10M Martensite Research Articles

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.

High-temperature magnetic shape memory actuation in a Ni–Mn–Ga single crystal

We report a Ni–Mn–Ga single crystal that demonstrates giant magnetic field-induced strain of 7.3% at room temperature and of 4.5% at 80 °C. This is the highest temperature at which strain caused by twin boundary motion induced by an external magnetic field has been reported. A dedicated testing bench was constructed for the high-temperature experiments. X-ray diffraction reveals that at room temperature the material possesses a mixture of 5M and of 7M martensites, while at 80 °C the 5M structure predominates.

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.

Single phase boundary actuation of a ferromagnetic shape memory foil

The novel mechanism of temperature-gradient-induced single phase boundary actuation is presented for a single crystalline ferromagnetic shape memory alloy (FSMA) foil. It is shown that applying a temperature gradient along the FSMA foil specimen results in the formation and propagation of a martensite–austenite phase boundary from the hot to the cold side, allowing for reproducible strain–temperature characteristics. The selection of martensite variants upon phase transformation is controlled by simultaneously applying a bias magnetic field, which determines the maximum strain response. Single phase boundary actuation is demonstrated for a Ni–Mn–Ga foil of 100μm thickness with 10M martensite structure at room temperature. A small temperature gradient of 5Kmm–1 and a bias field along the temperature gradient of 120mT are sufficient to achieve the maximum possible strain of 4.1%, corresponding to the length difference of the short c-axis of tetragonal martensite and the axis of cubic austenite. For a bias magnetic field in the perpendicular direction, the maximum strain change is −1.9%.

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.

Transformation behaviors, structural and magnetic characteristics of Ni—Mn—Ga films on MgO (001)

Ferromagnetic Ni—Mn—Ga films were fabricated by depositing on MgO (001) substrates at temperatures from 673 K to 923 K. Microstructure, crystal structure, martensitic transformation behavior, and magnetic properties of the films were studied. With increasing deposition temperature, the surface morphology of the films transforms from granular to continuous. The martensitic transformation temperature is not dependent on deposition temperature; while transformation behavior is affected substantially by deposition temperature. X-ray analysis reveals that the film deposited at 873 K has a 7M martensite phase, and its magnetization curve provides a typical step-increase, indicating the occurrence of magnetically induced reorientation (MIR). In situ magnetic domain structure observation on the film deposited at 873 K reflects that the martensitic transformation could be divided into two periods: nucleation and growth, in the form of stripe domains. The MIR occurs at the temperature at which martensitic transformation starts, and the switching field increases with the decrease of temperature due to damped thermal activation. The magnetically induced martensitic transformation is related to the difference of magnetization between martensite and austenite. A shift of martensite temperature of dT/dH = 0.43 K/T is observed, consistent with the theoretical value, 0.41 K/T.

Room temperature magneto-structural transition in Al for Sn substituted Ni–Mn–Sn melt spun ribbons

Martensitic and magnetic transformations in Ni48Mn39.5Sn12.5−xAlx (x=0, 1, 2, 3) Heusler alloy ribbons were investigated. It is demonstrated that both magnetic and structural transformations occur in all of the studied samples. It is also shown that substitution of Sn with Al causes the martensitic transformation (MT) and the reverse martensitic transformation (RMT) temperatures to increase to room temperature (ΔTMT=49K; ΔTRMT=43K), whereas the Curie temperature of martensite TCM decreases (ΔT=36K) and the Curie temperature of austenite TCA remains practically insensitive to Al introduction. This then allows to tune TCA and the MT temperature leading to their coincidence at ambient temperature. The austenite phase with the L21 type structure has been identified to exist in all the samples regardless of composition. On the other hand the structure of martensite has been shown to be sensitive to composition. It has been determined as the 10M martensite with (32¯) stacking sequence in Al free samples and the 4O martensite with the stacking periodicity (31¯) in Al containing samples. In addition, the splitting of the field cooling (FC) and the field heating (FH) thermo-magnetic curves at low (50Oe) magnetic field and below the TCM has been attributed to intermartensitic transition. The application of large magnetic field (50kOe) has shown the existence of two distinct ferromagnetic states with a considerable hysteresis loop. The properties of these materials make them promising for magnetocaloric applications.

