Isotropic nanocrystalline Sm(Fe,Co)11.3Ti0.7 magnets modified with B and Zr

Rare-earth-lean Sm(Fe,Co,Ti)12 alloys with the ThMn12 crystal structure and less than one Ti atom per formula unit have the potential of exceptionally powerful permanent magnets, but all prior attempts to develop high coercivity in bulk alloys, especially coercivity combined with crystallographic texture, have fallen short of the expectations. This study was aimed at improvement of the currently best Sm(Fe,Co,Ti)12 magnets prepared through melt-spinning which are inherently isotropic. Modifications of the alloys with B and Zr, already demonstrated in earlier studies to be effective separately, have been implemented simultaneously. A systematic study of Sm1.1-x(Fe,Co)11.3-yTi0.7By alloys melt-spun at a tangential speed of 50 m/s and annealed at 600–950 °C allowed for monitoring the continuous evolution of the two consecutive crystal structures, those of the TbCu7 and ThMn12 types. Zirconium was found to facilitate the formation of the 1:12 structure at the expense of the 1:7, whereas boron has the opposite effect, at certain concentrations completely suppressing the 1:12. When the two alloying elements are introduced simultaneously, they inhibit growth of the 1:12 crystallites at annealing temperatures higher than 800 °C, thus allowing for the development of a higher coercivity. Because of instrumental limitations, bulk magnets were prepared through a two-step process – compaction of the melt-spun ribbons at 650 °C and additional treatment at a higher temperature – and they were characterized by a reduced, 90–93%, density. Nevertheless, an isotropic Sm0.9Zr0.2(Fe,Co)10.8Ti0.7B0.5 magnet exhibited fair values of the remanence (7.4 kG), maximum energy product (8.5 MGOe) and coercivity (5.4 kOe), as well as high Curie temperature of 525 °C and remarkably small temperature coefficient of the coercivity, −0.25%/°C.