Microstructure and its correlation to magnetic properties in 2:17 type (Sm,Gd)-Co-Fe-Cu-Zr alloys

Studies on the evolution of microstructure and phase transformation at various stages of thermal processing have been carried out in 2:17 type (Sm,Gd)-Co-Fe-Cu-Zr alloys. Microstructural studies reveal that during the solution heat treatment stage, a lamellar structure is formed and its volume fraction is found to be more at 1463 K for alloys without Gd, while at 1473 K for alloys with Gd = 25 wt% of Sm. It appears that the formation of the lamellar structure is closely associated with the phase transformation from Th₂Ni₁₇ → (TbCu₇ + Th₂Zn₁₇) structure during the solution heat treatment. Magnetic measurements in isothermally aged (1123 K) samples showed that the replacement of 25% of Sm by Gd reduces the energy product from 180 to 136 kJ/m³. The Gd substitution is also found to reduce the temperature coefficient of magnetisation from 350 to 225 ppm/°C in the temperature range of 30–100°C.