Effect of intermartensite transformation on twinning stress in Ni-Mn-Ga 10 M martensite

Intermartensite transformations (IMTs) and temperature dependence of very low twinning stress associated with Type II twin boundaries were studied experimentally in four magnetic shape memory alloys with composition Ni50Mn25 + xGa25 – x (at. %), where x was between 3.2 and 3.9. The slightly different x or Mn content resulted in slightly different martensite transformation temperatures between 309 and 328 K, but in significantly different intermartensite transformation temperatures between 10 K and 251 K, with the transformation temperature being approximately proportional to x. In all four alloys, the initial twinning stress, about 0.1 MPa at room temperature, increases sharply when approaching the IMT at lower temperatures, up to 1 MPa and more. The effect is explained by the instability of 10M martensite near the IMT from which it follows that the twinning stress dependence shall be flat for x≤2.7. That is experimentally confirmed by an additional measurement on Ni50Mn27.7Ga22.3 alloy (x = 2.7), which exhibits just slowly increasing twinning stress with decreasing temperature, with the maximum of only 0.2 MPa at 1.7 K.

Magnetic-field-induced reverse transformation in a NiCoMnSn high temperature ferromagnetic shape memory alloy

The magnetic field induced reverse martensitic transformation was studied in polycrystalline Ni40Co10Mn41Sn9 with a high martensitic transformation start temperature (Ms, about 410K). The reverse transformation start temperature (As) dropped by about 34K when exposed to a magnetic field of 7T. At room temperature, the microstructure consisted of 14M martensite and γ phase.

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.

Coupling of Micromagnetic and Structural Properties Across the Martensite and Curie Temperatures in Miniaturized Ni‐Mn‐Ga Ferromagnetic Shape Memory Alloys

AbstractMicromagnetic structure evolution in Ni‐Mn‐Ga ferromagnetic shape memory thin films is investigated by means of temperature dependent magnetic force microscopy (TD‐MFM). The center of interest is the magnetic properties of epitaxial Ni‐Mn‐Ga thin films on MgO substrates across thermally induced phase transitions. Experimental results are discussed within the framework of competing magnetic interactions arising in stressed thin ferromagnetic films. Measurements on 14M martensite specimens are supplemented by three‐dimensional micromagnetic simulations. Corresponding calculated MFM micrographs are compared to experimental data. The influence of twin variant dimension and orientation on micromagnetic domain formation and wall structure is depicted from a theoretical point of view. A micromagnetic model system of partial flux closure is proposed and calculated analytically to estimate a stress induced magneto crystalline anisotropy constant in austenite Ni‐Mn‐Ga.

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.

Diffraction study of bending-induced polysynthetic twins in 10M modulated Ni-Mn-Ga martensite

Detailed conventional and synchrotron x-ray and electron backscatter diffraction experiments were performed to investigate bending-induced polysynthetic twins in a Ni−Mn−Ga single crystal with five-layered (10M) martensite crystal structure. It was proved that the twin microstructure produced in such a way is composed of type I twins. The type I twins exhibit a relatively high twinning stress of about 0.8 MPa, which limits the performance of bending-induced twins in magnetic actuation.

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.

First‐principles investigation of magnetic property and defect formation energy in Ni‐Mn‐Ga ferromagnetic shape memory alloy

AbstractThe magnetic properties of the different phases of Ni2MnGa and the defect formation energies in off‐stoichiometric Ni2MnGa are systematically investigated by means of the first‐principles calculations within the framework of density functional theory using the Vienna Ab Initio Software Package. The individual magnetic moment of each constituent atom keeps constant in austenite and NM martensite, whereas those of Ni and Mn in the 5M and 7M martensites oscillate according to the atomic position. The formation energies of the various point defects (antisite and vacancy) have been systematically calculated. Furthermore, the magnetic structures of the defective structures have been shown in detail. © 2012 Wiley Periodicals, Inc.

