Reorientation Of Martensite Variants Research Articles

Martensite Variant Reorientation of NiMnGa/Silicone Composites Containing Polystyrene Foam Particles

Effect of elastic modulus of matrix on ferromagnetic motion of NiMnGa particles was investigated for NiMnGa particles embedded silicone matrix composites with or without containing polystyrene form particles (PFPs), which are regarded as pores. NiMnGa single crystal was fabricated by a floating zone method and a cube-shape particle was fabricated with the surface orientation parallel to [100], [010] and [001] directions at the parent phase state. The elastic modulus of matrix polymer was controlled by changing the number of PFPs. It was found that the elastic modulus of silicone was decreased with increasing the volume fraction of PFPs. The magnetic-field-induced martensite variant reorientation (MVR) was evaluated by the magnetization curves obtained using a vibration sample magnetometer, and the magnetic field required for the start of martensite variant reorientation was reduced by increasing the number of PFPs. The control of elastic modulus of matrix polymer is important and a dominant factor for the magnetic motion of NiMnGa/polymer composites.

Numerical Investigations of Thermally Induced Transformation, Reorientation of Martensite Variants and Shape Memory Effect Using Constitutive Model for Accommodation of Transformation Strain

To confirm the constitutive model developed by the present authors, example calculations are conducted for the transformation behavior of shape memory alloys. The constitutive model describes the transformation behavior of the material by calculating the transformation stress and strain of transformation systems of micro structures in the material. In the calculating, the accommodation mechanism acting on microstructures of the material is also taken into consideration. Computational results by the constitutive model for these loading paths are revealed to be reasonable in comparing with experimental observations.

Determination of the orientation relationship between austenite and incommensurate 7M modulated martensite in Ni–Mn–Ga alloys

For Ni–Mn–Ga ferromagnetic shape memory alloys, a large magnetic-field-induced strain could be reached through the reorientation of martensitic variants in the martensite state. Owing to the collective and displacive nature of the austenite to martensite transformation, a certain orientation relationship (OR) between the parent and the product phase is required to minimize the transformation strain and the strain energy generated, which brings about self-accommodating groups of martensitic variants with specific orientation correlations. In this work, the microstructural and crystallographic characteristics of martensitic variants in a polycrystalline Ni 50Mn 30Ga 20 alloy were investigated by electron backscatter diffraction analysis. With accurate orientation measurement on inherited martensitic variants, the local orientations of parent austenite grains were predicted using four classical OR for the martensitic transformation. Furthermore, a specific OR, namely the Pitsch relation with (1 0 1) A//(1 2 ¯ 10 ¯ ) 7M and [1 0 1 ¯ ] A//[ 10 ¯ 10 ¯ 1] 7M, was unambiguously determined by considering the magnitude of discontinuity between the lattices of the product and parent phases and the structural modulation of the incommensurate 7M modulated martensite. The present procedure to determine the OR, without recourse to the presence of retained austenite, is in general applicable to a variety of materials with modulated superstructure for insight into their martensitic transformation processes.

Microstructure and shape-memory characteristics of Ni 56Mn 25− xCo xGa 19 ( x = 4, 8) high-temperature shape-memory alloys

NiMnCoGa alloys were studied as candidate of high-temperature shape-memory alloys with Mn substituted by Co. The results show that the hot workability and room-temperature ductility of NiMnGa alloys can be greatly improved by forming ductile γ phase through Co addition. When the amount of γ phase is about 19%, the tensile stress and strain are 491 MPa and 8.2%, respectively. When the amount of γ phase increased to about 43%, the tensile stress and strain are 729.3 MPa and 14.1%, respectively. The reversible strain due to shape-memory effect is 2.1% under a prestrain of 5.3% for x = 4, and the shape-memory ability almost disappears for x = 8 due to large amount of γ phase, which seriously hampers the reorientation of martensitic variants and/or detwinning.

Twin-variant reorientation-induced large magnetoresistance effect in Ni50Mn29Ga21 single crystal

We report a significant magnetoresistance (MR) effect arisen from magnetic field-induced reorientation of martensitic twin variants in a ferromagnetic shape memory Ni50Mn29Ga21 single crystal. The measured electrical resistivity shows large anisotropy and the measured MR value is as large as 25% over the wide temperature range of 230-315 K at a moderate magnetic field of 1.2 T. It is found that a proper combination of the initial state of martensitic twin variants and the direction and magnitude of applied magnetic field can give rise to either positive or negative MR value of ∼25%, thus allowing a periodic modulation of the MR effect in response to varying the spatial angle between the directions of applied magnetic field and electric current for every 180°.

