Preparation of anisotropic NdFeB magnets with different Nd contents by hot deformation (die-upsetting) using hot-pressed HDDR powders

In this work HDDR powders were used for the preparation of highly densified anisotropic NdFeB magnets by the die-upsetting process. HDDR powders with different Nd contents (14 at %, 16 at % and 18 at %) and different grain sizes were used in order to determine the most favourable composition and microstructure for a production of crack-free magnets with maximum texture and optimum magnetic properties. The powders were hot compacted in vacuum at 725 degrees C and 150 MPa. Then the samples were hot deformed in an argon atmosphere at 750 degrees C with a strain rate of 10(-3) s(-1). Using HDDR powders with grain sizes around 0.3 mu m, corresponding to a coercivity mu(O1)H(c) of 1 T, a Nd content of 14 at % and a deformation degree of 0.9, a remanence of 1.2 T and a coercivity of 0.7 T were obtained. However a large amount of cracks were present in the die-upset sample with 14 at % Nd. By increasing the Nd content suppression of the cracks in the die-upset samples was achieved and an increase of the coercivity was observed, but the degree of texture and the remanence of the sample an reduced. For a Nd content of 16 at %, crack-free magnets with a coercivity of 0.95 T and a remanence of 1 T could be produced. (C) 1998 Elsevier Science S.A.