Properties of NiMnGa Alloys Ultra Rapidly Solidified by Suction Casting

Ni₂MnGa alloys are obtained by suction casting in water chilled cylindrical copper moulds 50 mm long and 1-4 mm in diameter; the microstructure and the magnetic properties are then investigated as functions of the cylinder diameter. The solidification substructures are characterized by scanning electron microscopy and confocal microscopy. The martensitic transformation temperatures, determined by magnetization vs. temperature measurements, are lower than those measured in bulk Ni₂MnGa single crystals (202 K). The demagnetization curves measured from saturation in the martensitic state show two steps: the first is at about+130 mT and the larger second one at relatively large inverse fields (about -300 mT), except in cylinders 4 mm in diameter, where these steps overlap at low inverse fields. These steps are likely to arise from a demagnetization mechanism involving field induced twin boundary motion in a few martensite variants,indicating a ferromagnetic shape memory effect in the cylinders.