Microstructure and Hard Magnetic Properties of Sm1-xZrx(Fe,Co)11.3-yTi0.7By Ingots and Thick Melt-Spun Ribbons

Abstract

Revived interest on SmFe12-based compounds of the ThMn12 type as the permanent magnet materials has brought into focus the Sm1-xZrx(Fe,Co)11+δTi1-δ alloys remarkable by their high Curie temperature, large magnetization and strong magnetic anisotropy. However, attempts to develop practically useful coercivity in these alloys has encountered considerable difficulties. As most of the ongoing efforts rely on pulverization or on creation of a nanostructure in powders or melt-spun ribbons, relatively unexplored remain the massive cast alloys and "thick" ribbons melt-spun at a low solidification rate, even though they can be utilized as magnets without the need for costly compaction. In this study, alloys of Sm1.1-xZrx(Fe0.8Co0.2)11.3-yTi0.7By with 0 ≤ x ≤ 0.6 and 0 ≤ y ≤ 0.9 were subjected to structural, microscopic, thermomagnetic and isothermal magnetic characterization in samples immediately after arc-melting and after melt-spinning at a low wheel speed of 0.75–6.0 m/s. Slower solidification rates and Zr were found to generally promote the ThMn12-type crystal structure, whereas higher solidification rates and B tend to suppress the 1:12 structure in favor of the TbCu7-type one. When added simultaneously, B and Zr can alter the morphology of the 1:12 phase thus noticeably affecting the coercivity (Fig. 1). In particular, this alloying was found to generate in the arc-melted ingots a microstructure of isolated 1:12 crystallites 1–3 μm in size (Fig. 2); a better control of the minority phases, at least some of which are ferromagnetic, would be needed, however, to obtain in the alloy with such microstructure a Hc greater than 0.5–0.7 kOe. A moderately accelerated solidification can refine the 1:12 crystallites and increase the Hc; a Sm0.7Zr0.4(Fe,Co)10.8Ti0.7B0.5 alloy exhibited a Hc of 1.5 kOe and a maximum energy product (BH)max of 2.5 MGOe when it was melt-spun at 1.5 m/s into a 0.26-mm-thick ribbon. However, a more rapid solidification was found to suppress the 1:12 phase in favor of the magnetically softer 1:7 phase. After annealing at 800–900 °C, the alloys with the large B and Zr substitutions develop reasonably high Hc and (BH)max values even if melt-spun at the moderate wheel speed of 6 m/s. The respective values for the Sm0.7Zr0.4(Fe,Co)10.8Ti0.7B0.5 alloy were 4.1 kOe and 7.8 MGOe, whereas for a very Sm-lean Sm0.5Zr0.6(Fe,Co)10.6Ti0.7B0.7 alloy they were 3.6 kOe and 7.4 MGOe. In comparison, the parent Sm1.1(Fe,Co)11.3Ti0.7 alloy subjected to the same processing developed a Hc of only 0.4 kOe. The work was supported by the U.S. Department of Energy under Grant DE-FG02-90ER45413. **