Influence of Cooling Rate on Constituent Phases and Distribution of Elements in (Nd,Dy)-Fe-B Magnet Alloys

Abstract

Substitution of Dy for Nd is a widely used method to improve the thermal stability of sintered NdFeB magnets. However, there are serious concerns over the stable supply of Dy due to its looming shortage and rising prices. Therefore, it is essential to develop (Nd,Dy)FeB magnets with reduced Dy content. In this study, we investigated the influence of casting conditions of Nd_10.85Dy_3.21Fe_80.12B_5.82 alloys prepared by mold casting and strip casting on the constituent phases and elemental distribution to improve the magnet coercivity. Microstructural observations revealed the formation of primary crystals (α-Fe) at all selected positions in the mold cast samples. In contrast, no primary crystals were found in the strip cast samples. To obtain sintered (Nd,Dy)FeB magnets with high coercivity, the optimum thickness of the starting alloy was found to be 0.3 mm. The melting behavior was studied in a temperature range from 773 K to 1173 K and the melting temperature increased after pre-heating at 973 K and 1073 K for the strip cast samples. These results indicate that cooling below 900 K during strip casting influenced the melting behavior of the alloy. Furthermore, it could affect the densification during sintering of the magnets.