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Heczko O.,ASCR Institute of Physics Prague | Kopecky V.,ASCR Institute of Physics Prague | Sozinov A.,AdaptaMat Ltd | Straka L.,Aalto University
Applied Physics Letters | Year: 2013

Magnetic shape memory effect or magnetically induced structure reorientation (MIR) occurred down to 1.7 K in 10 M martensite with composition of Ni50.0Mn27.5Ga22.5 exhibiting no intermartensite transformation. The reorientation of the martensite microstructure was mediated by the motion of single Type II twin boundary. In contrast with weak thermal dependence of Type II boundary, MIR with Type I boundary in the same alloy showed strong thermal dependence resembling normal thermal activation process and the effect disappeared below 220 K. Thus the type of the boundary is decisive for MIR at low temperatures. © 2013 AIP Publishing LLC.


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.


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.


Straka L.,AdaptaMat Ltd. | Straka L.,Aalto University | Lanska N.,AdaptaMat Ltd. | Ullakko K.,AdaptaMat Ltd. | Sozinov A.,AdaptaMat Ltd.
Applied Physics Letters | Year: 2010

Ni-Mn-Ga single crystals with a twinning stress of about 0.1 MPa were studied. They showed a tendency to stay in a single variant state and to retain only one or very few twin boundaries during martensite reorientation induced by an external stress or magnetic field. This makes the crystals problematic for application in a magnetic actuator. To solve the issue, we introduced many parallel twin boundaries into the crystals by bending. However, this twin microstructure was not stable under cycling load. Additionally, it exhibited a twinning stress of 0.8 MPa-about ten times higher than a crystal with a single boundary. © 2010 American Institute of Physics.


Sozinov A.,Adaptamat Ltd | Lanska N.,Adaptamat Ltd | Soroka A.,Adaptamat Ltd | Zou W.,Adaptamat Ltd
Applied Physics Letters | Year: 2013

Magnetic field-induced strain (MFIS) of 12% is reported in ferromagnetic Ni46Mn24Ga22Co4Cu4 martensite exhibiting non-modulated (NM) tetragonal crystal structure with lattice parameter ratio c / a > 1. The strain was measured at ambient temperature in a magnetic field of the order of 1 T. The twinning stress σ T W and the magnetic stress σ M A G were also measured and the condition for a giant MFIS observation σ T W < σ M A G was confirmed. The MFIS was achieved in NM Ni46Mn24Ga22Co 4Cu4 martensite by considerable lowering of the σ T W value as compared to the values for NM martensites in ternary Ni-Mn-Ga system. © 2013 American Institute of Physics.


Sozinov A.,AdaptaMat Ltd. | Lanska N.,AdaptaMat Ltd. | Soroka A.,AdaptaMat Ltd. | Straka L.,Aalto University
Applied Physics Letters | Year: 2011

Twin relationships and stress-induced reorientation were studied in Ni 2Mn1.14Ga0.86 single crystal with five-layered modulated martensite crystal structure. Very low twinning stress of about 0.1 MPa was found for twin boundaries which deviated a few degrees from the (01 1 ) crystallographic plane. However, twin boundaries oriented exactly parallel to the (01 1 ) plane exhibited considerably higher level of twinning stress, above 1 MPa. X-ray diffraction experiments and calculations based on approximation of the martensite crystal lattice as a tetragonal lattice with a slight monoclinic distortion identified the two different kinds of twin interfaces as type II and type I twin boundaries. © 2011 American Institute of Physics.


Straka L.,Aalto University | Straka L.,AdaptaMat Ltd. | Hanninen H.,Aalto University | Soroka A.,AdaptaMat Ltd. | Sozinov A.,AdaptaMat Ltd.
Journal of Physics: Conference Series | Year: 2011

Twinning stress or mechanical hysteresis associated with the twin boundary motion is one of the most essential parameters which determine the actuating performance of magnetic shape memory alloys. Recent effort at AdaptaMat Ltd. to decrease the twinning stress resulted in a consistent production of Ni-Mn-Ga magnetic shape memory single crystals with the twinning stress of about 0.1 MPa, which is much lower than previously reported. In this work, the mechanical and magnetomechanical response of the developed crystals is discussed in detail and the importance of adjustment of the twin microstructure for obtaining an optimal actuating behavior is illustrated.


Chulist R.,TU Dresden | Straka L.,Aalto University | Lanska N.,AdaptaMat Ltd. | Soroka A.,AdaptaMat Ltd. | And 2 more authors.
Acta Materialia | Year: 2013

Mobile type I and type II twin boundaries mediating the magnetic field-induced strain in five-layered modulated (10M) Ni-Mn-Ga martensite were analyzed by electron backscatter diffraction. Taking into account the slight monoclinic distortion of the pseudo-tetragonal lattice, the electron backscatter diffraction study reveals domains of 0.01-1 mm thickness adjacent to the type I and type II twin boundaries. The domains differing in the modulation direction are {1 0 0) compound twins and their effect on twinning stress is discussed. Detailed analysis of type II twin boundary reveals that the domains are further internally twinned by compound {1 1 0) twins 1-15 μm in size. An additional example of a complex twin microstructure combining type I and type II twin boundaries is presented. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


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 | 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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