Surface tension measurement of Nd − Fe − B − X melts (X = O, Cu, Ga, In) as simulated substances of Nd-rich phase in Nd magnet

Abstract The microstructure of a neodymium (Nd)- iron (Fe)- boron (B) sintered magnet must be appropriately controlled in the sintering and annealing processes to improve the performance of the Nd magnet. In this study, the surface tension of molten Nd alloys containing copper (Cu), gallium (Ga), or indium (In) as a simulated substance of the Nd-rich phase was measured to investigate the formation mechanism of the microstructure from the perspective of physicochemical properties of the Nd-rich phase. The surface tension of the molten Nd–Fe alloy at the liquidus gradually decreased with an increase in Nd concentration, accompanied by a decrease in temperature. This indicates that lower temperatures are advantageous for the wetting-infiltration of the liquid Nd-rich phase into the interspace between the Nd2Fe14B grains in the magnet. While considering the dependence of the surface tension of the Nd–Fe–B ternary eutectic on the oxygen (O) concentration, no significant difference in the surface tension was determined in the investigated range of O concentrations (600–3000 mass ppm O). The surface tension of the ternary eutectic alloy did not significantly change with the substitution by Cu, which implies that the change in surface characteristics due to the addition of Cu is not the primary reason for the improvement in wetting of the Nd-rich phase. The surface tension of the ternary eutectic alloy increased with the substitution by Ga, but decreased with the substitution by In.