitic variants, which plays a vital role in bridging the austenite to NM martensite transformation. This result is expected to provide useful information for the design of these new functional materials.

MICROSTRUCTURE AND PHASE TRANSFORMATIONOF Ni43Co7Mn41Sng HIGH TEMPERATURE SHAPEMEMORY ALLOY RIBBON

NiCoMnSn shape memory alloy (SMA) is expected to be a promising high temperature SMA. However, the brittleness has become a big obstacle for its practical application. It is known that, grain refining is effective in improving the ductility of a specific metallic alloy. The aim of this work is to investigate the effect of melt–spinning on grain refinement and martensitic transformation and provide a guideline for the development of NiCoMnSn SMA. Ni43Co7Mn41Sn9 high temperature SMA ribbon was prepared by a single–roll melt–spinning method. The microstructure and martensitic transformation were investigated by means of OM, SEM, TEM, XRD and DSC, respectively. The experimental results showed that, the ribbon had a chemical composition close to the master alloy and exhibited a thermoelastic martensitic transformation at about 160 . The grains in the as–spun ribbon, ranging from 2 μm to 18 μm, were remarkably refined compared with the master alloy. In the as– prepared ribbon, most of the columnar grains grew along the direction vertical to the ribbon plane. At room temperature, non–modulated martensite (tetragonal structure) consisting of twin substructure is determined in the ribbon after relieving the internal stress. Transformation temperatures were lowered by 30 after heat treatment at 400 for 1 h and then kept nearly constant with the increase of heat treatment temperatures. * 51101040, Y4100618 HEUCF201310016 ! : 2013–04–02, ! : 2013–05–17 # : ! , , 1976 AE , DOI: 10.3724/SP.J.1037.2013.00155 8 ! : Ni43Co7Mn41Sn9 #$ %#!#$$%&$% 977&

Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys

We report on the vibrational properties of the ferromagnetic shape memory alloy system Ni-Mn-Ga in its stoichiometric Ni${}_{2}$MnGa and off-stoichiometric Ni${}_{49}$Mn${}_{32}$Ga${}_{19}$ compositions. Elastic and inelastic neutron scattering measurements at different temperatures are presented with a focus on the austenite phase and compared to first-principles calculations. The overall behavior of the full phonon dispersion is similar for both compositions with remarkable exceptions for the TA${}_{2}$[$\ensuremath{\xi}\ensuremath{\xi}0$] acoustic branch and optical phonon branches. Less dispersion is found in the optical phonons for Ni${}_{49}$Mn${}_{32}$Ga${}_{19}$ in the whole reciprocal space when compared to Ni${}_{2}$MnGa and is explained by the occupation of regular Ga sites by excess Mn atoms. A pronounced softening in the TA${}_{2}$[$\ensuremath{\xi}\ensuremath{\xi}0$] phonon branch within the austenite phase is observed in both samples when approaching the martensitic transition. Its location in reciprocal space reveals the martensitic transition mechanism. The austenite $L$2${}_{1}$ structure transforms to the tetragonal modulated martensite structure by shuffling (110) planes in the [1$\overline{1}$0] direction, similarly to what has been observed at the martensitic transitions of the ${d}^{1}$ and ${d}^{2}$ transition metals. Whereas the temperature dependence of the softening of the TA${}_{2}$[$\ensuremath{\xi}\ensuremath{\xi}0$] phonons in the stoichiometric sample coincides perfectly with the magnetic and structural transitions, this is not the case for the off-stoichiometric sample. Here the relation between the magnetic ordering and the vibrational properties is still an open question.

Evidence for a monoclinic incommensurate superstructure in modulated martensite

Structural modulation of martensitic phases is regarded as a prerequisite for magnetically or electrically induced reversible strains in shape-memory alloys. Controversy surrounds the crystal structure of modulated martensite. Here, we explore this critical issue through combined spatially resolved microstructural and crystallographic characterizations of a polycrystalline Ni53Mn22Ga25 alloy, by comparing the high-resolution Kikuchi patterns with those predicted according to various hypotheses—7M(IC) or nanotwin combination structure—on the modulated martensite. Detailed analysis has demonstrated that the modulated martensite plates may possess a monoclinic incommensurate superstructure, other than the nanotwinned tetragonal non-modulated structure. Such an incommensurate superstructure can generate a more favorable plate interface configuration for field-driven twinning/detwinning, capable of producing large reversible actuation strain. Moreover, the thermodynamically metastable modulated martensite may transform into the non-modulated martensite by further local lattice distortion, which would degrade the magnetic shape-memory effect. By taking into account the microstructure–property correlation, the ambiguity concerning the modulated martensitic structure could be clarified, which is essential to the understanding of the stability of modulated martensitic phases and their functionality as shape-memory materials.

