MICROSTRUCTURE AND TEMPERATURE STABILITY OF SINTERED ND-DY-FE-CO-MO-AL-B MAGNETS

Simultaneous additions of Co and Mo or V, respectively, to Nd-Dy-Fe-Al-B alloys improve the temperature stability of sintered magnets. The intrinsic coercivity at 150-degrees-C amounts to 9 kA/cm for magnets with a Dy-content of 3 at.%. Hence magnets with a load line, B/mu(rectangle)H = - 2, may be operated at temperatures up to 200-degrees-C. The additions of Mo or of V result in the precipitation of a tetragonal Mo2FeB2 compound, a = 0.58 nm, c = 0.32 nm, or of a tetragonal V3-xFexB2 compound, x = 0.2...1.2, within the (Nd,Dy)2(Fe,Co,Al)14B grains. The dimensions of the precipitates range from 10 to 600 nm. Besides Mo2FeB2- or V3-xFexB2-grains, respectively, with dimensions up to 10 mum were observed within the Nd-rich constituents. The intrinsic coercivity, however, is determined by nucleation of reversed domains. The addition of Co results in a partial replacement of the Nd-Fe solid solution by the Nd3Co compound. Corrosion tests revealed, that the corrosion behaviour is not improved significantly and depends much more on the surface treatment.