Tailoring martensitic transformation and martensite structure of NiMnIn alloy by Ga doping In

The martensitic transformation and the martensite crystal structure of alloys Ni50Mn34In16−xGax (x=0, 2, 4, 6, 8, 10, 12, 14, 16) have been investigated in this study. The studied alloys, with cubic L21 phase (x=0, 2), seven-layered modulated (14M) martensite phase (x=4, 6, 8), and non-modulated (NM) martensite phase (x=10, 12, 14, 16), exhibit martensitic transformation with critical temperature increasing linearly from 238K for x=0 to 486K for x=16. The linearity coefficient is estimated to be ∼15K per at.% of Ga. With regards to the lattice parameters, a elongates, whereas b and c contract as the austenite phase transformed to 14M martensite phase. While, it turns to c elongates, a and b contract as austenite transformed to NM martensite phase. It is found that the unit cell volume decreases with increasing Ga content, which contributes to the increase of transformation temperature with Ga content. A maximum strain of 12% is predicted through the lattice distortion in 14M martensite. Magnetic measurement shows the Curie temperature for martensite increases with Ga content.

Microstructure and magnetocaloric effect of melt-spun Ni52Mn26Ga22 ribbon

Microstructural features and magnetocaloric properties of Ni52Mn26Ga22 melt-spun ribbons were studied. Results show that there are four types of differently oriented variants of seven-layered modulated (7M) martensite at room temperature, being twin-related one another and clustered in colonies. Due to the coupled magnetic and structural transformations between parent austenite and 7M martensite, the melt-spun ribbons exhibit a significant magnetocaloric effect. At an applied magnetic field of 5 T, an absolute maximum value of the isothermal magnetic entropy change of 11.4 J kg−1 K−1 is achieved with negligible hysteresis losses.

TEM Study of Ni-Mn-Co-In Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloys (FSMA) are relatively new smart materials group. Recently, new FSMA from NiMnX (X=Sb, Sn, In, Co+In) systems are considered as alternative to the well known NiMnGa alloys. Four alloys of the following compositions: Ni43Mn35Co4In18, Ni41Mn35Co4In20, Ni42Mn35Co5In18, Ni40Mn35Co5In20 were studied in order to determine microstructure, phase composition and martensitic transformation temperatures versus their chemical composition. Structure of the alloys was studied by optical and transmission electron microscopy (TEM). All of the studied alloys showed macrostructure consisting of radially oriented columnar grains in the direction perpendicular to the casting axis. The structure of the phases occurred in the studied alloys depended on the cobalt and indium content. For the alloys containing 20 at. % of In at room temperature only L21 parent phase was observed whereas for those containing 18 at. % of In either single phase 14M modulated martensite or mixture of 14M martensite and L21 parent phase were seen. DSC measurements showed in studied alloys single-state martensitic transformation. Decrease In content of 2 at.% caused about 80°C fall of martensitic transformation temperatures. Curie temperature Tc increases of 20°C with 1 at% rise of the cobalt content.

Transmission Electron Microscopy of Twins in 10M Martensite in Ni–Mn–Ga Ferromagnetic Shape Memory Alloy

The combination of the 10M martensite plate variants in Ni–Mn–Ga ferromagnetic shape memory alloy was investigated by conventional transmission electron microscopy (CTEM) and electron diffraction experiments. There were four plate variants commonly designated as A, B, C and D, and three fundamental plate combinations can be identified in a given plate group, namely A : B (C : D) of Type II, A : C (B : D) of {1\\bar{2}\\bar{5}} Type I and A : D (B : C) of {105} compound twins in the martensites. The boundary structure of these twins was also observed in the edge-on state by high resolution electron microscopy (HREM). The boundary of the Type I and the compound twins was not sharp in comparison to that in other shape memory alloys. There were neither ledge nor step structures at the irrational boundary in the Type II twin.