Compression Behavior and Texture Development of Polymer Matrix Composites Based on NiMnGa Ferromagnetic Shape Memory Alloy Particles

Deformation behavior and texture development of NiMnGa ferromagnetic shape memory alloy (FSMA) particles embedded polymer composites were investigated by compression tests and X-ray diffraction pole-figure analysis (XRD-PF). Both the NiMnGa/silicone and NiMnGa/epoxy composites exhibited a characteristic three-stage deformation which is often seen in shape memory alloys due to martensite variant reorientation. XRD-PF revealed that (004) texture was developed in the compressed NiMnGa/silicone due to the retention of martensite variant reorientation. On the other hand, significant texture change was not recognized in the compressed NiMnGa/epoxy. Rearranged martensite variants was kept after unloading in silicone matrix having low Young's modulus, but reverse reorientation occurred in epoxy matrix with high Young's modulus. The rearrangement behavior of martensite variants is strongly affected by elastic properties of matrix.

A 3D finite strain phenomenological constitutive model for shape memory alloys considering martensite reorientation

Most devices based on shape memory alloys experience both finite deformations and non-proportional loading conditions in engineering applications. This motivates the development of constitutive models considering finite strain as well as martensite variant reorientation. To this end, in the present article, based on the principles of continuum thermodynamics with internal variables, a three-dimensional finite strain phenomenological constitutive model is proposed taking its basis from the recent model in the small strain regime proposed by Panico and Brinson (J Mech Phys Solids 55:2491–2511, 2007). In the finite strain constitutive model derivation, a multiplicative decomposition of the deformation gradient into elastic and inelastic parts, together with an additive decomposition of the inelastic strain rate tensor into transformation and reorientation parts is adopted. Moreover, it is shown that, when linearized, the proposed model reduces exactly to the original small strain model.

Self-activated reversibility in the magnetically induced reorientation of martensitic variants in ferromagnetic Ni-Mn-Ga films

Magnetically induced reorientation (MIR) of twin variants for [101]-oriented martensite has been studied for Ni-Mn-Ga thin films deposited on (001) YSZ substrates. The MIR effect displayed by these films is self-reversible, is characterized by a small increase in the magnetization, and shows a temperature-dependent activation field. These properties can be described in terms of a magnetic microstructure comprised of MIR-active and MIR-inactive twin variants, where the degree of activity depends on the relative orientations of the externally applied magnetic field and the easy axis of the magnetization of the variants. The MIR-active variants are responsible for the detwinning, while the MIR-inactive variants exert a restoring force on the MIR-active variants as they reorient. This interplay between the active and inactive variants is highly advantageous as it allows for the complete self-activation of the reversible MIR effect, a response which is desired for actuation applications. The results further demonstrate that the magnetic domain structure formed in the films is inherited from a substrate-induced crystallographic texture and microstructure. It is also suggested that a tailored MIR response could prove possible through an optimization of the film texture and microstructure.

Influence of magnetization rotation on martensite reorientation in magnetic shape memory alloy

A large field-induced strain of magnetic shape memory alloy is developed by the martensite variant reorientation. It is widely recognized that the martensite reorientation in a magnetic shape memory alloy (MSMA) can develop if the magnetic field is large enough. However, it has been shown in the literature that the magnetization rotation may block variant reorientation via energy minimization approach. In this paper, based on a micromechanical model associated with the thermodynamic theory, authors show that there are some limits for the martensite reorientation, which is hindered by the magnetization rotation. Some useful conclusions are obtained.

Crystallographic orientation and stress-amplitude dependence of damping in the martensite phase in textured Ti–Nb–Al shape memory alloy

The low-frequency damping (tanδ) of a strongly textured β-titanium shape memory alloy (SMA) was investigated by a dynamic mechanical analysis (DMA) in a tensile mode together with a tensile test and a crystallographic analysis of the transformation. In addition to the high background of tanδ in martensite, a broad tanδ peak with a relaxation-like character was found in the martensite phase similar to the Ti–Ni SMAs. There was a critical stress for the broad peak that corresponds to the stress to complete the martensite variant reorientation, whereas the high background of tanδ had no critical stress. This implies that the broad peak is generated through the long-range motion of twin boundaries. A guideline to estimate the orientation dependence of damping in textured SMAs is also proposed using the interaction energy between the external stress and the transformation.

