Synthesis of Fe–Co–Nd–B Amorphous Alloys with Glass Transition and their Crystallization-Induced Hard Magnetic Properties

Abstract

The glass transition followed by a supercooled liquid region was observed in the composition range of 5 to 57%Co, 2 to 4%Nd and 18 to 30 at%B in melt-spun Fe-Co-Nd-B amorphous alloys. The largest value of the supercooled liquid region defined by the difference between the glass transition temperature (T g ) and the crystallization temperature (T,), Δ T x (= T x - T g ), was 40 K for Fe 66.5 Co 10 Nd 3.5 B 20 and the T g and T x values are 802 K and 842 K, respectively. The crystallized structure consists of Fe 3 B, Nd 2 Fe 14 B, α-Fe and Nd 2 Fe 23 B 3 phases and their grain sizes after annealing for 420 s at 933 K are 20, 20, 70 and 10 nm, respectively. The interparticle spacing of the Nd 2 Fe 14 B phase are evaluated to be less than 50 nm for the 10%Co alloy. The magnetization at an applied field of 1256 kA/m (I 1256 ), remanence (B r ), intrinsic coercive force (iH c ) and maximum energy product (BH) max of the Fe 66.5 Co 10 Nd 3.5 B 20 alloy subjected to an optimum annealing treatment (933 K, 420 s) are 1.46 T, 1.25 T, 187 kA/m and 86 kJ/m 3 , respectively. The rather good hard magnetic properties are interpreted to result from the exchange magnetic coupling among Nd 2 Fe 14 B, Fe 3 B and α-Fe phases. The achievement of the rather good hard magnetic properties in the crystallized state of the Fe 66.5 Co 10 Nd 3.5 B 20 amorphous alloy with the large ΔT x of 40 K is expected to enable the future fabrication of a bulk hard magnetic material by the simple process of the formation of a bulk amorphous alloy followed by optimum crystallization.