Obtaining of Ni-Mn-Ga magnetic shape memory alloy by annealing electrochemically deposited Ga/Mn/Ni layers

Ni-Mn-Ga thin films are promising candidates for MicroElectroMechanical Systems. Triple-layers of nickel, manganese, and gallium were electrodeposited from chemical solutions on to tungsten and molybdenum refractory metal substrates. These layered films were subsequently annealed at 800 to 900°C to form a Ni-Mn-Ga Heusler alloy by diffusion. To evaluate the quality of the film, the magnetization of the Ni-Mn-Ga film was measured and normalized by the magnetization of nickel, yielding the relative magnetization. Due to the formation of Ni-Mn-Ga during annealing, the relative magnetization was approximately 2 times larger than the tri-layered as-plated film. These results are comparable to bulk Ni-Mn-Ga reference samples. X-ray diffraction measurements confirmed that the material was present as a mixture of L21-ordered austenite as well as modulated 10M and non modulated martensite with manganese oxide impurities.

Phase stability of Ni2(Mn1−xFex)Ga: A first-principles study

Ni${}_{2}$(Mn${}_{1\ensuremath{-}x}$Fe${}_{x}$)Ga ferromagnetic shape memory alloy shows unusual composition-dependent martensitic transformation temperature (${T}_{\mathrm{M}}$), namely, ${T}_{\mathrm{M}}$ decreases with increasing $e/a$ ratio. In hope of understanding this unusual behavior, we investigated the composition dependence of the heat of formation (${H}_{\mathrm{f}}$) and shear elastic modulus (${C}^{\ensuremath{'}}$) of the cubic austenite as well as the stability of the five-layer modulated (5M) tetragonal martensite relative to the austenite, using first-principles exact muffin-tin orbital method in combination with coherent potential approximation. Our calculations demonstrated that ${H}_{\mathrm{f}}$ of the austenite increases with the Fe content $x$. ${C}^{\ensuremath{'}}$ increases slightly with $x$ up to 0.05 but decreases thereafter. The composition dependence of both ${H}_{\mathrm{f}}$ and ${C}^{\ensuremath{'}}$ cannot fully account for the trend of ${T}_{\mathrm{M}}$ against $x$ although such correlations have been proposed in literature for other Ni-Mn-Ga based alloys. The structure of 5M martensite phase of Ni${}_{2}$(Mn${}_{1\ensuremath{-}x}$Fe${}_{x}$)Ga with $0<x<0.2$ is determined by optimizing both the shear (changing $c/a$) and wavelike shuffle of atoms in (110) planes along [1$\overline{1}$0] direction adopting the experimentally determined modulation function. The energy difference $\ensuremath{\Delta}{E}_{\mathrm{AM}}$ between the austenite and 5M phases decreases with increasing $x$ up to 0.05, following the lower $\ensuremath{\Delta}{E}_{\mathrm{AM}}$ corresponding to lower ${T}_{\mathrm{M}}$ rule. However, with $x$ larger than 0.05, $\ensuremath{\Delta}{E}_{\mathrm{AM}}$ increases, against the experimental ${T}_{\mathrm{M}}\ensuremath{\sim}x$ behavior. We propose that, if taking the temperature effect and the spin-orbital coupling into account, the $\ensuremath{\Delta}{E}_{\mathrm{AM}}\ensuremath{\sim}x$ curve might be altered and may explain the unusual composition dependence of Ni${}_{2}$(Mn${}_{1\ensuremath{-}x}$Fe${}_{x}$)Ga.

Domain Models of Ferromagnetic Shape Memory Materials

AbstractMagnetic shape‐memory materials are multiferroics that combine ferroelastic and ferromagnetic order. The diffusionless transformation processes in these materials unroll by the creation of crystallographic domains or twinned microstructures that are coupled to magnetic domain patterns. Miniaturization of the crystallographic domains to the scale of lattice unit cells is afforded by the concept of adaptive modulated martensite. A ferroelastic model for these crystallographic domain patterns is introduced for the exemplary case of the Ni2MnGa Heusler alloy. The pseudo‐microscopic model incorporates data from ab initio calculations and phonon dispersion. The twin‐boundary energy for the diffuse wall of this model are very low of the order 1 mJ · m−2 fulfilling a key requirement for the formation of adaptive martensite in this material. An extension of the Ginzburg–Landau phase‐field model for large strain gradients is introduced to model atomically sharp twin‐boundaries.

Effect of Trivalent Additions and Processing on Structural and Magnetic Transitions in Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory Ni50Mn30Ga15Al5-xBx (x = 0, 1, and 4) alloys were prepared by vacuum arc melting and subsequent heat-treatment as well as by melt spinning to investigate the effect of trivalent element additions in ternary Ni-Mn-Ga alloys. The heat-treated alloys containing Al were reported to possess a modulated martensite structure, however alloy containing both Al and B showed a loss of modulated structure in martensite formed. The rapidly solidified alloys on the other hand showed the formation of a similar modulated structure without composition change in alloys containing Al and the alloys containing Al and B. In addition, the former showed a presence of an amorphous phase with latter showing crystalline boron rich phases. The magnetisation of the B containing alloys in both the processing technique was however very low, showing lower magnetic exchange interaction in such alloys. Defence Science Journal, 2012, 62(4), pp.252-260 , DOI:http://dx.doi.org/10.14429/dsj.62.1279