Microstructure of high-remanence Nd-Fe-B alloys with low-rare-earth content

The microstructure and magnetic properties of bonded and fully dense magnets produced from melt-spun ribbons of the composition RE4.5TM76Ga1B18.5 have been investigated, where RE refers to a mixture of Nd and Dy, and TM refers to a mixture of Fe and Co. Results show that annealing overquenched ribbons with about 3 wt. % Dy and 3 wt. % Co at 700 °C can produce materials with Hci≂4 kOe, Br≂11.5 kG, and (BH)max≂14.5 MGOe, the latter being comparable to those of commercial ribbons containing three times as much Nd. The microstructure of annealed ribbons consists of about 30% Nd2Fe14B grains, 65% Fe3B grains, and 5% α-Fe grains. The grains of all the phases are nearly spherical. The grain diameters are in the 30–50 nm range. Hot-pressed magnets made from overquenched ribbons are fully isotropic, with magnetic and microstructural characteristics similar to those of the annealed ribbons. The high remanence and low coercivity correlate well with the microstructure consisting of a uniform mixture of the hard Nd2Fe14B phase and soft α-Fe and Fe3B phases.