CONICET | Buscador de Institutos y Recursos Humanos

Ni-Mn-Ga alloys are active and multifunctional materials which combine the properties of ferromagnetism with those of a reversible martensitic transformation. On cooling, they undergo a martensitic transformation from a ferromagnetic high-temperature cubic L21 phase to a low symmetry martensitic phase. Recoverable strains induced by magnetic fields, of up to 10%, have been reported for single crystals close to the stoichiometric composition, due to the rearrangement of martensite variants in the ferromagnetic martensitic phase. Magnetic-field-induced-strains (MFIS) found in polycrystalline alloys are remarkably smaller because of the randomly oriented grains and twin variants. Larger MFIS can be generated in coarse-grained, highly-textured, polycrystalline samples. In this sense, the suction casting rapid solidification technique is used in this investigation to produce Ni2MnGa bulk polycrystalline alloys in the form of cylinders of 1 and 2 mm in diameter, and tubes of 2 and 3 mm outer diameters and wall thickness of about 250 µm. A detailed study of the microstructure and crystallographic texture of the as-cast samples is presented.Rods and tubes are characterized by X-ray diffraction, confocal microscopy, electron backscattered diffraction and transmission electron microscopy. At room temperature all samples are in cubic L21 ordered austenitic phase. EBSD maps of rods? cross sections show small and equiaxed grains on the outer surface close to the mold, and toward the center, larger columnar grains grown in the radial direction. For the tubes, columnar grains are mainly observed. A larger population of small and equiaxed grains is found at the outer surface and for sample T2. A strong texture with the [100] direction parallel to the radial direction is observed, in complete agreement with X-ray diffraction results.