Temperature dependence of the crystal structures and phase fractions of secondary phases in a Nd-Fe-B sintered magnet

The constituent phases of a Nd14.2Fe79.4B6.3Cu0.1 sintered magnet and their crystal structures at ambient and elevated temperatures have been determined by synchrotron X-ray diffraction. At room temperature, dhcp-Nd, NdOx, hcp-Nd2O3, and Nd5Fe18B18 (Nd1.1Fe4B4) secondary phases have been identified in addition to the Nd2Fe14B main phase. The dhcp-Nd phase melts around 600 degrees C, corresponding to the eutectic temperature for the ternary Nd2Fe14B, Nd and Nd5Fe18B18 system. Below 600 degrees C, the variation of the lattice constants of the dhcp-Nd phase with temperature is strongly influenced by an internal stress from the spontaneous magnetostriction of the Nd2Fe14B phase. The difference of the lattice constants of the dhcp-Nd phase compared to those of pristine Nd metal is also significant, which is possibly associated with the inclusion of interstitial oxygen atoms at the 4f Wyckoff positions in the dhcp-Nd crystal. The validity of this scenario is confirmed by first-principles calculations of the equilibrium lattice constants of three-, four-, and nine-unit-cell supercells, in which different numbers of oxygen atoms quasi randomly occupy some of the unoccupied 4f-sites. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.