Reorientation of Ni–Mn–Ga martensite in rotating magnetic field

Reorientation of martensitic variants in the Ni52.0Mn24.4Ga23.6 single crystalline disk under magnetic field rotating in the disk plane has been studied. The reorientation proved to be possible in the fields exceeding 130kAm−1 and started when the angle between the magnetic moment of the reorienting martensite variant and the [100] crystallographic direction reached 560. The theoretical analysis of experimental results pointed to the comparatively low value of magnetocrystalline anisotropy constant (50kJm−3) and the possibility of the reversible reorientation of martensitic variants under the co-operative action of the external magnetic field and internal magnetostatic field of the specimen.

905 変態ひずみアコモデーションモデルによる形状記憶効果のシミュレーション(OS9-2 形状記憶材料の特性と応用)

To demonstrate the ability of the coristitutive model which was developed by the authors, example calculations are conducted for the transformation behavior of shape memory alloys. The problem calculated here is described as below. The polycrystalline Ti-Ni shape memory alloy with austenitic phase is first set to be under the stress free condition. The material is cooled down below the martensite finish temperature to occur the martensite phase transformation. The orientations of martensite variants are randomly distributed in the space so as to accomplish the state of the material free from the macroscopic strain. Then uniaxial tensile loading is applied to cause martensite variant reorientation. The material shows the inelastic stress-strain curve during this loading path. Then the load is removed where the residual strain of transformation remains at the stress free state. Next, the material is heated up to above the reverse transformation finish temperature. The residual strain disappears during this process and the shape memory effect is realized. Computational results by the constitutive model for these loading paths are revealed to be reasonable in comparing with experimental observations.

Large superplastic strain in non-modulated epitaxial Ni–Mn–Ga films

The phase transformation and superplastic characteristics of free-standing epitaxial Ni–Mn–Ga stripes are reported. The stripes are prepared by micromachining a 1μm thick Ni–Mn–Ga film sputter-deposited on a single crystalline MgO (100) substrate using optical lithography and a Chromium-based sacrificial layer technology. The stripes are oriented at angles of 0 and 45 degrees with respect to the Ni–Mn–Ga unit cell. Electrical resistance versus temperature characteristics reveal a reversible thermally induced phase transformation between 169∘C and 191∘C. Stress-strain measurements are performed with the stress applied along the [100]Ni−Mn−Ga as well as [110]Ni−Mn−Ga direction. Depending on the orientation, the twinning stress ranges between 25 and 30MPa, respectively. For the [100]Ni−Mn−Ga and [110]Ni−Mn−Ga directions, superplastic behaviour with a strain plateau of 12% and 4% are observed, respectively, indicating stress-induced reorientation of non-modulated martensite variants.

Recent Progress in FSMA Microactuator Developments

The giant magneto-strain effect is particularly attractive for actuator applications in micro- and nanometer dimensions as it enables contact-less control of large deformations, which can hardly be achieved by other actuation principles in small space. Two different approaches are being pursued to develop ferromagnetic shape memory (FSMA) microactuators based on the magnetically induced reorientation of martensite variants: (1) the fabrication of free-standing epitaxial Ni-Mn-Ga thin film actuators in a bottom-up manner by magnetron sputtering, substrate release and integration technologies and (2) the top-down approach of thickness reduction of bulk Ni-Mn-Ga single crystals to foil specimens of decreasing thicknesses (200 – 40 μm) and subsequent integration. This review describes the fabrication technologies, procedures for thermo-mechanical training adapted to the quasi-two-dimensional geometries of film and foil specimens as well as the performance characteristics of state-of-the art actuators after processing and training.

Large magnetoelectric effect from mechanically mediated magnetic field-induced strain effect in Ni–Mn–Ga single crystal and piezoelectric effect in PVDF polymer

Abstract Magnetoelectric (ME) effect is observed in a bilayered composite fabricated by laminating one ferromagnetic shape memory Ni–Mn–Ga single-crystal plate with one piezoelectric PVDF polymer film. The reported ME effect originates from the mechanically mediated magnetic field-induced strain effect in the Ni–Mn–Ga plate and piezoelectric effect in the PVDF film. The induced ME voltage shows a good linear response to the applied ac magnetic field in the field range of 0–5 Oe. A large ME coefficient ( α E ) of 1.24 V/(cm Oe) is obtained at an optimal bias magnetic field of 5.1 kOe due to the reversible reorientation of martensitic twin variants in the Ni–Mn–Ga plate. Based on the combination of pseudo-piezomagnetism in the Ni–Mn–Ga plate and piezoelectricity in the PVDF film, the ME effect in the bilayered composite is predicted and found to agree with the experimental results.

