Abstract
In this work, the effects of sintering temperature, cooling rate after sintering and post-sintering annealing treatments on the microstructure and magnetic properties of a Nd 11.9MM 2.9Fe 73.9Co 3.3Ni 1.1B 6.9-type magnet have been investigated. It was found that the best magnetic properties were only obtained at a narrow range of sintering temperature at 1333–1353 K. Porosity was found to be minimized with increasing sintering temperature up to 1333 K. However, some grain growth occurred by increasing the sintering temperature from 1333 to 1353 K. Further increase of sintering temperature resulted in a significant decrease in the magnetic properties due to appearance of α-iron in the microstructure of the magnets and/or grain growth. The dependence of intrinsic coercivity ( H cJ) on the cooling rate (after sintering) and post-sintering annealing treatments has been related to electrical conductivity (EC) of the magnets; thus an increase in the EC resulted in a significant improvement in the H cJ values. This could be due to reduction of residual stresses within the specimens during slow cooling after sintering/post-sintering annealing treatments. The compatibility between the variations of the H cJ and EC with the cooling rates (after sintering) or a post-sintering annealing treatment at around 873 K confirms this mater.
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