Compression-induced texture change in NiMnGa-polymer composites observed by synchrotron radiation

Abstract

Composites consisting of magnetic shape memory ͑MSM͒ particles embedded in a polyester matrix were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni 50.9 Mn 27.1 Ga 22.0 ͑at. %͒ fibers. The crystal structure of the martensite is tetragonal ͑5M͒ with c Ͻ a = b. Magnetic characterization of these composites shows indirect evidence for stress induced twin boundary motion in the MSM particles, as the compressed composite is easy to magnetize in the direction of compression and more difficult to magnetize in the perpendicular directions. The texture of all the embedded MSM particles is investigated before and after compression by means of synchrotron radiation. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After a 30% compression ͑height reduction͒, the MSM particles have a ͑004͒-fiber texture in the direction of compression. This is unambiguous evidence for stress induced twin boundary motion within the MSM particles. The term magnetic shape memory ͑MSM͒ effect refers to a magnetic field induced structural phase transformation. At the same time, also a magnetic field induced twin boundary motion is commonly referred to as MSM effect. 1-3 Large magnetic field induced strains can be achieved in MSM alloys, e.g., in the Heusler alloy Ni 2 MnGa up to 10%. 4 Besides employing single and polycrystals to utilize the MSM effect in actuators or dampers, also composites, i.e., MSM particles embedded in a polymer matrix, were proposed. 5,6 It was shown previously that compressed composite samples can be magnetized easier in the compressed direction and it is more difficult to magnetize them in the perpendicular directions. 6,7 This behavior was regarded as an indication for stress induced twin boundary motion within the MSM particles.In order to verify this, the texture of the MSM particles before and after compression is investigated in this work. The texture is measured by using diffraction of high-energy synchrotron radiation. The advantage of this method is that the texture of the whole sample volume is measured.The MSM particles used were prepared by mortar grinding of crucible melt-extracted and subsequently annealed Ni 50.9 Mn 27.1 Ga 22.0 ͑at. %͒ fibers. The preparation of the composite was done by mixing these MSM particles with a two component polyester resin ͑about 20 vol % MSM particles͒ followed by a vacuum treatment to remove air bubbles. Curing of the polyester matrix was done without a magnetic field applied. The composite was disk shaped with a height of 2.79 mm and a diameter of about 9 mm. A cylindrical sample with a diameter of 3.03 mm and a height of 2.79 mm was cut out from the middle of the composite. Detailed information on preparation of MSM particles and composites are given by Scheerbaum et al. 6 For the synchrotron texture measurements the beam line BW5 ͑about 100 keV͒ at DESY-HASYLAB in Hamburg ͑Germany͒ was used. The synchrotron radiation...

In-situ studies of stress- and magnetic-field-induced phase transformation in a polymer-bonded Ni–Co–Mn–In composite

“…Thus, both a good ductility and a considerable MFIS are expected. Scheerbaum et al [11][12][13] have successfully prepared a composite using polyester resin and Ni-Mn-Ga FSMPs. They confirmed that stress-induced twin boundary motion still occurred within the Ni-Mn-Ga particles.…”

“…But the single crystals are of high cost and not easy to shape. A possible way of overcoming these problems is to embed ferromagnetic shape-memory particles (FSMPs) in a stiffness-matched matrix [9][10][11][12][13]. The ideal situation is that if every particle embedded in the matrix is a single crystal in the austenitic state, then the rigidity of grain boundaries arising from incompatible orientations can be reduced.…”

Comparative Study of Structural and Magnetic Properties of Bulk and Powder Ni$_{52}$Fe$_{17}$Ga$_{27}$Co$_{4}$ Magnetic Shape Memory Alloy

“…Among these alloys, the Ni-Fe-Ga-Co system is regarded to be a promising candidate due to its significantly improved ductility by introducing the second phase (of disordered fcc structure) to the matrix as well as its high magnetocrystalline anisotropy energy [8], [9]. More importantly, the critical stress for twin boundary motion in a single variant state is very low (2)(3) in Ni-Fe-Ga-Co alloys. As a result, a relatively large MFIS of 0.7% in a Ni Fe Ga Co alloy with seven-layered modulation period (called 7 M) martensite was obtained at room temperature [9].…”

Phase transition of Ni–Mn–Ga alloy powders prepared by vibration ball milling

“…Nevertheless, Ni-Mn-Ga alloys are very brittle, which remarkably impedes their practical applications. To overcome the problem, several forms based on the alloy have been proposed and investigated, including thin films [5][6][7][8][9], ribbons [10][11][12] and composites consisting of ductile polymer matrix and Ni-Mn-Ga alloy particles [13][14][15][16][17][18][19][20]. Among these forms, the composites prepared by mixing Ni-Mn-Ga alloy particles and polymer matrix exhibit great advantages with good formability and low cost of production, compared with the thin films and ribbons which are often restricted by their dimension in applications.…”