A COMPARISON OF THE MAGNETIC-PROPERTIES AND DEFORMATION-BEHAVIOR OF ND-FE-B MAGNETS MADE FROM MELT-SPUN, MECHANICALLY ALLOYED AND HDDR POWDERS

Commercial melt-spun and HDDR Nd-Fe-B powders as well as mechanically alloyed Nd-Fe-B powders have been used for hot pressing and subsequent die upsetting. A comparison of all three types of magnets with respec t to their magnetic properties and deformation behaviour is discussed in this paper. The highest values of remanence, B-r = 1.24 T, and ener gy density, (BH)(max) = 283 kJ m(-3), found for the die-upset melt-spu n material can be explained in terms of its small Nd content of 14 at% and its high degree of texture connected with the strong shape anisot ropy of the deformed nanocrystalline grains. Fracture surfaces of this material have coarse grains at the former flake boundaries which resu lts in a coercivity, mu(oj)H(c), of only 1 T. An energy density of 268 kJ m(-3) has been measured for a die-upset mechanically alloyed mater ial. In spite of a Nd content of 16 at% in this alloy, a remanence of 1.2 T and a coercivity of 1.6 T have been attained. The large coercivi ty is due to (i) a very homogeneous distribution of the Nd-rich interg ranular phase, (ii) a grain size of only 100 nm x 500 nm observed afte r die upsetting and (iii) the fact that there is no formation of facet ed grains during hot deformation. The microstructure of HDDR magnets ( 14 at% Nd) has larger average grain sizes and some faceted grains. Con sequently these magnets have the smallest values of the degree of text ure, remanence, B-r = 1.1 T, and energy density, (BH)(max) = 207 kJ m( -3). By addition of further elements the coercivity of this material c ould be held at mu(oj)H(c) = 1.4 T After thermal demagnetization the t hree types of hot-deformed Nd-Fe-B magnets have a relatively large ini tial susceptibility, which is due to the presence of classical magneti c domains as well as interaction domains. For the investigated fine-gr ained Nd-Fe-B materials the deformation mechanism can be described by using a solution-precipitation creep model, governed by interface-reac tion-controlled creep.