Microstructure, crystallization, and coercivity of rare earth-iron-boron amorphous alloy ribbons

Abstract

Amorphous alloys of rare earth-iron-boron develop high coercivity when crystallized around 650 °C to 700 °C. These alloys are located in the iron-rich corner of the ternary system, and the alloys contain 10 to 15 at. pct rare earth (R). In the amorphous state, isomorphous substitution of different rare earth atoms occurs. The short-range Fe-Fe and R-Fe environments in amorphous ribbons are similar to those in Fe3B and R6Fe23 (within 6 A), respectively. Beyond 6 A, the R-Fe environment appears similar to R2Fe14B. On nonisothermal heating of these alloy ribbons, a stress-relieving process occurs around 460 °C. The crystallization of α-Fe and R2Fe14B occurs at around 520 °C and near 600 °C, respectively. The Fe3B and R6Fe23 phases crystallize next. Depending on the alloy composition, the Fe3B crystallizes between 600 °C and 650 °C, and R6Fe23 crystallizes between 650 °C and 680 °C. The presence of rare earth atoms, 10 to 15 at. pct, significantly raises the crystallization temperatures of a-Fe and Fe3B. The favorable short-range Fe-Fe and R-Fe environments may be responsible for the nucleation and growth (crystallization) of Fe3B and R6Fe23 in the ternary alloys. The high coercivity of the annealed ribbons containing 10 at. pct Tb, Dy, and Ho is related to the single magnetic domain nature of the small crystallized grains of Fe3B and R6Fe23. For the annealed alloy ribbons with 12 to 15 at. pct rare earth, the high coercivity is related to the ternary hard phase, R2Fe14B.