Effect of wheel speed and subsequent annealing on the microstructure and magnetic properties of nanocomposite Pr2Fe14B/α-Fe magnets

Nanocomposite Pr2Fe14B/alpha-Fe magnets have been synthesized by melt-spinning a Pr8Fe86B6 alloy at low wheel speed in the range from 10 to 21.7 m/s. Microstructural and magnetic studies showed that there is an optimum wheel speed (about 16.7 m/s) at which a uniform Pr2Fe14B/alpha-Fe microstructure with fine alpha-Fe grains is developed directly from the melt. Lower speed gives larger grains for both the 2 : 14 : 1 and alpha-Fe while higher speed leads to the appearance of an amorphous phase which will result in a Pr2Fe14B/alpha-Fe structure with larger alpha-Fe grains after a subsequent crystallization annealing. Magnetic properties obtained under the optimum-quenching followed by a subsequent annealing are M-s = 176.6 emu/g, M-r = 118.2 emu/g, M-r/M-s = 0.67, H-c = 5.4 kOe and (BH)(m) = 12.6 MGOe. The coercivity and energy product are about 20% higher than those obtained by over-quenching and then annealing due to the refinement of alpha-Fe size which leads to an enhanced exchange coupling between Pr2Fe14B and alpha-Fe. The fine alpha-Fe development is associated with its solidification behavior characterized by higher nucleation rate and lower growth rate during the rapid solidification. (C) 1999 Elsevier Science B.V. All rights reserved.