Microstructure and micro-texture evolution in rapidly solidified melt-spun Ni50Mn28Ga22 ribbons

Abstract

Microstructure and texture play an important role in determining the magneto-mechanical properties of NiMnGa based Heusler alloys. The Ni50Mn28Ga22 melt-spun ribbons were prepared at wheel speeds of 1300 and 1600 RPM. The as-spun ribbons were annealed at different temperatures and for different time periods. X-ray phase analysis of the ribbons showed the presence of modulated structures, which was confirmed from the appearance of the three (202) peaks in almost all the 1300 RPM melt-spun and annealed ribbons. Phase analysis indicates that these twins (martensites) have both tetragonal and orthorhombic structures. Precipitates were observed along the grain boundaries and in the grain interior after annealing. Auger spectroscopy revealed that the precipitates were manganese-rich. TEM micrographs confirm that the annealed specimens have both 5 M and 7 M modulated structures, according to the satellite spots observed in the electron diffraction patterns. As-spun ribbons produced at 1300 and 1600 RPM had a predominantly 〈001〉 ‖ND fibre texture. 〈001〉 ‖ND fibre texture was also observed in the ribbons produced at 1300 RPM annealed at lower temperatures, such as 400 °C. 〈111〉 ‖ND texture was obtained in the melt-spun ribbons produced at 1300 RPM and annealed at 1000 °C for 5 h.