Annealing effect on coercivity and microhardness in sintered (Nd, Tb)(15)(Fe, Co, Al)(78)B-7 magnets

The variation of the coercivity and microhardness with annealing (475-1150 degrees C/3.6 ks) of sintered (Nd, Tb)(15)(Fe, Co, Al)(78)B-7 permanent mag net have been investigated. The data indicate that the coercivity reaches t he mininum and the maximum at 600 degrees C and 775 degrees C, respectively . Optical and scanning electron microscopy and magnetic domain structure ob servations give no hints to any change in microstructure. Fitting of the Mo ssbauer spectra of permanent magnets for different coercivity (H-i(c)) valu es revealed that magnetic hyperfine fields (B-Hf) on all Fe sites are reduc ed with the increase in H-i(c) value and the higher value of the H-i(c) cor responds to the higher value of the Mossbauer line width. The microhardness changes in the matrix Nd2Fe14B phase have been correlated with changes in the coercivity.