Microstructure of the NiMnGa Heusler Alloys Prepared by Suction Casting and Melt-Spinning

The microstructure of the Ni50Mn25Ga25 Heusler alloy (HA) rod cast to a copper mould and melt-spun ribbon was studied. According to X-ray diffraction analyses in both cases, the solidified alloy is a single-phase. In suction casting, the radial columnar crystals grow by cellular or dendritic growth, which leads to chemical inhomogeneity of the whole cast rod on the microscopic scale. The melt-spun ribbon exhibits a homogeneous alloy at the wheel side and an inhomogeneous alloy on the free surface side of the ribbon formed by cellular and dendritic growth. At both types of casting, the columnar crystals growth along the crystal direction. According to texture analyses by EBSD, the fine grain structure at the wheel side of the ribbon exhibits no texture, while the columnar grain structure on the free surface side exhibits the fibre texture with a declination by about 10 degrees in the spinning direction. Melt-spinning leads to significant refinement of the HA microstructure.