Hard magnetic properties of Fe–Co–Nd–Dy–B nanocrystalline alloys containing residual amorphous phase

Abstract

The glass transition and supercooled liquid region were observed in the composition range of 4 to 56 %Co, 2 to 4 %Nd, and 18 to 30 at%B in melt-spun Fe–Co–Nd–0.5 at%Dy–B amorphous alloys. The largest value of the supercooled liquid region defined by the difference between the glass transition temperature (Tg) and the crystallization temperature (Tx), ΔTx(=Tx−Tg), was 47 K for Fe67Co9.5Nd3Dy0.5B20 and the Tg and Tx are 798 and 845 K, respectively. The crystallized structure consists of Fe3B, Nd2Fe14B, α-Fe, and remaining amorphous phases in the annealing temperature up to about 903 K for 420 s and changes to Fe3B, Nd2Fe14B, and α-Fe phases in the higher temperature range. The grain sizes after annealing at 903 K are about 20 nm for Fe3B, 10 nm for Nd2Fe14B, 30 nm for α-Fe, and 5 nm for the remaining amorphous phase. The interparticle spacing of the Nd2Fe14B phase is less than 40 nm. The maximum energy product (BH)max was obtained for the alloy containing the residual amorphous phase subjected to annealing for 420 s at 903 K. The magnetization at a field of 1256 kA/m, remanence, intrinsic coercive force, and (BH)max are 1.60 T, 1.36 T, 227 kA/m, and 110 kJ/m3, respectively. The hard magnetic properties are interpreted to result from the exchange magnetic coupling among Nd2Fe14B, Fe3B, α-Fe, and remaining amorphous phases with ferromagnetism at room temperature. The formation of the finely mixed structure caused by the residual amorphous phase is concluded to result in the good hard magnetic properties with (BH)max above 100 kJ/m3 for the B-rich-type Fe–Co–Nd–Dy–B alloy.