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Pinneker V.,Karlsruhe Institute of Technology | Gueltig M.,Karlsruhe Institute of Technology | Sozinov A.,Adaptamat Ltd. | Kohl M.,Karlsruhe Institute of Technology
Acta Materialia | Year: 2014

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 100 μm thickness with 10 M martensite structure at room temperature. A small temperature gradient of 5 K mm-1 and a bias field along the temperature gradient of 120 mT 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%. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Kohl M.,Karlsruhe Institute of Technology | Krevet B.,Karlsruhe Institute of Technology | Yeduru S.R.,Karlsruhe Institute of Technology | Ezer Y.,Adaptamat Ltd. | Sozinov A.,Adaptamat Ltd.
Smart Materials and Structures | Year: 2011

A novel foil actuator of 15 × 3mm 2 lateral dimensions is presented making use of the magnetic shape memory (MSM) effect. The actuation material is a Ni-Mn-Ga foil of 200νm thickness that has been fabricated by cutting of a bulk Ni-Mn-Ga(100) single crystal consisting of 10M martensite variants at room temperature. Stress-strain experiments on tensile test structures demonstrate that the stress needed for reorientation of martensite variants is about 1.2MPa. The low twinning stress allows magnetic-field-induced variant switching, the basic mechanism of MSM actuation. A Ni-Mn-Ga foil actuator is fabricated by lithography and hybrid integration. The actuator shows a maximum magneto-strain of 4.9%, which is limited by the constraints of fixation and loading. Upon tensile loading at 1.5MPa, linear actuation cycles are generated with an actuation stroke of 2.2%. The foil actuator is used as a benchmark system for modeling the coupled magneto-mechanical behavior of MSM actuation. We present finite element simulations based on a thermodynamic Gibbs free-energy model that qualitatively describes the observed tensile stress-dependence of magneto-strain. © 2011 IOP Publishing Ltd.

Chulist R.,TU Dresden | Straka L.,Aalto University | Sozinov A.,Adaptamat Ltd. | Lippmann T.,Helmholtz Center Geesthacht | Skrotzki W.,TU Dresden
Scripta Materialia | Year: 2013

Synchrotron X-ray diffraction experiments were performed to investigate modulation reorientation by an external stress in a 10M Ni-Mn-Ga single crystal. The initial crystal, with a single c-axis, exhibited two modulation directions, indicating {1 0 0) compound twins. Due to monoclinic distortion of the pseudo-tetragonal lattice, a compression along 〈110] with constraint on the c-axis resulted in a single modulation along the stress axis, i.e. the elimination of {1 0 0) compound twins. Subsequent perpendicular compression reoriented the modulation towards the new loading axis. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Straka L.,Aalto University | Heczko O.,ASCR Institute of Physics Prague | Seiner H.,Czech Institute of Thermomechanics | Lanska N.,Adaptamat Ltd. | And 5 more authors.
Acta Materialia | Year: 2011

The huge strains that Ni-Mn-Ga magnetic shape memory alloys can achieve are usually described in a tetragonal unit cell approximation of a five-layered modulated (10 M) crystal structure. Here we analyze the impact of a slight orthorhombic and monoclinic distortion of the 10 M structure in Ni 50.2Mn28.3Ga21.5at.% single crystal. Combining dedicated experiments to probe the microstructure, structure and mechanical properties with calculation using elastic continuum theory, we prove the existence of fine a/b-laminates within modulation macrotwins of the order of 100 micrometers in size. This complex twin microstructure containing a Type II macrotwin interface is associated with an extraordinarily low twinning stress of between 0.05 and 0.3 MPa, while Type I twins exhibit twinning stress of about 1 MPa. The findings provide important guidelines for designing the martensitic microstructure for more efficient actuators. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Straka L.,Aalto University | Soroka A.,Adaptamat Ltd. | Seiner H.,Czech Institute of Thermomechanics | Hanninen H.,Aalto University | Sozinov A.,Adaptamat Ltd.
Scripta Materialia | Year: 2012

We measure the temperature dependence of twinning stress in alloys with composition between Ni50Mn28Ga22 and Ni50Mn29Ga21 in the temperature range 288-334 K. For Type I twins, the twinning stress increases approximately linearly (0.04 MPa K -1) with decreasing temperature. For Type II twins, it is temperature independent and is about 0.2 MPa. The dependences are about the same in all alloys when presented as a function of the difference between the test temperature and the martensite transformation temperature. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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