A bi-crystal aggregate model of pseudoelastic behaviour of shape-memory alloy polycrystals

A multi-scale model of stress-induced phase transformation and martensite variant reorientation in shape memory alloy (SMA) polycrystals is developed. It is proposed to include neighbouring-grain interaction in a simple manner by introducing an intermediate bi-crystal level into the sequential averaging scheme for SMA. The constitutive relationships are defined by specifying the free energy and dissipation functions. At the level of a single grain, the rate-independent dissipation function is used that incorporates the dissipation due to forward and reverse austenite-to-martensite transformation as well as reorientation of martensite variants. The global response of the model is simulated numerically by minimizing the total incremental energy supply. Specific examples are calculated for a NiTi polycrystal for proportional and non-proportional loading paths.

Magnetically induced martensite transition in freestanding epitaxial Ni–Mn–Ga films

The martensitic transformation in freestanding Ni–Mn–Ga films obtained by epitaxial growth on NaCl (001) is analyzed. A temperature-magnetic field phase diagram reveals that the martensitic phase, exhibiting a higher magnetization compared to austenite, is favored by an external field. A shift of martensite temperature of dT/dH=0.36 K/T is observed, in good agreement with the value expected from a Clausius–Clapeyron equation. The practicality and energy input for actuation using magnetically induced martensitic transition is compared with a magnetically induced reorientation of martensitic variants.

Abrupt Softening Phenomenon in a Shape Memory CuAlNi Single Crystal under Uniaxial Compression

Abrupt softening phenomenon was observed in a shape memory CuAlNi single crystal upon uniaxial compression. Sudden martensite variant(s) reorientation was found to be the reason behind this anomalous behavior. The significance of this finding is twofold. On one hand, it clearly demonstrates that the stress induced transformation can follow a sequence of the phase transformation (martensitic transformation, austenite to martensite) and then reorientation (from one martensite variant to another). On the other hand, the anomalous softening provides a good evidence for explaining the propagation of the phase transformation front.

Modeling the Coupled Strain and Magnetization Response of Magnetic Shape Memory Alloys under Magnetomechanical Loading

This article is concerned with the modeling of the magnetic shape memory alloy (MSMA) constitutive response caused by the reorientation of martensitic variants under mechanical and magnetic fields. The presented model is able to better capture the complexity of the magnetomechanical MSMA behavior by accounting not only for the mechanism of field-induced variant reorientation, but also the magnetization rotation away from magnetic easy axes and the magnetic domain wall motion at low stress and magnetic field levels. Following the general formulation of the model, reduced versions of the constitutive equations are derived for three specific loading cases: (1) magnetic-field-induced variant reorientation at constant stress; (2) stress-induced variant reorientation at constant magnetic field; (3) variant reorientation under collinear magnetic field and stress with perpendicular bias field. For each of these cases the nonlinear and hysteretic strain and magnetization response of MSMAs are predicted and compared to experimental data where available. The relation of critical stresses and magnetic fields for the activation of the reorientation process are visualized in a variant reorientation diagram. The captured loading-history-dependent macroscopic material response is explained in detail by connecting it to the evolution of the crystallographic and magnetic microstructure as represented by a set of internal state variables.

Stress-induced martensite variant reorientation in magnetic shape memory Ni–Mn–Gasingle crystal studied by neutron diffraction

Stress-induced martensite variant reorientation in magnetic shape memory Ni–Mn–Gasingle crystal was studied in situ by the neutron diffraction technique. Principles ofdetermination of individual tetragonal martensitic variants in shape memory alloys areexplained. Using neutron diffraction we show that the macroscopic strain originates solelyfrom the martensite structure reorientation or variant redistribution. Neutron diffractionalso reveals that the reorientation of martensite is not fully completed even at a stressvalue of 25 MPa, which is about 20 times larger than the mean stress needed forreorientation. Only one twinning system is active during the reorientation process.