A correlation between intrinsic coercivity-electrical conductivity-thermaltreatment in a Nd11.9MM2.9Fe73.9Co3.3Ni1.1B6.9-type magnet

In this work, the effects of sintering temperature, cooling rate after sint ering and post-sintering annealing treatments on the microstructure and mag netic properties of a Nd11.9MM2.9Fe73.9Co3.3Ni1.1B6.9-type magnet have been investigated. It was found that the best magnetic properties were only obt ained at a narrow range of sintering temperature at 1333-1353 K. Porosity w as found to be minimized with increasing sintering temperature up to 1333 K . However, some grain growth occurred by increasing the sintering temperatu re from 1333 to 1353 K. Further increase of sintering temperature resulted in a significant decrease in the magnetic properties due to appearance of o r-iron in the microstructure of the magnets and/or grain growth. The depend ence of intrinsic coercivity (H-cJ) on the cooling rate (after sintering) a nd post-sintering annealing treatments has been related to electrical condu ctivity (EC) of the magnets; thus an increase in the EC resulted in a signi ficant improvement in the H-cJ values. This could be due to reduction of re sidual stresses within the specimens during slow cooling after sintering/po st-sintering annealing treatments. The compatibility between the variations of the H-cJ and EC with the cooling rates (after sintering) or a post-sint ering annealing treatment at around 873 K confirms this mater. (C) 2001 Els evier Science B.V. All rights reserved.