Cast and Rolling Textures of NiMnGa Alloys
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1050-BB09-03
Cast and Rolling Textures of NiMnGa Alloys Robert Chulist1, Martin Poetschke2, Andrea Boehm3, Heinz-Guenter Brokmeier4, Ulf Garbe5, Thomas Lippmann6, Carl-Georg Oertel1, and Werner Skrotzki1 1 Institut für Strukturphysik, TU Dresden, Zellescher Weg 16, Dresden, 01062, Germany 2 Leibniz-Institut für Festkoerper- und Werkstoffforschung, Dresden, 01069, Germany 3 Fraunhofer-Institut fuer Werkzeugmaschinen und Umformtechnik, Dresden, 01187, Germany 4 GKSS Forschungszentrum, Geesthacht, 21494, Germany 5 GKSS Aussenstation, ZWE FRM II, Garching, 85747, Germany 6 GKSS Forschungszentrum, Geesthacht, 21502, Germany ABSTRACT The texture of two polycrystalline NiMnGa magnetic shape memory alloys fabricated by directional solidification and hot rolling has been measured with high-energy synchrotron radiation and neutron diffraction. At room temperature the alloys used are composed of a modulated (7M) and non-modulated (NM) martensitic structure. The texture of the directionally solidified alloy for all phases is a pronounced fibre texture with preferentially aligned along the growth direction. In the directionally solidified case a variant selection took place with [001] dominating. The texture of the hot rolled alloy shows a weak recrystallization texture with {111} and aligned parallel to the rolling plane and rolling direction, respectively. The texture results are discussed with respect to material, processing and phase transformations including variant selection. INTRODUCITION NiMnGa alloys close to the stoichiometric composition Ni2MnGa belong to the quite new family of ferromagnetic shape memory alloys. These alloys are characterized by magnetically induced reorientation (MIR) resulting from the motion of twin boundaries under the influence of a magnetic field1,2. Plastic deformation of NiMnGa single crystals by twin boundary motion leads to a maximum magnetic field induced strain (MFIS) of about 10%. To our knowledge, until now MFIS has not been reported for NiMnGa polycrystals. However, since for broad technical applications growth of single crystals is quite time consuming and cost intensive it is necessary to investigate polycrystalline samples on their suitability for MFIS. To approach MFIS of single crystals the crystallographic texture of polycrystalline samples is of particular concern. Thus, the present work focuses on texture analyses of NiMnGa polycrystals fabricated with two different techniques: directional solidification and plastic deformation by hot rolling. To examine the texture of coarse grained NiMnGa alloys in a statistically relevant way diffraction of synchrotron radiation and neutrons has been applied. The texture results are discussed with respect to the material, processing, phase transformations and MFIS. During cooling from high temperatures NiMnGa alloys experience different phase transitions, from B2 (cubic) via L21 (cubic, austenite) to martensitic structures (7M modulated, orthorhombic, space group Fmmm; 5M modulated and non-modulated, both tetragonal, space group I4mmm)3-5.
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