Peculiarities of forming of magnetic hardening in sintered Nd15Fe76.2Ti1.0Al0.8B7 permanent magnet by ageing

Variations of the intrinsic coercivity (H-i(c)) with ageing (at 293 to 1173 K for 0.45 to 57.6 ks) of sintered Nd15Fe76.2Ti1.0Al0.8B7 have been invest igated. The tests indicated minimum and maximum of intrinsic coercivity (H- i(c)min and H-i(c)max) at temperatures 725 K and 1025 K, respectively. The observed behaviour of H-i(c) has been compared with microhardness measureme nts and microstructure observations (SEM, EDAX, domain structure (DS) and r oom temperature Mossbauer spectroscopy). SEM and DS investigations gave hin ts to a change in microstructure. Fitting of the Mossbauer spectra of perma nent magnets for different H-i(c) has shown that the magnetic hyperfine fie ld B-hf of 8j(2) and 16k(2) sites is reduced with increasing H-i(c) which s eems to be connected with the replacement of Fe atoms in their sites by Al atoms during ageing. Moreover the higher value of H-i(c) corresponds to the higher value of the line width. The H-i(c)max corresponds to the microhard ness H-V(max) for the Nd-2 Fe14B grain phase. The results suggest that the coercivity of investigated permanent magnet increases during ageing not onl y by further diffusion of Al into Nd2Fe14B phase, but also by better distri bution of elements in the main phase and especially in grain boundary phase s and removing the stress between the phases of the permanent magnet.