Rs. Mottram et al., The effects of blending additions of copper and cobalt to Nd16Fe76B8 milled powder to produce sintered magnets, J MAGN MAGN, 234(1), 2001, pp. 80-89
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 e
ffective and simple way of adding copper or cobalt and copper to the compos
ition. This allows the composition and hence properties of the finished mag
nets to be adjusted subsequent to the casting and milling of the basic allo
y. Additions of more than 0.4 at% Cu resulted in poor densification. The co
ercivities of the magnets containing Cu were very much dependent on the hea
t 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 s
ame heat treatment decreased the coercivity significantly for an addition o
f 0.15 at%, Cu. Examination of the microstructure showed that Cu was affect
ing the grain size and nature of the grain boundary phases. A combined addi
tion 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 rem
anence. The combined addition led to the formation of Nd3Co as the main gra
in boundary phase. The optimal heat treatment for magnets containing both C
o and Cu was 900 degreesC for 10 h followed by 500 degreesC for 1 h and a r
apid cool. However, the lower temperature treatment on its own also yielded
excellent properties. (C) 2001 Elsevier Science B.V. All rights reserved.