Thermodynamics of Oxygen in Molten Nd–Pr–Fe–B Magnet Alloy at High Temperature

Neodymium magnets are widely used as high-performance magnets, but there is a problem that thermal demagnetization is relatively significant because of low Curie temperature. Prevention of oxidation for the Nd-rich phases in the grain boundary phase to which improve coercivity is required. Since oxygen dissolution occurs mainly during melting in alloy preparation process, it is necessary to know the affinity between the molten magnet alloy and oxygen. For the proper understanding of this process in practical use, neodymium magnets are made by replacing some of the Nd with Pr in order to cut costs, so the investigation of the system including Pr is required. In this study, Nd–Pr–Fe–B molten alloys and Nd2O3 crucibles were equilibrated in a sealed system, and oxygen solubility was investigated as a function of alloy compositions. It is confirmed that the affinity between the alloy and oxygen increases as the rare earth concentration increases. On the other hand, little effect of Pr ratio was found on the affinity between the molten Nd–Pr–Fe–B magnet and oxygen. It indicates that Nd and Pr have the similar effects on the affinity of the Nd–Pr–Fe–B magnet to oxygen. In addition, as a way to utilize the thermodynamic data obtained in this report, we did calculations regarding deoxidation to prevent oxidation of the Nd-rich phase.