A 1% magnetostrain in polycrystalline 5M Ni–Mn–Ga

Gaitzsch, Uwe; Pötschke, Martin; Roth, S.; Rellinghaus, Bernd; Schultz, L.

doi:10.1016/j.actamat.2008.09.017

Cited by 148 publications

(60 citation statements)

References 25 publications

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“…Notably, the concept of transformation twinning is widely adopted for the elucidation of structural changes during martensitic transformation. Although the formation of twinned martensitic variants is driven by a deformation from the parent phase and may not have any relation to the simple shear deformation defined by the twinning shear, the detwinning process can be well predicted by these elements, especially for the newly developed ferromagnetic shape memory alloys (Gaitzsch et al, 2009;Li et al, 2010). In such a case, the twinned martensitic variants always form regular arrays of alternate lamellae with fixed thickness and the twin boundaries are highly glissile, where the detwinning shear determines the shape memory performance.…”

Section: Introductionmentioning

confidence: 99%

A general method to determine twinning elements

Zhang

Esling

et al. 2010

J Appl Cryst

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The fundamental theory of crystal twinning has been long established, leading to a significant advance in understanding the nature of this physical phenomenon. However, there remains a substantial gap between the elaborate theory and the practical determination of twinning elements. This paper proposes a direct and simple method -valid for any crystal structure and based on the minimum shear criterion -to calculate various twinning elements from the experimentally determined twinning plane for Type I twins or the twinning direction for Type II twins. Without additional efforts, it is generally applicable to identify and predict possible twinning modes occurring in a variety of crystalline solids. Therefore, the present method is a promising tool to characterize twinning elements, especially for those materials with complex crystal structure.

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Section: Introductionmentioning

confidence: 99%

A general method to determine twinning elements

Zhang

Esling

et al. 2010

J Appl Cryst

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“…[2] This exceeds the strain obtainable from piezoelectric or magnetostrictive materials, currently used in actuators, by more than one order of magnitude, and opens new opportunities for applications. Advanced materials have been developed based on new compositions [3][4][5] or on innovative routes of fabrication resulting in foams, [6] fibers, [7] textured polycrystals, [8] or composites.[9] Thin MSM films with these high strains would be of significant benefit for use in microactuators. However, preparation of films exhibiting such high strains has not been achieved so far.…”

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confidence: 99%

Stray‐Field‐Induced Actuation of Free‐Standing Magnetic Shape‐Memory Films

et al. 2009

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Magnetic shape-memory (MSM) alloys, such as NiMnGa, [1] reach maximum strains of close to 10% in single crystals. [2] This exceeds the strain obtainable from piezoelectric or magnetostrictive materials, currently used in actuators, by more than one order of magnitude, and opens new opportunities for applications. Advanced materials have been developed based on new compositions [3][4][5] or on innovative routes of fabrication resulting in foams, [6] fibers, [7] textured polycrystals, [8] or composites.[9] Thin MSM films with these high strains would be of significant benefit for use in microactuators. However, preparation of films exhibiting such high strains has not been achieved so far. In the present work, we demonstrate a new and enhanced thermal actuation mode, which is achieved by the use of freestanding, epitaxially grown NiMnGa films. This new mode utilizes a variant selection by magnetic stray-field energy within the martensite state. It allows for reversible actuation similar to the conventional two-way shape-memory effect, yet neither training nor an external magnetic field is required. A novel path to the realization of sub-micrometer, miniaturized microactuators based on MSM films is established. Due to the finite size of magnetic domains, this actuation mode is a unique feature of small systems.The novel mode uses both the ferromagnetic and martensitic properties of MSM alloys. Since it involves features of known actuation modes, these are described in the following. Shapememory alloys exhibit a diffusionless transition from a hightemperature cubic austenite phase to a low-temperature martensite phase of lower symmetry.[10] The martensitic phase may, as in the present case, exhibit a tetragonal structure with two identical long crystal a axes and one short c axis. Consequently, three different orientations of the martensitic c axis with respect to the cubic austenite cell are possible. Neighboring unit cells of identical orientation form so-called martensitic variants. Adjacent variants with different orientations are connected by twin boundaries. This special microstructure allows an easy deformation by changing the fractions of martensitic variants through twin-boundary motion. Heating to the cubic austenite state restores the original shape, which is the so-called one-way shape-memory effect. To obtain the two-way shape-memory effect, several cycles of mechanical training and heating are required. This presumably leads to the formation of microstructural defects, which can act as nucleation sites during martensite formation, memorizing the uneven variant distribution and hence the macroscopic shape.[11] For this actuation mode, temperature is the control parameter.Martensitic materials, which also exhibit ferromagnetic order, allow two additional actuation modes. The first type utilizes the coupling between crystal structure and spontaneous magnetization. In a magnetic field, the phase with the higher magnetic moment is energetically favored, leading to a field-dependent shift of the martensitic p...

