Crystal quality boosts responsiveness of magnetic shape memory single crystals

Rolfs, Katharina; Mecklenburg, A.; Guldbakke, Jan Magnus; Wimpory, Robert C.; Raatz, Annika; Hesselbach, J.; Schneider, R.

doi:10.1016/j.jmmm.2008.10.023

Cited by 18 publications

(16 citation statements)

References 8 publications

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“…Figure 4 shows a stress-strain characteristic measured in tensile mode along the X-direction of the foil specimen at 323 K. Before this measurement, a magnetic field of 0.85 Tesla has been applied in Ydirection to induce a c-axis orientation of the martensite variants in Y-direction. A very low twinning stress of about 0.3 MPa is observed, which is comparable to the results of the corresponding Ni-Mn-Ga bulk single crystal [13]. We therefore conclude that the tensile training method allows the complete recovery of the low twinning stress of the bulk reference.…”

Section: Thermo-mechanical Trainingsupporting

confidence: 83%

“…Various -ray diffraction pattern using Cu-K radiation of the Ni-Mn-Ga foil of narrow (400) diffraction peak at room temperature, which is due to 5M tetragonal martensite [11]. This result is similar to the martensite structure of the initial bulk single crystal [13].…”

Section: Ni-mn-ga Foil Specimensmentioning

confidence: 55%

“…The twinning stress tw is determined to be 5.5 MPa. The corresponding Ni-Mn-Ga bulk single crystal specimens exhibit a much lower twinning stress of 1.1 MPa in compression mode, which even further decreases upon heating to about 0.3 MPa at 323 K [13]. Obviously, the preparation of the foils introduces defects, which impede the mobility of twin boundaries.…”

Section: Ni-mn-ga Foil Specimensmentioning

confidence: 99%

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Magnetic-Field-Induced Reorientation in Single Crystalline Ni-Mn-Ga Foil Actuators

Khelfaoui¹,

Reddy²,

Kohl

et al. 2009

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations

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Abstract. The coupled magnetic and mechanical properties of single crystalline Ni-Mn-Ga foil actuators are investigated for decreasing thicknesses in the range from 200 6 The foils exhibit a narrow martensitic phase transformation above room temperature below the Curie temperature of 369 K. Single crystalline Ni-Mn-Ga beam actuators are fabricated by laser micromachining. As-prepared specimens show enlarged twinning stress and thus no measurable magneto strain. Therefore, several training methods are adapted to the quasi-two-dimensional geometry of the foils. As compression training can no longer be applied in lateral direction of thin foils, more sophisticated magneto-thermo-mechanical treatments are required. We show that the low twinning stress of the bulk reference specimens can be recovered. Magnetization experiments and the direct observation of magneto strain on trained foil actuators show different degrees of reorientation up to 1% depending on the training method. The observed partial reorientation effect is discussed in terms of the foil geometry and training parameters.

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Section: Thermo-mechanical Trainingsupporting

confidence: 83%

Section: Ni-mn-ga Foil Specimensmentioning

confidence: 55%

Section: Ni-mn-ga Foil Specimensmentioning

confidence: 99%

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Magnetic-Field-Induced Reorientation in Single Crystalline Ni-Mn-Ga Foil Actuators

Khelfaoui¹,

Reddy²,

Kohl

et al. 2009

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations

Self Cite

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“…The resulting stress versus tensile strain characteristic is shown in Fig. 9 for a single crystalline foil of 200 µm thickness at 323 K. As a major result, we find a strong reduction of twinning stress σ tw to about 0.3 MPa, which is comparable to the bulk value at 323 K [25]. We therefore conclude that the tensile training method allows the complete recovery of the low twinning stress of the bulk reference.…”

Section: Thermo-mechanical Trainingmentioning

confidence: 59%

“…8(b). At higher temperature, the twinning stress further decreases to about 0.3 MPa at 323 K [25]. Obviously, the preparation of the foils introduces defects, which impede the mobility of twin boundaries.…”

Section: Single-crystalline Ni-mn-ga Foil Actuatorsmentioning

confidence: 99%

Recent Progress in FSMA Microactuator Developments

Kohl

Reddy

Khelfaoui

et al. 2009

MSF

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Abstract. 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.

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Key Properties of NiMnGa Based Single Crystals Grown with the SLARE Technique

et al. 2012

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Magnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of NiMnGa based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of NiMnGa, NiMnGaFe, and NiMnGaCo of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. NiMnGa alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as NiMnGa with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.

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Crystal quality boosts responsiveness of magnetic shape memory single crystals

Cited by 18 publications

References 8 publications

Magnetic-Field-Induced Reorientation in Single Crystalline Ni-Mn-Ga Foil Actuators

Magnetic-Field-Induced Reorientation in Single Crystalline Ni-Mn-Ga Foil Actuators

Recent Progress in FSMA Microactuator Developments

Key Properties of NiMnGa Based Single Crystals Grown with the SLARE Technique

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