Whole process metallurgical behavior of the high-abundance rare-earth elements LRE (La, Ce and Y) and the magnetic performance of Nd0.75LRE0.25-Fe-B sintered magnets

Balanced usages of the high-abundance rare-earth elements, such as La, Ce and Y, have attracted tremendous interests of the industrial and scientific societies because of the economic and environmental merits. One of the key issues is of understanding the whole process metallurgical behavior of the high-abundance rare-earth elements in order to optimize the microstructures for a high magnetic performance of the RE-Fe-B sintered magnets, which contain a high concentration of the high-abundance rare-earth elements. In this work, we systematically characterized the metallurgical behaviors during the whole process of strip casting, sintering and annealing for the magnets (Nd0.75LRE0.25)30.5FebalAl0.1Cu0.1B1 (LRE = La, Ce and Y, wt. %). During the strip casting (SC), La and Ce mainly distribute in the grain boundary phase while Y distributes uniformly in the 2:14:1 matrix phase grains. Under the sintering and annealing, La is squeezed out from the 2:14:1 matrix phase to segregate in the grain boundaries, while Ce and Y migrate from the grain boundary to the 2:14:1 matrix phase. The crystalline structures of matrix phase and grain boundary phase of Nd-La/Ce/Y-Fe-B are tetragonal phase (space group P42/mnm) and hcp-Re2O3. The investigations of the grain boundary of Nd-La/Ce/Y-Fe-B magnets demonstrate that the grain boundary of the magnet with Ce is thicker than that of magnets with La and Y. The different metallurgical behaviors of elements La, Ce and Y are attributed to the different solidification temperature and substitution energy of Re2Fe14B phases. Core-shell grain structure was formed in the case of Y-containing magnets. Elemental analysis demonstrates that Y prefers to locate in the core of the grain boundary and Nd distributes in the shell of the grain boundary.