Thermodynamics of Oxygen in Molten Nd–Fe–B Alloy for Production of Low-Oxygen Nd–Fe–B Magnet

Abstract

Oxidation of the Nd-rich phase in Nd–Fe–B magnet alloys is known to deteriorate their coercivity. Therefore, it is important to prevent the dissolution of oxygen into the Nd–Fe–B magnet alloy during the manufacturing process. This requires the knowledge of the properties of oxygen in Nd–Fe–B magnet alloys, which has not been investigated yet. In this study, the oxygen solubility of Nd–Fe–B alloy at 1673 K was measured, and the effect of boron concentration on the affinity between oxygen and Nd–Fe–B alloy was analyzed. The standard Gibbs energy for the dissolution of oxygen into the molten Nd–Fe–B alloy at 1673 K was derived from experimental results, which indicated that the addition of boron into the molten Nd–Fe alloy decreased the affinity between oxygen and the alloy. The thermodynamic cause of this effect of boron is discussed, and a method to suppress oxygen dissolution into Nd–Fe–B alloy during the melt manufacturing process is suggested.

Fig. 3 Dependence of oxygen solubility of molten Nd–Fe–B alloys at 1673 K on boron concentration.