The production and characterization of bonded, hot-pressed and die-upset HDDR magnets

Abstract

The recently developed hydrogenation, disproportionation, desorption and recombination (HDDR) process has now been extended to the production of hot-pressed isotropic magnets. These magnets have been produced from cast alloy with a composition Nd 16Fe 76B 8 and exhibit intrinsic coercivities of about 1200 kA m −1, remanences of around 690 mT and BH (max) values of approximately 90 kJ m −3. Subsequent die upsetting indicates that increased remanences can be obtained by using a suitable height reduction ratio. Careful studies of the microstructures of the isotropic hot-pressed magnets reveal that they are similar to the HDDR material in its powder form, i.e. they consist of Nd 2Fe 14B grains predominantly in the range 0.1⩽ × ⩽1.0 μm with a very small number of grains in the range 10 ⩽ × ⩽ 35 μm. These larger grains exhibit a very regular morphology and appear to be the result of a very rapid growth of some of the smaller submicron grains. The starting composition of the cast material ensures that the magnets have about 12% neodymium rich intergranular material and this low melting point constituent has been found to be of great assistance in the densification process. However, scanning and transmission electron microscopy studies show that the neodymium rich material is very coarsely distributed throughout the hot-pressed magnet and little evidence can be found for its presence between the grains of Nd 2Fe 14B. The addition of small amounts of zirconium to the basic Nd 16Fe 76B 8 alloy was investigated to determine its effect on coercivity and grain orientation. Highly oriented powder with good coercivity has been produced from material containing 0.1 at.% Zr (Nd 16Fe 76.9B 8Zr 0.1). Larger zirconium additions resulted in significant losses in coercivity. Bonded magnets with a remanence of about 150 mT, an intrinsic coercivity of 740 kA m −1 and an energy product of about 150 kJ m −3 have been produced from the anisotropic HDDR powder.