The effects of blending additions of copper and cobalt to Nd16Fe76B8 milled powder to produce sintered magnets

A blending process involving the mixing of powders of NdFeB and pure copper and pure cobalt has been developed. This process has been shown to be an effective and simple way of adding copper or cobalt and copper to the composition. This allows the composition and hence properties of the finished magnets to be adjusted subsequent to the casting and milling of the basic alloy. Additions of more than 0.4 at% Cu resulted in poor densification. The coercivities of the magnets containing Cu were very much dependent on the heat treatment. A treatment of 500 degreesC for 1 h followed by rapid cooling, yielded an excellent coercivity for 0.25at% addition of Cu; however, the same heat treatment decreased the coercivity significantly for an addition of 0.15 at%, Cu. Examination of the microstructure showed that Cu was affecting the grain size and nature of the grain boundary phases. A combined addition of 0.25 at% Cu and Co allowed Co to be added without excessive loss in coercivity. Increased Co content led to improved Curie temperature and remanence. The combined addition led to the formation of Nd3Co as the main grain boundary phase. The optimal heat treatment for magnets containing both Co and Cu was 900 degreesC for 10 h followed by 500 degreesC for 1 h and a rapid cool. However, the lower temperature treatment on its own also yielded excellent properties. (C) 2001 Elsevier Science B.V. All rights reserved.