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“…A strong texture, large grains and a mechanical training can decrease s twin . [4,5] The main disadvantage of single-and polycrystals is their complex preparation.An alternative to single-and polycrystals are Ni-Mn-Ga polymer composites. [6][7][8] Single crystalline particles embedded in a stiffness-matched polymer matrix allow overcoming the disadvantages.…”

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confidence: 99%

“…A strong texture, large grains and a mechanical training can decrease s twin . [4,5] The main disadvantage of single-and polycrystals is their complex preparation.…”

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Reversible Magnetic Field Induced Strain in Ni₂MnGa‐Polymer‐Composites

et al. 2011

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Magnetic shape memory (MSM) alloys are a new class of materials for sensor and actuator applications. Ni-Mn-Ga single crystals show large magnetic-field induced strain (MFIS) in moderate magnetic fields below 1 T caused by reorientation of martensitic variants by twin boundary motion. The maximum possible strain e ¼ 1-(c/a) is given by the lattice parameters a and c of the martensite unit cell resulting in e ¼ 6% for 5M martensite and e ¼ 11% for 7M martensite. [1][2][3] However, the main disadvantage of single crystals is their high brittleness. One possible solution is the synthesis of textured polycrystalline Ni-Mn-Ga for which a MFIS of 1% has been reported. [4] Due to constraints by grain boundaries, the twinning stress s twin for moving twin boundaries is still much higher for polycrystals (s twin > 15 MPa) compared to that in single crystals (s twin < 2 MPa). A strong texture, large grains and a mechanical training can decrease s twin . [4,5] The main disadvantage of single-and polycrystals is their complex preparation.An alternative to single-and polycrystals are Ni-Mn-Ga polymer composites. [6][7][8] Single crystalline particles embedded in a stiffness-matched polymer matrix allow overcoming the disadvantages. A thin polymer film between the single crystalline particles allows the particles to strain und reduces s twin . Another advantage of composites is the decrease of eddy currents due to a non-conducting polymer matrix, which enables high frequency applications, and the simple preparation of textured bulk materials.In previous works, we showed that melt-extracted and subsequently annealed fibres exhibit a bamboo-like grain structure and an MFIS of 1%. [6] However, only small amount of grains in the fibre are active. Different crystallographic orientations of grains as well as still existing grain boundaries hinder activation of the whole fibre. A separation of fibres into nearly single crystalline particles by breaking them alongComposite materials consisting of magnetic shape memory alloy particles and a polymer matrix combine the advantages of both material classes: the high achievable magnetic field induced strain (MFIS) of 6% of Ni-Mn-Ga with a ductile matrix. Engineering the particle-matrix interface as well as matching stiffness of polymer matrix is of importance for achieving high reversible MFIS to use this material as actuator or damper. We investigated those properties for Ni 50.9 Mn 27.1 Ga 22.0 and Ni 50.3 Mn 24.6 Ga 25.1 polymer composites. Particles were produced by gently crushing melt-extracted and subsequently annealed fibres. At room temperature, the Ni 50.9 Mn 27.1 Ga 22.0 particles exhibit a 5M martensitic structure, while the Ni 50.3 Mn 24.6 Ga 25.1 particles are austenitic. These particles were embedded into the polymer, either a stiff epoxy resin or a soft polyurethane. In response to an external applied magnetic field, the particles tend to relocate within the polyurethane due to its very low Young's modulus and magnetostatic interaction between particles. Slightly stiffer pol...

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A 1% magnetostrain in polycrystalline 5M Ni–Mn–Ga

Cited by 148 publications

References 25 publications

A general method to determine twinning elements

A general method to determine twinning elements

Stray‐Field‐Induced Actuation of Free‐Standing Magnetic Shape‐Memory Films

Reversible Magnetic Field Induced Strain in Ni₂MnGa‐Polymer‐Composites

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A 1% magnetostrain in polycrystalline 5M Ni–Mn–Ga

Cited by 148 publications

References 25 publications

A general method to determine twinning elements

A general method to determine twinning elements

Stray‐Field‐Induced Actuation of Free‐Standing Magnetic Shape‐Memory Films

Reversible Magnetic Field Induced Strain in Ni2MnGa‐Polymer‐Composites

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Reversible Magnetic Field Induced Strain in Ni₂MnGa‐Polymer‐Composites