Crystallization kinetics of amorphous Nd_3.6Pr_5.4Fe₈₃Co₃B₅ and preparation of α-Fe/Nd₂Fe₁₄B nanocomposite magnets by controlled melt-solidification technique

Abstract

The crystallization kinetics of amorphous Nd_3.6Pr_5.4Fe₈₃Co₃B₅ and the preparation of α-Fe/Nd₂Fe₁₄ B nanocomposite magnets by controlled melt-solidification of Nd_3.6Pr_5.4Fe₈₃Co₃B₅ was investigated by employing DTA, XRD, and TEM. The results show that a metastable intermediate phase Nd₈Fe₂₇B₂₄ prior to α-Fe and Nd₂Fe₁₄B phases is crystallized as the amorphous Nd_3.6Pr_5.4Fe₈₃Co₃B₅ is heated to 1 223 K. The crystallization activation energy of α-Fe and Nd₈Fe₂₇B₂₄ phases is larger at the beginning stage of crystallization, and then it decreases with crystallized fraction x for the former and has little change when x is below 70% for the latter, which essentially results in an α-Fe/Nd₂Fe₁₄B microstructure with a relatively coarse grain size about 20–60 nm and a non-uniform distribution of grain size in the annealed alloy. The α-Fe/Nd₂Fe₁₄B nanocomposite magnets with a small average grain size about 14 nm and a quite uniform grain size distribution were prepared by controlled melt-solidification of Nd_3.6Pr_5.4Fe₈₃Co₃B₅ at a wheel speed of 20 m · s⁻¹ during melt-spinning. The magnets show a high maximum energy product of (BH)max=194 kJ · m⁻³, which is nearly twice of that of the nanocomposite magnets made by annealing the amorphous Nd_3.6Pr_5.4Fe₈₃Co₃B₅ precursor alloy.

Keywords

nanocomposite magnet / nanocrystalline alloy / amorphous alloy

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