Effects of post-sintering annealing on magnetic properties of Nd-Fe-B sintered magnets

Dependence of the coercive force of (Nd, Dy)-Fe-B magnets on annealing time, temperature and the order of annealing step were investigated. The two schemes of heat treatment were used: HT1 - cooling at 925 deg. C to 20 deg. C before annealing and HT2 - cooling at 925 deg. C to annealing temperature. The annealing temperature dependences of Hci for HT1 and HT2 are substantially different. These dependences reveal three phase's transformations in microstructure of magnets, which corresponded the maxima of Hci around 450 deg. C, 535 deg. C, 590 deg. C. The correlation between annealing behavior of Hci and hypothetic metastable Fe-Nd diagram was found. We supposed that below eutectic point at 640 deg. C (L[rlhar2]A+Nd) the series of consecutive eutectoid decompositions of A phase take place. The origin of new Fe-rich phases occurs preferentially at the surface of Nd2Fe14B grains. The size, morphology and number of initial precipitates are determined by temperature of annealing. At high temperature the separate large precipitates form ''roughness'' on the surfaces of Nd2Fe14B grains and decrease Hci. At low temperature continuous thin layers make 'smoothing' better and increase Hci