Lattice strain and performance of sintered Nd-Fe-B magnets regulated by pressing mode and annealing process

In this paper, the lattice strain and internal stresses in sintered Nd-Fe-B magnets were explored. The in-situ X-ray diffraction results revealed the variation of lattice strain with the temperature, suggesting that the internal stresses caused by thermal expansion and lattice mismatch as well as the magnetostriction effect of 2:14:1 phase near the Curie temperature. The lattice strain fields of 2:14:1 phase were characterized by Kernel Average Misorientation based on Electron Back Scatter Diffraction as well as geometric phase analysis based on high resolution transmission electron microscopy. The lattice strain and internal stresses in magnets can be regulated by various pressing modes as well as the cooling rates of annealing. A discrete element method was utilized to simulate the density and stress distribution in Nd-Fe-B green compacts with rubber mold pressing and steel mold pressing, respectively. It was demonstrated that the lower density and more uniform distribution were achieved, which led to a lower stress level in rubber mold compact. It was further verified by experimental results that the lattice stresses in rubber mold pressing magnet were obviously lower than those in steel mold pressing magnet, meanwhile the bending strength of the former is improved. The lattice strain can be declined and the bending strength of magnet can be enhanced by slowing down the cooling rate of annealing. The research can help us to better understand and regulate the lattice strain and internal stresses, providing an insight of the development of high performance Nd-Fe-B magnets.