Phase boundary structure in Nd–Fe–B sintered magnets

The interface between tetragonal Nd2Fe14B phase and Nd-rich grain boundary phase in Nd–Fe–B sintered magnets was investigated with high resolution transmission electron microscope. Selected area diffraction patterns of the Nd-rich phase can be indexed by face-centered-cubic (fcc) structure with the lattice constant of a=0.548 nm. The orientation relationships between the two phases contacted directly with each other approximately agree with either of the following formulas: (110)tetra.//(110)fcc, [001]tetra.//[22̄3]fcc, or (110)tetra.//(010)fcc, [001]tetra.//[1̄02]fcc. These preferred orientations would be the results which minimize the phase boundary energy during postsintering heat treatment. In addition, it may change the local crystal field as well as the strain at the outermost Nd3+ ion site of the tetragonal grain both of which affect the coercivity of the magnet.