An in situ neutron diffraction study of magnetic hardening in Fe3B/Nd2Fe14B nanocomposite magnets induced by rapid thermal annealing

Neutron diffraction patterns were acquired in situ from a melt-spun amorphous Fe74Nd5B18Cr3 alloy upon heating at constant rates of 1.7×10−2 and 1.3 K/s. The diffraction patterns revealed the following decomposition reaction with two different intermediate routes: amorphous→Fe3B/Nd2Fe23B3 (at 1.7×10−2 K/s) or Fe3B/Nd2Fe14B (at 1.3 K/s)→Fe/Fe2B/Nd1.1Fe4B4. This confirms that the changeover between the two ternary compounds in the metastable intermediate state is the direct cause of magnetic hardening induced by rapid heating. The effect of group IVA-VIA metals on the magnetic hardness of Fe77Nd5B18 was investigated and only the 3d transition-metal additives appeared to be effective in enhancing the coercivity. The heterogeneous nucleation of Nd2Fe14B on tetragonal-Fe3B precipitates promoted by the 3d transition-metal additives is proposed as a possible mechanism of magnetic hardening.