Microstructure and Hard Magnetic Properties of Sm_1-xZr_x(Fe,Co)_11.3-yTi_0.7B_y Ingots and Thick Melt-Spun Ribbons

Permanent magnets made from Sm(Fe,Co) ₁₂ -based compounds are being actively pursued through nanostructuring and powder metallurgy. This study was aimed at the development of hard magnetic properties in bulk as-cast alloys and in melt-spun alloys for very low wheel speeds. Slower solidification rates and alloying with Zr promote the tetragonal ThMn ₁₂ -type crystal structure, whereas higher solidification rates and alloying with B replace the ThMn ₁₂ structure type with the TbCu ₇ structure type. When introduced simultaneously, Zr and B dramatically reduce the alloy solidification rates required for both the refinement of the 1:12 crystallites and their replacement with the 1:7 phase. In bulk arc-melted alloys, this allowed for a microstructure of separated 1:12 crystallites 1– $3~\mu \text{m}$ in size, although, because of the ferromagnetic nature of a minority phase, the coercivity of these fine-grained alloys reached only 0.73 kOe. A moderately accelerated solidification further refined the 1:12 crystallites and increased the coercivity; a Sm _0.7 Zr _0.4 (Fe,Co) _10.8 Ti _0.7 B _0.5 alloy exhibited a coercivity of 1.5 kOe and a maximum energy product of 3.4 MGOe when it was melt-spun into a 0.26 mm-thick ribbon. A more rapid solidification suppressed the 1:12 phase, and after annealing at 800 °C–850 °C, the alloys modified with Zr and B developed reasonably high coercivity and maximum energy product even when melt-spun at a wheel speed of 6 m/s. For the above-mentioned alloy, these values were 4.1 kOe and 7.8 MGOe, respectively. A similarly processed very-Sm-lean Sm _0.5 Zr _0.6 (Fe,Co) _10.6 Ti _0.7 B _0.7 alloy exhibited a remanence of 8.8 kG and an energy product of 7.4 MGOe.