A comparison of the magnetic properties and deformation behaviour 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 respect to their magnetic properties and deformation behaviour is discussed in this paper. The highest values of remanence, B-r = 1.24 T, and energy density, (BH)(max) = 283 kJ m(-3), found for the die-upset melt-spun 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 anisotropy of the deformed nanocrystalline grains. Fracture surfaces of this material have coarse grains at the former flake boundaries which results 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 material. 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 coercivity is due to (i) a very homogeneous distribution of the Nd-rich intergranular phase, (ii) a grain size of only 100 nm x 500 nm observed after die upsetting and (iii) the fact that there is no formation of faceted grains during hot deformation. The microstructure of HDDR magnets (14 at% Nd) has larger average grain sizes and some faceted grains. Consequently these magnets have the smallest values of the degree of texture, 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 could be held at mu(oj)H(c) = 1.4 T After thermal demagnetization the three types of hot-deformed Nd-Fe-B magnets have a relatively large initial susceptibility, which is due to the presence of classical magnetic domains as well as interaction domains. For the investigated fine-grained Nd-Fe-B materials the deformation mechanism can be described by using a solution-precipitation creep model, governed by interface-reaction-controlled creep.